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Drivers API

Callback API vs Core API

The Callback API:

• Starts a transaction, executes the specified operations, and commits (or aborts on error).
• Automatically incorporates error handling logic for "TransientTransactionError" and "UnknownTransactionCommitResult".

The Core API:

• Requires explicit call to start the transaction and commit the transaction.
• Does not incorporate error handling logic for "TransientTransactionError" and "UnknownTransactionCommitResult", and instead provides the flexibility to incorporate custom error handling for these errors.

Callback API

The new callback API incorporates logic:

• To retry the transaction as a whole if the transaction encounters a "TransientTransactionError".
• To retry the commit operation if the commit encounters an "UnknownTransactionCommitResult".

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Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

copy

# For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
# uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
# For a sharded cluster, connect to the mongos instances; e.g.
# uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'

client = MongoClient(uriString)
wc_majority = WriteConcern("majority", wtimeout=1000)

# Prereq: Create collections.
client.get_database(
    "mydb1", write_concern=wc_majority).foo.insert_one({'abc': 0})
client.get_database(
    "mydb2", write_concern=wc_majority).bar.insert_one({'xyz': 0})

# Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.
def callback(session):
    collection_one = session.client.mydb1.foo
    collection_two = session.client.mydb2.bar

    # Important:: You must pass the session to the operations.
    collection_one.insert_one({'abc': 1}, session=session)
    collection_two.insert_one({'xyz': 999}, session=session)

# Step 2: Start a client session.
with client.start_session() as session:
    # Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).
    session.with_transaction(
        callback, read_concern=ReadConcern('local'),
        write_concern=wc_majority,
        read_preference=ReadPreference.PRIMARY)

Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

copy

/*
  For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
  String uri = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/admin?replicaSet=myRepl";
  For a sharded cluster, connect to the mongos instances; e.g.
  String uri = "mongodb://mongos0.example.com:27017,mongos1.example.com:27017:27017/admin";
 */

final MongoClient client = MongoClients.create(uri);

/*
    Create collections.
 */

client.getDatabase("mydb1").getCollection("foo")
        .withWriteConcern(WriteConcern.MAJORITY).insertOne( new Document("abc", 0));
client.getDatabase("mydb2").getCollection("bar")
        .withWriteConcern(WriteConcern.MAJORITY).insertOne( new Document("xyz", 0));

/* Step 1: Start a client session. */

final ClientSession clientSession = client.startSession();

/* Step 2: Optional. Define options to use for the transaction. */

TransactionOptions txnOptions = TransactionOptions.builder()
        .readPreference(ReadPreference.primary())
        .readConcern(ReadConcern.LOCAL)
        .writeConcern(WriteConcern.MAJORITY)
        .build();

/* Step 3: Define the sequence of operations to perform inside the transactions. */

TransactionBody txnBody = new TransactionBody<String>() {
    public String execute() {
        MongoCollection<Document> coll1 = client.getDatabase("mydb1").getCollection("foo");
        MongoCollection<Document> coll2 = client.getDatabase("mydb2").getCollection("bar");

        /*
           Important:: You must pass the session to the operations..
         */

        coll1.insertOne(clientSession, new Document("abc", 1));
        coll2.insertOne(clientSession, new Document("xyz", 999));

        return "Inserted into collections in different databases";
    }
};
try {
    /*
        Step 4: Use .withTransaction() to start a transaction,
        execute the callback, and commit (or abort on error).
     */

    clientSession.withTransaction(txnBody, txnOptions);
} catch (RuntimeException e) {
    // some error handling
} finally {
    clientSession.close();
}

Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

copy

  // For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
  // const uri = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
  // For a sharded cluster, connect to the mongos instances; e.g.
  // const uri = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'

  const client = new MongoClient(uri);
  await client.connect();

  // Prereq: Create collections.

  await client.db('mydb1').collection('foo').insertOne({ abc: 0 }, { w: 'majority' });

  await client.db('mydb2').collection('bar').insertOne({ xyz: 0 }, { w: 'majority' });

  // Step 1: Start a Client Session
  const session = client.startSession();

  // Step 2: Optional. Define options to use for the transaction
  const transactionOptions = {
    readPreference: 'primary',
    readConcern: { level: 'local' },
    writeConcern: { w: 'majority' }
  };

  // Step 3: Use withTransaction to start a transaction, execute the callback, and commit (or abort on error)
  // Note: The callback for withTransaction MUST be async and/or return a Promise.
  try {
    await session.withTransaction(async () => {
      const coll1 = client.db('mydb1').collection('foo');
      const coll2 = client.db('mydb2').collection('bar');

      // Important:: You must pass the session to the operations

      await coll1.insertOne({ abc: 1 }, { session });
      await coll2.insertOne({ xyz: 999 }, { session });
    }, transactionOptions);
  } finally {
    await session.endSession();
    await client.close();
  }

Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

copy

/*
 * For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
 * uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
 * For a sharded cluster, connect to the mongos instances; e.g.
 * uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'
 */

$client = new \MongoDB\Client($uriString);

// Prerequisite: Create collections.
$client->selectCollection(
    'mydb1',
    'foo',
    [
        'writeConcern' => new \MongoDB\Driver\WriteConcern(\MongoDB\Driver\WriteConcern::MAJORITY, 1000),
    ]
)->insertOne(['abc' => 0]);

$client->selectCollection(
    'mydb2',
    'bar',
    [
        'writeConcern' => new \MongoDB\Driver\WriteConcern(\MongoDB\Driver\WriteConcern::MAJORITY, 1000),
    ]
)->insertOne(['xyz' => 0]);

// Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.

$callback = function (\MongoDB\Driver\Session $session) use ($client) {
    $client
        ->selectCollection('mydb1', 'foo')
        ->insertOne(['abc' => 1], ['session' => $session]);

    $client
        ->selectCollection('mydb2', 'bar')
        ->insertOne(['xyz' => 999], ['session' => $session]);
};

// Step 2: Start a client session.

$session = $client->startSession();

// Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).

