MySQL replication in action - Part 3: all-masters P2P topology

Previous episodes:

• MySQL replication monitoring 101
• MySQL replication in action - Part 1: GTID & Co
• MySQL replication in action - Part 2 - Fan-in topology

In the previous article, we saw the basics of establishing replication from multiple origins to the same destination. By extending that concept, we can deploy more complex topologies, such as the point-to-point (P2P) all-masters topology, a robust and fast way of moving data.

Introduction to P2P all-masters topology

A P2P (Point-to-point) topology is a kind of deployment where replication happens in a single step from the producer to the consumers. For example, in a master/slave topology, replication from the master (producer) reaches every slave (consumer) in one step. This is simple P2P replication. If we use a hierarchical deployment, where every slave that is connected to the master is also replicating to one or more slaves, we will have a 2-step replication (Figure 1). Similarly, in circular replication, we have as many steps as the number of nodes minus one (Figure 2.)
Figure 1 - Hierarchical replication depth of processing

Changed defaults between MySQL 5.6 and 5.7

MySQL 5.7 comes with many changes. Some of them are better explained than others.

I wanted to see how many changes I could get by comparing SHOW VARIABLES in MySQL 5.6 and 5.7.
The most notable ones are:

• binlog_format: the default is now ROW. This variable affects the format of the binary log, whether you use it as a backup complement or for replication, the change means bigger binary logs and possibly side effects.
• binlog_error_action now defaults to ABORT_SERVER. If the server cannot write to the binary log, rather than continuing its work without logging, it shuts down. This could be a desirable course of action, but better be prepared for the eventuality.
• innodb_strict_mode is enabled by default, which is probably a good thing, but it means that previously accepted events will now generate an error instead than a warning.
• sql_mode is now STRICT by default. While many well prepared users will be pleased with this change, which was advocated as best practice by some DBAs, the practical outcome is that several exiting applications may break because of unclean input.
• sync_binlog, which affects data safety but also server performance is enabled.

A few hacks to simulate mysqldump --ignore-database

A few days ago, Ronald Bradford asked for a mysqldump –ignore-database option.

As a workaround, he proposes:

mysqldump --databases `mysql --skip-column-names \
   -e "SELECT GROUP_CONCAT(schema_name SEPARATOR ' ') \
   FROM information_schema.schemata WHERE schema_name \
   NOT IN ('mysql','performance_schema','information_schema');" \
   >` >/mysql/backup/rds2.sql

It's a clever solution, but unfortunately it only works if you have a handful of schemas. If your databases happens to have several dozens (or hundreds or thousands) of schemas (which is where you need this option more), then the output will be truncated to the length of group_concat_max_len (by default, 1024.)

There are two alternative methods.

The all-shell method

This method lets shell commands filter the wanted databases. In its simplest way, it goes

DATABASE_LIST=$(mysql -NBe 'show schemas' | grep -wv 'mysql\|personnel\|buildings')
mysqldump --all-databases $DATABASE_LIST

Notice that, when you use --all-databases, information_schema and performance_schema are filtered off by default.

This method works, because the default length of the command line in Unix is much longer than group_concat_max_len:

$ getconf ARG_MAX
131072  # Linux

$ getconf ARG_MAX
262144  # Mac OSX

(in Windows it's much shorter: 8191, but since I haven't used Windows for ages, I don't really care).

A more elaborate method would require a scripting wrapper around the above instructions, but I think that as it is, it's simple enough to be remembered.

The options file method

If the length of all your database names combined is more than the maximum allowed by the shell, and all you want is filtering a few databases off a huge list, there is still hope.

Let's assume that you want to ignore mysql, personnel, and buildings from your backup.

echo '[mysqldump]' > mydump.cnf
mysql -NBe "select concat('ignore-table=', table_schema, '.', table_name) \
  from information_schema.tables \
  where table_schema in ('mysql', 'personnel', 'buildings')" \
  >> mydump.cnf

Now the options file looks like this:

[mysqldump]
ignore-table=mysql.db
ignore-table=mysql.host
ignore-table=mysql.user
[...]

What we need to do is tell mysqldump to get its information from this options file, and it will duly skip all the tables that are listed in there.

mysqldump --defaults-file=./mydump.cnf  -u $DBUSER -p$DBPWD --all-databases

There are two drawbacks with this approach:

• There will be a DROP DATABASE IF EXISTS and CREATE DATABASE for each of the excluded schemas, although no tables will be dumped.
• This method only works with --default-options-file. Theoretically, it should also work with --default-extra-file, but it doesn't. Therefore, if you are relying on an options file for connection parameters, they should be added to this file or listed in the command line.

Summing up, there is no perfect solution, but there are enough hacks available that you probably would find what suits you.

Evolution of MySQL metadata

I was looking at the latest MySQL versions, and I happened to notice that there has been a great increment in the number of metadata tables, both in the information_schema and performance_schema databases. So I made a simple count of both schemas in the various versions, and draw a graph. The advance looks straightforward.

version	Information_schema	performance_schema
5.0.92	17	0
5.1.54	28	0
5.1.54 with innodb plugin	35	0
5.5.8	37	17
5.6.2	48	23

The difference between 5.0 and 5.6 is staggering. We came from 17 to 71 metadata total tables. A stacked bar chart helps visualize the changes.



I noticed, BTW, that MySQL 5.0.92, which is not in active support, was released without the binaries for Mac OSX. If this kind of edition is limited to the versions in extended support, that's fine with me. I hope the habit does not contaminate the regular builds.