> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt

# 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](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-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"`](https://www.mongodb.com/docs/manual/reference/read-concern-local.md#mongodb-readconcern-readconcern.-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.](https://www.mongodb.com/docs/manual/core/transactions.md#std-label-transactions-read-concern)

You can [create collections and indexes](https://www.mongodb.com/docs/manual/core/transactions.md#std-label-transactions-create-collections-indexes) inside a transaction. If [explicitly](https://www.mongodb.com/docs/manual/core/transactions-operations.md#std-label-transactions-operations-ddl-explicit) creating a collection or an index, the transaction must use read concern [`"local"`](https://www.mongodb.com/docs/manual/reference/read-concern-local.md#mongodb-readconcern-readconcern.-local-). If you [implicitly](https://www.mongodb.com/docs/manual/core/transactions-operations.md#std-label-transactions-operations-ddl-implicit) 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.

![Timeline of a write operation to a three member replica set](/images/read-concern-write-timeline.svg)

| 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"`](https://www.mongodb.com/docs/manual/reference/read-concern-local.md#mongodb-readconcern-readconcern.-local-) read concern would see at time `T`.

![Timeline of a write operation to a three member replica set.](/images/read-concern-write-timeline.svg)

| 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 |