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"local"

Read Concern "local"

A query with read concern "local" returns data from the instance with no guarantee that the data has been written to a majority of the replica set members (i.e. may be rolled back).

Read concern "local" is the default for:

• read operations against primary
• read operations against secondaries if the reads are associated with causally consistent sessions.

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.

Availability

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

Read Concern "local" and Transactions

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.

Starting in MongoDB 4.4, with feature compatibility version (fcv) "4.4" or greater, you can create collections and indexes inside a transaction. If explicitly creating a collection or an index, the transaction must use read concern "local". Implicit creation of a collection can use any of the read concerns available for transactions.

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 "local" read concern would see at time T.

Read Target	Time T	State of Data
Primary	After t0	Data reflects Write0.
Secondary1	Before t1	Data reflects Writeprev
Secondary1	After t1	Data reflects Write0
Secondary2	Before t2	Data reflects Writeprev
Secondary2	After t2	Data reflects Write0

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