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Index Builds on Populated Collections

Starting in MongoDB 4.2, index builds use an optimized build process holds an exclusive lock on the collection at the beginning and end of the index build. The rest of the build process yields to interleaving read and write operations. For a detailed description of index build process and locking behavior, see Index Build Process.

Starting in MongoDB 4.4, index builds on a replica set or sharded cluster build simultaneously across all data-bearing replica set members. The primary requires a minimum number of data-bearing voting members (i.e. commit quorum), including itself, that must complete the build before marking the index as ready for use. A “voting” member is any replica set member where members[n].votes is greater than 0. See Index Builds in Replicated Environments for more information.

Behavior

Comparison to Foreground and Background Builds

Previous versions of MongoDB supported building indexes either in the foreground or background. Foreground index builds were fast and produced more efficient index data structures, but required blocking all read-write access to the parent database of the collection being indexed for the duration of the build. Background index builds were slower and had less efficient results, but allowed read-write access to the database and its collections during the build process.

Starting in MongoDB 4.2, index builds obtain an exclusive lock on only the collection being indexed during the start and end of the build process to protect metadata changes. The rest of the build process uses the yielding behavior of background index builds to maximize read-write access to the collection during the build. 4.2 index builds still produce efficient index data structures despite the more permissive locking behavior.

The optimized index build performance is at least on par with background index builds. For workloads with few or no updates received during the build process, optimized index builds can be as fast as a foreground index build on that same data.

Use db.currentOp() to monitor the progress of ongoing index builds.

MongoDB ignores the background index build option if specified to createIndexes or its shell helpers createIndex() and createIndexes().

Constraint Violations During Index Build

For indexes that enforce constraints on the collection, such as unique indexes, the mongod checks all pre-existing and concurrently-written documents for violations of those constraints after the index build completes. Documents that violate the index constraints can exist during the index build. If any documents violate the index constraints at the end of the build, the mongod terminates the build and throws an error.

For example, consider a populated collection inventory. An administrator wants to create a unique index on the product_sku field. If any documents in the collection have duplicate values for product_sku, the index build can still start successfully. If any violations still exist at the end of the build, the mongod terminates the build and throws an error.

Similarly, an application can successfully write documents to the inventory collection with duplicate values of product_sku while the index build is in progress. If any violations still exist at the end of the build, the mongod terminates the build and throws an error.

To mitigate the risk of index build failure due to constraint violations:

  • Validate that no documents in the collection violate the index constraints.
  • Stop all writes to the collection from applications that cannot guarantee violation-free write operations.

Sharded Collections

For a sharded collection distributed across multiple shards, one or more shards may contain a chunk with duplicate documents. As such, the create index operation may succeed on some of the shards (i.e. the ones without duplicates) but not on others (i.e. the ones with duplicates). To avoid leaving inconsistent indexes across shards, you can issue the db.collection.dropIndex() from a mongos to drop the index from the collection.

To mitigate the risk of this occurrence, before creating the index:

  • Validate that no documents in the collection violate the index constraints.
  • Stop all writes to the collection from applications that cannot guarantee violation-free write operations.

Index Build Impact on Database Performance

Index Builds During Write-Heavy Workloads

Building indexes during time periods where the target collection is under heavy write load can result in reduced write performance and longer index builds.

Consider designating a maintenance window during which applications stop or reduce write operations against the collection. Start the index build during this maintenance window to mitigate the potential negative impact of the build process.

Insufficient Available System Memory (RAM)

createIndexes supports building one or more indexes on a collection. createIndexes uses a combination of memory and temporary files on disk to complete index builds. The default limit on memory usage for createIndexes is 200 megabytes (for versions 4.2.3 and later) and 500 (for versions 4.2.2 and earlier), shared between all indexes built using a single createIndexes command. Once the memory limit is reached, createIndexes uses temporary disk files in a subdirectory named _tmp within the --dbpath directory to complete the build.

You can override the memory limit by setting the maxIndexBuildMemoryUsageMegabytes server parameter. Setting a higher memory limit may result in faster completion of index builds. However, setting this limit too high relative to the unused RAM on your system can result in memory exhaustion and server shutdown.

If the host machine has limited available free RAM, you may need to schedule a maintenance period to increase the total system RAM before you can modify the mongod RAM usage.

Index Builds in Replicated Environments

Requires featureCompatibilityVersion 4.4+

Each mongod in the replica set or sharded cluster must have featureCompatibilityVersion set to at least 4.4 to start index builds simultaneously across replica set members.

MongoDB 4.4 running featureCompatibilityVersion: "4.2" builds indexes on the primary before replicating the index build to secondaries.

