Exploring memcached caches

Both Koha and Evergreen use memcached to cache user sessions and data that would be expensive to continually fetch and refetch from the database. For example, Koha uses memcached to cache MARC frameworks, while Evergreen caches search results, bibliographic added content, search suggestions, and other data.

Even though the data that gets cached is transitory, at times it can be useful to look at it. For example, you may need to check to see if some stale data is present in the cache, or you may want to capture some statistics about user sessions that would otherwise be lost when the cache expires.

The library libMemcached include several command-line utilities for interrogating a memcached server. We’ll look at memcdump and memccat.

memcdump prints a list of keys that are (or were, since the data may have expired) stored in a memcached server. Here’s an example of the sorts of keys you might see in an Evergreen system:

memcdump --servers

The --servers bit tells memcdump to check memcached running on the local server.

A list of keys, however, doesn’t tell you much. To see the value that’s stored under that key, use memccat. Here’s an example of looking at a user session record in Koha (assuming you’ve set the SessionStorage system preference to use memcached):

memccat --servers KOHA78c879b9942dee326710ce8e046acede
_SESSION_ATIME: '1363060711'
_SESSION_CTIME: '1363060711'
_SESSION_ID: 78c879b9942dee326710ce8e046acede
branch: CPL
branchname: Centerville
cardnumber: cat
emailaddress: ''
firstname: ''
flags: 1
id: cat
lasttime: '1363060711'
number: 51
surname: cat

And here’s an example of an Evergreen user session cached object:

memccat --servers oils_auth_8682b1017b7b27035576fecbfc7715c4

We’ll let the YAMLites and JSONistas square off outside, and take a look at a final example. This is an excerpt a cached catalog search result in Evergreen:

memccat --servers open-ils.search_4b81a8a59544e8c7e9fdcda357d7b05f
{"0":{"summary":{"checked":630,"visible":"546","excluded":84,"deleted":0,"total":630,"complex_query":1},"results":[["74093"],["130197"], ...., ["880940"],["574457"]]}}

There are other tools that let you manipulate the cache, including memcrm to remove keys and memccp to load key/value pairs into memcached.

For a complete list of the command-line tools provided by libMemcached, check out its documentation. To install them on Debian or Ubuntu, run apt-get install libmemcached-tools. Note that the Debian package renames the tools from ‘memdump’ to ‘memcdump’, ‘memcat’ to ‘memccat’, etc., to avoid a naming conflict with another package.

CC BY-SA 4.0 Exploring memcached caches by Galen Charlton is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.