Ongoing MariaDB development: filtering and rewrites in mysqlbinlog

The main focus of MariaDB staff has been the MariaDB 5.1.38 release in recent weeks, but this doesn’t mean that we’ve abandoned everything else for it. There are several non-release projects going on, one of which is adding binlog filtering and markup capabilities.

In order to see how the new features fit in, let’s first look at what binlog filtering options are already present in MySQL (and so, MariaDB):

As long as MySQL had only statement-based replication, one could work around the blank space in mysqlbinlog column by processing mysqlbinlog output with perl/awk/etc scripts. With Row-Based Replication, mysqlbinlog’s output contains events that look like this:

BINLOG '
vjrjShMBAAAAJwAAAPcCAAAAABIAAAAAAAAAAmQyAAJ0MgABAwAB
vjrjShkBAAAAIgAAABkDAAAQABIAAAAAAAEAAf/+AgAAAA==
'/*!*/;

which practically prevents one from doing any processing on it with perl/awk or similar tools. We’ve got a request to fix this, and set to add the following filtering capabilities:

At this moment, MWL#36 has already been coded and pushed into mariadb-5.2 tree. The rest of the tasks will hopefully follow.

Some implementation notes

MWL#36’s --replicate-rewrite-db has the same limitations as slave’s --replicate-rewrite-db: cross-database updates and CREATE/DROP/ALTER DATABASE statements are not rewritten. We were lucky that slave’s replicate-rewrite-db had these limitations and we could follow them. The thing is, since the slave parses the queries, it is relatively easily for it to walk the parse tree and rewrite the database name wherever necessary (and thus provide handling all kinds of statements). If this was implemented on the slave, it would be very difficult to repeat in mysqlbinlog, since mysqlbinlog has no SQL parser and so is not able to reliably find references to database name in SQL statement text.

It seems we won’t be able to dodge this problem in MWL#40: Table-level filtering, though. According to the manual, replicate-ignore-table “Tells the slave thread to not replicate any statement that updates the specified table, even if any other tables might be updated by the same statement (source). In order to copy this behaviour, mysqlbinlog will need to be able to tell which tables are affected by each of the statements it processes. As written in the worklog entry, so far we see three reliable ways to do that

• Include MySQL’s SQL parser into mysqlbinlog
• Have the master annotate the statements with easily-parseable information about which tables are updated in the statement.
• Do not try to solve the problem on mysqlbinlog side, delay it until the point where we do understand SQL. As the first option, let the server support @@ignore_tables session variable and then let mysqlbinlog print SET ignore_table=... as the first statement in its output.

All three approaches have certain drawbacks. The first seems like an overkill and will have no potential to ever be able to work for VIEWs. The second will increase the size of binary logs and won’t work for un-annotated binary logs produced by legacy servers. The third approach requires this @@ignore_tableshack in the server and doesn’t really do filtering, which might be a nuisance when one does some additional processing on mysqlbinlog‘s output … I’m still swaying.