Memcached Module

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)
5.1. 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

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

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

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:

4. Parameters

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 10800s (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

  • $mct(key)

  • $mcinc(key)

  • $mcdec(key)

  • $mctex(key)

Exported pseudo-variables are documented at