# MySQL/Replication
## What is replication

Replication means that data written on a master MySQL will be sent to separate server and executed there.
Applications:
* backups
* spread read access on multiple servers for scalability
* failover/HA
Replication types:
* Asynchronous replication (basic master/slave)
* Semi-asynchronous replication (asynchronous replication + enforce 1 slave replication before completing queries)
Replication configurations:
* standard: master->slave
* dual master: master<->master
In Master-Master replication both hosts are masters and slaves at the same time. ServerA replicates to serverB which replicates to serevrA. There are no consistency checks and even with auto_increment_increment/auto_increment_offset configured both servers should not be used for concurrent writes.

## Asynchronous replication

That's the most simple replication. A master writes a binary log file, and slaves can read this log file (possibly selectively) to replay the query statements. It's asynchronous, which mean the master and slaves may have different states at a specific point of time; also this setup can survive a network disconnection.

### Configuration on the master

In /etc/mysql/my.cnf, in the [mysqld] section:
* Define a server identifier (detects loops?); customarily we'll use 1 for the server, but it can be different:
* Replication is based on binary logs, so enable them:
Create a new user for the slave to connect with:
Verify your server identifier:

### Configuration on each slave

In /etc/mysql/my.cnf, in the [mysqld] section:
* Define a server identifier, different than the master (and different than the other slaves):
* Verify with:
* You can also declare the slave hostname to the master (cf. SHOW SLAVE HOSTS below):
Declare the master:
If setting up replication from backup, specify start point (add to previous command):
Start the replication:
This will create a file named master.info in your data directory, typically /var/lib/mysql/master.info; this file will contain the slave configuration and status.
TODO:

### Check the replication
#### On the slave

On a slave, type:
Or more for a more readable (line-based) output:
Example:
Check in particular:
You can inspect the asynchronous nature of the replication:
See also:

#### On the master

You can see a connection from the slave in the process list.
If you enabled report-host, the slave is also visible in:

### Consistency

Note that this replication is a simple replay, similar to feeding a mysqldump output to the mysql client. Consequently, to maintain the consistency:
* Avoid writing critical data on the slave
* Start the replication with identical initial data on both the master and the slave
* To test: we suspect it would be best to use the same version of MySQL on the master and slaves

### Fixing

By default, replicate will stop if it meets an error. This can happen if your master and slaves were not consistent in the beginning, or due to a network error causing a malformed query.
In this case, you'll get a trace in the system log (typically /var/log/syslog):
The best way is to reset the replication entirely.
You can also fix the mistake manually, and then ask MySQL to skip 1 statement this way:
You can set SQL_SLAVE_SKIP_COUNTER to any number, e.g. 100. Beware that in this case, it will skip both valid and invalid statements, not only errors.
Another way to fix broken replication is to use Maatkit tools.
* mk-slave-restart (to restart replication on slave if there are more errors and SQL_SLAVE_SKIP_COUNTER can't help)
* mk-table-checksum (to perform checksumming of tables on master and slave)
* mk-table-sync (to sync slave with master based on stats generated by mk-table-checksum)

### Uninstalling

To erase the replication:
* Type:
* Note: at this point, MySQL paused the slave and replaced the configuration with default values. The master.info file was also removed.
* Restart MySQL to clear all configuration.
Warning: STOP SLAVE will stop replication. It can be started manually again or (by default) it will automatically resume if you restart the MySQL server. To avoid auto start of replication during process of startup, add to your configuration file:
If you want to stop the replication for good (and use the server for another purpose), you need to reset the configuration as explained above.
At this point your slave configuration should be completely empty:

## SQL Hints

Some hints can be placed in comment before each request concerning the replication. For example via the PHP Mysqlnd plugin (for native driver).
* MYSQLND_MS_MASTER_SWITCH: forces the request execution on the master.
* MYSQLND_MS_SLAVE_SWITCH: forces the request execution on the slave.
* MYSQLND_MS_LAST_USED_SWITCH: forces the request execution on the last used server.

## Federated tables

As an alternative to the replication and clustering, the MySQL Federated storage engine allows to create a table on a server which synchronizes with the same on another one.

## References

* ↑ https://dev.mysql.com/doc/connectors/en/apis-php-mysqlnd-ms.quickstart.sqlhints.html