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 the
primary and secondaries.
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.
You can create collections and indexes inside a transaction. If
explicitly creating a
collection or an index, the transaction must use read concern
"local". If you implicitly create a collection, you can
use any of the read concerns available for transactions.
On a replica set, even if a transaction uses read concern local, you
might observe stronger read isolation where the operation reads from a
snapshot at the point in time that the transaction was opened.
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 acknowledgment 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 "local" read concern would see at
time T.
Read Target | Time T | State of Data |
|---|---|---|
Primary | After t 0 | Data reflects Write 0. |
Secondary 1 | Before t 1 | Data reflects Write prev |
Secondary 1 | After t 1 | Data reflects Write 0 |
Secondary 2 | Before t 2 | Data reflects Write prev |
Secondary 2 | After t 2 | Data reflects Write 0 |