$transactionOptions = [
    'readConcern' => new \MongoDB\Driver\ReadConcern(\MongoDB\Driver\ReadConcern::LOCAL),
    'writeConcern' => new \MongoDB\Driver\WriteConcern(\MongoDB\Driver\WriteConcern::MAJORITY, 1000),
    'readPreference' => new \MongoDB\Driver\ReadPreference(\MongoDB\Driver\ReadPreference::RP_PRIMARY),
];

\MongoDB\with_transaction($session, $callback, $transactionOptions);

Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

copy

# For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
# uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
# For a sharded cluster, connect to the mongos instances; e.g.
# uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'

client = AsyncIOMotorClient(uriString)
wc_majority = WriteConcern("majority", wtimeout=1000)

# Prereq: Create collections.
await client.get_database(
    "mydb1", write_concern=wc_majority).foo.insert_one({'abc': 0})
await client.get_database(
    "mydb2", write_concern=wc_majority).bar.insert_one({'xyz': 0})

# Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.
async def callback(my_session):
    collection_one = my_session.client.mydb1.foo
    collection_two = my_session.client.mydb2.bar

    # Important:: You must pass the session to the operations.
    await collection_one.insert_one({'abc': 1}, session=my_session)
    await collection_two.insert_one({'xyz': 999}, session=my_session)

# Step 2: Start a client session.
async with await client.start_session() as session:
    # Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).
    await session.with_transaction(
        callback, read_concern=ReadConcern('local'),
        write_concern=wc_majority,
        read_preference=ReadPreference.PRIMARY)

Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

copy

static bool
with_transaction_example (bson_error_t *error)
{
   mongoc_client_t *client = NULL;
   mongoc_write_concern_t *wc = NULL;
   mongoc_read_concern_t *rc = NULL;
   mongoc_read_prefs_t *rp = NULL;
   mongoc_collection_t *coll = NULL;
   bool success = false;
   bool ret = false;
   bson_t *doc = NULL;
   bson_t *insert_opts = NULL;
   mongoc_client_session_t *session = NULL;
   mongoc_transaction_opt_t *txn_opts = NULL;

   /* For a replica set, include the replica set name and a seedlist of the
    * members in the URI string; e.g.
    * uri_repl = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:" \
    *    "27017/?replicaSet=myRepl";
    * client = mongoc_client_new (uri_repl);
    * For a sharded cluster, connect to the mongos instances; e.g.
    * uri_sharded =
    * "mongodb://mongos0.example.com:27017,mongos1.example.com:27017/";
    * client = mongoc_client_new (uri_sharded);
    */

   client = get_client ();

   /* Prereq: Create collections. */
   wc = mongoc_write_concern_new ();
   mongoc_write_concern_set_wmajority (wc, 1000);
   insert_opts = bson_new ();
   mongoc_write_concern_append (wc, insert_opts);
   coll = mongoc_client_get_collection (client, "mydb1", "foo");
   doc = BCON_NEW ("abc", BCON_INT32 (0));
   ret = mongoc_collection_insert_one (
      coll, doc, insert_opts, NULL /* reply */, error);
   if (!ret) {
      goto fail;
   }
   bson_destroy (doc);
   mongoc_collection_destroy (coll);
   coll = mongoc_client_get_collection (client, "mydb2", "bar");
   doc = BCON_NEW ("xyz", BCON_INT32 (0));
   ret = mongoc_collection_insert_one (
      coll, doc, insert_opts, NULL /* reply */, error);
   if (!ret) {
      goto fail;
   }

   /* Step 1: Start a client session. */
   session = mongoc_client_start_session (client, NULL /* opts */, error);
   if (!session) {
      goto fail;
   }

   /* Step 2: Optional. Define options to use for the transaction. */
   txn_opts = mongoc_transaction_opts_new ();
   rp = mongoc_read_prefs_new (MONGOC_READ_PRIMARY);
   rc = mongoc_read_concern_new ();
   mongoc_read_concern_set_level (rc, MONGOC_READ_CONCERN_LEVEL_LOCAL);
   mongoc_transaction_opts_set_read_prefs (txn_opts, rp);
   mongoc_transaction_opts_set_read_concern (txn_opts, rc);
   mongoc_transaction_opts_set_write_concern (txn_opts, wc);

   /* Step 3: Use mongoc_client_session_with_transaction to start a transaction,
    * execute the callback, and commit (or abort on error). */
   ret = mongoc_client_session_with_transaction (
      session, callback, txn_opts, NULL /* ctx */, NULL /* reply */, error);
   if (!ret) {
      goto fail;
   }

   success = true;
fail:
   bson_destroy (doc);
   mongoc_collection_destroy (coll);
   bson_destroy (insert_opts);
   mongoc_read_concern_destroy (rc);
   mongoc_read_prefs_destroy (rp);
   mongoc_write_concern_destroy (wc);
   mongoc_transaction_opts_destroy (txn_opts);
   mongoc_client_session_destroy (session);
   mongoc_client_destroy (client);
   return success;
}

/* Define the callback that specifies the sequence of operations to perform
 * inside the transactions. */
static bool
callback (mongoc_client_session_t *session,
          void *ctx,
          bson_t **reply,
          bson_error_t *error)
{
   mongoc_client_t *client = NULL;
   mongoc_collection_t *coll = NULL;
   bson_t *doc = NULL;
   bool success = false;
   bool ret = false;

   client = mongoc_client_session_get_client (session);
   coll = mongoc_client_get_collection (client, "mydb1", "foo");
   doc = BCON_NEW ("abc", BCON_INT32 (1));
   ret =
      mongoc_collection_insert_one (coll, doc, NULL /* opts */, *reply, error);
   if (!ret) {
      goto fail;
   }
   bson_destroy (doc);
   mongoc_collection_destroy (coll);
   coll = mongoc_client_get_collection (client, "mydb2", "bar");
   doc = BCON_NEW ("xyz", BCON_INT32 (999));
   ret =
      mongoc_collection_insert_one (coll, doc, NULL /* opts */, *reply, error);
   if (!ret) {
      goto fail;
   }

   success = true;
fail:
   mongoc_collection_destroy (coll);
   bson_destroy (doc);
   return success;
}

Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

copy

// The mongocxx::instance constructor and destructor initialize and shut down the driver,
// respectively. Therefore, a mongocxx::instance must be created before using the driver and
// must remain alive for as long as the driver is in use.
mongocxx::instance inst{};

// For a replica set, include the replica set name and a seedlist of the members in the URI
// string; e.g.
// uriString =
// 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
// For a sharded cluster, connect to the mongos instances; e.g.
// uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'
mongocxx::client client{mongocxx::uri{"mongodb://localhost/?replicaSet=replset"}};

write_concern wc_majority{};
wc_majority.acknowledge_level(write_concern::level::k_majority);

read_concern rc_local{};
rc_local.acknowledge_level(read_concern::level::k_local);

read_preference rp_primary{};
rp_primary.mode(read_preference::read_mode::k_primary);

