p_usrloc - Distributed databases

Jonas Appel

   1&1 Internet AG

Henning Westerholt

   1&1 Internet AG

Marius Zbihlei

   1&1 Internet AG

   Copyright © 2007 1&1 Internet AG
     __________________________________________________________________

   Table of Contents

   1. User's Guide

        1. Overview
        2. Dependencies

              2.1. Kamailio Modules
              2.2. External Libraries or Applications

        3. Parameters

              3.1. write_db_url (string)
              3.2. read_db_url (string)
              3.3. reg_db_table (string)
              3.4. id_column (string)
              3.5. num_column (string)
              3.6. url_column (string)
              3.7. status_column (string)
              3.8. failover_time_column (string)
              3.9. spare_flag_column (string)
              3.10. error_column (string)
              3.11. risk_group_column (string)
              3.12. expire_time (int)
              3.13. db_err_threshold (int)
              3.14. failover_level (int)
              3.15. db_retry_interval (int)
              3.16. db_use_transactions (int)
              3.17. db_transaction_level (string)
              3.18. write_on_master_db (int)
              3.19. write_on_db (int)
              3.20. connection_expires (int)
              3.21. alg_location (int)
              3.22. domain_db(str)
              3.23. default_db_type(str)

        4. default_db_url(str)
        5. Changes from usrloc module

              5.1. db_mode (integer)
              5.2. db_url

        6. Installation & Running

              6.1. Database setup

                    6.1.1. Configuration Table

              6.2. Maintenance
              6.3. Additional configuration

   2. Developer's Guide

        1. load_ul_db_api(ul_db_api_t * api)
        2. int (* ul_db_insert_t) (str * table, str * first, str * second,
                db_key_t* _k, db_val_t* _v, int _n)

        3. int (* ul_db_update_t) (str * table, str * first, str * second,
                db_key_t* _k, db_op_t * _op, db_val_t* _v, db_key_t* _uk,
                db_val_t* _uv, int _n, int _un);

        4. int (* ul_db_insert_update_t) (str * table, str * first, str *
                second, db_key_t* _k, db_val_t* _v, int _n)

        5. int (* ul_db_replace_t) (str * table, str * first, str *
                second, db_key_t* _k, db_val_t* _v, int _n)

        6. int (* ul_db_delete_t) (str * table, str * first, str * second,
                db_key_t* _k, db_op_t* _o, db_val_t* _v, int _n)

        7. int (* ul_db_query_t) (str * table, str * first, str * second,
                db_con_t *** _r_h, db_key_t* _k, db_op_t* _op, db_val_t*
                _v, db_key_t* _c, int _n, int _nc, db_key_t _o, db_res_t**
                _r);

        8. int (* ul_db_free_result_t)(db_con_t ** dbh, db_res_t * res);

   List of Examples

   1.1. Set write_db_url parameter
   1.2. Set read_db_url parameter
   1.3. Set reg_db_table parameter
   1.4. Set id_column parameter
   1.5. Set num_column parameter
   1.6. Set url_column parameter
   1.7. Set status_column parameter
   1.8. Set failover_time_column parameter
   1.9. Set spare_flag_column parameter
   1.10. Set error_column parameter
   1.11. Set risk_group_column parameter
   1.12. Set expire_time parameter
   1.13. Set db_err_threshold parameter
   1.14. Set failover_level parameter
   1.15. Set db_retry_interval parameter
   1.16. Set db_use_transactions parameter
   1.17. Set db_transaction_level parameter
   1.18. Set write_on_master_db parameter
   1.19. Set write_on_db parameter
   1.20. Set connection_expires parameter
   1.21. Set alg_location parameter
   1.22. Set domain_db parameter
   1.23. Set default_db_type parameter
   1.24. Set default_db_type parameter
   1.25. Set db_mode parameter
   1.26. Example database content - reg_table (locdb) table

