16.1.3.7 Stored Function Examples to Manipulate GTIDs

This section provides examples of stored functions (see Chapter 23, Stored Objects) which you can create using some of the built-in functions provided by MySQL for use with GTID-based replication, listed here:

GTID_SUBSET(): Shows whether one GTID set is a subset of another.
GTID_SUBTRACT(): Returns the GTIDs from one GTID set that are not in another.
WAIT_FOR_EXECUTED_GTID_SET(): Waits until all transactions in a given GTID set have been executed.

See Section 12.18, “Functions Used with Global Transaction Identifiers (GTIDs)”, more more information about the functions just listed.

Note that in these stored functions, the delimiter command has been used to change the MySQL statement delimiter to a vertical bar, like this:

mysql> delimiter |

All of the stored functions shown in this section take string representations of GTID sets as arguments, so GTID sets must always be quoted when used with them.

This function returns nonzero (true) if two GTID sets are the same set, even if they are not formatted in the same way:

CREATE FUNCTION GTID_IS_EQUAL(gs1 LONGTEXT, gs2 LONGTEXT)
  RETURNS INT
  RETURN GTID_SUBSET(gs1, gs2) AND GTID_SUBSET(gs2, gs1)
|

This function returns nonzero (true) if two GTID sets are disjoint:

CREATE FUNCTION GTID_IS_DISJOINT(gs1 LONGTEXT, gs2 LONGTEXT)
RETURNS INT
  RETURN GTID_SUBSET(gs1, GTID_SUBTRACT(gs1, gs2))
|

This function returns nonzero (true) if two GTID sets are disjoint and sum is their union:

CREATE FUNCTION GTID_IS_DISJOINT_UNION(gs1 LONGTEXT, gs2 LONGTEXT, sum LONGTEXT)
RETURNS INT
  RETURN GTID_IS_EQUAL(GTID_SUBTRACT(sum, gs1), gs2) AND
         GTID_IS_EQUAL(GTID_SUBTRACT(sum, gs2), gs1)
|

This function returns a normalized form of the GTID set, in all uppercase, with no whitespace and no duplicates, with UUIDs in alphabetic order and intervals in numeric order:

CREATE FUNCTION GTID_NORMALIZE(gs LONGTEXT)
RETURNS LONGTEXT
  RETURN GTID_SUBTRACT(gs, '')
|

This function returns the union of two GTID sets:

CREATE FUNCTION GTID_UNION(gs1 LONGTEXT, gs2 LONGTEXT)
RETURNS LONGTEXT
  RETURN GTID_NORMALIZE(CONCAT(gs1, ',', gs2))
|

This function returns the intersection of two GTID sets.

CREATE FUNCTION GTID_INTERSECTION(gs1 LONGTEXT, gs2 LONGTEXT)
RETURNS LONGTEXT
  RETURN GTID_SUBTRACT(gs1, GTID_SUBTRACT(gs1, gs2))
|

This function returns the symmetric difference between two GTID sets, that is, the GTIDs that exist in gs1 but not in gs2, as well as the GTIDs that exist in gs2 but not in gs1.

CREATE FUNCTION GTID_SYMMETRIC_DIFFERENCE(gs1 LONGTEXT, gs2 LONGTEXT)
RETURNS LONGTEXT
  RETURN GTID_SUBTRACT(CONCAT(gs1, ',', gs2), GTID_INTERSECTION(gs1, gs2))
|

This function removes from a GTID set all the GTIDs with the specified origin, and returns the remaining GTIDs, if any. The UUID is the identifier used by the server where the transaction originated, which is normally the value of server_uuid.

CREATE FUNCTION GTID_SUBTRACT_UUID(gs LONGTEXT, uuid TEXT)
RETURNS LONGTEXT
  RETURN GTID_SUBTRACT(gs, CONCAT(UUID, ':1-', (1 << 63) - 2))
|

This function acts as the reverse of the previous one; it returns only those GTIDs from the GTID set that originate from the server with the specified identifier (UUID).

CREATE FUNCTION GTID_INTERSECTION_WITH_UUID(gs LONGTEXT, uuid TEXT)
RETURNS LONGTEXT
  RETURN GTID_SUBTRACT(gs, GTID_SUBTRACT_UUID(gs, uuid))
|

Example 16.1 Verifying that a replica is up to date

The built-in functions GTID_SUBSET() and GTID_SUBTRACT() can be used to check that a replica has applied at least every transaction that a source has applied.

