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Read Concern "majority"
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"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.
Warning
If you are using a three-member primary-secondary-arbiter (PSA)
architecture, the write concern "majority" can cause
performance issues if a secondary is unavailable or lagging. See
Mitigate Performance Issues with PSA Replica Set for advice on how to mitigate these
issues.
Example
Consider the following timeline of a write operation Write 0 to a three member replica set:
Note
For simplification, the example assumes:
All writes prior to Write 0 have been successfully replicated to all members.
Write prev is the previous write before Write 0.
No other writes have occured after Write 0.
Time | Event | Most Recent Write | Most Recent w: "majority" write |
|---|---|---|---|
t 0 | Primary applies Write 0 | Primary: Write 0 Secondary 1: Write prev Secondary 2: Write prev | Primary: Write prev Secondary 1: Write prev Secondary 2: Write prev |
t 1 | Secondary 1 applies write 0 | Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write prev | Primary: Write prev Secondary 1: Write prev Secondary 2: Write prev |
t 2 | Secondary 2 applies write 0 | Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0 | Primary: Write prev Secondary 1: Write prev Secondary 2: Write prev |
t 3 | Primary is aware of successful replication to Secondary 1 and sends acknowledgement to client | Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0 | Primary: Write 0 Secondary 1: Write prev Secondary 2: Write prev |
t 4 | Primary is aware of successful replication to Secondary 2 | Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0 | Primary: Write 0 Secondary 1: Write prev Secondary 2: Write prev |
t 5 | Secondary 1 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: "majority" write | Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0 | Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write prev |
t 6 | Secondary 2 receives notice (through regular replication mechanism) to update its snapshot of its most recent w: "majority" write | Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0 | Primary: Write 0 Secondary 1: Write 0 Secondary 2: Write 0 |
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 t 3 | Data reflects Write prev |
Primary | After t 3 | Data reflects Write 0 |
Secondary 1 | Before t 5 | Data reflects Write prev |
Secondary 1 | After t 5 | Data reflects Write 0 |
Secondary 2 | Before or at t 6 | Data reflects Write prev |
Secondary 2 | After t 6 | Data reflects Write 0 |
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, in order to read your own writes you must issue
your write operation with { w: "majority" }
write concern, and then issue your read operation with
primary read preference, and either
"majority" or "linearizable" read concern.
Primary-Secondary-Arbiter Replica Sets
Starting in MongoDB 5.0,
enableMajorityReadConcern and
--enableMajorityReadConcern cannot be changed
and are always set to true due to storage engine improvements.
In earlier versions of MongoDB,
enableMajorityReadConcern and
--enableMajorityReadConcern are configurable and can be set
to false to prevent storage cache pressure from immobilizing a
deployment with a three-member primary-secondary-arbiter (PSA)
architecture.
If you are using a three-member primary-secondary-arbiter (PSA)
architecture, the write concern "majority" can cause
performance issues if a secondary is unavailable or lagging. See
Mitigate Performance Issues with PSA Replica Set for advice on how to mitigate these
issues.