Chapter 1. User's Guide

   Table of Contents

   1. Overview
   2. Dependencies

        2.1. Kamailio Modules
        2.2. External Libraries or Applications

   3. Parameters

        3.1. write_db_url (string)
        3.2. read_db_url (string)
        3.3. reg_db_table (string)
        3.4. id_column (string)
        3.5. num_column (string)
        3.6. url_column (string)
        3.7. status_column (string)
        3.8. failover_time_column (string)
        3.9. spare_flag_column (string)
        3.10. error_column (string)
        3.11. risk_group_column (string)
        3.12. expire_time (int)
        3.13. db_err_threshold (int)
        3.14. failover_level (int)
        3.15. db_retry_interval (int)
        3.16. db_use_transactions (int)
        3.17. db_transaction_level (string)
        3.18. write_on_master_db (int)
        3.19. write_on_db (int)
        3.20. connection_expires (int)
        3.21. alg_location (int)
        3.22. domain_db(str)
        3.23. default_db_type(str)

   4. default_db_url(str)
   5. Changes from usrloc module

        5.1. db_mode (integer)
        5.2. db_url

   6. Installation & Running

        6.1. Database setup

              6.1.1. Configuration Table

        6.2. Maintenance
        6.3. Additional configuration

1. Overview

   The p_usrloc module is built upon the usrloc module and provides the
   same usrloc export to the registrar module (for example). The usrloc
   module must not be loaded at the same time with the p_usrloc module.
   The parameters and the interface for the p_usrloc module are thus
   almost the same, so in this document only extra parameters (specific
   only to p_usrloc module) and differences are noted. For a complete
   description of the paramters that are inherited from usrloc see the
   page here. Note that the modparam statement must still refer to the
   p_usrloc module.

   The goal of the p_usrloc module is to provide partionioned user
   location service to Kamailio/SER. This provides three major benefits:
     * Redundancy Failure of a location database does not mean that the
       location service is down
     * Load Balancing DB performance is greatly increased as queries are
       automatically split on a pool of ID(number configurable at
       runtime). Furthermore, a read only db handler can be used to
       further increase the spead of the "select" queries.
     * Failover A faulty DB server is detected and taken out. When it
       recovers, the module makes sure that no stale contacts are returned
       from it.

   Write queries are replicated to two or more databases, depending on
   some distribution keys, e.g. the username and domain the subscribers.
   The number of databases used for replication is configurable at compile
   time by modifying the DB_NUM define. Its main field of application is
   for storing and retrieving location data. The actual database for this
   subscriber is choosen from a set that is specified in the master
   database. Read queries are executed on one database of the set. If this
   database is not available, the other database would be used. This
   failure is recorded in the master database.

   Connections to the distributed databases are setup prior the actual
   query is executed. The module maintains a cache of the database handles
   to provide better performance. This handles are periodically checked if
   there are still valid and if needed updated from the master database.
   As default only one key is used for the database lookup, but the
   additional usage of a second one is possible.

   Changes in the master database are autodetected. For better performance
   it is possible to use one read-only instance, that is locally
   installed, and write only errors and other status information to a
   central master database. It is also possible to specify different
   failover schemas and even use transaction to provide additional data
   safety. This module could further try to minimise the impact of a
   database error with the usage of spare databases that are automatically
   activated. It also supports the manual deactivation of redundant
   databases for maintenance purposes.

Warning

   The p_usrloc module still has some missing feature, like automatic
   expiry of contacts and dumping of all users via the fifo cmd.

2. Dependencies

   2.1. Kamailio Modules
   2.2. External Libraries or Applications

2.1. Kamailio Modules

   The following modules must be loaded before this module:
     * a database module necessary to connect to the master database and
       to the distributed databases.

2.2. External Libraries or Applications

   The following libraries or applications must be installed before
   running Kamailio with this module loaded:
     * None.

3. Parameters

   3.1. write_db_url (string)
   3.2. read_db_url (string)
   3.3. reg_db_table (string)
   3.4. id_column (string)
   3.5. num_column (string)
   3.6. url_column (string)
   3.7. status_column (string)
   3.8. failover_time_column (string)
   3.9. spare_flag_column (string)
   3.10. error_column (string)
   3.11. risk_group_column (string)
   3.12. expire_time (int)
   3.13. db_err_threshold (int)
   3.14. failover_level (int)
   3.15. db_retry_interval (int)
   3.16. db_use_transactions (int)
   3.17. db_transaction_level (string)
   3.18. write_on_master_db (int)
   3.19. write_on_db (int)
   3.20. connection_expires (int)
   3.21. alg_location (int)
   3.22. domain_db(str)
   3.23. default_db_type(str)

3.1. write_db_url (string)

   The url to the master database where errors are written to.