To perform this check with GTID_SUBSET(), execute the following statement on the replica:

SELECT GTID_SUBSET(source_gtid_executed, replica_gtid_executed);

If the returns value is 0 (false), this means that some GTIDs in source_gtid_executed are not present in replica_gtid_executed, and that the replica has not yet applied transactions that were applied on the source, which means that the replica is not up to date.

To perform the same check with GTID_SUBTRACT(), execute the following statement on the replica:

SELECT GTID_SUBTRACT(source_gtid_executed, replica_gtid_executed);

This statement returns any GTIDs that are in source_gtid_executed but not in replica_gtid_executed. If any GTIDs are returned, the source has applied some transactions that the replica has not applied, and the replica is therefore not up to date.

Example 16.2 Backup and restore scenario

The stored functions GTID_IS_EQUAL(), GTID_IS_DISJOINT(), and GTID_IS_DISJOINT_UNION() can be used to verify backup and restore operations involving multiple databases and servers. In this example scenario, server1 contains database db1, and server2 contains database db2. The goal is to copy database db2 to server1, and the result on server1 should be the union of the two databases. The procedure used is to back up server2 using mysqldump, then to restore this backup on server1.

Provided that mysqldump was run with --set-gtid-purged set to ON or AUTO (the default), the output contains a SET @@GLOBAL.gtid_purged statement which adds the gtid_executed set from server2 to the gtid_purged set on server1. gtid_purged contains the GTIDs of all the transactions that have been committed on a given server but which do not exist in any binary log file on the server. When database db2 is copied to server1, the GTIDs of the transactions committed on server2, which are not in the binary log files on server1, must be added to gtid_purged for server1 to make the set complete.

The stored functions can be used to assist with the following steps in this scenario:

Use GTID_IS_EQUAL() to verify that the backup operation computed the correct GTID set for the SET @@GLOBAL.gtid_purged statement. On server2, extract that statement from the mysqldump output, and store the GTID set into a local variable, such as $gtid_purged_set. Then execute the following statement:
```
server2> SELECT GTID_IS_EQUAL($gtid_purged_set, @@GLOBAL.gtid_executed); 
```
If the result is 1, the two GTID sets are equal, and the set has been computed correctly.
Use GTID_IS_DISJOINT() to verify that the GTID set in the mysqldump output does not overlap with the gtid_executed set on server1. Having identical GTIDs present on both servers causes errors when copying database db2 to server1. To check, on server1, extract and store gtid_purged from the output into a local variable as done previously, then execute the following statement:
```
server1> SELECT GTID_IS_DISJOINT($gtid_purged_set, @@GLOBAL.gtid_executed);
```
If the result is 1, there is no overlap between the two GTID sets, so no duplicate GTIDs are present.
Use GTID_IS_DISJOINT_UNION() to verify that the restore operation resulted in the correct GTID state on server1. Before restoring the backup, on server1, obtain the existing gtid_executed set by executing the following statement:
```
server1> SELECT @@GLOBAL.gtid_executed;
```
Store the result in a local variable $original_gtid_executed, as well as the set from gtid_purged in another local variable as described previously. When the backup from server2 has been restored onto server1, execute the following statement to verify the GTID state:
```
server1> SELECT 
      ->   GTID_IS_DISJOINT_UNION($original_gtid_executed,
      ->                          $gtid_purged_set,
      ->                          @@GLOBAL.gtid_executed); 
```
If the result is 1, the stored function has verified that the original gtid_executed set from server1 ($original_gtid_executed) and the gtid_purged set that was added from server2 ($gtid_purged_set) have no overlap, and that the updated gtid_executed set on server1 now consists of the previous gtid_executed set from server1 plus the gtid_purged set from server2, which is the desired result. Ensure that this check is carried out before any further transactions take place on server1, otherwise the new transactions in gtid_executed cause it to fail.

Example 16.3 Selecting the most up-to-date replica for manual failover

The stored function GTID_UNION() can be used to identify the most up-to-date replica from a set of replicas, in order to perform a manual failover operation after a source server has stopped unexpectedly. If some of the replicas are experiencing replication lag, this stored function can be used to compute the most up-to-date replica without waiting for all the replicas to apply their existing relay logs, and therefore to minimize the failover time. The function can return the union of gtid_executed on each replica with the set of transactions received by the replica, which is recorded in the Performance Schema replication_connection_status table. You can compare these results to find which replica's record of transactions is the most up to date, even if not all of the transactions have been committed yet.