Starting with MongoDB 4.4, index builds on a replica set or sharded cluster build simultaneously across all data-bearing replica set members. For sharded clusters, the index build occurs only on shards containing data for the collection being indexed. The primary requires a minimum number of data-bearing voting members (i.e commit quorum), including itself, that must complete the build before marking the index as ready for use.

The build process is summarized as follows:

  1. The primary receives the createIndexes command and immediately creates a “startIndexBuild” oplog entry associated with the index build.

  2. The secondaries start the index build after they replicate the “startIndexBuild oplog entry.

  3. Each member “votes” to commit the build once it finishes indexing data in the collection.

  4. Secondary members continue to process any new write operations into the index while waiting for the primary to confirm a quorum of votes.

  5. When the primary has a quorum of votes, it checks for any key constraint violations such as duplicate key errors.

    • If there are no key constraint violations, the primary completes the index build, marks the index as ready for use, and creates an associated “commitIndexBuild” oplog entry.
    • If there are any key constraint violations, the index build fails. The primary aborts the index build and creates an associated “abortIndexBuild” oplog entry.
  6. The secondaries replicate the “commitIndexBuild” oplog entry and complete the index build.

    If the secondaries instead replicate an “abortIndexBuild” oplog entry, they abort the index build and discard the build job.

For sharded clusters, the index build occurs only on shards containing data for the collection being indexed.

For a more detailed description of the index build process, see Index Build Process.

By default, index builds use a commit quorum of "votingMembers", or all data-bearing voting members. To start an index build with a non-default commit quorum, specify the commitQuorum parameter to createIndexes or its shell helpers db.collection.createIndex() and db.collection.createIndexes().

To modify the commit quorum required for an in-progress simultaneous index build, use the setIndexCommitQuorum command.

Note

Index builds can impact replica set performance. For workloads which cannot tolerate performance decrease due to index builds, consider performing a rolling index build process. Rolling index builds take at most one replica set member out at a time, starting with the secondary members, and builds the index on that member as a standalone. Rolling index builds require at least one replica set election.

Build Failure and Recovery

Interrupted Index Builds on a Primary mongod

If the primary mongod shuts down during the index build, the build progress is lost. The mongod automatically recovers the index build and restarts it from the beginning.

Interrupted Index Builds on a Secondary mongod

If a secondary shuts down during the index build, the index build job is persisted. Restarting the mongod automatically recovers the index build and restarts it from the beginning.

Prior to MongoDB 4.4, the startup process stalls behind any recovered index builds. The secondary could fall out of sync with the replica set and require resynchronization. Starting in MongoDB 4.4, the mongod can perform the startup process while the recovering index builds.

If you restart the mongod as a standalone (i.e. removing or commenting out replication.replSetName or omitting --replSetName), the mongod cannot restart the index build. The build remains in a paused state until it is manually dropped.

Interrupted Index Builds on Standalone mongod

If the mongod shuts down during the index build, the index build job and all progress is lost. Restarting the mongod does not restart the index build. You must re-issue the createIndex() operation to restart the index build.

Rollbacks during Build Process

Starting in version 4.4, MongoDB can pause an in-progress index build to perform a rollback.

  • If the rollback does not revert the index build, MongoDB restarts the index build after completing the rollback.
  • If the rollback reverts the index build, you must re-create the index or indexes after the rollback completes.

Prior to MongoDb 4.4, rollbacks could start only after all in-progress index builds finished.

Index Consistency Checks for Sharded Collections

A sharded collection has an inconsistent index if the collection does not have the exact same indexes (including the index options) on each shard that contains chunks for the collection. Although inconsistent indexes should not occur during normal operations, inconsistent indexes can occur, such as:

  • When a user is creating an index with a unique key constraint and one shard contains a chunk with duplicate documents. In such cases, the create index operation may succeed on the shards without duplicates but not on the shard with duplicates.
  • When a user is creating an index across the shards in a rolling manner (i.e. manually building the index one by one across the shards) but either fails to build the index for an associated shard or incorrectly builds an index with different specification.

Starting in MongoDB 4.4 (and in MongoDB 4.2.6), the config server primary periodically checks for index inconsistencies across the shards for sharded collections. To configure these periodic checks, see enableShardedIndexConsistencyCheck and shardedIndexConsistencyCheckIntervalMS.

The command serverStatus returns the field shardedIndexConsistency to report on index inconsistencies when run on the config server primary.

To check if a sharded collection has inconsistent indexes, see Find Inconsistent Indexes across Shards.

Monitor In Progress Index Builds

To see the status of an index build operation, you can use the db.currentOp() method in the mongo shell. To filter the current operations for index creation operations, see Active Indexing Operations for an example.

The msg field includes a percentage-complete measurement of the current stage in the index build process.

