The different consistency levels provide flexibility to both DBAs, who can use them to set up their infrastructure; and to developers who can use the consistency level that best suits their application's requirements. The following scenarios show how to choose a consistency guarantee level based on how you use your group:

You can choose the scope for which the consistency level is enforced by setting group_replication_consistency with session or global scope. This is important because consistency levels can have a negative impact on group performance they apply globally.

To enforce the consistency level for the current session, use session scope, like this:

> SET @@SESSION.group_replication_consistency= 'BEFORE';

To enforce the consistency level for all sessions, use global scope, as shown here:

> SET @@GLOBAL.group_replication_consistency= 'BEFORE';

The possibility of setting the consistency level on specific sessions enables you to take advantage of scenarios such as those listed here:

In sum, you do not need to run all transactions with the same specific consistency level, especially if only some transactions actually require it.

You should be aware that all read/write transactions are always ordered in Group Replication, so even when you set the consistency level to AFTER for the current session, this transaction waits until its changes are applied on all members, which means waiting for this and all preceding transactions that could be in the secondaries' queues. In other words, the consistency level AFTER waits for everything up to and including this transaction.

Another way to classify the consistency levels is in terms of impact on the group, that is, the repercussions that the consistency levels have on the other members.

The BEFORE consistency level, apart from being ordered on the transaction stream, only impacts on the local member. That is, it does not require coordination with the other members and does not have repercussions on their transactions. In other words, BEFORE only impacts the transactions on which it is used.

The AFTER and BEFORE_AND_AFTER consistency levels do have side-effects on concurrent transactions executed on other members. These consistency levels make the other members transactions wait if transactions with the EVENTUAL consistency level start while a transaction with AFTER or BEFORE_AND_AFTER is executing. The other members wait until the AFTER transaction is committed on that member, even if the other member's transactions have the EVENTUAL consistency level. In other words, AFTER and BEFORE_AND_AFTER impact all ONLINE group members.

To illustrate this further, imagine a group with 3 members, M1, M2 and M3. On member M1 a client issues:

> SET @@SESSION.group_replication_consistency= AFTER;
> BEGIN;
> INSERT INTO t1 VALUES (1);
> COMMIT;

Then, while the above transaction is being applied, on member M2 a client issues:

> SET SESSION group_replication_consistency= EVENTUAL;

In this situation, even though the second transaction's consistency level is EVENTUAL, because it starts executing while the first transaction is already in the commit phase on M2, the second transaction has to wait for the first transaction to finish the commit and only then can it execute.

You can only use the consistency levels BEFORE, AFTER and BEFORE_AND_AFTER on ONLINE members, attempting to use them on members in other states causes a session error.

Transactions whose consistency level is not EVENTUAL hold execution until a timeout, configured by wait_timeout value is reached, which defaults to 8 hours. If the timeout is reached an ER_GR_HOLD_WAIT_TIMEOUT error is thrown.

This section describes how a group's consistency level impacts on a single-primary group that has elected a new primary. Such a group automatically detects failures and adjusts the view of the members that are active, in other words the membership configuration. Furthermore, if a group is deployed in single-primary mode, whenever the group's membership changes there is a check performed to detect if there is still a primary member in the group. If there is none, a new one is selected from the list of secondary members. Typically, this is known as the secondary promotion.

Given the fact that the system detects failures and reconfigures itself automatically, the user may also expect that once the promotion takes place, the new primary is in the exact state, data-wise, as that of the old one. In other words, the user may expect that there is no backlog of replicated transactions to be applied on the new primary once he is able to read from and write to it. In practical terms, the user may expect that once his application fails-over to the new primary, there would be no chance, even if temporarily, to read old data or write into old data records.

When flow control is activated and properly tuned on a group, there is only a small chance of transiently reading stale data from a newly elected primary immediately after the promotion, as there should not be a backlog, or if there is one it should be small. Moreover, you might have a proxy or middleware layers that govern application accesses to the primary after a promotion and enforce the consistency criteria at that level. You can specify the behavior of the new primary once it is promoted using the group_replication_consistency variable, which controls whether a newly elected primary blocks both reads and writes until after the backlog is fully applied. If the group_replication_consistency variable was set to BEFORE_ON_PRIMARY_FAILOVER on a newly elected primary which has backlog to apply, and transactions are issued against the new primary while it is still applying the backlog, incoming transactions are blocked until the backlog is fully applied. This prevents the following anomalies:

To summarize, when group_replication_consistency is set to BEFORE_ON_PRIMARY_FAILOVER you are choosing to prioritize consistency over availability, because reads and writes are held whenever a new primary is elected. This is the trade-off you have to consider when configuring your group. It should also be remembered that if flow control is working correctly, backlog should be minimal. Note that the higher consistency levels BEFORE, AFTER, and BEFORE_AND_AFTER also include the consistency guarantees provided by BEFORE_ON_PRIMARY_FAILOVER.

To guarantee that the group provides the same consistency level regardless of which member is promoted to primary, all members of the group should have BEFORE_ON_PRIMARY_FAILOVER (or a higher consistency level) persisted to their configuration. For example, on each member issue:

> SET PERSIST group_replication_consistency='BEFORE_ON_PRIMARY_FAILOVER';

This ensures that the members all behave in the same way, and that the configuration is persisted after a restart of the member.

A transaction cannot be on-hold forever, and if the time held exceeds wait_timeout it returns an ER_GR_HOLD_WAIT_TIMEOUT error.

Although all writes are held when using BEFORE_ON_PRIMARY_FAILOVER consistency level, not all reads are blocked to ensure that you can still inspect the server while it is applying backlog after a promotion took place. This is useful for debugging, monitoring, observability and troubleshooting. Some queries that do not modify data are allowed, such as the following: