# &adminguide;

## 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.

### 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
    ...

### 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)");
    ...

### Set custom expire time when adding an entry

    ...
    $mct(test=>10) = 1;
    xlog("stored value is $mct(test)");
    # sleep 10 seconds
    xlog("stored value is $mct(test)"); # will return <null>
    ...

### 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 set up, as no dedicated server needs to be provided. But when a distributed storage facility is necessary, or one want to separate the storage from the SIP server, this module could be used.

## Implementation notes

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

### 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.

### 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 sufficiently 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.

## Dependencies

### &kamailio; Modules

The following modules must be loaded before this module:

- No dependencies on other &kamailio; modules.

### External Libraries or Applications

The following libraries or applications must be installed before running &kamailio; with this module loaded:

- the libmemcached library.

- the memcached server implementation.

## Parameters

### `servers` (str)

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

Default value is localhost:11211.

#### Set `servers` parameter

    ...
    modparam("memcached", "servers", "localhost:11211")
    ...

### `expire` (integer)

The default expire value of entries in memcached in seconds. The maximal value is 2592000 (about 30 days). 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.

It is possible to override this default value when adding a key with the mct pseudo-variable, or later on by setting a different timeout for an existing key with the mctex pseudo-variable.

Default value is 10800s (3h).

#### Set `expire` parameter

    ...
    modparam("memcached", "expire", 10800)
    ...

### `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).

#### Set `mode` parameter

    ...
    modparam("memcached", "mode", 0)
    ...

### `timeout` (integer)

The timeout for the memcache servers access in milliseconds.

Default value is 5000 (5s).

#### Set `timeout` parameter

    ...
    modparam("memcached", "timeout", 10000)
    ...

### `memory` (integer)

The memory mode for the memcached client library. The library can use the system memory manager or the internal memory manager from &kamailio;. The system memory manager configuration is the default, most implementations (like other projects) probably use this approach as well. The internal memory configuration should be faster and protects better against memory leaks that could bring down your server, as the available memory pool is limited by the &kamailio; configuration.

Default value is 0 (use system memory manager).

#### Set `memory` parameter

    ...
    modparam("memcached", "memory", 1)
    ...

### `stringify` (integer)

The string mode for the memcached module. By default the module checks the flags for each returned value from the memcached library to decide to evaluate it as string or numerical value. If you need interoperability with existing applications that are not able to set this flag, you can force the module to evaluate all values as strings.

Default value is 0 (don't force string values).

#### Set `stringify` parameter

    ...
    modparam("memcached", "stringify", 1)
    ...

### Exported pseudo-variables

- $mct(key)

- $mct(key=>expiry)

- $mcinc(key)

- $mcdec(key)

- $mctex(key)

Exported pseudo-variables are documented at &kamwikilink;.