Terminate In Progress Index Builds

Use the dropIndexes command or its shell helpers dropIndex() or dropIndexes() to terminate an in-progress index build. See Abort In-Progress Index Builds for more information.

Do not use killOp to terminate an in-progress index builds in replica sets or sharded clusters.

Index Build Process

The following table describes each stage of the index build process:

Stage Description
Lock The mongod obtains an exclusive X lock on the the collection being indexed. This blocks all read and write operations on the collection, including the application of any replicated write operations or metadata commands that target the collection. The mongod does not yield this lock.
Initialization

The mongod creates three data structures at this initial state:

  • The initial index metadata entry.
  • A temporary table (“side writes table”) that stores keys generated from writes to the collection being indexed during the build process.
  • A temporary table (“constraint violation table”) for all documents that may cause a key generation error. Key generation errors occur when a document has invalid keys for the indexed fields. For example, a document with duplicate field values when building a unique index or malformed GeoJSON objects when building a 2dsphere index.
Lock The mongod downgrades the exclusive X collection lock to an intent exclusive IX lock. The mongod periodically yields this lock to interleaving read and write operations.
Scan Collection

For each document in the collection, the mongod generates a key for that document and dumps the key into an external sorter.

If the mongod encounters a key generation error while generating a key during the collection scan, it stores that key in the constraint violation table for later processing.

If the mongod encounters any other error while generating a key, the build fails with an error.

Once the mongod completes the collection scan, it dumps the sorted keys into the index.

Process Side Writes Table

The mongod drains the side write table using first-in-first-out priority.

If the mongod encounters a key generation error while processing a key in the side write table, it stores that key in the constraint violation table for later processing.

If the mongod encounters any other error while processing a key, the build fails with an error.

For each document written to the collection during the build process, the mongod generates a key for that document and stores it in the side write table for later processing. The mongod uses a snapshot system to set a limit to the number of keys to process.

Vote and Wait for Commit Quorum

A mongod that is not part of a replica set skips this stage.

Starting in MongoDB 4.4, the mongod submits a “vote” to the primary to commit the index. Specifically, it writes the “vote” to an internal replicated collection on the primary.

If the mongod is the primary, it waits until it has a commit quorum of votes (all voting data-bearing members by default) before continuing the index build process.

If the mongod is a secondary, it waits until it replicates either a “commitIndexBuild” or “abortIndexBuild” oplog entry:

  • If the mongod replicates a “commitIndexBuild” oplog entry, it finishes draining the side writes table and moves to the next stage in the index build process.
  • If the mongod replicates an “abortIndexBuild” oplog entry, it aborts the index build and discards the build job.

While waiting for commit quorum, the mongod adds any additional keys generated from write operations to the collection being indexed to the side writes table and periodically drains the table.

Lock The mongod upgrades the intent exclusive IX lock on the collection to a shared S lock. This blocks all write operations to the collection, including the application of any replicated write operations or metadata commands that target the collection.
Finish Processing Temporary Side Writes Table

The mongod continues draining remaining records in the side writes table. The mongod may pause replication during this stage.

If the mongod encounters a key generation error while processing a key in the side write table, it stores that key in the constraint violation table for later processing.

If the mongod encounters any other error while processing a key, the build fails with an error.

Lock The mongod upgrades the shared S lock on the collection to an exclusive X lock on the collection. This blocks all read and write operations on the collection, including the application of any replicated write operations or metadata commands that target the collection. The mongod does not yield this lock.
Drop Side Write Table

The mongod applies any remaining operations in the side writes table before dropping it.

If the mongod encounters a key generation error while processing a key in the side write table, it stores that key in the constraint violation table for later processing.

If the mongod encounters any other error while processing a key, the build fails with an error.

At this point, the index includes all data written to the collection.

Process Constraint Violation Table

If the mongod is the primary, it drains the constraint violation table using first-in-first-out priority.

  • If no keys in the constraint violation table produce a key generation error or if the table is empty, the mongod drops the table and creates a “commitIndexBuild” oplog entry. Secondaries can complete the associated index build after replicating the oplog entry.
  • If any key in the constraint violation table still produces a key generation error, the mongod aborts the build and throws an error. The mongod creates an associated “abortIndexBuild” oplog entry to indicate that secondaries should abort and discard the index build job.

If the mongod is a secondary, it drops the constraint violation table. Since the primary must successfully drain the constraint violation table prior to creating the “commitOplogEntry” oplog entry, the secondary can safely assume that no violations exist.

Mark the Index as Ready The mongod updates the index metadata to mark the index as ready for use.
Lock The mongod releases the X lock on the collection.

See also

FAQ: Concurrency