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Config Database

The collections in the config database support:



The schema of the config database is internal and may change between releases of MongoDB. The config database is not a dependable API, and users should not write data to the config database in the course of normal operation or maintenance.


You cannot perform read/write operations to the collections in the config database inside a multi-document transaction.

Collections to Support Sharded Cluster Operations

To access the config database and view the list of collections that support sharding operations, connect a mongo shell to a mongos instance in a sharded cluster and issue the following:

use config

show collections


If running with access control, ensure you have privileges that grant listCollections action on the database.

The config database is mainly for internal use, and during normal operations you should never manually insert or store data in it. However, if you need to verify the write availability of the config server for a sharded cluster, you can insert a document into a test collection (after making sure that no collection of that name already exists):


Modification of the config database on a functioning system may lead to instability or inconsistent data sets. If you must modify the config database, use mongodump to create a full backup of the config database.

db.testConfigServerWriteAvail.insert( { a : 1 } )

If the operation succeeds, the config server is available to process writes.

Future releases of the server may include different collections in the config database, so be careful when selecting a name for your test collection.

MongoDB uses the following collections in the config database to support sharding:


The changelog collection stores a document for each change to the metadata of a sharded collection.


The following example displays a single record of a chunk split from a changelog collection:

 "_id" : "<hostname>-<timestamp>-<increment>",
 "server" : "<hostname><:port>",
 "clientAddr" : "",
 "time" : ISODate("2012-12-11T14:09:21.039Z"),
 "what" : "split",
 "ns" : "<database>.<collection>",
 "details" : {
    "before" : {
       "min" : {
          "<database>" : { $minKey : 1 }
       "max" : {
          "<database>" : { $maxKey : 1 }
       "lastmod" : Timestamp(1000, 0),
       "lastmodEpoch" : ObjectId("000000000000000000000000")
    "left" : {
       "min" : {
          "<database>" : { $minKey : 1 }
       "max" : {
          "<database>" : "<value>"
       "lastmod" : Timestamp(1000, 1),
       "lastmodEpoch" : ObjectId(<...>)
    "right" : {
       "min" : {
          "<database>" : "<value>"
       "max" : {
          "<database>" : { $maxKey : 1 }
       "lastmod" : Timestamp(1000, 2),
       "lastmodEpoch" : ObjectId("<...>")

Each document in the changelog collection contains the following fields:


The value of changelog._id is: <hostname>-<timestamp>-<increment>.


The hostname of the server that holds this data.


A string that holds the address of the client, a mongos instance that initiates this change.


A ISODate timestamp that reflects when the change occurred.


Reflects the type of change recorded. Possible values include:

  • dropCollection
  • dropCollection.start
  • dropDatabase
  • dropDatabase.start
  • moveChunk.start
  • moveChunk.commit
  • split
  • multi-split

Namespace where the change occurred.


A document that contains additional details regarding the change. The structure of the details document depends on the type of change.


The chunks collection stores a document for each chunk in the cluster. Consider the following example of a document for a chunk named\"cat\":

   "_id" : "\"cat\"",
   "lastmod" : Timestamp(2, 1),
   "lastmodEpoch" : ObjectId("5d8aa9fbe7a2f52c300e8e6f"),
   "ns" : "",
   "min" : {
         "animal" : "cat"
   "max" : {
         "animal" : "dog"
   "shard" : "shard0004",
   "history" : [ { "validAfter" : Timestamp(1569368571, 27), "shard" : "shard0004" } ]

These documents store the range of values for the shard key that describe the chunk in the min and max fields. Additionally the shard field identifies the shard in the cluster that “owns” the chunk.


The collections collection stores a document for each sharded collection in the cluster. Given a collection named pets in the records database, a document in the collections collection would resemble the following:

   "_id" : "records.pets",
   "lastmod" : ISODate("1970-01-16T15:00:58.107Z"),
   "dropped" : false,
   "key" : {
         "a" : 1
   "unique" : false,
   "lastmodEpoch" : ObjectId("5078407bd58b175c5c225fdc"),
   "uuid" :  UUID("f8669e52-5c1b-4ea2-bbdc-a00189b341da")

The databases collection stores a document for each database in the cluster.

For each database, the corresponding document displays the name, the database’s primary shard , the database’s sharding enabled status, and a version.

{ "_id" : "test", "primary" : "shardA", "partitioned" : true, "version" : { "uuid" : UUID("516a5f79-5eb9-4844-8ee9-b8e9de91b760"), "lastMod" : 1 } }
{ "_id" : "hr", "primary" : "shardA", "partitioned" : false, "version" : { "uuid" : UUID("8e39d61d-6259-4c33-a5ed-bcd2ae317b6f"), "lastMod" : 1 } }
{ "_id" : "reporting", "primary" : "shardB", "partitioned" : false, "version" : { "uuid" : UUID("07c63242-51b3-460c-865f-a67b3372d792"), "lastMod" : 1 } }

The method sh.status() returns this information in the Databases section.


