MySQL supports delayed replication such that a replica server deliberately executes transactions later than the source by at least a specified amount of time. This section describes how to configure a replication delay on a replica, and how to monitor replication delay.
In MySQL 8.1, the method of delaying replication
depends on two timestamps,
Replication Delay Timestamps); delayed
replication is measured using these timestamps. If either the
immediate source or replica is not using these timestamps, the
implementation of delayed replication from MySQL 5.7 is used (see
Delayed Replication). This section
describes delayed replication between servers which are all using
The default replication delay is 0 seconds. Use a
REPLICATION SOURCE TO
SOURCE_DELAY= statement to
set the delay to
N seconds. A
transaction received from the source is not executed until at
N seconds later than its commit
on the immediate source. The delay happens per transaction (not
event as in previous MySQL versions) and the actual delay is
imposed only on
anonymous_gtid_log_event. The other events in
the transaction always follow these events without any waiting
time imposed on them.
Performance Schema table contains the
DESIRED_DELAY column which shows the delay
configured using the
SOURCE_DELAY option. The
Performance Schema table contains the
REMAINING_DELAY column which shows the number
of delay seconds remaining.
Delayed replication can be used for several purposes:
To protect against user mistakes on the source. With a delay you can roll back a delayed replica to the time just before the mistake.
To test how the system behaves when there is a lag. For example, in an application, a lag might be caused by a heavy load on the replica. However, it can be difficult to generate this load level. Delayed replication can simulate the lag without having to simulate the load. It can also be used to debug conditions related to a lagging replica.
To inspect what the database looked like in the past, without having to reload a backup. For example, by configuring a replica with a delay of one week, if you then need to see what the database looked like before the last few days' worth of development, the delayed replica can be inspected.
MySQL 8.1 provides a new method for measuring delay (also referred to as replication lag) in replication topologies that depends on the following timestamps associated with the GTID of each transaction (instead of each event) written to the binary log.
original_commit_timestamp: the number of microseconds since epoch when the transaction was written (committed) to the binary log of the original source.
immediate_commit_timestamp: the number of microseconds since epoch when the transaction was written (committed) to the binary log of the immediate source.
The output of mysqlbinlog displays these
timestamps in two formats, microseconds from epoch and also
TIMESTAMP format, which is based on the user
defined time zone for better readability. For example:
#170404 10:48:05 server id 1 end_log_pos 233 CRC32 0x016ce647 GTID last_committed=0 \ sequence_number=1 original_committed_timestamp=1491299285661130 immediate_commit_timestamp=1491299285843771 # original_commit_timestamp=1491299285661130 (2017-04-04 10:48:05.661130 WEST) # immediate_commit_timestamp=1491299285843771 (2017-04-04 10:48:05.843771 WEST) /*!80001 SET @@SESSION.original_commit_timestamp=1491299285661130*//*!*/; SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:1'/*!*/; # at 233
As a rule, the
always the same on all replicas where the transaction is
applied. In source-replica replication, the
original_commit_timestamp of a transaction in
the (original) source’s binary log is always the same as its
immediate_commit_timestamp. In the
replica’s relay log, the
immediate_commit_timestamp of the transaction
are the same as in the source’s binary log; whereas in its own
binary log, the transaction’s
immediate_commit_timestamp corresponds to
when the replica committed the transaction.
In a Group Replication setup, when the original source is a
member of a group, the
original_commit_timestamp is generated when
the transaction is ready to be committed. In other words, when
it finished executing on the original source and its write set
is ready to be sent to all members of the group for
certification. When the original source is a server outside the
preserved. The same
for a particular transaction is replicated to all servers in the
group, and to any replica outside the group that is replicating
from a member. Each recipient of the transaction also stores the
local commit time in its binary log using
View change events, which are exclusive to Group Replication, are a special case. Transactions containing these events are generated by each group member but share the same GTID (so, they are not first executed in a source and then replicated to the group, but all members of the group execute and apply the same transaction). Group members set local timestamp values for transactions associated with view change events.
One of the most common ways to monitor replication delay (lag)
in previous MySQL versions was by relying on the
Seconds_Behind_Master field in the output of
SHOW REPLICA STATUS. However,
this metric is not suitable when using replication topologies
more complex than the traditional source-replica setup, such as
Group Replication. The addition of
original_commit_timestamp to MySQL 8 provides
a much finer degree of information about replication delay. The
recommended method to monitor replication delay in a topology
that supports these timestamps is using the following
Performance Schema tables.
replication_connection_status: current status of the connection to the source, provides information on the last and current transaction the connection thread queued into the relay log.
replication_applier_status_by_coordinator: current status of the coordinator thread that only displays information when using a multithreaded replica, provides information on the last transaction buffered by the coordinator thread to a worker’s queue, as well as the transaction it is currently buffering.
replication_applier_status_by_worker: current status of the thread(s) applying transactions received from the source, provides information about the transactions applied by the replication SQL thread, or by each worker thread when using a multithreaded replica.
Using these tables you can monitor information about the last transaction the corresponding thread processed and the transaction that thread is currently processing. This information comprises:
a transaction’s GTID
immediate_commit_timestamp, retrieved from the replica’s relay log
the time a thread started processing a transaction
for the last processed transaction, the time the thread finished processing it
In addition to the Performance Schema tables, the output of
REPLICA STATUS has three fields that show:
SQL_Delay: A nonnegative integer indicating the replication delay configured using
CHANGE REPLICATION SOURCE TO SOURCE_DELAY=, where
Nis measured in seconds.
Waiting until MASTER_DELAY seconds after master executed event, this field contains an integer indicating the number of seconds left of the delay. At other times, this field is
Replica_SQL_Running_State: A string indicating the state of the SQL thread (analogous to
Replica_IO_State). The value is identical to the
Statevalue of the SQL thread as displayed by
When the replication SQL thread is waiting for the delay to
elapse before executing an event,
PROCESSLIST displays its
Waiting until MASTER_DELAY seconds after
master executed event.