Config Database
On this page
The collections in the config
database support:
Causally consistent sessions for standalones, replica sets, and sharded clusters and retryable writes for replica sets and sharded clusters.
Restrictions
Starting in MongoDB 5.0, non-transaction reads are not allowed on
the config.transactions
collection with the following
read concerns and options:
"majority"
and the afterClusterTime option is setWhen using a MongoDB Driver and
"majority"
within a causally consistent session
Important
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.
Note
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 mongosh
to a
mongos
instance in a sharded cluster and issue the following:
use config show collections
Note
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):
Warning
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.insertOne( { 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:
config.changelog
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
The
changelog
collection stores a document for each change to the metadata of a sharded collection.Example
The following example displays a single record of a chunk split from a
changelog
collection:{ "_id" : "<hostname>-<timestamp>-<increment>", "server" : "<hostname><:port>", "clientAddr" : "127.0.0.1:63381", "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("<...>") }, "owningShard" : "<value>" } } Each document in the
changelog
collection contains the following fields:config.changelog.clientAddr
A string that holds the address of the client, a
mongos
instance that initiates this change.
config.changelog.time
A ISODate timestamp that reflects when the change occurred.
config.chunks
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
The
config.chunks
collection stores a document for each chunk in the cluster. The following example shows a document:{ _id: ObjectId('65a954c0de11596e08e7c1dc'), uuid: UUID('a4479215-a38d-478f-a82b-e5e95d455e55'), min: { a: Long('121204345') }, max: { a: Long('993849349') }, shard: 'shard01', lastmod: Timestamp({ t: 1, i: 0 }), history: [ { validAfter: Timestamp({ t: 1705596095, i: 14 }), shard: 'shard01' } ] } In the document:
_id
is the chunk identifier.min
andmax
are the range of values for the chunk's shard key.shard
is the name of the shard that stores the chunk in the cluster.
Tip
To find the chunks in a collection, retrieve the collection's
uuid
identifier from theconfig.collections
collection. Then, useuuid
to retrieve the matching document with the sameuuid
from theconfig.chunks
collection.
config.collections
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
The
collections
collection stores a document for each sharded collection in the cluster. Given a collection namedpets
in therecords
database, a document in thecollections
collection would resemble the following:{ "_id" : "records.pets", "lastmod" : ISODate("2021-07-21T15:48:15.193Z"), "timestamp": Timestamp(1626882495, 1), "key" : { "a" : 1 }, "unique" : false, "lastmodEpoch" : ObjectId("5078407bd58b175c5c225fdc"), "uuid" : UUID("f8669e52-5c1b-4ea2-bbdc-a00189b341da") }
config.csrs.indexes
New in version 7.0.
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
The
indexes
collection stores a document for each global index available on a shard.Each document in the collection contains the following fields:
FieldData TypeDescriptionname
StringName of the global index.keyPattern
DocumentIndex key specification.options
DocumentProvides information on specified index options, including whether the index is a global index.lastmod
TimestampTimestamp that provides information on when the index was last modified and the index version.collectionUUID
UUIDUUID of the sharded collection.indexCollectionUUID
UUIDUUID of the secondary collection that tracks the global index.
config.databases
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
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 , and a version.
{ "_id" : "test", "primary" : "shardA", "version" : { "uuid" : UUID("516a5f79-5eb9-4844-8ee9-b8e9de91b760"), "timestamp" : Timestamp(1626894204, 1), "lastMod" : 1 } } { "_id" : "hr", "primary" : "shardA", "version" : { "uuid" : UUID("8e39d61d-6259-4c33-a5ed-bcd2ae317b6f"), "timestamp" : Timestamp(1626895015, 1), "lastMod" : 1 } } { "_id" : "reporting", "primary" : "shardB", "version" : { "uuid" : UUID("07c63242-51b3-460c-865f-a67b3372d792"), "timestamp" : Timestamp(1626895826, 1), "lastMod" : 1 } } The method
sh.status()
returns this information in the Databases section.
config.migrationCoordinators
New in version 4.4.
