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"majority"

Read Concern "majority"

For read operations not associated with multi-document transactions, read concern "majority" guarantees that the data read has been acknowledged by a majority of the replica set members (i.e. the documents read are durable and guaranteed not to roll back).

For operations in multi-document transactions, read concern "majority" provides its guarantees only if the transaction commits with write concern “majority”. Otherwise, the "majority" read concern provides no guarantees about the data read in transactions.

Regardless of the read concern level, the most recent data on a node may not reflect the most recent version of the data in the system.

Performance

Each replica set member maintains, in memory, a view of the data at the majority-commit point; the majority-commit point is calculated by the primary. To fulfill read concern "majority", the node returns data from this view and is comparable in performance cost to other read concerns.

Availability

Read concern "majority" is available for use with or without causally consistent sessions and transactions.

You can disable read concern "majority" for a deployment with a three-member primary-secondary-arbiter (PSA) architecture; however, this has implications for change streams (in MongoDB 4.0 and earlier only) and transactions on sharded clusters. For more information, see Disable Read Concern Majority.

Example

Consider the following timeline of a write operation Write₀ to a three member replica set:

Note

For simplification, the example assumes:

• All writes prior to Write₀ have been successfully replicated to all members.
• Write_prev is the previous write before Write₀.
• No other writes have occured after Write₀.

Time	Event	Most Recent Write	Most Recent w: “majority” write
t0	Primary applies Write0	Primary: Write0 Secondary1: Writeprev Secondary2: Writeprev	Primary: Writeprev Secondary1: Writeprev Secondary2: Writeprev
t1	Secondary1 applies write0	Primary: Write0 Secondary1: Write0 Secondary2: Writeprev	Primary: Writeprev Secondary1: Writeprev Secondary2: Writeprev
t2	Secondary2 applies write0	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Writeprev Secondary1: Writeprev Secondary2: Writeprev
t3	Primary is aware of successful replication to Secondary1 and sends acknowledgement to client	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Write0 Secondary1: Writeprev Secondary2: Writeprev
t4	Primary is aware of successful replication to Secondary2	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Write0 Secondary1: Writeprev Secondary2: Writeprev
t5	Secondary1 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: “majority” write	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Write0 Secondary1: Write0 Secondary2: Writeprev
t6	Secondary2 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: “majority” write	Primary: Write0 Secondary1: Write0 Secondary2: Write0	Primary: Write0 Secondary1: Write0 Secondary2: Write0

Then, the following tables summarizes the state of the data that a read operation with "majority" read concern would see at time T.

Read Target	Time T	State of Data
Primary	Before t3	Data reflects Writeprev
Primary	After t3	Data reflects Write0
Secondary1	Before t5	Data reflects Writeprev
Secondary1	After t5	Data reflects Write0
Secondary2	Before or at t6	Data reflects Writeprev
Secondary2	After t6	Data reflects Write0

Storage Engine Support

Read concern "majority" is available for the WiredTiger storage engine.

Tip

The serverStatus command returns the storageEngine.supportsCommittedReads field which indicates whether the storage engine supports "majority" read concern.

Read Concern "majority" and Transactions

Note

You set the read concern at the transaction level, not at the individual operation level. To set the read concern for transactions, see Transactions and Read Concern.

For operations in multi-document transactions, read concern "majority" provides its guarantees only if the transaction commits with write concern “majority”. Otherwise, the "majority" read concern provides no guarantees about the data read in transactions.

Read Concern "majority" and Aggregation

Starting in MongoDB 4.2, you can specify read concern level "majority" for an aggregation that includes an $out stage.

In MongoDB 4.0 and earlier, you cannot include the $out stage to use "majority" read concern for the aggregation.

Read Your Own Writes

Changed in version 3.6.

Starting in MongoDB 3.6, you can use causally consistent sessions to read your own writes, if the writes request acknowledgement.

Prior to MongoDB 3.6, you must have issued your write operation with { w: "majority" } write concern and then use either "majority" or "linearizable" read concern for the read operations to ensure that a single thread can read its own writes.

Disable Read Concern Majority

For 3-Member Primary-Secondary-Arbiter Architecture

You can disable read concern "majority" if you have a three-member replica set with a primary-secondary-arbiter (PSA) architecture or a sharded cluster with a three-member PSA shards.

Note

If you are using a deployment other than a 3-member PSA, you do not need to disable read concern majority.

With a three-member PSA architecture, the cache pressure will increase if any data bearing node is down. To prevent the storage cache pressure from immobilizing a deployment with a PSA architecture, you can disable read concern by setting either:

• --enableMajorityReadConcern command line option to false.
• replication.enableMajorityReadConcern configuration file setting to false.

To check if read concern “majority” is disabled, You can run db.serverStatus() on the mongod instances and check the storageEngine.supportsCommittedReads field. If false, read concern “majority” is disabled.

Important

In general, avoid disabling "majority" read concern unless necessary. However, if you have a three-member replica set with a primary-secondary-arbiter (PSA) architecture or a sharded cluster with a three-member PSA shard, disable to prevent the storage cache pressure from immobilizing the deployment.

Change Streams

Disabling "majority" read concern disables support for Change Streams for MongoDB 4.0 and earlier. For MongoDB 4.2+, disabling read concern "majority" has no effect on change streams availability.

Transactions

Disabling "majority" read concern affects support for transactions on sharded clusters. Specifically:

• A transaction cannot use read concern "snapshot" if the transaction involves a shard that has disabled read concern “majority”.
• A transaction that writes to multiple shards errors if any of the transaction’s read or write operations involves a shard that has disabled read concern "majority".

However, it does not affect transactions on replica sets. For transactions on replica sets, you can specify read concern "majority" (or "snapshot" or "local" ) for multi-document transactions even if read concern "majority" is disabled.

Rollback Considerations

Disabling "majority" read concern prevents collMod commands which modify an index from rolling back. If such an operation needs to be rolled back, you must resync the affected nodes with the primary node.

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