// Prereq: Create collections.

auto foo = client["mydb1"]["foo"];
auto bar = client["mydb2"]["bar"];

try {
    options::insert opts;
    opts.write_concern(wc_majority);

    foo.insert_one(make_document(kvp("abc", 0)), opts);
    bar.insert_one(make_document(kvp("xyz", 0)), opts);
} catch (const mongocxx::exception& e) {
    std::cout << "An exception occurred while inserting: " << e.what() << std::endl;
    return EXIT_FAILURE;
}

// Step 1: Define the callback that specifies the sequence of operations to perform inside the
// transactions.
client_session::with_transaction_cb callback = [&](client_session* session) {
    // Important::  You must pass the session to the operations.
    foo.insert_one(*session, make_document(kvp("abc", 1)));
    bar.insert_one(*session, make_document(kvp("xyz", 999)));
};

// Step 2: Start a client session
auto session = client.start_session();

// Step 3: Use with_transaction to start a transaction, execute the callback,
// and commit (or abort on error).
try {
    options::transaction opts;
    opts.write_concern(wc_majority);
    opts.read_concern(rc_local);
    opts.read_preference(rp_primary);

    session.with_transaction(callback, opts);
} catch (const mongocxx::exception& e) {
    std::cout << "An exception occurred: " << e.what() << std::endl;
    return EXIT_FAILURE;
}

return EXIT_SUCCESS;

Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

copy

// For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
// string uri = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl";
// For a sharded cluster, connect to the mongos instances; e.g.
// string uri = "mongodb://mongos0.example.com:27017,mongos1.example.com:27017/";
var client = new MongoClient(connectionString);

// Prereq: Create collections.
var database1 = client.GetDatabase("mydb1");
var collection1 = database1.GetCollection<BsonDocument>("foo").WithWriteConcern(WriteConcern.WMajority);
collection1.InsertOne(new BsonDocument("abc", 0));

var database2 = client.GetDatabase("mydb2");
var collection2 = database2.GetCollection<BsonDocument>("bar").WithWriteConcern(WriteConcern.WMajority);
collection2.InsertOne(new BsonDocument("xyz", 0));

// Step 1: Start a client session.
using (var session = client.StartSession())
{
    // Step 2: Optional. Define options to use for the transaction.
    var transactionOptions = new TransactionOptions(
        readPreference: ReadPreference.Primary,
        readConcern: ReadConcern.Local,
        writeConcern: WriteConcern.WMajority);

    // Step 3: Define the sequence of operations to perform inside the transactions
    var cancellationToken = CancellationToken.None; // normally a real token would be used
    result = session.WithTransaction(
        (s, ct) =>
        {
            collection1.InsertOne(s, new BsonDocument("abc", 1), cancellationToken: ct);
            collection2.InsertOne(s, new BsonDocument("xyz", 999), cancellationToken: ct);
            return "Inserted into collections in different databases";
        },
        transactionOptions,
        cancellationToken);
}

Note

For the Perl driver, see the Core API usage example instead.

Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

copy

# For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
# uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
# For a sharded cluster, connect to the mongos instances; e.g.
# uri_string = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'

client = Mongo::Client.new(uri_string, write_concern: {w: :majority, wtimeout: 1000})

# Prereq: Create collections.

client.use('mydb1')['foo'].insert_one(abc: 0)
client.use('mydb2')['bar'].insert_one(xyz: 0)

# Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.

callback = Proc.new do |my_session|
  collection_one = client.use('mydb1')['foo']
  collection_two = client.use('mydb2')['bar']

  # Important: You must pass the session to the operations.

  collection_one.insert_one({'abc': 1}, session: my_session)
  collection_two.insert_one({'xyz': 999}, session: my_session)
end

#. Step 2: Start a client session.

session = client.start_session

# Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).

session.with_transaction(
  read_concern: {level: :local},
  write_concern: {w: :majority, wtimeout: 1000},
  read: {mode: :primary},
  &callback)

Note

For the Scala driver, see the Core API usage example instead.

The example uses the new callback API for working with transactions, which starts a transaction, executes the specified operations, and commits (or aborts on error). The new callback API incorporates retry logic for "TransientTransactionError" or "UnknownTransactionCommitResult" commit errors.

Important

• Recommended. Use the MongoDB driver updated for the version of your MongoDB deployment. For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
• When using the drivers, each operation in the transaction must be associated with the session (i.e. pass in the session to each operation).
• Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
• In MongoDB 4.2 and earlier, you cannot create collections in transactions. Write operations that result in document inserts (e.g. insert or update operations with upsert: true) must be on existing collections if run inside transactions.
• Starting in MongoDB 4.4, you can create collections in transactions implicitly or explicitly. You must use MongoDB drivers updated for 4.4, however. See Create Collections and Indexes In a Transaction for details.

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// WithTransactionExample is an example of using the Session.WithTransaction function.
func WithTransactionExample() {
	ctx := context.Background()
	// For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
	// uri := "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl"
	// For a sharded cluster, connect to the mongos instances; e.g.
	// uri := "mongodb://mongos0.example.com:27017,mongos1.example.com:27017/"
	var uri string

	clientOpts := options.Client().ApplyURI(uri)
	client, err := mongo.Connect(ctx, clientOpts)
	if err != nil {
		panic(err)
	}
	defer func() { _ = client.Disconnect(ctx) }()

	// Prereq: Create collections.
	wcMajority := writeconcern.New(writeconcern.WMajority(), writeconcern.WTimeout(1*time.Second))
	wcMajorityCollectionOpts := options.Collection().SetWriteConcern(wcMajority)
	fooColl := client.Database("mydb1").Collection("foo", wcMajorityCollectionOpts)
	barColl := client.Database("mydb1").Collection("bar", wcMajorityCollectionOpts)

	// Step 1: Define the callback that specifies the sequence of operations to perform inside the transaction.
	callback := func(sessCtx mongo.SessionContext) (interface{}, error) {
		// Important: You must pass sessCtx as the Context parameter to the operations for them to be executed in the
		// transaction.
		if _, err := fooColl.InsertOne(sessCtx, bson.D{{"abc", 1}}); err != nil {
			return nil, err
		}
		if _, err := barColl.InsertOne(sessCtx, bson.D{{"xyz", 999}}); err != nil {
			return nil, err
		}

		return nil, nil
	}

	// Step 2: Start a session and run the callback using WithTransaction.
	session, err := client.StartSession()
	if err != nil {
		panic(err)
	}
	defer session.EndSession(ctx)

	result, err := session.WithTransaction(ctx, callback)
	if err != nil {
		panic(err)
	}
	fmt.Printf("result: %v\n", result)
}

Core API

The core transaction API does not incorporate retry logic for errors labeled:

• "TransientTransactionError". If an operation in a transaction returns an error labeled "TransientTransactionError", the transaction as a whole can be retried.

