Memcached Module

Henning Westerholt

   Copyright © 2009 Henning Westerholt
     __________________________________________________________________

   Table of Contents

   1. Admin Guide

        1. Overview
        2. Implementation notes

              2.1. Error behaviour
              2.2. Data safety
              2.3. Size restrictions

        3. Dependencies

              3.1. Kamailio Modules
              3.2. External Libraries or Applications

        4. Parameters

              4.1. servers (str)
              4.2. expire (integer)
              4.3. mode (integer)
              4.4. timeout (integer)

        5.

              5.1. Exported pseudo-variables

   List of Examples

   1.1. Storing and retrieving entries
   1.2. Using atomic operations
   1.3. Modifying expire time for existing entries
   1.4. Set servers parameter
   1.5. Set expire parameter
   1.6. Set mode parameter
   1.7. Set timeout parameter

Chapter 1. Admin Guide

   Table of Contents

   1. Overview
   2. Implementation notes

        2.1. Error behaviour
        2.2. Data safety
        2.3. Size restrictions

   3. Dependencies

        3.1. Kamailio Modules
        3.2. External Libraries or Applications

   4. Parameters

        4.1. servers (str)
        4.2. expire (integer)
        4.3. mode (integer)
        4.4. timeout (integer)

   5.

        5.1. Exported pseudo-variables

1. Overview

   The module provides access to the distributed hash table memcached.
   This hash table is stored in memory and can can be accessed via a
   pseudo-variable: $mct(key). Entries are stored and retrieved from an
   external server application.

   The “key” can be a static string and also any existing pseudo-variable.
   Further interfaces to the functionality provided from memcached are
   also provided, like access to the atomic increment and decrement
   operations.

   Example 1.1. Storing and retrieving entries
...
$mct(test) = 1;
xlog("stored value is $mct(test)");
$mct(test) = $null; # delete it
xlog("stored value is $mct(test)"); # will return <null> or empty string
...

   Example 1.2. Using atomic operations
...
$mct(cnt) = 1;
$mcinc(cnt) = 1; # increment by 1
xlog("counter is now $mct(cnt)");
$mcdec(cnt) = 1; # decrement by 1
xlog("counter is now $mct(cnt)");
...

   Example 1.3. Modifying expire time for existing entries
...
$mct(test) = 1;
xlog("stored value is $mct(test)");
$mctex(test) = 10; # set expire time to 10 seconds
# sleep 10 seconds
xlog("stored value is $mct(test)"); # will return <null>
...

   This module is an addition to the existing htable functionality, not a
   replacement. In smaller architectures or installations where only one
   instance needs access to the hash table the htable module is easier to
   setup, as no dedicated server needs to be provided. But when a
   distributed storage facilility is necessary, or one want to separate
   the storage from the SIP server, this module could be used.

2. Implementation notes

   2.1. Error behaviour
   2.2. Data safety
   2.3. Size restrictions

   Important notes about made assumptions and adaptions that were
   necessary for the proper integration of this library into Kamailio.

2.1. Error behaviour

   The used memcache library has a rather conservative approach to error
   handling. In the default configuration each error with severity above
   “WARN” will lead to a immediately exit (or even an abort) of the
   execution, causing also a shutdown of Kamailio. This is of course not
   optimal from a availability point of view. Therefore in the internal
   error handler this is overriden. This means that we err a bit on the
   side of data integrity in order to get a sufficient availability. But
   even with this changes some problems in the memcache server can lead to
   problems in the library and subsequently also in the server.

2.2. Data safety

   Don't store data in memcached that you don't also have somewhere else.
   This system was designed as fast cache, and not for persistent storage.
   The memcached server can crash, machines can reboot or are restarted.
   If the memcache storage pool gets fulls, it starts to drop the least
   used items, even if they are not yet expired. So don't store any data
   in it where it would be a problem when it disappear from one moment to
   the other.

2.3. Size restrictions

   The maximum key length that is supported from memcached is 250
   characters. In order to support longer keys in the Kamailio
   configuration script they are hashed with MD5. This should normally be
   safe against collisions, as the value space is sufficient large enough.

   The maximum value size that is supported is 1MB. The reason for this is
   the internal memory manager used from memcached. But normally this
   restriction should be not a problem in the SIP environment where this
   module is used.

3. Dependencies

   3.1. Kamailio Modules
   3.2. External Libraries or Applications

3.1. Kamailio Modules

   The following modules must be loaded before this module:
     * No dependencies on other Kamailio modules.

3.2. External Libraries or Applications

   The following libraries or applications must be installed before
   running Kamailio with this module loaded:
     * the libmemcache library.
     * the memcached server implementation.

4. Parameters

   4.1. servers (str)
   4.2. expire (integer)
   4.3. mode (integer)
   4.4. timeout (integer)

4.1. servers (str)

   The servers to connect to. At the moment only one server is supported.

   Default value is “localhost:11211”.

   Example 1.4. Set servers parameter
...
modparam("memcached", "servers", "localhost:11211")
...

4.2. expire (integer)

   The default expire value of all entries in memcached in seconds. The
   maximal value is 2592000 (about 30 days). You can also specify an
   arbitrary unix time stamp in the future. A value of zero means that no
   automatic expiration is done, memcached will then delete the least used
   items when the cache gets full.

   Please note that memcached implements lazy caching, that means items
   are only deleted when they requested (they are of course not delivered
   to the client), or on insertion of new entries when the cache is full.
   Items can also be deleted before there expire time when the available
   space in memory is exhausted.

   Its possible to override this default value later on by setting a
   different timeout for this key with the mctex pseudo-variable.

   Default value is “10800”s (3h).

   Example 1.5. Set expire parameter
...
modparam("memcached", "expire", 10800)
...

4.3. mode (integer)

   The used storage mode for the memcached module for write access to the
   hash table. A value of “0” means to set (overwrite) the old value, with
   a value of “1” the module will not overwrite it. Here every entry to
   the hash table could be written only once, subsequent inserts will
   fail.

   Default value is “0” (overwrite).

   Example 1.6. Set mode parameter
...
modparam("memcached", "mode", 0)
...

4.4. timeout (integer)

   The timeout for the memcache servers access in milliseconds.

   Default value is “5000” (5s).

   Example 1.7. Set timeout parameter
...
modparam("memcached", "timeout", 10000)
...

   5.1. Exported pseudo-variables

5.1. Exported pseudo-variables

     * $mct(key)
     * $mcinc(key)
     * $mcdec(key)
     * $mctex(key)

   Exported pseudo-variables are documented at
   http://www.kamailio.org/dokuwiki/.
