To be able to safely configure the replication mode of an online server it is important to understand some key concepts of replication. This section explains these concepts and is essential reading before attempting to modify the replication mode of an online server.
The modes of replication available in MySQL rely on different techniques for identifying transactions which are logged. The types of transactions used by replication are as follows:
GTID transactions are identified by a global transaction identifier (GTID) in the form
UUID:NUMBER
. Every GTID transaction in a log is always preceded by aGtid_log_event
. GTID transactions can be addressed using either the GTID or using the file name and position.Anonymous transactions do not have a GTID assigned, and MySQL ensures that every anonymous transaction in a log is preceded by an
Anonymous_gtid_log_event
. In previous versions, anonymous transactions were not preceded by any particular event. Anonymous transactions can only be addressed using file name and position.
When using GTIDs you can take advantage of GTID auto-positioning and automatic fail-over, as well as useWAIT_FOR_EXECUTED_GTID_SET()
,session_track_gtids
, and monitor replicated transactions using Performance Schema tables.
Transactions in a relay log that was received from a source running a previous version of MySQL may not be preceded by any particular event at all, but after being replayed and logged in the replica's binary log, they are preceded with anAnonymous_gtid_log_event
.
The ability to configure the replication mode online means that thegtid_mode
andenforce_gtid_consistency
variables are now both dynamic and can be set from a top-level statement by an account that has privileges sufficient to set global system variables. SeeSystem Variable Privileges. In MySQL 5.6 and earlier, both of these variables could only be configured using the appropriate option at server start, meaning that changes to the replication mode required a server restart. In all versionsgtid_mode
could be set toON
orOFF
, which corresponded to whether GTIDs were used to identify transactions or not. Whengtid_mode=ON
it is not possible to replicate anonymous transactions, and whengtid_mode=OFF
only anonymous transactions can be replicated. Whengtid_mode=OFF_PERMISSIVE
thennewtransactions are anonymous while permitting replicated transactions to be either GTID or anonymous transactions. Whengtid_mode=ON_PERMISSIVE
thennewtransactions use GTIDs while permitting replicated transactions to be either GTID or anonymous transactions. This means it is possible to have a replication topology that has servers using both anonymous and GTID transactions. For example a source withgtid_mode=ON
could be replicating to a replica withgtid_mode=ON_PERMISSIVE
. The valid values forgtid_mode
are as follows and in this order:
OFF
OFF_PERMISSIVE
ON_PERMISSIVE
ON
It is important to note that the state ofgtid_mode
can only be changed by one step at a time based on the above order. For example, ifgtid_mode
is currently set toOFF_PERMISSIVE
, it is possible to change toOFF
orON_PERMISSIVE
but not toON
. This is to ensure that the process of changing from anonymous transactions to GTID transactions online is correctly handled by the server. When you switch betweengtid_mode=ON
andgtid_mode=OFF
, the GTID state (in other words the value ofgtid_executed
) is persistent. This ensures that the GTID set that has been applied by the server is always retained, regardless of changes between types ofgtid_mode
.
The fields related to GTIDs display the correct information regardless of the currently selectedgtid_mode
. This means that fields which display GTID sets, such asgtid_executed
,gtid_purged
,RECEIVED_TRANSACTION_SET
in thereplication_connection_status
Performance Schema table, and the GTID related results ofSHOW REPLICA STATUS
(or before MySQL 8.0.22,SHOW SLAVE STATUS
), now return the empty string when there are no GTIDs present. Fields that display a single GTID, such asCURRENT_TRANSACTION
in the Performance Schemareplication_applier_status_by_worker
table, now displayANONYMOUS
when GTID transactions are not being used.
Replication from a source usinggtid_mode=ON
provides the ability to use GTID auto-positioning, configured using theSOURCE_AUTO_POSITION
of theCHANGE REPLICATION SOURCE TO
statement (from MySQL 8.0.23), or theMASTER_AUTO_POSITION
option of theCHANGE MASTER TO
statement (before MySQL 8.0.23). The replication topology being used impacts on whether it is possible to enable auto-positioning or not, as this feature relies on GTIDs and is not compatible with anonymous transactions. It is strongly recommended to ensure there are no anonymous transactions remaining in the topology before enabling auto-positioning, see2.4.2节,“使交易GTID在线”.
The valid combinations ofgtid_mode
and auto-positioning on source and replica are shown in the following table, where the source'sgtid_mode
is shown on the horizontal and the replica'sgtid_mode
is on the vertical. The meaning of each entry is as follows:
Table 2.1 Valid Combinations of Source and Replica gtid_mode
Source |
Source |
Source |
Source |
|
---|---|---|---|---|
Replica |
Y |
Y |
N |
N |
Replica |
Y |
Y |
Y |
Y* |
Replica |
Y |
Y |
Y |
Y* |
Replica |
N |
N |
Y |
Y* |
The currently selectedgtid_mode
also impacts on thegtid_next
variable. The following table shows the behavior of the server for the different values ofgtid_mode
andgtid_next
. The meaning of each entry is as follows:
ANONYMOUS
: generate an anonymous transaction.Error
: generate an error and fail to executeSET GTID_NEXT
.UUID:NUMBER
: generate a GTID with the specified UUID:NUMBER.New GTID
: generate a GTID with an automatically generated number.
Table 2.2 Valid Combinations of gtid_mode and gtid_next
binary log on |
binary log off |
|
|
|
---|---|---|---|---|
|
ANONYMOUS |
ANONYMOUS |
ANONYMOUS | Error |
|
ANONYMOUS |
ANONYMOUS |
ANONYMOUS |
UUID:NUMBER |
|
New GTID |
ANONYMOUS |
ANONYMOUS |
UUID:NUMBER |
New GTID |
ANONYMOUS |
Error |
UUID:NUMBER |
When the binary log is off andgtid_next
is set toAUTOMATIC
, then no GTID is generated. This is consistent with the behavior of previous versions.