  To handle "TransientTransactionError", applications should explicitly incorporate retry logic for the error.

• "UnknownTransactionCommitResult". If the commit returns an error labeled "UnknownTransactionCommitResult", the commit can be retried.

  To handle "UnknownTransactionCommitResult", applications should explicitly incorporate retry logic for the error.

The following example incorporates logic to retry the transaction for transient errors and retry the commit for unknown commit error:

• Python
• Java (Sync)
• Node.js
• PHP
• Motor
• Other
  • C
  • C++11
  • C#
  • Perl
  • Ruby
  • Scala
  • Go

Important

To associate read and write operations with a transaction, you must pass the session to each operation in the transaction.

copy

def run_transaction_with_retry(txn_func, session):
    while True:
        try:
            txn_func(session)  # performs transaction
            break
        except (ConnectionFailure, OperationFailure) as exc:
            # If transient error, retry the whole transaction
            if exc.has_error_label("TransientTransactionError"):
                print("TransientTransactionError, retrying "
                      "transaction ...")
                continue
            else:
                raise

def commit_with_retry(session):
    while True:
        try:
            # Commit uses write concern set at transaction start.
            session.commit_transaction()
            print("Transaction committed.")
            break
        except (ConnectionFailure, OperationFailure) as exc:
            # Can retry commit
            if exc.has_error_label("UnknownTransactionCommitResult"):
                print("UnknownTransactionCommitResult, retrying "
                      "commit operation ...")
                continue
            else:
                print("Error during commit ...")
                raise

# Updates two collections in a transactions

def update_employee_info(session):
    employees_coll = session.client.hr.employees
    events_coll = session.client.reporting.events

    with session.start_transaction(
            read_concern=ReadConcern("snapshot"),
            write_concern=WriteConcern(w="majority"),
            read_preference=ReadPreference.PRIMARY):
        employees_coll.update_one(
            {"employee": 3}, {"$set": {"status": "Inactive"}},
            session=session)
        events_coll.insert_one(
            {"employee": 3, "status": {
                "new": "Inactive", "old": "Active"}},
            session=session)

        commit_with_retry(session)

# Start a session.
with client.start_session() as session:
    try:
        run_transaction_with_retry(update_employee_info, session)
    except Exception as exc:
        # Do something with error.
        raise

Important

To associate read and write operations with a transaction, you must pass the session to each operation in the transaction.

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void runTransactionWithRetry(Runnable transactional) {
    while (true) {
        try {
            transactional.run();
            break;
        } catch (MongoException e) {
            System.out.println("Transaction aborted. Caught exception during transaction.");

            if (e.hasErrorLabel(MongoException.TRANSIENT_TRANSACTION_ERROR_LABEL)) {
                System.out.println("TransientTransactionError, aborting transaction and retrying ...");
                continue;
            } else {
                throw e;
            }
        }
    }
}

void commitWithRetry(ClientSession clientSession) {
    while (true) {
        try {
            clientSession.commitTransaction();
            System.out.println("Transaction committed");
            break;
        } catch (MongoException e) {
            // can retry commit
            if (e.hasErrorLabel(MongoException.UNKNOWN_TRANSACTION_COMMIT_RESULT_LABEL)) {
                System.out.println("UnknownTransactionCommitResult, retrying commit operation ...");
                continue;
            } else {
                System.out.println("Exception during commit ...");
                throw e;
            }
        }
    }
}

void updateEmployeeInfo() {

    MongoCollection<Document> employeesCollection = client.getDatabase("hr").getCollection("employees");
    MongoCollection<Document> eventsCollection = client.getDatabase("reporting").getCollection("events");

    TransactionOptions txnOptions = TransactionOptions.builder()
            .readPreference(ReadPreference.primary())
            .readConcern(ReadConcern.MAJORITY)
            .writeConcern(WriteConcern.MAJORITY)
            .build();

    try (ClientSession clientSession = client.startSession()) {
        clientSession.startTransaction(txnOptions);

        employeesCollection.updateOne(clientSession,
                Filters.eq("employee", 3),
                Updates.set("status", "Inactive"));
        eventsCollection.insertOne(clientSession,
                new Document("employee", 3).append("status", new Document("new", "Inactive").append("old", "Active")));

        commitWithRetry(clientSession);
    }
}


void updateEmployeeInfoWithRetry() {
    runTransactionWithRetry(this::updateEmployeeInfo);
}

Important

To associate read and write operations with a transaction, you must pass the session to each operation in the transaction.

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async function commitWithRetry(session) {
  try {
    await session.commitTransaction();
    console.log('Transaction committed.');
  } catch (error) {
    if (error.hasErrorLabel('UnknownTransactionCommitResult')) {
      console.log('UnknownTransactionCommitResult, retrying commit operation ...');
      await commitWithRetry(session);
    } else {
      console.log('Error during commit ...');
      throw error;
    }
  }
}

async function runTransactionWithRetry(txnFunc, client, session) {
  try {
    await txnFunc(client, session);
  } catch (error) {
    console.log('Transaction aborted. Caught exception during transaction.');

    // If transient error, retry the whole transaction
    if (error.hasErrorLabel('TransientTransactionError')) {
      console.log('TransientTransactionError, retrying transaction ...');
      await runTransactionWithRetry(txnFunc, client, session);
    } else {
      throw error;
    }
  }
}

async function updateEmployeeInfo(client, session) {
  session.startTransaction({
    readConcern: { level: 'snapshot' },
    writeConcern: { w: 'majority' },
    readPreference: 'primary'
  });

  const employeesCollection = client.db('hr').collection('employees');
  const eventsCollection = client.db('reporting').collection('events');

  await employeesCollection.updateOne(
    { employee: 3 },
    { $set: { status: 'Inactive' } },
    { session }
  );
  await eventsCollection.insertOne(
    {
      employee: 3,
      status: { new: 'Inactive', old: 'Active' }
    },
    { session }
  );

  try {
    await commitWithRetry(session);
  } catch (error) {
    await session.abortTransaction();
    throw error;
  }
}

return client.withSession(session =>
  runTransactionWithRetry(updateEmployeeInfo, client, session)
);

Important

To associate read and write operations with a transaction, you must pass the session to each operation in the transaction.

