DB Cassandra Module

Anca Vamanu

   <anca.vamanu@1and1.ro>

Boudewyn Ligthart

   <bligthart@btlnet.co.uk>

Edited by

Anca Vamanu

   <anca.vamanu@1and1.ro>

   Copyright © 2012 1&1 Internet AG
     __________________________________________________________________

   Table of Contents

   1. Admin Guide

        1. Overview
        2. Dependencies

              2.1. SIP Router Modules
              2.2. External Libraries or Applications

        3. Parameters

              3.1. schema_path (string)

        4. Functions
        5. Installation
        6. Table schema
        7. Limitations

   List of Examples

   1.1. Set schema_path parameter

Chapter 1. Admin Guide

   Table of Contents

   1. Overview
   2. Dependencies

        2.1. SIP Router Modules
        2.2. External Libraries or Applications

   3. Parameters

        3.1. schema_path (string)

   4. Functions
   5. Installation
   6. Table schema
   7. Limitations

1. Overview

   Db_cassandra is one of the Kamailio database modules. It does not
   export any functions executable from the configuration scripts, but it
   exports a subset of functions using the database API, and thus, other
   modules can use it as a database driver, instead of, for example, the
   Mysql module.

   The storage backend is a Cassandra cluster and this module provides an
   SQL interface to be used by other modules for storing and retrieving
   data. Because Cassandra is a NoSQL distributed system, there are
   limitations on the operations that can be performed. The limitations
   concern the indexes on which queries are performed, as it is only
   possible to have simple conditions (equality comparison only) and only
   two indexing levels. These issues will be explained in an example
   below.

   Cassandra DB is especially suited for storing large amounts of data or
   data that requires distribution, redundancy or replication. One usage
   example is a distributed location system in a platform that has a
   cluster of SIP Router servers, with several proxies and registration
   servers accessing the same location database. This was actually the
   main use case we had in mind when implementing this module. Please NOTE
   that it has only been tested with the usrloc, auth_db and domain
   modules.

   You can find a configuration file example for this usage in the module
   - kamailio_cassa.cfg.

   Because the module has to do the translation from SQL to Cassandra
   NoSQL queries, the schemas for the tables must be known by the module.
   You will find the schemas for location, subscriber and version tables
   in utils/kamctl/dbcassandra directory. You have to provide the path to
   the directory containing the table definitions by setting the module
   parameter schema_path.

   There is no need to configure a table metadata in Cassandra cluster.
   You only need to define a keyspace with the name of the database and
   for each table a column family inside that keyspace with the name of
   the table. The comparator and validators should be either UTF8Type or
   ASCIIType. Example:
   ...
   create keyspace kamailio;
   use kamailio;
   create column family 'location' with comparator='UTF8Type' and
default_validation_class='UTF8Type' and key_validation_class='UTF8Type';
   ...

   Special attention was given to performance in Cassandra. Therefore, the
   implementation uses only the native row indexing in Cassandra and no
   secondary indexes, because they are costly. Instead, we simulate a
   secondary index by using the column names and putting information in
   them, which is very efficient. Also, for deleting expired records, we
   let Cassandra take care of this with its own mechanism (by setting the
   TTL for columns).

   The module supports raw queries. However these queries must follow the
   CQL (Cassandra Query Language) syntax. The queries can be issued in the
   script by means of the AVPOPS module. Keep in mind that when passing
   back the results from the database only the first row is used to set
   the AVP variables. (default AVPOPS behaviour) The script lines below
   can be used as an example for issuing the query towards an cassandra
   instance.(This example will work once the column family `location` is
   configured correctly in the cassandra keyspace)
   ...
   $var(dballowed)="select * from location where key = 'userx' limit 1;";
   avp_db_query("$var(dballowed)");
   xlog("L_INFO","Got result here: [$avp(i:1)] [$avp(i:2)] [$avp(i:3)].\n");
   ...

2. Dependencies

   2.1. SIP Router Modules
   2.2. External Libraries or Applications

2.1. SIP Router Modules

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

2.2. External Libraries or Applications

   The following libraries or applications must be installed before
   running SIP Router with this module loaded:
     * Thrift library (tested with version 0.6.1 and version 0.7.0). You
       can download it from http://archive.apache.org/dist/thrift .

   The implementation was tested with Cassandra version 1.0.1 and version
   1.1.6. I used the sourced from DataStax Community Edition
   (http://www.datastax.com/download/community).

