Memcached failover

memcachednet

We have 2 memcached servers configured and use the Eniym client. When one of the server is down, it appears this server is added to the deadServers list (ServerPool.cs) and tries to resurrect the server every 10seconds (we have configured deadTimeOut to be 10seconds). Attempting to connect to the failed server causes a TCP timeout, the pages take a long time to load which results in bad user experience.

1) What is the standard way of resolving this issue? There are some posts about removing the server from the deadServers list. Is it okay to do this?

2) What is the recommended deadTimeOut setting (I understand by default it's 2 mins and we've changed it to 10seconds in our implementation)

3) Am I correct in my understanding that the cached data is not replicated across Server 1 and Server 2? If Server 1 is down, then it goes to the database to fetch these values (and it doesn't really check Server2)?

Any help is really appreciated.

Best Answer

As a general rule, it's normally expected that you just accept that the cache may or may not have what you want.
- It depends on the scenario, but it sounds like you might benefit from a higher one. There's no great loss having it higher (2-5 minutes).
- Yes. Memcache will usually cache the values again on Server 2 (after fetching from the DB because Server 1's cache is unavailable).

You probably also lower your TCP timeout being used to reconnect to the possibly-dead server.

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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.

Memcached memory usage

As Mike said, you can look at the line including the "STAT bytes" to see memory usage:

$ echo "stats" | nc -w 1 <host> <port> | awk '$2 == "bytes" { print $2" "$3 }'

Best Answer

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Memcached memory usage

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