dbHash (database command)

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.

Warning

The dbHash command obtains a shared (S) lock on the database, which prevents writes until the command completes.

Compatibility

This command is available in deployments hosted in the following environments:

MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Important

This command is not supported in M0 and Flex clusters. For more information, see Unsupported Commands.

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

The command has the following syntax:

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

Command Fields

The command takes the following fields:

Field	Type	Description
`dbHash`	Any type	The command to run. Specify any value.
`collections`	array	Optional. An array of collection names in the current database. Specify each collection by name only. Do not qualify a name with the database, such as `test.orders`. To return the hash values for all collections in the database, specify the empty array.

Behavior

Non-Existent Collection

If a collection in the collections array is non-existent, dbHash does not return a hash value for that collection. A database-qualified name such as test.orders never matches, because dbHash compares each entry against the collection names in the current database.

If no name in the collections array matches a collection, dbHash returns the md5 value of an empty input, d41d8cd98f00b204e9800998ecf8427e. Before you compare md5 values across deployments, confirm that the collections field of the return document lists each collection that you expect.

Restrictions

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.

{
   <collection1>: <hash1>,
   <collection2>: <hash2>,
   ...
}

capped

An array that lists the capped collections.

uuids

A document with the collections and their corresponding UUID values.

{
   <collection1>: <UUID1>,
   <collection2>: <UUID2>,
   ...
}

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:

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

The operation returns the following document:

{
   "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" : Long("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:

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

The operation returns the following document:

{
   "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" : Long("6567898567824900097")
      }
   }
}

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