3. Parameters

   3.1. schema_path (string)

3.1. schema_path (string)

   The directory where the files with the table schemas are located. This
   directory has to contain the subdirectories corresponding to the
   database name (name of the directory = name of the database). These
   directories, in turn, contain the files with the table schemas. See the
   schemas in utils/kamctl/dbcassandra directory.

   Example 1.1. Set schema_path parameter
   ...
   modparam("db_cassandra", "schema_path",
               "/usr/local/kamailio/etc/kamctl/dbcassandra")
   ...

4. Functions

   NONE

5. Installation

   Because the db_cassandra module dependes on an external library, it is
   not compiled and installed by default. You can use one of these
   options:
     * - edit the "Makefile" and remove "db_cassandra" from
       "excluded_modules" list. Then follow the standard procedure to
       install SIP Router: "make all; make install".
     * - from command line, run: 'make all include_modules="db_cassandra";
       make install include_modules="db_cassandra"'.

6. Table schema

   The module must know the table schema for the tables that will be used.
   You must configure the path to the schema directory by setting the
   schema_path parameter.

   A table schema document has the following structure:
     * First row: the name and type of the columns in the form name(type)
       separated by spaces. The possible types are: string, int, double
       and timestamp.
       Thetimestamp type has a special meaning. Only one column of this
       type can be defined for a table, and it should contain the expiry
       time for that record. If defined this value will be used to compute
       the ttl for the columns and Cassandra will automatically delete the
       columns when they expire. Because we want the ttl to have meaning
       for the entire record, we must ensure that when the ttl is updated,
       it is updated for all columns for that record. In other words, to
       update the expiration time of a record, an insert operation must be
       performed from the point of view of the db_cassandra module
       ("insert" in Cassandra means "replace if exists or insert new
       record otherwise"). So, if you define a table with a timestamp
       column, the update operations on that table that also update the
       timestamp must update all columns. So, these update operations must
       in fact be insert operations.
     * Second row: the columns that form the row key separated by space.
     * Third row: the columns that form the secondary key separated by
       space.

   Below you can see the schema for the location table (when use_domain
   not set):

   ...
   callid(string) cflags(int) contact(string) cseq(int) expires(timestamp) flags
(int) last_modified(int) methods(int) path(string) q(double) received(string) so
cket(string) user_agent(string) username(string) ruid(string) instance(string) r
eg_id(int)
   username
   contact
   ...

   Observe first that the row key is the username and the secondary index
   is the contact. We have also defined a timestamp column - expires.

   If you need to use the domain part of the AOR also (you have set
   use_domain parameter for usrloc in the script), you should include the
   domain column in the list of columns and in the primary key. The schema
   will then look like this:
   ...
   callid(string) cflags(int) contact(string) cseq(int) domain(string) expires(t
imestamp) flags(int) last_modified(int) methods(int) path(string) q(double) rece
ived(string) socket(string) user_agent(string) username(string) ruid(string) ins
tance(string) reg_id(int)
   username domain
   contact
   ...

   Notice that a key (primary or secondary) can be composed from more
   columns, in which case you have to specify them separated by space.

   To understand why the schema looks like this, we must first see which
   queries are performed on the location table. (The 'callid' condition
   was ignored as it doesn't really have a well defined role in the SIP
   RFC).
     * When Invite received, lookup location: select where username='..'.
     * When Register received, update registration: update where
       username='..' and contact='..'.

   So, the relation between these keys is the following:
     * The unique key for a table is actually the combination of row key +
       secondary key.
     * A row defined by a row key will contain more records with different
       secondary keys.

   The timestamp column that leaves the Cassandra cluster to deal with
   deleting the expired records. For this to work right we needed to
   modify a bit the behavior of usrloc module and replace update sql query
   performed at re-registration with an insert sql query (so that all
   columns are updated and the new timestamp is set for all columns). This
   behavior is enabled by setting a parameter in the usrloc module
   db_update_as_insert:

   ...
   modparam("usrloc", "db_update_as_insert", 1)
   ...

   Also you should disable in usrloc module the timer routine that checks
   for expired records. You can do this by setting the timer interval to
   0. timer_interval:

   ...
   modparam("usrloc", "timer_interval", 0)
   ...

   The alternative would have been to define an index on the expire column
   and run a external job to periodically delete the expired records.
   However, obviously, this would be more costly.

7. Limitations

   The module can be used only when the queries use only one index, which
   is also the unique key, or have two indexes that form the unique key
   like in the usrloc usage.
