We're considering moving our php session data to redis.
The setup looks simple. Just set the following in php.ini and restart apache. It should be all set.:
session.save_handler = redis
session.save_path = "tcp://host1:6379"
If possible I would like our users not to notice the migration. Is it possible to move the session data to redis without losing any existing session data?
There is no out-of-the-box solution available right now for what you are asking, but writing a custom script for this task can actually be fairly simple.
Esentially, phpredis stores session data in redis as strings with the following key name format: PHPREDIS_SESSION:$sessionid, where $sessionid is the php id of the session, the one retrievable via session_id(). The session data is "encoded" as a php-session serialized variable (which is a slightly different format to the common php serialize/unserialize, see session_encode).
Now that we know this, there are two possibilities to migrate session data stored in files:
Iterate through every session file (the actual path is set at session.save_path in your php.ini), read the data and write it back to redis. The files themselves store the php-session serialized representation of the session data, which means the content can be copied as it is directly to redis, and the filenames have the following pattern: sess_$sessionid, where $sessionid is, you guessed it, the id you'll want to use for your redis keys.
Migrate the data progressively by staying with file based sessions for now, but populating redis in real time as session data is being used, until the amount of sessions stored in redis looks good enough to do the switch. This could be achieved by doing something like:
$redis->set("PHPREDIS_SESSION:".session_id(), session_encode());
Right before each script ends. This method may add a little bit of overhead depending on the amount of data in session and how session_encode works.
Possible, yes, easy, not so much.
AFAIK, phpredis does not have a migration script, so you'd have to write one yourself. You may want to take a look at Cm_RedisSession's script that does something similar for that redis module.
Just created such a script in bash and added to my repo.
If you are using symfony, you can use a command like this:
yml configuration:
parameters:
redis_address: "localhost"
project_name : "ACME_"
snc_redis:
clients:
default:
type: predis
alias: default
dsn: redis://%redis_address%
logging: '%kernel.debug%'
session:
type: predis
alias: session
dsn: redis://%redis_address%/1
logging: true
session:
client: session
prefix: '%project_name%PHPREDIS_SESSION'
ttl: 7776000 # 90 days
symfony command:
<?php
// Command: app/console acme:migrate:session:files:to:redis --env=dev
namespace Acme\AppBundle\Command;
use Symfony\Component\Console\Input\InputInterface;
use Symfony\Component\Console\Output\OutputInterface;
use Symfony\Bundle\FrameworkBundle\Command\ContainerAwareCommand;
use Symfony\Component\Finder\Finder;
class MigrateSessionFilesToRedisCommand extends ContainerAwareCommand {
protected $env;
protected function configure() {
$this->setName('acme:migrate:session:files:to:redis')
->setDescription("Migrate Session Files To Redis")
->setHelp("Migrate Session Files To Redis");
}
protected function execute(InputInterface $input, OutputInterface $output) {
$sessionPath = realpath( sprintf('%s%s', $this->getContainer()->getParameter('kernel.root_dir'), '/sessions') );
$prefix = 'ACME_PHPREDIS_SESSION';
$redis = $this->getContainer()->get('snc_redis.session');
$finder = new Finder();
$finder->files()->in($sessionPath);
foreach ($finder as $file) {
$realPath = $file->getRealpath();
$sessionId = str_replace( 'sess_', '', $file->getRelativePathname() );
$redis->append( sprintf('%s:%s', $prefix, $sessionId) , file_get_contents( $realPath ) );
}
}
}
Note: Replace "ACME" with your Project ID/Name and Set The right Session Path where files are stored.
Here is my super simple script for importing session data to redis:
#!/bin/bash
export REDISCLI_AUTH=my-supper-strong-password-4-redis-server
TTL=$(( 24 * 3600 ))
for i in sess_*; do
ex=$(( $(date +%s) - $(stat -c %Y "$i") + $TTL ))
k="PHPREDIS_SESSION:${i:5}"
v=$(cat "$i")
echo "SET $k of len:${#v} EX $ex"
redis-cli SET "$k" "$v" EX $ex
done
I did not test it thoroughly, so use it caution.