The
migrationCoordinators
collection exists on each shard and stores a document for each in-progress chunk migration from this shard to another shard. The chunk migration fails if the document cannot be written to a majority of the members of the shard's replica set.When a migration is complete, the document describing the migration is deleted from the collection.
config.mongos
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
The
mongos
collection stores a document for eachmongos
instance affiliated with the cluster. The cluster maintains this collection for reporting purposes.Each document in the
mongos
collection contains these fields:FieldData TypeDescription_id
StringadvisoryHostFQDNs
Array of stringsArray of themongos
's fully qualified domain names (FQDNs).created
DateWhen the
mongos
was started.New in version 5.2.
mongoVersion
StringVersion of MongoDB that themongos
is running.ping
Datemongos
instances send pings to the config server every 30 seconds. This field indicates the last ping time.up
NumberLongNumber of seconds themongos
has been up as of the last ping.waiting
BooleanAs of MongoDB 3.4, this field is alwaystrue
and is only present for backward compatibility.The following document shows the status of the
mongos
running onexample.com:27017
.[ { _id: 'example.com:27017', advisoryHostFQDNs: [ "example.com" ], created: ISODate("2021-11-22T16:32:13.708Z"), mongoVersion: "5.2.0", ping: ISODate("2021-12-15T22:09:23.161Z"), up: Long("2007429"), waiting: true } ]
config.rangeDeletions
The
rangeDeletions
collection exists on each shard and stores a document for each chunk range that contains orphaned documents. The chunk migration fails if the document cannot be written to a majority of the members of the shard's replica set.When the orphaned chunk range is cleaned up, the document describing the range is deleted from the collection.
config.settings
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
The
settings
collection holds the following sharding configuration settings:Range size. To change range size, see Modify Range Size in a Sharded Cluster. The specified
chunksize
value is in megabytes.Balancer settings. To change the balancer settings, including balancer status, see Manage Sharded Cluster Balancer.
Autosplit:
Starting in MongoDB 6.1, automatic chunk splitting is not performed. This is because of balancing policy improvements. Auto-splitting commands still exist, but do not perform an operation. For details, see Balancing Policy Changes.
In MongoDB versions earlier than 6.1:
balancerStart
also enables auto-splitting for the sharded cluster.balancerStop
also disables auto-splitting for the sharded cluster.To enable or disable the autosplit flag, use the corresponding
sh.enableAutoSplit()
method orsh.disableAutoSplit()
method.
Example documents in the
settings
collection:{ "_id" : "chunksize", "value" : 64 } { "_id" : "balancer", "mode" : "full", "stopped" : false }
config.shards
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
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" ] }
config.tags
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
The
tags
collection holds documents for each zone range in the cluster. The documents in thetags
collection resemble the following:{ "_id" : { "ns" : "records.users", "min" : { "zipcode" : "10001" } }, "ns" : "records.users", "min" : { "zipcode" : "10001" }, "max" : { "zipcode" : "10281" }, "tag" : "NYC" }
config.version
Tip
Internal MongoDB Metadata
The config database is internal: applications and administrators should not modify or depend upon its content in the course of normal operation.
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 thedb.getCollection()
method. For example, to retrieve the collection's document:db.getCollection("version").find()
Collections to Support Sessions
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.
Warning
Do not manually modify or drop these collections.
To access these collections for a mongod
or
mongos
instance, connect mongosh
to the
instance.
config.system.sessions
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
ormongos
instance, the record of the session initially exists only in-memory on the instance. Periodically, the instance will sync its cached sessions to thesystem.sessions
collection; at which time, they are visible to all members of the deployment.To view records in the
system.sessions
collection, use$listSessions
.Warning
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'ssystem.sessions
collection during the add process.
config.transactions
The
transactions
collection stores records used to support retryable writes and transactions for replica sets and sharded clusters.Warning
Do not manually modify or drop the
transactions
collection.