Codeigniter: Separating reads and writes for scaling MySQL

Sukumar — Thu, 06 Aug 2009 09:41:12 +0000

Generally websites average a ratio of 9:1 or more for reads:writes for their applications which makes MySQL replication as one of the ways to scale you web application. The simplest configuration is to separate reads and writes with all the reads coming from the slave servers.

MySQL Database replication

We can implement this in codeigniter using database groups.
Below is a sample execution.

Create two active groups – one for read and one for write.

File: system/application/config/database.php

$active_group = "write";
$active_record = TRUE;

$db['read']['hostname'] = "localhost";
$db['read']['username'] = "root";
$db['read']['password'] = "";
$db['read']['database'] = "read_database_name";
$db['read']['dbdriver'] = "mysql";
$db['read']['dbprefix'] = "";
$db['read']['pconnect'] = TRUE;
$db['read']['db_debug'] = FALSE;
$db['read']['cache_on'] = FALSE;
$db['read']['cachedir'] = "";
$db['read']['char_set'] = "utf8";
$db['read']['dbcollat'] = "utf8_general_ci";

$db['write']['hostname'] = "localhost";
$db['write']['username'] = "root";
$db['write']['password'] = "";
$db['write']['database'] = "write_database_name";
$db['write']['dbdriver'] = "mysql";
$db['write']['dbprefix'] = "";
$db['write']['pconnect'] = TRUE;
$db['write']['db_debug'] = FALSE;
$db['write']['cache_on'] = FALSE;
$db['write']['cachedir'] = "";
$db['write']['char_set'] = "utf8";
$db['write']['dbcollat'] = "utf8_general_ci";

Create separate database connections to access read and write databases separately where needed (constructor is generally a good place for this).

$read_db = $this->load->database('read', TRUE);
$write_db = $this->load->database('write', TRUE);

You can now go about running queries in the usual way.

/* For reads */
$query = $read_db->get('table_name');

foreach ($query->result() as $row)
{
    echo $row->title;
}

/* For writes */
$data = array(
               'title' => $title,
               'name' => $name,
               'date' => $date
            );

$write_db->insert('mytable', $data);

In case you want to try this on an application you are already running, you can leave the “default” connection group intact and create only the “read” connection group. Use it where you think you are running read heavy queries.

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How to skip MySQL replication counter

Sukumar — Tue, 07 Oct 2008 17:47:59 +0000

There are such times when MySQL replication stops when you run certain updates on the master and the slave fails. Like for example you may have run a create table or an alter table on the master and further inserts but these DDLs get skipped and the inserts into these non-existent tables cause the slave to error out and stop.

A simple way out of this is to run the missing DDLs on the slave and push the counter by a step. This is not recommended as this might cause data inconsistency.

Use the below sql to skip the counter by 1.

SET GLOBAL SQL_SLAVE_SKIP_COUNTER = 1; START SLAVE;

Just remember to use the value 1 for any SQL statement that does not use AUTO_INCREMENT or LAST_INSERT_ID(), otherwise you will need to use the value 2. Statements that use AUTO_INCREMENT or LAST_INSERT_ID() take up 2 events in the binary log.

To skip lots of duplicate errors, you can set this in my.cnf

slave-skip-errors = 1062

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