   Default value is "mysql://openser:openserrw@localhost/openser"

   Example 1.1. Set write_db_url parameter
...
modparam("p_usrloc", "write_db_url", "mysql://username:password@localhost/databa
sename")
...

3.2. read_db_url (string)

   The url to the master database where the distribution data is read
   from. It is seperated from write access, so for better performance, a
   local replicate can be used for read access.

   Default value is mysql://openser:openserrw@localhost/openser .

   Example 1.2. Set read_db_url parameter
...
modparam("p_usrloc", "read_db_url", "mysql://user:passwd@localhost/db")
...

3.3. reg_db_table (string)

   Specifies the table where the information about the distributed
   databases is stored.

   Default value is "locdb".

   Example 1.3. Set reg_db_table parameter
...
modparam("p_usrloc", "reg_db_table", "locdb")
...

3.4. id_column (string)

   Specifies the column where the id of a distributed database is stored.

   Default value is "id".

   Example 1.4. Set id_column parameter
...
modparam("p_usrloc", "id_column", "id")
...

3.5. num_column (string)

   Specifies the column where the associated number of the distributed
   database is stored. For each distributed database ID there must be at
   least two databases available, the databases above the second are
   ignored.

   Default value is "no".

   Example 1.5. Set num_column parameter
...
modparam("p_usrloc", "num_column", "nr")
...

3.6. url_column (string)

   Specifies the column where the url of the distributed database is
   stored.

   Default value is "url".

   Example 1.6. Set url_column parameter
...
modparam("p_usrloc", "url_column", "url")
...

3.7. status_column (string)

   Specifies the column where the status of the distributed database is
   stored.

   Default value is "status".

   Example 1.7. Set status_column parameter
...
modparam("p_usrloc", "status_column", "status")
...

3.8. failover_time_column (string)

   Specifies the column where the failover time of the location databases
   is stored. This field is set to the current time when a databases is
   turned off or turned on.

   Default value is "failover".

   Example 1.8. Set failover_time_column parameter
...
modparam("p_usrloc", "failover_time_column", "fail")
...

3.9. spare_flag_column (string)

   Specifies the column where the spare flag of the location databases is
   stored. It is possible to declare an entry in the p_usrloc table as
   spare which is used in a failover. Due to the fact that the data is
   stored in two databases and it takes the spare the complete expire time
   to be up to date, it is not very useful.

   Default value is "spare".

   Example 1.9. Set spare_flag_column parameter
...
modparam("p_usrloc", "spare_flag_column", "spare")
...

3.10. error_column (string)

   Specifies the column where the errors of the distributed databases are
   stored. Each call to db_handle_error increases the error counter. After
   exceeding the error threshold, the database's status is set to off.

   Default value is "errors".

   Example 1.10. Set error_column parameter
...
modparam("p_usrloc", "error_column", "error")
...

3.11. risk_group_column (string)

   Specifies the column where the risk group of the distributed databases
   is stored. All databases on one host are in the same risk group. This
   is only useful when using spares and prevents the module from taking a
   spare which shares the same risk as the broken database.

   Default value is "rg".

   Example 1.11. Set risk_group_column parameter
...
modparam("p_usrloc", "risk_group_column", "rg")
...

3.12. expire_time (int)

   Specifies the time (in seconds) when a contact expires, used for
   resetting the failover time of a reactivated database. It should be
   equal or greater than the contact expiration time of the registrar
   module.

   Default value is "3600".

   Example 1.12. Set expire_time parameter
...
modparam("p_usrloc", "expire_time", "3600")
...

3.13. db_err_threshold (int)

   Specifies the error value on which a database shall be turned of.

   Default value is "50".

   Example 1.13. Set db_err_threshold parameter
...
modparam("p_usrloc", "db_err_threshold", "50")
...

