Parallel Rollback client¶
On servers having several processors or processor cores, it may be possible to reduce rollback elapse time by paralleling the operation on multiple threads of execution. For this purpose, E-Maj delivers a specific client to run as a command. It activates E-Maj rollback functions though several parallel connections to the database.
To run a rollback in parallel, E-Maj spreads tables and sequences to process for one or several tables groups into “sessions”. Each session is then processed in its own thread.
However, in order to guarantee the integrity of the global operation, the rollback of all sessions is executed inside a single transaction.
To build the most balanced sessions as possible, E-Maj takes into account:
- the number of sessions specified by the user in its command,
- statistics about rows to rollback, as reported by the emaj_log_stat_group() function,
- foreign key constraints that link several tables between them, 2 updated tables linked by a foreign key constraint being affected into the same session.
The command to run parallel rollbacks is written in php. As a consequence, php software and its PostgreSQL interface has to be installed on the server that executes the command (which is not necessarily the same as the one that hosts the PostgreSQL instance).
Rolling back each session on behalf of a unique transaction implies the use of two phase commit. As a consequence, the max_prepared_transaction parameter of the postgresql.conf file must be adjusted. As the default value of this parameter equals 0, it must be modified by specifying a value at least equal to the maximum number of sessions that will be used.
The command that performs a parallel rollback has the following syntax:
emajParallelRollback.php -g <group(s).name> -m <mark> -s <number.of.sessions> [OPTIONS]...
The general options are:
- -l specifies that the requested rollback is a logged rollback
- -a specifies that the requested rollback is allowed to reach a mark set before an alter group operation
- -v displays more information about the execution of the processing
- –help only displays a command help
- –version only displays the software version
And the connection options are:
- -d <database to connect to>
- -h <host to connect to>
- -p <ip-port to connect to>
- -U <connection role to use>
- -W <password associated to the role>, if needed
To replace some or all these parameters, the usual PGDATABASE, PGPORT, PGHOST and/or PGUSER environment variables can be used.
To specify a list of tables groups in the -g parameter, separate the name of each group by a comma.
The supplied connection role must be either a superuser or a role having emaj_adm rights.
For safety reasons, it is not recommended to use the -W option to supply a password. It is rather advisable to use the .pgpass file (see PostgreSQL documentation).
To allow the rollback operation to work, the tables group or groups must be in LOGGING state. The supplied mark must also correspond to the same point in time for all groups. In other words, this mark must have been set by the same emaj_set_mark_group() function call.
The ‘EMAJ_LAST_MARK’ keyword can be used as mark name, meaning the last set mark.
It is possible to monitor the multi-session rollback operations with the same tools as for mono-session rollbacks.
In order to test the emajParallelRollback.php command, the E-Maj extension supplies a test script, emaj_prepare_parallel_rollback_test.sql. It prepares an environment with two tables groups containing some tables and sequences, on which some updates have been performed, with intermediate marks. Once this script has been executed under psql, the command displayed at the end of the script can be simply run.
./php/emajParallelRollback.php -d mydb -g myGroup1 -m Mark1 -s 3
logs on database mydb and executes a rollback of group myGroup1 to mark Mark1, using 3 parallel sessions.
./php/emajParallelRollback.php -d mydb -g "myGroup1,myGroup2" -m Mark1 -s 3 -l
logs on database mydb and executes a logged rollback of both groups myGroup1 and myGroup2 to mark Mark1, using 3 parallel sessions.