Working with MySQL Binary Logs

Published: May 19, 2018


Binary Logs are a useful feature in MySQL. Mainly intended for master / slave replication setups and point-in-time recovery they contain records of all changes to the MySQL database, including schema alterations and table creations, but also INSERT, UPDATE and DELETE statements. This makes them extremely useful in offering system audit-ability to do things like forensic analysis in the case of a security breach or answer questions like “why does this product keep getting disabled on my website?”.

This post answers some common questions you might have while working with them…

Checking If Binary Logs Are Enabled

The log_bin system variable will tell you if binary logs are enabled.

mysql> SHOW VARIABLES LIKE 'log_bin';

This value cannot be changed at runtime and requires a MySQL restart to change.

While the log_bin system variable reports whether or not binary logging is enabled, the --log-bin server option specifies the location to the binary location. This can be done in [mysqld] stanza of /etc/my.cnf as follows…

log-bin = /var/lib/mysql/bin-log

Doing so will enable binary logging (there are some other options you’ll want to set as well…make sure to read this full post).

Finding The Binary Logs

The log_bin_basename system variable will tell you where the bin logs are located

mysql> SHOW VARIABLES LIKE 'log_bin_basename';

As mentioned above in “Checking If Binary Logs Are Enabled”, this value is set with the --log-bin server option. Again, typically in /etc/my.cnf you’ll see something like this…

log-bin = /var/lib/mysql/bin-log

The SHOW BINARY LOGS statement can be used to get a list of binary log files on the server.


How Binary Logs Are Rotated

The max_binlog_size variable can be consulted and changed at runtime to control the maximum size per binlog file

mysql> SHOW VARIABLES LIKE 'max_binlog_size'

The default value is 1073741824 bytes… around 1G, which is also the maximum value.

The expire_logs_days variable can be consulted and changed at runtime to control the number of days binary logs are stored for. This defaults to 0, meaning no automatic removal. This can also be added to /etc/my.cnf

expire_logs_days = 14

Binary Log Formats

There are 3 options for binary log format…

  • Row
  • Statement
  • Mixed

The specifics of these formats are detailed in the “Replication Formats” page from the MySQL documentation. However, from a high level…

  • Statement-based - Logs the actual SQL queries that are executed
  • Row-based - Logs events that indicate how table rows are changed
  • Mixed - Uses statement-based by default, but under certain circumstances switches to row-based.

MySQL defaults to row-based logging as of v5.7.7 (previously it was statement-based), however for the audit-ability use case statement-based logging is more convenient.

The binlog_format variable can be consulted and changed at runtime to control binary log format.

mysql> SHOW VARIABLES LIKE 'binlog_format';

It can also be set in my.cnf as follows

binlog-format = STATEMENT

Reading The Binary Logs

The mysqlbinlog command can be used to read the contents of the bin log. The output can be piped to commands such as tail, head, grep or less.

$ sudo mysqlbinlog /var/lib/mysql/bin-log | tail

The article “15 mysqlbinlog Command Examples for MySQL Binary Log Files” documents some of the useful flags the mysqlbinlog utility offers such as --start-datetime and --end-datetime. The man page is also provides some useful information.

Translating Row-based Binary Logs to SQL

The -v (--verbose) flag can be used to translate row-based MySQL binary logs to “pseudo-sql”.

$ sudo mysqlbinlog -v /var/lib/mysql/bin-log | less

Enabling The Binary Logs

As mentioned in “Checking If Binary Logs Are Enabled” enabling binary logging requires a mysql restart.

Typically you’ll specify the required startup options within the [mysqld] stanza of the /etc/my.cnf file. For example…


# Other options
# go here

## Replication
binlog-format = STATEMENT
expire_logs_days = 14
log-bin = /var/lib/mysql/bin-log

Note that per MySQL’s “Server System Variables” documentation in MySQL 5.7 the server-id is required if binary logging is enabled. Otherwise MySQL will refuse to start.

Test your changes in a non-production environment first and schedule a maintenance window for making them in production!

Max Chadwick Hi, I'm Max!

I'm a software developer who mainly works in PHP, but loves dabbling in other languages like Go and Ruby. Technical topics that interest me are monitoring, security and performance. I'm also a stickler for good documentation and clear technical writing.

During the day I lead a team of developers and solve challenging technical problems at Something Digital where I mainly work with the Magento platform. I've also spoken at a number of events.

In my spare time I blog about tech, work on open source and participate in bug bounty programs.

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