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private function runTransactionWithRetry3(callable $txnFunc, \MongoDB\Client $client, \MongoDB\Driver\Session $session)
{
    while (true) {
        try {
            $txnFunc($client, $session);  // performs transaction
            break;
        } catch (\MongoDB\Driver\Exception\CommandException $error) {
            $resultDoc = $error->getResultDocument();

            // If transient error, retry the whole transaction
            if (isset($resultDoc->errorLabels) && in_array('TransientTransactionError', $resultDoc->errorLabels)) {
                continue;
            } else {
                throw $error;
            }
        } catch (\MongoDB\Driver\Exception\Exception $error) {
            throw $error;
        }
    }
}

private function commitWithRetry3(\MongoDB\Driver\Session $session)
{
    while (true) {
        try {
            $session->commitTransaction();
            echo "Transaction committed.\n";
            break;
        } catch (\MongoDB\Driver\Exception\CommandException $error) {
            $resultDoc = $error->getResultDocument();

            if (isset($resultDoc->errorLabels) && in_array('UnknownTransactionCommitResult', $resultDoc->errorLabels)) {
                echo "UnknownTransactionCommitResult, retrying commit operation ...\n";
                continue;
            } else {
                echo "Error during commit ...\n";
                throw $error;
            }
        } catch (\MongoDB\Driver\Exception\Exception $error) {
            echo "Error during commit ...\n";
            throw $error;
        }
    }
}

private function updateEmployeeInfo3(\MongoDB\Client $client, \MongoDB\Driver\Session $session)
{
    $session->startTransaction([
        'readConcern' => new \MongoDB\Driver\ReadConcern("snapshot"),
        'readPrefernece' => new \MongoDB\Driver\ReadPreference(\MongoDB\Driver\ReadPreference::RP_PRIMARY),
        'writeConcern' => new \MongoDB\Driver\WriteConcern(\MongoDB\Driver\WriteConcern::MAJORITY),
    ]);

    try {
        $client->hr->employees->updateOne(
            ['employee' => 3],
            ['$set' => ['status' => 'Inactive']],
            ['session' => $session]
        );
        $client->reporting->events->insertOne(
            ['employee' => 3, 'status' => [ 'new' => 'Inactive', 'old' => 'Active']],
            ['session' => $session]
        );
    } catch (\MongoDB\Driver\Exception\Exception $error) {
        echo "Caught exception during transaction, aborting.\n";
        $session->abortTransaction();
        throw $error;
    }

    $this->commitWithRetry3($session);
}

private function doUpdateEmployeeInfo(\MongoDB\Client $client)
{
    // Start a session.
    $session = $client->startSession();

    try {
        $this->runTransactionWithRetry3([$this, 'updateEmployeeInfo3'], $client, $session);
    } catch (\MongoDB\Driver\Exception\Exception $error) {
        // Do something with error
    }
}

Note

For Motor, see the Callback API instead.

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/* takes a session, an out-param for server reply, and out-param for error. */
typedef bool (*txn_func_t) (mongoc_client_session_t *,
                            bson_t *,
                            bson_error_t *);


/* runs transactions with retry logic */
bool
run_transaction_with_retry (txn_func_t txn_func,
                            mongoc_client_session_t *cs,
                            bson_error_t *error)
{
   bson_t reply;
   bool r;

   while (true) {
      /* perform transaction */
      r = txn_func (cs, &reply, error);
      if (r) {
         /* success */
         bson_destroy (&reply);
         return true;
      }

      MONGOC_WARNING ("Transaction aborted: %s", error->message);
      if (mongoc_error_has_label (&reply, "TransientTransactionError")) {
         /* on transient error, retry the whole transaction */
         MONGOC_WARNING ("TransientTransactionError, retrying transaction...");
         bson_destroy (&reply);
      } else {
         /* non-transient error */
         break;
      }
   }

   bson_destroy (&reply);
   return false;
}


/* commit transactions with retry logic */
bool
commit_with_retry (mongoc_client_session_t *cs, bson_error_t *error)
{
   bson_t reply;
   bool r;

   while (true) {
      /* commit uses write concern set at transaction start, see
       * mongoc_transaction_opts_set_write_concern */
      r = mongoc_client_session_commit_transaction (cs, &reply, error);
      if (r) {
         MONGOC_INFO ("Transaction committed");
         break;
      }

      if (mongoc_error_has_label (&reply, "UnknownTransactionCommitResult")) {
         MONGOC_WARNING ("UnknownTransactionCommitResult, retrying commit ...");
         bson_destroy (&reply);
      } else {
         /* commit failed, cannot retry */
         break;
      }
   }

   bson_destroy (&reply);

   return r;
}


/* updates two collections in a transaction and calls commit_with_retry */
bool
update_employee_info (mongoc_client_session_t *cs,
                      bson_t *reply,
                      bson_error_t *error)
{
   mongoc_client_t *client;
   mongoc_collection_t *employees;
   mongoc_collection_t *events;
   mongoc_read_concern_t *rc;
   mongoc_write_concern_t *wc;
   mongoc_transaction_opt_t *txn_opts;
   bson_t opts = BSON_INITIALIZER;
   bson_t *filter = NULL;
   bson_t *update = NULL;
   bson_t *event = NULL;
   bool r;

   bson_init (reply);

   client = mongoc_client_session_get_client (cs);
   employees = mongoc_client_get_collection (client, "hr", "employees");
   events = mongoc_client_get_collection (client, "reporting", "events");

   rc = mongoc_read_concern_new ();
   mongoc_read_concern_set_level (rc, MONGOC_READ_CONCERN_LEVEL_SNAPSHOT);
   wc = mongoc_write_concern_new ();
   mongoc_write_concern_set_w (wc, MONGOC_WRITE_CONCERN_W_MAJORITY);
   txn_opts = mongoc_transaction_opts_new ();
   mongoc_transaction_opts_set_read_concern (txn_opts, rc);
   mongoc_transaction_opts_set_write_concern (txn_opts, wc);