3.14. failover_level (int)

   Specifies the manner a failover is done. Following modes are supported:
     * 1 - Just turn off the broken database
     * 2 - Try to find a spare, if none found, just turn off the broken
       database

   Default value is "1".

   Example 1.14. Set failover_level parameter
...
modparam("p_usrloc", "failover_level", "1")
...

3.15. db_retry_interval (int)

   Specifies the interval (in seconds) in which a timer process shall
   check the availability of the databases and try to reconnect to broken
   ones. It don't make sense to choose a lower value as 10. It's necessary
   to provide a writeable master database, otherwise this check stays
   disabled.

   Default value is "10".

   Example 1.15. Set db_retry_interval parameter
...
modparam("p_usrloc", "db_retry_interval", "10")
...

3.16. db_use_transactions (int)

   Specifies if transactions should be used (set to 1) to reach a higher
   data consistency. Keep in mind that this will probably decrease
   performance.

   Default value is "0".

   Example 1.16. Set db_use_transactions parameter
...
modparam("p_usrloc", "db_use_transactions", "0")
...

3.17. db_transaction_level (string)

   Specifies the isolation level on which transactions are performed.
   Possible values: Modes supported by the database backend. In order to
   activate transaction the db_use_transactions parameter must be also
   set.

   Default value is "READ UNCOMMITED".

   Example 1.17. Set db_transaction_level parameter
...
modparam("p_usrloc", "db_transaction_level", "READ UNCOMMITED")
...

3.18. write_on_master_db (int)

   Sets the write access to the master database. If set to 0, no write
   operations are permitted on the master database.

   Default value is "0".

   Example 1.18. Set write_on_master_db parameter
...
modparam("p_usrloc", "write_on_master_db", "0")
...

3.19. write_on_db (int)

   Sets the write access to the distributed databases. If set to 0, no
   write operations are permitted on the databases.

   Default value is "0".

   Example 1.19. Set write_on_db parameter
...
modparam("p_usrloc", "write_on_db", "0")
...

3.20. connection_expires (int)

   Specifies the period (in seconds) after a database connection expires.
   Usage of a too small value will probably decrease the performance.

   Default value is "300".

   Example 1.20. Set connection_expires parameter
...
modparam("p_usrloc", "connection_expires", "300")
...

3.21. alg_location (int)

   Specify the way the distribution of the subscriptions are computed. At
   the moment the only way is to use the CRC32 algorithm to compute the
   location ID. Any integer value is fine.

   Default value is "0".

   Example 1.21. Set alg_location parameter
...
modparam("p_usrloc", "alg_location", 1)
...

3.22. domain_db(str)

   Specify the way the lookup is made. In can be either partitioned or
   single. For example, if you have a location table that is large and
   needs to be partitioned, and a smaller table cfa that is ok to be on
   only the master db(so there is no need to have it distributed), you can
   set this parameter to "location=cluster,cfa=single". This means that a
   call to
lookup(location)

   will be done via the partition databases configured via the
   reg_db_table parameter, but a call to
lookup(cfa)

   will be done on only the master database (as with usrloc module)

   Default value is "location=cluster,cfa=single".

   Example 1.22. Set domain_db parameter
...
modparam("p_usrloc", "domain_db", "location=cluster,cfa=single")
...

3.23. default_db_type(str)

   In case a domain (like location,cfa) is not matched by a domain_db
   definition, the type is configured by using this parameter. Accepted
   values are single and cluster.

   Default value is "single".

   Example 1.23. Set default_db_type parameter
...
modparam("p_usrloc", "default_db_type", "cluster")
...

4. default_db_url(str)

   The URL of the default database for Location domains (for domains that
   are single). This must be configured if they are use.

   Default value is "DEFAULT_DB_URL".

   Example 1.24. Set default_db_type parameter
...
modparam("p_usrloc", "default_db_url", "mysql://ser:ser@localhost/ser")
...

