Selectively Skipping Replication of Binlog Events
The terms master and slave have historically been used in replication, and MariaDB has begun the process of adding primary and replica synonyms. The old terms will continue to be used to maintain backward compatibility - see MDEV-18777 to follow progress on this effort.
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Normally, all changes that are logged as events in the binary log are also replicated to all replicas (though still subject to filtering by replicate-do-db, replicate-ignore-db, and similar options). However, sometimes it may be desirable to have certain events be logged into the binlog, but not be replicated to all or a subset of replicas, where the distinction between events that should be replicated or not is under the control of the application making the changes.
This could be useful if an application does some replication external to the server outside of the built-in replication, or if it has some data that should not be replicated for whatever reason.
This is possible with the following system variables.
Primary Session Variable: skip_replication
When the skip_replication variable is set to true, changes are logged into the binary log with the flag @@skip_replication set. Such events will not be replicated by replicas that run with
--replicate-events-marked-for-skip set different from its default of REPLICATE.
| Variable Name | skip_replication |
|---|---|
| Scope | Session only |
| Access Type | Dynamic |
| Data Type | bool |
| Default Value | OFF |
The skip_replication option only has effect if binary logging is enabled
and sql_log_bin is true.
Attempting to change @@skip_replication in the middle of a transaction will
fail; this is to avoid getting half of a transaction replicated while the other
half is not replicated. Be sure to end any current transaction with
COMMIT/ROLLBACK before changing the variable.
Replica Option: --replicate-events-marked-for-skip
The replicate_events_marked_for_skip option tells the replica whether to replicate events that are marked with
the @@skip_replication flag. Default is REPLICATE, to ensure that all
changes are replicated to the replica. If set to FILTER_ON_SLAVE, events so
marked will be skipped on the replica and not replicated. If set to
FILTER_ON_MASTER, the filtering will be done on the primary, saving on
network bandwidth as the events will not be received by the replica at all.
| Variable Name | replicate_events_marked_for_skip |
|---|---|
| Scope | Global |
| Access Type | Dynamic |
| Data Type | enum: REPLICATE | FILTER_ON_SLAVE | FILTER_ON_MASTER |
| Default Value | REPLICATE |
Note: replicate_events_marked_for_skip is a dynamic variable (it can be
changed without restarting the server), however the replica threads must be
stopped when it is changed, otherwise an error will be thrown.
When events are filtered due to @@skip_replication, the filtering happens
on the primary side; in other words, the event is never sent to the replica. If
many events are filtered like this, a replica can sit a long time without
receiving any events from the primary. This is not a problem in itself, but must
be kept in mind when inquiring on the replica about events that are filtered. For
example START SLAVE UNTIL <some position> will stop when the first event
that is not filtered is encountered at the given position or beyond. If the
event at the given position is filtered, then the replica thread will only stop
when the next non-filtered event is encountered. In effect, if an event is
filtered, to the replica it appears that it was never written to the binlog on
the primary.
Note that when events are filtered for a replica, the data in the database will
be different on the replica and on the primary. It is the responsibility of the
application to replicate the data outside of the built-in replication or
otherwise ensure consistency of operation. If this is not done, it is possible
for replication to encounter, for example,
UNIQUE contraint violations or
other problems which will cause replication to stop and require manual
intervention to fix.
The session variable @@skip_replication can be changed without requiring
special privileges. This makes it possible for normal applications to control
it without requiring SUPER privileges. But it must be kept in mind when using
replicas with --replicate-events-marked-for-skip set different
from REPLICATE, as it allows any connection to do changes that are not
replicated.
skip_replication and sql_log_bin
@@sql_log_bin and @@skip_replication are somewhat
related, as they can both be used to prevent a change on the primary from being
replicated to the replica. The difference is that with @@skip_replication,
changes are still written into the binlog, and replication of the events is
only skipped on replicas that explicitly are configured to do so, with
--replicate-events-marked-for-skip different from
REPLICATE. With @@sql_log_bin, events are not logged into the binlog,
and so are not replicated by any replica.
skip_replication and the Binlog
When events in the binlog are marked with the @@skip_replication flag, the
flag will be preserved if the events are dumped by the mariadb-binlog
program and re-applied against a server with the
mariadb client program. Similarly, the
BINLOG statement will preserve the flag from the
event being replayed. And a replica which runs with
--log-slave-updates and does not filter events
(--replicate-events-marked-for-skip=REPLICATE) will also
preserve the flag in the events logged into the binlog on the replica.
See Also
- Using SQL_SLAVE_SKIP_COUNTER - How to skip a number of events on the replica
