Read consistency for write forwarding
You can control the degree of read consistency on a DB cluster. The read consistency
level determines how long the DB cluster waits before each read operation to ensure that some
or all changes are replicated from the writer. You can adjust the read consistency level to
make sure that all forwarded write operations from your session are visible in the DB cluster
before any subsequent queries. You can also use this setting to make sure that queries on the
DB cluster always see the most current updates from the writer. This setting also applies to
queries submitted by other sessions or other clusters. To specify this type of behavior for
your application, choose a value for the aurora_replica_read_consistency DB
parameter or DB cluster parameter.
Important
Always set the aurora_replica_read_consistency DB parameter or DB
cluster parameter when you want to forward writes. If you don't, then Aurora
doesn't forward writes. This parameter has an empty value by default, so choose a
specific value when you use this parameter. The aurora_replica_read_consistency
parameter only affects DB clusters or instances that have write forwarding enabled.
As you increase the consistency level, your application spends more time waiting for changes to be propagated between DB instances. You can choose the balance between fast response time and making sure that changes made in other DB instances are fully available before your queries run.
You can specify the following values for the aurora_replica_read_consistency parameter:
-
EVENTUAL– Results of write operations in the same session aren't visible until the write operation is performed on the writer DB instance. The query doesn't wait for the updated results to be available. Thus it might retrieve the older data or the updated data, depending on the timing of the statements and the amount of replication lag. This is the same consistency as for Aurora MySQL DB clusters that don't use write forwarding. -
SESSION– All queries that use write forwarding see the results of all changes made in that session. The changes are visible regardless of whether the transaction is committed. If necessary, the query waits for the results of forwarded write operations to be replicated. -
GLOBAL– A session sees all committed changes across all sessions and instances in the DB cluster. Each query might wait for a period that varies depending on the amount of session lag. The query proceeds when the DB cluster is up-to-date with all committed data from the writer, as of the time that the query began.
For information about the configuration parameters involved in write forwarding, see Configuration parameters for write forwarding.
Note
You can also use aurora_replica_read_consistency as a session variable, for example:
mysql> set aurora_replica_read_consistency = 'session';
Examples of using write forwarding
The following examples show the effects of the aurora_replica_read_consistency parameter on running
INSERT statements followed by SELECT statements. The results can differ, depending on the
value of aurora_replica_read_consistency and the timing of the statements.
To achieve higher consistency, you might wait briefly before issuing the SELECT statement. Or Aurora can
automatically wait until the results finish replicating before proceeding with SELECT.
For information on setting DB parameters, see Parameter groups for Amazon Aurora.
Example with aurora_replica_read_consistency set to EVENTUAL
Running an INSERT statement, immediately followed by a SELECT statement, returns a value for
COUNT(*) with the number of rows before the new row is inserted. Running the SELECT again
a short time later returns the updated row count. The SELECT statements don't wait.
mysql> select count(*) from t1; +----------+ | count(*) | +----------+ | 5 | +----------+ 1 row in set (0.00 sec) mysql> insert into t1 values (6); select count(*) from t1; +----------+ | count(*) | +----------+ | 5 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from t1; +----------+ | count(*) | +----------+ | 6 | +----------+ 1 row in set (0.00 sec)
Example with aurora_replica_read_consistency set to SESSION
A SELECT statement immediately after an INSERT waits until the changes from the
INSERT statement are visible. Subsequent SELECT statements don't wait.
mysql> select count(*) from t1; +----------+ | count(*) | +----------+ | 6 | +----------+ 1 row in set (0.01 sec) mysql> insert into t1 values (6); select count(*) from t1; select count(*) from t1; Query OK, 1 row affected (0.08 sec) +----------+ | count(*) | +----------+ | 7 | +----------+ 1 row in set (0.37 sec) +----------+ | count(*) | +----------+ | 7 | +----------+ 1 row in set (0.00 sec)
With the read consistency setting still set to SESSION, introducing a brief wait after performing an
INSERT statement makes the updated row count available by the time the next SELECT
statement runs.
mysql> insert into t1 values (6); select sleep(2); select count(*) from t1; Query OK, 1 row affected (0.07 sec) +----------+ | sleep(2) | +----------+ | 0 | +----------+ 1 row in set (2.01 sec) +----------+ | count(*) | +----------+ | 8 | +----------+ 1 row in set (0.00 sec)
Example with aurora_replica_read_consistency set to GLOBAL
Each SELECT statement waits for all data changes, as of the start time of the statement, to be visible
before performing the query. The wait time for each SELECT statement varies, depending on the amount of
replication lag.
mysql> select count(*) from t1; +----------+ | count(*) | +----------+ | 8 | +----------+ 1 row in set (0.75 sec) mysql> select count(*) from t1; +----------+ | count(*) | +----------+ | 8 | +----------+ 1 row in set (0.37 sec) mysql> select count(*) from t1; +----------+ | count(*) | +----------+ | 8 | +----------+ 1 row in set (0.66 sec)