dbHash¶

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Definition
Behavior
Return Document
Examples

Definition¶

dbHash¶

Returns the hash values of the collections in a database and an MD5 value for these collections. dbHash is useful to compare databases across mongod instances, such as across members of replica sets.

dbHash has the following syntax:

copy

db.runCommand ( { dbHash: 1, collections: [ <collection1>, ... ] } )

Field Type Description

dbHash Any type The command to run. Specify any value.

collections

array

Optional. An array of collection names.

Either specify the collections for which to return the hash values, or omit or specify an empty array to return the hash values for all collections in the database.

Behavior¶

Non-Existent Collection¶

If a collection in the collections array is non-existent, dbHash does not return a hash value for that collection.

Restrictions¶

MongoDB drivers automatically set afterClusterTime for operations associated with causally consistent sessions. Starting in MongoDB 4.2, the dbHash command no longer support afterClusterTime. As such, dbHash cannot be associated with causally consistent sessions.

Return Document¶

The command returns a document with the following fields:

Field	Description
host	The host and port of the `mongod` instance on which the command is run.
`collections`	A document with the collections and their corresponding hash values. copy { <collection1>: <hash1>, <collection2>: <hash2>, ... }
`capped`	An array that lists the capped collections. New in version 4.0.
`uuids`	A document with the collections and their corresponding UUID values. copy { <collection1>: <UUID1>, <collection2>: <UUID2>, ... } New in version 4.0.
`md5`	The aggregate hash value for these collections.
`timeMillis`	Number of milliseconds to generate the hash.
`ok` `operationTime` `$clusterTime`	Returned with every command. See also Response for details.

Examples¶

Return Hash Values for All Collections in a Database¶

The following example returns the hash value for all collections in the database test:

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use test
db.runCommand( { dbHash: 1 } )

The operation returns the following document:

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{
   "host" : "myHostName.local:27017",
   "collections" : {
      "foo" : "d27b769230edc551d869060ec3fb68bd",
      "inventory" : "ec3d821581ea1bd3aa8196c94b946874",
      "log" : "d41d8cd98f00b204e9800998ecf8427e",
      "orders" : "0242c0a128c284ea9576a34db2306c12",
      "restaurants" : "5dc9b88091c36f0d529567b5b6e3fc92",
      "zipcodes" : "31ede812bf397509a87359c65bf2a08c"
   },
   "capped" : [
      "log"
   ],
   "uuids" : {
      "foo" : UUID("469592fe-3bfe-425e-975f-cedbe0c4741d"),
      "inventory" : UUID("0830e0ad-cc24-4fc7-80d0-8e22fe45e382"),
      "log" : UUID("4be024ff-711b-4ab8-836b-dee662e090f1"),
      "orders" : UUID("755be489-745f-400c-ac3b-f27525ad0108"),
      "restaurants" : UUID("520b56ec-3276-4904-b6e5-286bc9bfa648"),
      "zipcodes" : UUID("12e97b70-c174-40af-a178-5d83a241fe20")
   },
   "md5" : "0cb7417ae9d9eb865000b4debdc671da",
   "timeMillis" : 53,
   "ok" : 1,
   "operationTime" : Timestamp(1529208582, 4),
   "$clusterTime" : {
      "clusterTime" : Timestamp(1529208582, 4),
      "signature" : {
         "hash" : BinData(0,"X3MmevDqUgCVvN1AhnT+fiOL/Lc="),
         "keyId" : NumberLong("6567898567824900097")
      }
   }
}

Return Hash Values for Specified Collections in a Database¶

The following example returns the hash value for the collections inventory and orders in the database test:

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use test
db.runCommand( { dbHash: 1, collections: [ "inventory", "orders" ] } )

The operation returns the following document:

copy

{
   "host" : "myHostName.local:27017",
   "collections" : {
      "inventory" : "ec3d821581ea1bd3aa8196c94b946874",
      "orders" : "0242c0a128c284ea9576a34db2306c12"
   },
   "capped" : [ ],
   "uuids" : {
      "inventory" : UUID("0830e0ad-cc24-4fc7-80d0-8e22fe45e382"),
      "orders" : UUID("755be489-745f-400c-ac3b-f27525ad0108")
   },
   "md5" : "cb4676f316ff2ff29c701a5edd18afe3",
   "timeMillis" : 0,
   "ok" : 1,
   "operationTime" : Timestamp(1529208801, 1),
   "$clusterTime" : {
      "clusterTime" : Timestamp(1529208801, 1),
      "signature" : {
         "hash" : BinData(0,"I4z4a4Mgs+tcx0MP5xIU8DYAMB0="),
         "keyId" : NumberLong("6567898567824900097")
      }
   }
}

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