MySQL 8.4 Release Notes
When a replication source server shuts down and restarts, its
MEMORY tables become empty. To
replicate this effect to replicas, the first time that the
source uses a given MEMORY table
after startup, it logs an event that notifies replicas that the
table must be emptied by writing a
DELETE or
TRUNCATE TABLE statement for that
table to the binary log. This generated event is identifiable by
a comment in the binary log, and if GTIDs are in use on the
server, it has a GTID assigned. The statement is always logged
in statement format, even if the binary logging format is set to
ROW, and it is written even if
read_only or
super_read_only mode is set on the server.
Note that the replica still has outdated data in a
MEMORY table during the interval
between the source's restart and its first use of the table. To
avoid this interval when a direct query to the replica could
return stale data, you can set the
init_file system variable to
name a file containing statements that populate the
MEMORY table on the source at startup.
When a replica server shuts down and restarts, its
MEMORY tables become empty. This
causes the replica to be out of synchrony with the source and
may lead to other failures or cause the replica to stop:
Row-format updates and deletes received from the source may fail with
Can't find record in '.memory_table'Statements such as
INSERT INTO ... SELECT FROMmay insert a different set of rows on the source and replica.memory_table
The replica also writes a DELETE
or TRUNCATE TABLE statement to
its own binary log, which is passed on to any downstream
replicas, causing them to empty their own
MEMORY tables.
The safe way to restart a replica that is replicating
MEMORY tables is to first drop or
delete all rows from the MEMORY
tables on the source and wait until those changes have
replicated to the replica. Then it is safe to restart the
replica.
An alternative restart method may apply in some cases. When
binlog_format=ROW, you can
prevent the replica from stopping if you set
replica_exec_mode=IDEMPOTENT
before you start the replica again. This allows the replica to
continue to replicate, but its
MEMORY tables still differ from
those on the source. This is acceptable if the application logic
is such that the contents of MEMORY
tables can be safely lost (for example, if the
MEMORY tables are used for
caching).
replica_exec_mode=IDEMPOTENT
applies globally to all tables, so it may hide other replication
errors in non-MEMORY tables.
(The method just described is not applicable in NDB Cluster,
where replica_exec_mode is
always IDEMPOTENT, and cannot be changed.)
The size of MEMORY tables is
limited by the value of the
max_heap_table_size system
variable, which is not replicated (see
Section 19.5.1.39, “Replication and Variables”). A change in
max_heap_table_size takes effect for
MEMORY tables that are created or updated
using ALTER TABLE
... ENGINE = MEMORY or TRUNCATE
TABLE following the change, or for all
MEMORY tables following a server
restart. If you increase the value of this variable on the
source without doing so on the replica, it becomes possible for
a table on the source to grow larger than its counterpart on the
replica, leading to inserts that succeed on the source but fail
on the replica with Table is full errors.
This is a known issue (Bug #48666). In such cases, you must set
the global value of
max_heap_table_size on the
replica as well as on the source, then restart replication. It
is also recommended that you restart both the source and replica
MySQL servers, to ensure that the new value takes complete
(global) effect on each of them.
See Section 18.3, “The MEMORY Storage Engine”, for more
information about MEMORY tables.