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dbHash

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  • Definition
  • Compatibility
  • Syntax
  • Command Fields
  • 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.

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, M2, and M5 clusters or in serverless instances. For more information, see Unsupported Commands.

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.

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

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" : 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:

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" : NumberLong("6567898567824900097")
      }
   }
}

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