   r = mongoc_client_session_start_transaction (cs, txn_opts, error);
   if (!r) {
      goto done;
   }

   r = mongoc_client_session_append (cs, &opts, error);
   if (!r) {
      goto done;
   }

   filter = BCON_NEW ("employee", BCON_INT32 (3));
   update = BCON_NEW ("$set", "{", "status", "Inactive", "}");
   /* mongoc_collection_update_one will reinitialize reply */
   bson_destroy (reply);
   r = mongoc_collection_update_one (
      employees, filter, update, &opts, reply, error);

   if (!r) {
      goto abort;
   }

   event = BCON_NEW ("employee", BCON_INT32 (3));
   BCON_APPEND (event, "status", "{", "new", "Inactive", "old", "Active", "}");

   bson_destroy (reply);
   r = mongoc_collection_insert_one (events, event, &opts, reply, error);
   if (!r) {
      goto abort;
   }

   r = commit_with_retry (cs, error);

abort:
   if (!r) {
      MONGOC_ERROR ("Aborting due to error in transaction: %s", error->message);
      mongoc_client_session_abort_transaction (cs, NULL);
   }

done:
   mongoc_collection_destroy (employees);
   mongoc_collection_destroy (events);
   mongoc_read_concern_destroy (rc);
   mongoc_write_concern_destroy (wc);
   mongoc_transaction_opts_destroy (txn_opts);
   bson_destroy (&opts);
   bson_destroy (filter);
   bson_destroy (update);
   bson_destroy (event);

   return r;
}


void
example_func (mongoc_client_t *client)
{
   mongoc_client_session_t *cs;
   bson_error_t error;
   bool r;

   cs = mongoc_client_start_session (client, NULL, &error);
   if (!cs) {
      MONGOC_ERROR ("Could not start session: %s", error.message);
      return;
   }

   r = run_transaction_with_retry (update_employee_info, cs, &error);
   if (!r) {
      MONGOC_ERROR ("Could not update employee, permanent error: %s",
                    error.message);
   }

   mongoc_client_session_destroy (cs);
}

copy

using transaction_func = std::function<void(client_session & session)>;
auto run_transaction_with_retry = [](transaction_func txn_func, client_session& session) {
    while (true) {
        try {
            txn_func(session);  // performs transaction.
            break;
        } catch (const operation_exception& oe) {
            std::cout << "Transaction aborted. Caught exception during transaction."
                      << std::endl;
            // If transient error, retry the whole transaction.
            if (oe.has_error_label("TransientTransactionError")) {
                std::cout << "TransientTransactionError, retrying transaction ..."
                          << std::endl;
                continue;
            } else {
                throw oe;
            }
        }
    }
};

auto commit_with_retry = [](client_session& session) {
    while (true) {
        try {
            session.commit_transaction();  // Uses write concern set at transaction start.
            std::cout << "Transaction committed." << std::endl;
            break;
        } catch (const operation_exception& oe) {
            // Can retry commit
            if (oe.has_error_label("UnknownTransactionCommitResult")) {
                std::cout << "UnknownTransactionCommitResult, retrying commit operation ..."
                          << std::endl;
                continue;
            } else {
                std::cout << "Error during commit ..." << std::endl;
                throw oe;
            }
        }
    }
};

// Updates two collections in a transaction
auto update_employee_info = [&](client_session& session) {
    auto& client = session.client();
    auto employees = client["hr"]["employees"];
    auto events = client["reporting"]["events"];

    options::transaction txn_opts;
    read_concern rc;
    rc.acknowledge_level(read_concern::level::k_snapshot);
    txn_opts.read_concern(rc);
    write_concern wc;
    wc.acknowledge_level(write_concern::level::k_majority);
    txn_opts.write_concern(wc);

    session.start_transaction(txn_opts);

    try {
        employees.update_one(
            make_document(kvp("employee", 3)),
            make_document(kvp("$set", make_document(kvp("status", "Inactive")))));
        events.insert_one(make_document(
            kvp("employee", 3),
            kvp("status", make_document(kvp("new", "Inactive"), kvp("old", "Active")))));
    } catch (const operation_exception& oe) {
        std::cout << "Caught exception during transaction, aborting." << std::endl;
        session.abort_transaction();
        throw oe;
    }

    commit_with_retry(session);
};

auto session = client.start_session();
try {
    run_transaction_with_retry(update_employee_info, session);
} catch (const operation_exception& oe) {
    // Do something with error.
    throw oe;
}

copy

public void RunTransactionWithRetry(Action<IMongoClient, IClientSessionHandle> txnFunc, IMongoClient client, IClientSessionHandle session)
{
    while (true)
    {
        try
        {
            txnFunc(client, session); // performs transaction
            break;
        }
        catch (MongoException exception)
        {
            // if transient error, retry the whole transaction
            if (exception.HasErrorLabel("TransientTransactionError"))
            {
                Console.WriteLine("TransientTransactionError, retrying transaction.");
                continue;
            }
            else
            {
                throw;
            }
        }
    }
}

public void CommitWithRetry(IClientSessionHandle session)
{
    while (true)
    {
        try
        {
            session.CommitTransaction();
            Console.WriteLine("Transaction committed.");
            break;
        }
        catch (MongoException exception)
        {
            // can retry commit
            if (exception.HasErrorLabel("UnknownTransactionCommitResult"))
            {
                Console.WriteLine("UnknownTransactionCommitResult, retrying commit operation");
                continue;
            }
            else
            {
                Console.WriteLine($"Error during commit: {exception.Message}.");
                throw;
            }
        }
    }
}

// updates two collections in a transaction
public void UpdateEmployeeInfo(IMongoClient client, IClientSessionHandle session)
{
    var employeesCollection = client.GetDatabase("hr").GetCollection<BsonDocument>("employees");
    var eventsCollection = client.GetDatabase("reporting").GetCollection<BsonDocument>("events");

    session.StartTransaction(new TransactionOptions(
        readConcern: ReadConcern.Snapshot,
        writeConcern: WriteConcern.WMajority,
        readPreference: ReadPreference.Primary));

    try
    {
        employeesCollection.UpdateOne(
            session,
            Builders<BsonDocument>.Filter.Eq("employee", 3),
            Builders<BsonDocument>.Update.Set("status", "Inactive"));
        eventsCollection.InsertOne(
            session,
            new BsonDocument
            {
                { "employee", 3 },
                { "status", new BsonDocument { { "new", "Inactive" }, { "old", "Active" } } }
            });
    }
    catch (Exception exception)
    {
        Console.WriteLine($"Caught exception during transaction, aborting: {exception.Message}.");
        session.AbortTransaction();
        throw;
    }

