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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).

However, 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.

To use read concern level of "majority", replica sets must use WiredTiger storage engine.

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.

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.
  • Writeprev is the previous write before Write0.
  • No other writes have occured after Write0.
Timeline of a write operation to a three member replica set.
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.

Timeline of a write operation to a three member replica set.
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.

Causally Consistent Sessions

Read concern majority is available for use with causally consistent sessions.

Read Concern "majority" and Transactions

In MongoDB 4.0, all multi-documents transactions have snapshot isolation. In future versions of MongoDB, the server will optimize around your specified read concern (isolation level).

For "local" and "majority" read concern, MongoDB may provide stronger isolation guarantees than specified. If stronger guarantees are needed, be explicit as future versions of the server may include optimizations that remove stronger isolation guarantees than explicitly specified.

Note

You set the read concern at the transaction level, not at the individual operation level. To set the read concern for transactions, see Transaction Options (Read Concern/Write 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 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.