The lockpings collection keeps track of the active components in the sharded cluster. Given a cluster with a mongos running on, the document in the lockpings collection would resemble:

{ "_id" : "", "ping" : ISODate("2012-10-12T18:32:54.892Z") }

The locks collection stores the distributed locks. The primary of the config server replica set takes a lock by inserting a document into the locks collection.

   "_id" : "test.myShardedCollection",
   "state" : 2,
   "process" : "ConfigServer",
   "ts" : ObjectId("5be0b9ede46e4f441a60d891"),
   "when" : ISODate("2018-11-05T21:52:00.846Z"),
   "who" : "ConfigServer:Balancer",
   "why" : "Migrating chunk(s) in collection test.myShardedCollection"

As of version 3.4, the state field will always have a value 2 to prevent any legacy mongos instances from performing the balancing operation. The when field specifies the time when the config server member became the primary.

In version 3.4, when the balancer is active, the balancer takes a lock, as in the following 3.4 example:

   "_id" : "balancer",
   "state" : 2,
   "ts" : ObjectId("5be0bc6cb20effa83b15baa8"),
   "who" : "ConfigServer:Balancer",
   "process" : "ConfigServer",
   "when" : ISODate("2018-11-05T21:56:13.096Z"),
   "why" : "CSRS Balancer"

Starting in version 3.6, the balancer no longer takes a “lock”. If you have upgraded from 3.4 to 3.6, you may choose to delete any residual "_id" : "balancer" documents.


The mongos collection stores a document for each mongos instance affiliated with the cluster. mongos instances send pings to all members of the cluster every 30 seconds so the cluster can verify that the mongos is active. The ping field shows the time of the last ping, while the up field reports the uptime of the mongos as of the last ping. The cluster maintains this collection for reporting purposes.

The following document shows the status of the mongos running on

   "_id" : "",
   "advisoryHostFQDNs" : [
   "mongoVersion" : "4.2.0",
   "ping" : ISODate("2019-09-25T19:26:52.360Z"),
   "up" : NumberLong(50),
   "waiting" : true

The settings collection holds the following sharding configuration settings:

The following are some example documents in the settings collection:

{ "_id" : "chunksize", "value" : 64 }
{ "_id" : "balancer", "mode" : "full", "stopped" : false }
{ "_id" : "autosplit", "enabled" : true }

The shards collection represents each shard in the cluster in a separate document, as in the following:

{ "_id" : "shard0000", "host" : "localhost:30000", "state" : 1 }

If the shard is a replica set, the host field displays the name of the replica set, then a slash, then a comma-separated list of the hostnames of each member of the replica set, as in the following example:

{ "_id" : "shard0001", "host" : "shard0001/localhost:27018,localhost:27019,localhost:27020", "state" : 1 }

If the shard has zones assigned, this document has a tags field, that holds an array of the zones to which it is assigned, as in the following example:

{ "_id" : "shard0002", "host" : "localhost:30002", "state" : 1, "tags": [ "NYC" ] }

The tags collection holds documents for each zone range in the cluster. The documents in the tags collection resemble the following:

    "_id" : { "ns" : "records.users", "min" : { "zipcode" : "10001" } },
    "ns" : "records.users",
    "min" : { "zipcode" : "10001" },
    "max" : { "zipcode" : "10281" },
    "tag" : "NYC"

The version collection holds the current metadata version number. This collection contains only one document. For example:

{ "_id" : 1, "minCompatibleVersion" : 5, "currentVersion" : 6, "clusterId" : ObjectId("5d8bc01a690d8abbd2014ddd") }

To access the version collection, you must use the db.getCollection() method. For example, to retrieve the collection’s document:


Collections to Support Sessions

New in version 3.6.

Starting in MongoDB 3.6, the config database contains the internal collections to support causally consistent sessions for standalones, replica sets, and sharded clusters and retryable writes and transactions for replica sets and sharded clusters.


Do not manually modify or drop these collections.

To access these collections for a mongod or mongos instance, connect mongo shell to the instance.


The system.sessions collection stores session records that are available to all members of the deployment.

When a user creates a session on a mongod or mongos instance, the record of the session initially exists only in-memory on the instance. Periodically, the instance will sync its cached sessions to the system.sessions collection; at which time, they are visible to all members of the deployment.

To view records in the system.sessions collection, use $listSessions.


Do not manually modify or drop the system.sessions collection.

In a sharded cluster, the system.sessions collection is sharded.

  • When adding a shard to the sharded cluster, if the shard to add already contains its own system.sessions collection, MongoDB drops the new shard’s system.sessions collection during the add process.
  • Starting in version 4.2.7, MongoDB automatically splits the system.sessions collection into at least 1024 chunks and distributes the chunks uniformly across shards in the cluster.

The transactions collection stores records used to support retryable writes and transactions for replica sets and sharded clusters.


Do not manually modify or drop the transactions collection.