Memcached and PHP Sessions in multiple servers

Question

I will like to know how memcached manage cache for php sessions i mean. I would like to design a php app that scale out and in each http-PHP server include a memcached layer for (db,app cache and session caching), but if memcached dont replicate de data when a user come to webserver1 dont see the same session in webserver2.

memcached1 and memcached2 need to be replicated to handle php sessions thanks in advance.

regards.

Answer 1

Joseph.

I undertand your point, I already test the architecture with a separate memcached server (and redis too). My intentions is to "pack" the application server in a unit (docker) and the measure the load parameters to deploy a new instance, to scale out the infraestructure.

I found this.

https://www.digitalocean.com/community/tutorials/how-to-share-php-sessions-on-multiple-memcached-servers-on-ubuntu-14-04

thanks for your reply!

regards.

Answer 2

While I agree there is no question here we could try to help the OP understand how memcache works.

When you use memcache which is an in-memory cache how you set it up is determined upon your current infrastructure.

For instance if you only have 1 web server you could install memcache on that same machine along with the database layer being on that machine as well. This works for increasing performance of the site because the site can get data from memcache (in memory) rather than from the database (on disk, and slower to read). Using it in this manner is good but as your site requires better performance or scalability you would probably start up a cluster of web servers behind a load balancer.

This is when things can get a bit tricky. You have all these machines and you are thinking that you need to have memcache on every machine so how do we replicate these instances? The simple answer is you don't. If you have multiple web servers the best method is to put memcache on it's own server (or cluster behind a load balancer), this way every web server is hitting the same IP address for the memcache server(s).

You do not need to worry about keeping anything in sync because the way memcache works is it creates a hash that specifies which server the key has been assigned to (when you have a cluster of memcache servers).

Based on this question it would appear that you would need to do one of the following:

1.) Read up on system architecture
2.) Hire someone to architect your systems layer.

My best suggestion would be to use a single server for your memcache instance and set the web servers to use that for memcache rather than trying to run memcache on each of the web servers.