    CommitWithRetry(session);
}

public void UpdateEmployeeInfoWithTransactionRetry(IMongoClient client)
{
    // start a session
    using (var session = client.StartSession())
    {
        try
        {
            RunTransactionWithRetry(UpdateEmployeeInfo, client, session);
        }
        catch (Exception exception)
        {
            // do something with error
            Console.WriteLine($"Non transient exception caught during transaction: ${exception.Message}.");
        }
    }
}

Important

To associate read and write operations with a transaction, you must pass the session to each operation in the transaction.

copy

sub runTransactionWithRetry {
    my ( $txnFunc, $session ) = @_;

    LOOP: {
        eval {
            $txnFunc->($session); # performs transaction
        };
        if ( my $error = $@ ) {
            print("Transaction aborted-> Caught exception during transaction.\n");
            # If transient error, retry the whole transaction
            if ( $error->has_error_label("TransientTransactionError") ) {
                print("TransientTransactionError, retrying transaction ->..\n");
                redo LOOP;
            }
            else {
                die $error;
            }
        }
    }

    return;
}

sub commitWithRetry {
    my ($session) = @_;

    LOOP: {
        eval {
            $session->commit_transaction(); # Uses write concern set at transaction start.
            print("Transaction committed->\n");
        };
        if ( my $error = $@ ) {
            # Can retry commit
            if ( $error->has_error_label("UnknownTransactionCommitResult") ) {
                print("UnknownTransactionCommitResult, retrying commit operation ->..\n");
                redo LOOP;
            }
            else {
                print("Error during commit ->..\n");
                die $error;
            }
        }
    }

    return;
}

# Updates two collections in a transactions

sub updateEmployeeInfo {
    my ($session) = @_;
    my $employeesCollection = $session->client->ns("hr.employees");
    my $eventsCollection    = $session->client->ns("reporting.events");

    $session->start_transaction(
        {
            readConcern  => { level => "snapshot" },
            writeConcern => { w     => "majority" },
            readPreference => 'primary',
        }
    );

    eval {
        $employeesCollection->update_one(
            { employee => 3 }, { '$set' => { status => "Inactive" } },
            { session => $session},
        );
        $eventsCollection->insert_one(
            { employee => 3, status => { new => "Inactive", old => "Active" } },
            { session => $session},
        );
    };
    if ( my $error = $@ ) {
        print("Caught exception during transaction, aborting->\n");
        $session->abort_transaction();
        die $error;
    }

    commitWithRetry($session);
}

# Start a session
my $session = $client->start_session();

eval {
    runTransactionWithRetry(\&updateEmployeeInfo, $session);
};
if ( my $error = $@ ) {
    # Do something with error
}

$session->end_session();

Important

To associate read and write operations with a transaction, you must pass the session to each operation in the transaction.

copy

def run_transaction_with_retry(session)
  begin
    yield session # performs transaction
  rescue Mongo::Error => e
    puts 'Transaction aborted. Caught exception during transaction.'
    raise unless e.label?('TransientTransactionError')
    puts "TransientTransactionError, retrying transaction ..."
    retry
  end
end

def commit_with_retry(session)
  begin
    session.commit_transaction
    puts 'Transaction committed.'
  rescue Mongo::Error => e
    if e.label?('UnknownTransactionCommitResult')
      puts "UnknownTransactionCommitResult, retrying commit operation ..."
      retry
    else
      puts 'Error during commit ...'
      raise
    end
  end
end

# updates two collections in a transaction

def update_employee_info(session)
  employees_coll = session.client.use(:hr)[:employees]
  events_coll = session.client.use(:reporting)[:events]

  session.start_transaction(read_concern: { level: :snapshot },
                            write_concern: { w: :majority },
                            read: {mode: :primary})
  employees_coll.update_one({ employee: 3 }, { '$set' => { status: 'Inactive'} },
                            session: session)
  events_coll.insert_one({ employee: 3, status: { new: 'Inactive', old: 'Active' } },
                         session: session)
  commit_with_retry(session)
end

session = client.start_session

begin
  run_transaction_with_retry(session) do
    update_employee_info(session)
  end
rescue StandardError => e
  # Do something with error
  raise
end

Important

To associate read and write operations with a transaction, you must pass the session to each operation in the transaction.

copy

  def updateEmployeeInfo(database: MongoDatabase, observable: SingleObservable[ClientSession]): SingleObservable[ClientSession] = {
    observable.map(clientSession => {
      val employeesCollection = database.getCollection("employees")
      val eventsCollection = database.getCollection("events")

      val transactionOptions = TransactionOptions.builder()
        .readPreference(ReadPreference.primary())
        .readConcern(ReadConcern.SNAPSHOT)
        .writeConcern(WriteConcern.MAJORITY)
        .build()
      clientSession.startTransaction(transactionOptions)
      employeesCollection.updateOne(clientSession, Filters.eq("employee", 3), Updates.set("status", "Inactive"))
        .subscribe((res: UpdateResult) => println(res))
      eventsCollection.insertOne(clientSession, Document("employee" -> 3, "status" -> Document("new" -> "Inactive", "old" -> "Active")))
        .subscribe((res: Completed) => println(res))

      clientSession
    })
  }

  def commitAndRetry(observable: SingleObservable[Completed]): SingleObservable[Completed] = {
    observable.recoverWith({
      case e: MongoException if e.hasErrorLabel(MongoException.UNKNOWN_TRANSACTION_COMMIT_RESULT_LABEL) => {
        println("UnknownTransactionCommitResult, retrying commit operation ...")
        commitAndRetry(observable)
      }
      case e: Exception => {
        println(s"Exception during commit ...: $e")
        throw e
      }
    })
  }

  def runTransactionAndRetry(observable: SingleObservable[Completed]): SingleObservable[Completed] = {
    observable.recoverWith({
      case e: MongoException if e.hasErrorLabel(MongoException.TRANSIENT_TRANSACTION_ERROR_LABEL) => {
        println("TransientTransactionError, aborting transaction and retrying ...")
        runTransactionAndRetry(observable)
      }
    })
  }

  def updateEmployeeInfoWithRetry(client: MongoClient): SingleObservable[Completed] = {

    val database = client.getDatabase("hr")
    val updateEmployeeInfoObservable: Observable[ClientSession] = updateEmployeeInfo(database, client.startSession())
    val commitTransactionObservable: SingleObservable[Completed] =
      updateEmployeeInfoObservable.flatMap(clientSession => clientSession.commitTransaction())
    val commitAndRetryObservable: SingleObservable[Completed] = commitAndRetry(commitTransactionObservable)

    runTransactionAndRetry(commitAndRetryObservable)
  }
}

copy

	runTransactionWithRetry := func(sctx mongo.SessionContext, txnFn func(mongo.SessionContext) error) error {
		for {
			err := txnFn(sctx) // Performs transaction.
			if err == nil {
				return nil
			}

			log.Println("Transaction aborted. Caught exception during transaction.")

