Configuring GTID and binary logging

This tutorial demands a service restart since some flags here presented can not be dynamically changed

What is GTID and why do I need it? Directly from the MySQL documentation (excerpt taken as is with different jargons than used here, for master/slave we are using primary/replica):

A global transaction identifier (GTID) is a unique identifier created and associated with each transaction committed on the server of origin (the master). This identifier is unique not only to the server on which it originated, but is unique across all servers in a given replication topology.

GTID assignment distinguishes between client transactions, which are committed on the master, and replicated transactions, which are reproduced on a slave. When a client transaction is committed on the master, it is assigned a new GTID, provided that the transaction was written to the binary log. Client transactions are guaranteed to have monotonically increasing GTIDs without gaps between the generated numbers. If a client transaction is not written to the binary log (for example, because the transaction was filtered out, or the transaction was read-only), it is not assigned a GTID on the server of origin.

In theory you can use replication with only binary logging enabled, however replication with GTID is significantly more reliable. And while some providers don’t require it, at Google Cloud GTID is mandatory.

Representation

To represent a GTID a pair of coordinates are used, one is the server_uuid paired with the transaction_id which is an integer. Example of valid GTID:

GTID = 8b5dbf2a-45b4-11e8-81bc-42010a800002:25

To understand more how this impacts replication, I recommend reading the section GTID Format and Storage in the MySQL documentation.

Enabling GTID

Thankfully, to enable it you don’t need to do much, edit your mysqld.cnf file to support this variables:

server-id = 2 # Or any other number, we recommend to not be 1
log-bin = mysql-bin # Or any other valid value

gtid_mode = ON
enforce-gtid-consistency = true

Restart the database server to load up the new configuration with sudo service mysql restart.

Side effects

Some applications may cause errors due to the enforce-gtid-consistency flag. That happens because usually the application is trying to do a non-transactional action that also is not possible to replicate inside a transaction.

If you do the following:

START TRANSACTION;

CREATE TEMPORARY TABLE `tmp_users` ( id INTEGER );

COMMIT;

It is not a good practice I may add . You will get this error:

ERROR 1787 (HY000): Statement violates GTID consistency: CREATE TEMPORARY TABLE and DROP TEMPORARY TABLE can only be executed outside transactional context. These statements are also not allowed in a function or trigger because functions and triggers are also considered to be multi-statement transactions.

What are you basically doing is telling the database to create a connection, which is fine, however the following command is a CREATE TEMPORARY TABLE. This command is bound to the current connection, and because it won’t have a transaction_id it won’t be able to replicate the statement. Temporary tables are not replicated.

If your application happens to do that, all you need to do is remove the creation of temporary tables to outside of the transaction. Unfortunately Magento does not do that.

See something wrong in this tutorial? Please don’t hesitate to message me through the comments or the contact page.

How to setup a Replication User

A replication user is necessary to set up the relationship Primary/Replica. This is a short step but it needs a bit more of attention.

From the MySQL 5.7 documentation (highlights are my own):

Although you do not have to create an account specifically for replication, you should be aware that the replication user name and password are stored in plain text in the master info repository file or table (see Section 16.2.4.2, “Slave Status Logs”). Therefore, you may want to create a separate account that has privileges only for the replication process, to minimize the possibility of compromise to other accounts.

The following command specifically will allow replication from all databases and tables connecting from all hosts. For security reasons you may want to limit access to replication only to the IP address of the server doing the replication.

Log into the MySQL console using a user with GRANT privileges in the primary server and execute the following:

CREATE USER 'replication'@'%' IDENTIFIED BY 'mysupersecretpassword'
GRANT REPLICATION SLAVE ON *.* TO 'replication'@'%';

My advice is instead of using the % wildcard, set up the IP address of your replica.

This user will be added to the primary ’s MASTER_USER option, and in theory could be any user as long it also has REPLICATION SLAVE privileges. After that, the replica will connect to the primary and perform some kind of handshake with those credentials and if they match, theprimary will allow replication to occur.

See something wrong in this tutorial? Please don’t hesitate to message me through the comments or the contact page.

How to reset your `root` password on your MySQL server

You don’t need this tutorial if you have access to the root user or another one with SUPER and GRANT privileges.

The following instructions works for MySQL 5.7. You will need to stop the MySQL server and start it with mysqld_safe with the option skip-grant-tables:

sudo service mysql stop
sudo mysqld_safe --skip-grant-tables &
mysql -u root mysql

If you get an error on start, chances are there is no folder created for the mysqld_safe executable to run, on my tests I was able to solve by doing:

sudo mkdir /var/run/mysqld
sudo chown -R mysql:mysql /var/run/mysqld

And then trying to start the mysqld_safe process again.

After this, the MySQL console will pop up, and you need to set up a new password for root. The second line is necessary due to a MySQL bug #79027:

UPDATE mysql.user SET authentication_string=PASSWORD('mypassword') WHERE User='root';
UPDATE mysql.user SET plugin="mysql_native_password" WHERE User='root';
FLUSH PRIVILEGES;

Once finished, kill all MySQL processes and start the service again:

ps aux | grep mysql
sudo kill -9 [pid]
sudo service mysql start

Done, you have reset the root password! Make sure to keep it safe this time around!

See something wrong in this tutorial? Please don’t hesitate to message me through the comments or the contact page.