On each replica, compute the complete record of transactions by issuing the following statement:

SELECT GTID_UNION(RECEIVED_TRANSACTION_SET, @@GLOBAL.gtid_executed)
    FROM performance_schema.replication_connection_status
    WHERE channel_name = 'name';

You can then compare the results from each replica to see which one has the most up-to-date record of transactions, and use this replica as the new source.

Example 16.4 Checking for extraneous transactions on a replica

The stored function GTID_SUBTRACT_UUID() can be used to check whether a replica has received transactions that did not originate from its designated source or sources. If it has, there might be an issue with your replication setup, or with a proxy, router, or load balancer. This function works by removing from a GTID set all the GTIDs from a specified originating server, and returning the remaining GTIDs, if any.

For a replica with a single source, issue the following statement, giving the identifier of the originating source, which is normally the same as server_uuid:

SELECT GTID_SUBTRACT_UUID(@@GLOBAL.gtid_executed, server_uuid_of_source);

If the result is not empty, the transactions returned are extra transactions that did not originate from the designated source.

For a replica in a multisource topology, include the server UUID of each source in the function call, like this:

SELECT 
  GTID_SUBTRACT_UUID(GTID_SUBTRACT_UUID(@@GLOBAL.gtid_executed,
                                        server_uuid_of_source_1),
                                        server_uuid_of_source_2);

If the result is not empty, the transactions returned are extra transactions that did not originate from any of the designated sources.

Example 16.5 Verifying that a server in a replication topology is read-only

The stored function GTID_INTERSECTION_WITH_UUID() can be used to verify that a server has not originated any GTIDs and is in a read-only state. The function returns only those GTIDs from the GTID set that originate from the server with the specified identifier. If any of the transactions listed in gtid_executed from this server use the server's own identifier, the server itself originated those transactions. You can issue the following statement on the server to check:

SELECT GTID_INTERSECTION_WITH_UUID(@@GLOBAL.gtid_executed, my_server_uuid);

Example 16.6 Validating an additional replica in multisource replication

The stored function GTID_INTERSECTION_WITH_UUID() can be used to find out if a replica attached to a multisource replication setup has applied all the transactions originating from one particular source. In this scenario, source1 and source2 are both sources and replicas and replicate to each other. source2 also has its own replica. The replica also receives and applies transactions from source1 if source2 is configured with log_replica_updates=ON, but it does not do so if source2 uses log_replica_updates=OFF. Whichever the case, we currently want only to find out if the replica is up to date with source2. In this situation, GTID_INTERSECTION_WITH_UUID() can be used to identify the transactions that source2 originated, discarding the transactions that source2 has replicated from source1. The built-in function GTID_SUBSET() can then be used to compare the result with the gtid_executed set on the replica. If the replica is up to date with source2, the gtid_executed set on the replica contains all the transactions in the intersection set (the transactions that originated from source2).

To carry out this check, store the values of gtid_executed and the server UUID from source2 and the value of gtid_executed from the replica into user variables as follows:

    
source2> SELECT @@GLOBAL.gtid_executed INTO @source2_gtid_executed;

source2> SELECT @@GLOBAL.server_uuid INTO @source2_server_uuid;

replica> SELECT @@GLOBAL.gtid_executed INTO @replica_gtid_executed;

Then use GTID_INTERSECTION_WITH_UUID() and GTID_SUBSET() with these variables as input, as follows:

SELECT 
  GTID_SUBSET(
    GTID_INTERSECTION_WITH_UUID(@source2_gtid_executed,
                                @source2_server_uuid),
                                @replica_gtid_executed);

The server identifier from source2 (@source2_server_uuid) is used with GTID_INTERSECTION_WITH_UUID() to identify and return only those GTIDs from the set of GTIDs that originated on source2, omitting those that originated on source1. The resulting GTID set is then compared with the set of all executed GTIDs on the replica, using GTID_SUBSET(). If this statement returns nonzero (true), all the identified GTIDs from source2 (the first set input) are also found in gtid_executed from the replica, meaning that the replica has received and executed all the transactions that originated from source2.