Linux – Memcache Master and slaves

The short answer: Either one is what you are looking for, but my first choice would be memcache (the first one you listed), purely based on its correct use of nomenclature.

Now here's how I came to that conclusion:

Here is a quick backgrounder in naming conventions (for those unfamiliar), which explains the frustration by the question asker: For many *nix applications, the piece that does the backend work is called a "daemon" (think "service" in Windows-land), while the interface or client application is what you use to control or access the daemon. The daemon is most often named the same as the client, with the letter "d" appended to it. For example "imap" would be a client that connects to the "imapd" daemon.

This naming convention is clearly being adhered to by memcache when you read the introduction to the memcache module (notice the distinction between memcache and memcached in this excerpt):

Memcache module provides handy procedural and object oriented interface to memcached, highly effective caching daemon, which was especially designed to decrease database load in dynamic web applications.

The Memcache module also provides a session handler (memcache).

More information about memcached can be found at » http://www.danga.com/memcached/.

The frustration here is caused by the author of the PHP extension which was badly named memcached, since it shares the same name as the actual daemon called memcached. Notice also that in the introduction to memcached (the php module), it makes mention of libmemcached, which is the shared library (or API) that is used by the module to access the memcached daemon:

memcached is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.

This extension uses libmemcached library to provide API for communicating with memcached servers. It also provides a session handler (memcached).

Information about libmemcached can be found at » http://tangent.org/552/libmemcached.html.

In summary, both are functionally the same, but they simply have different authors, and the one is simply named more appropriately than the other.

Disclaimer: You'd be mad to listen to me without doing a tonne of testing AND getting a 2nd opinion from someone qualified - I'm new to this game.

The efficiency improvement idea proposed in this question won't work. The main mistake that I made was to think that the order that the memcached stores are defined in the pool dictates some kind of priority. This is not the case. When you define a pool of memached daemons (e.g. using session.save_path="tcp://192.168.0.1:11211, tcp://192.168.0.2:11211") you can't know which store will be used. Data is distributed evenly, meaning that a item might be stored in the first, or it could be the last (or it could be both if the memcache client is configured to replicate - note it is the client that handles replication, the memcached server does not do it itself). Either way will mean that using localhost as the first in the pool won't improve performance - there is a 50% chance of hitting either store.

Having done a little bit of testing and research I have concluded that you CAN share sessions across servers using memcache BUT you probably don't want to - it doesn't seem to be popular because it doesn't scale as well as using a shared database at it is not as robust. I'd appreciate feedback on this so I can learn more...

Ignore the following unless you have a PHP app:

Tip 1: If you want to share sessions across 2 servers using memcache:

Ensure you answered Yes to "Enable memcache session handler support?" when you installed the PHP memcache client and add the following in your /etc/php.d/memcache.ini file:

session.save_handler = memcache

On webserver 1 (IP: 192.168.0.1):

session.save_path="tcp://192.168.0.1:11211"

On webserver 2 (IP: 192.168.0.2):

session.save_path="tcp://192.168.0.1:11211"

Tip 2: If you want to share sessions across 2 servers using memcache AND have failover support:

Add the following to your /etc/php.d/memcache.ini file:

memcache.hash_strategy = consistent
memcache.allow_failover = 1

On webserver 1 (IP: 192.168.0.1):

session.save_path="tcp://192.168.0.1:11211, tcp://192.168.0.2:11211"

On webserver 2 (IP: 192.168.0.2):

session.save_path="tcp://192.168.0.1:11211, tcp://192.168.0.2:11211"

Notes:

• This highlights another mistake I made in the original question - I wasn't using an identical session.save_path on all servers.
• In this case "failover" means that should one memcache daemon fail, the PHP memcache client will start using the other one. i.e. anyone who had their session in the store that failed will be logged out. It is not transparent failover.

Tip 3: If you want to share sessions using memcache AND have transparent failover support:

Same as tip 2 except you need to add the following to your /etc/php.d/memcache.ini file:

memcache.session_redundancy=2

Notes:

• This makes the PHP memcache client write the sessions to 2 servers. You get redundancy (like RAID-1) so that writes are sent to n mirrors, and failed get's are retried on the mirrors. This will mean that users do not loose their session in the case of one memcache daemon failure.
• Mirrored writes are done in parallel (using non-blocking-IO) so speed performance shouldn't go down much as the number of mirrors increases. However, network traffic will increase if your memcache mirrors are distributed on different machines. For example, there is no longer a 50% chance of using localhost and avoiding network access.
  • Apparently, the delay in write replication can cause old data to be retrieved instead of a cache miss. The question is whether this matters to your application? How often do you write session data?
• memcache.session_redundancy is for session redundancy but there is also a memcache.redundancy ini option that can be used by your PHP application code if you want it to have a different level of redundancy.
• You need a recent version (still in beta at this time) of the PHP memcache client - Version 3.0.3 from pecl worked for me.