			// If transient error, retry the whole transaction
			if cmdErr, ok := err.(mongo.CommandError); ok && cmdErr.HasErrorLabel("TransientTransactionError") {
				log.Println("TransientTransactionError, retrying transaction...")
				continue
			}
			return err
		}
	}

	commitWithRetry := func(sctx mongo.SessionContext) error {
		for {
			err := sctx.CommitTransaction(sctx)
			switch e := err.(type) {
			case nil:
				log.Println("Transaction committed.")
				return nil
			case mongo.CommandError:
				// Can retry commit
				if e.HasErrorLabel("UnknownTransactionCommitResult") {
					log.Println("UnknownTransactionCommitResult, retrying commit operation...")
					continue
				}
				log.Println("Error during commit...")
				return e
			default:
				log.Println("Error during commit...")
				return e
			}
		}
	}

	// Updates two collections in a transaction.
	updateEmployeeInfo := func(sctx mongo.SessionContext) error {
		employees := client.Database("hr").Collection("employees")
		events := client.Database("reporting").Collection("events")

		err := sctx.StartTransaction(options.Transaction().
			SetReadConcern(readconcern.Snapshot()).
			SetWriteConcern(writeconcern.New(writeconcern.WMajority())),
		)
		if err != nil {
			return err
		}

		_, err = employees.UpdateOne(sctx, bson.D{{"employee", 3}}, bson.D{{"$set", bson.D{{"status", "Inactive"}}}})
		if err != nil {
			sctx.AbortTransaction(sctx)
			log.Println("caught exception during transaction, aborting.")
			return err
		}
		_, err = events.InsertOne(sctx, bson.D{{"employee", 3}, {"status", bson.D{{"new", "Inactive"}, {"old", "Active"}}}})
		if err != nil {
			sctx.AbortTransaction(sctx)
			log.Println("caught exception during transaction, aborting.")
			return err
		}

		return commitWithRetry(sctx)
	}

	return client.UseSessionWithOptions(
		ctx, options.Session().SetDefaultReadPreference(readpref.Primary()),
		func(sctx mongo.SessionContext) error {
			return runTransactionWithRetry(sctx, updateEmployeeInfo)
		},
	)
}

Driver Versions

For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2:

C 1.15.0 C# 2.9.0 Go 1.1

Java 3.11.0 Node 3.3.0 Perl 2.2.0

Python 3.9.0 Ruby 2.10.0 Scala 2.7.0

For transactions on MongoDB 4.0 replica sets, clients require MongoDB drivers updated for MongoDB 4.0 or later.

Java 3.8.0 Python 3.7.0 C 1.11.0

C# 2.7 Node 3.1.0 Ruby 2.6.0

Perl 2.0.0 PHP (PHPC) 1.5.0 Scala 2.4.0

Transaction Error Handling

Regardless of the database system, whether MongoDB or relational databases, applications should take measures to handle errors during transaction commits and incorporate retry logic for transactions.

"TransientTransactionError"

The individual write operations inside the transaction are not retryable, regardless of the value of retryWrites. If an operation encounters an error associated with the label "TransientTransactionError", such as when the primary steps down, the transaction as a whole can be retried.

• The callback API incorporates retry logic for "TransientTransactionError".
• The core transaction API does not incorporate retry logic for "TransientTransactionError". To handle "TransientTransactionError", applications should explicitly incorporate retry logic for the error.

"UnknownTransactionCommitResult"

The commit operations are retryable write operations. If the commit operation encounters an error, MongoDB drivers retry the commit regardless of the value of retryWrites.

If the commit operation encounters an error labeled "UnknownTransactionCommitResult", the commit can be retried.

• The callback API incorporates retry logic for "UnknownTransactionCommitResult".
• The core transaction API does not incorporate retry logic for "UnknownTransactionCommitResult". To handle "UnknownTransactionCommitResult", applications should explicitly incorporate retry logic for the error.

Driver Version Errors

On sharded clusters with multiple mongos instances, performing transactions with drivers updated for MongoDB 4.0 (instead of MongoDB 4.2) will fail and can result in errors, including:

Note

Your driver may return a different error. Refer to your driver’s documentation for details.

Error Code	Error Message
251	cannot continue txnId -1 for session ... with txnId 1
50940	cannot commit with no participants

For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), use the MongoDB drivers updated for MongoDB 4.2

Additional Information

mongo Shell Example

The following mongo shell methods are available for transactions:

• Session.startTransaction()
• Session.commitTransaction()
• Session.abortTransaction()

Note

The mongo shell example omits retry logic and robust error handling for simplicity’s sake. For a more practical example of incorporating transactions in applications, see Transaction Error Handling instead.

// Create collections:
db.getSiblingDB("mydb1").foo.insert( {abc: 0}, { writeConcern: { w: "majority", wtimeout: 2000 } } );
db.getSiblingDB("mydb2").bar.insert( {xyz: 0}, { writeConcern: { w: "majority", wtimeout: 2000 } } );

// Start a session.
session = db.getMongo().startSession( { readPreference: { mode: "primary" } } );

coll1 = session.getDatabase("mydb1").foo;
coll2 = session.getDatabase("mydb2").bar;

// Start a transaction
session.startTransaction( { readConcern: { level: "local" }, writeConcern: { w: "majority" } } );

// Operations inside the transaction
try {
   coll1.insertOne( { abc: 1 } );
   coll2.insertOne( { xyz: 999 } );
} catch (error) {
   // Abort transaction on error
   session.abortTransaction();
   throw error;
}

// Commit the transaction using write concern set at transaction start
session.commitTransaction();

session.endSession();

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