5.  Changes from usrloc module

   5.1. db_mode (integer)
   5.2. db_url

5.1. db_mode (integer)

   The p_usrloc module must utilize database for persistent contact
   storage. Only mode 3 is possible at this time. Because of the way other
   matching mode work, they make no sense on a distributed environment.
     * 3 - DB-Only scheme. No memory cache is kept, all operations being
       directly performed with the database. The timer deletes all expired
       contacts from database - cleans after clients that didn't
       un-register or re-register. The mode is useful if you configure
       more servers sharing the same DB without any replication at SIP
       level. The mode may be slower due the high number of DB operation.
       For example NAT pinging is a killer since during each ping cycle
       all nated contact are loaded from the DB; The lack of memory
       caching also disable the statistics exports.

   Default value is 3.

   Example 1.25. Set db_mode parameter
...
modparam("p_usrloc", "db_mode", 2)
...

5.2. db_url

   This parameters is now obsolete, and replaced by specific p_usrloc
   parameters

6. Installation & Running

   6.1. Database setup

        6.1.1. Configuration Table

   6.2. Maintenance
   6.3. Additional configuration

6.1. Database setup

6.1.1. Configuration Table

   Before running Kamailio with p_usrloc, you have to setup the master
   database table where the module will find data about the distributed
   databases. For that, if the table was not created by the installation
   script or you choose to install everything by yourself you can use the
   p_usrloc-create.sql SQL script in the database directories in the
   openser/scripts folder as template. Database and table name can be set
   with module parameters so they can be changed, but the name of the
   columns must be as they are in the SQL script. You can also find the
   complete database documentation on the project webpage,
   http://www.kamailio.org/docs/db-tables/kamailio-db-devel.html.

   Example 1.26. Example database content - reg_table (locdb) table
...
+----+----+------+--------+--------+---------------------+-------+----+
| id | no | url  | status | errors | failover            | spare | rg |
+----+----+------+--------+--------+---------------------+-------+----+
|  1 |  1 | URL1 |      1 |      0 | 1900-01-01 00:00:01 |     0 |  0 |
|  1 |  2 | URL2 |      1 |      0 | 1900-01-01 00:00:01 |     0 |  0 |
|  2 |  1 | URL3 |      1 |      0 | 1900-01-01 00:00:01 |     0 |  0 |
|  2 |  2 | URL4 |      1 |      0 | 1900-01-01 00:00:01 |     0 |  0 |
+----+----+------+--------+--------+---------------------+-------+----+
...

   The URLs are omitted for a better overview, but you can use standard
   Kamailio database URLs like mysql://openser:openserrw@localhost/openser
   Databases don't need to be on different hosts, e.g. for testing
   purposes.

   This table contains two database groups. The first with id one, and the
   second with the id two.

6.2. Maintenance

   The module supports the decativation of redundant databases for
   maintenance reasons. This can be done by setting the status column of
   the respective database in the p_usrloc to the value "2". This setting
   is autodetected from all modules on the server cluster. Changes in the
   locdb table are periodically polled with help of a timer process. After
   one minute the all read and write traffic is removed from the
   deactivated database, and maintenance can be done.

   In order to activate the database again, after the maintenance has been
   finished, the respective status column needs to be set to "0". This is
   autodetected as well, the first module that noticed the change will set
   the status column to "1" and setting the failover column to the current
   time and date. Write requests are now transfered again to the database,
   but no reads are done yet.

   After the configured expire_time has been passed, i.e. every contact
   has been inserted at least one time in the database the respective
   failover time column is set to the default value again. The database is
   then also used to read contacts from, the cluster is in normal
   operation with full redundancy again.

6.3. Additional configuration

   As this module is only used internally from other modules, there is no
   additional configuration except for the module parameter setup
   necessary.

Chapter 2. Developer's Guide

   Table of Contents

   1. load_ul_db_api(ul_db_api_t * api)
   2. int (* ul_db_insert_t) (str * table, str * first, str * second,
          db_key_t* _k, db_val_t* _v, int _n)

   3. int (* ul_db_update_t) (str * table, str * first, str * second,
          db_key_t* _k, db_op_t * _op, db_val_t* _v, db_key_t* _uk,
          db_val_t* _uv, int _n, int _un);

   4. int (* ul_db_insert_update_t) (str * table, str * first, str *
          second, db_key_t* _k, db_val_t* _v, int _n)

   5. int (* ul_db_replace_t) (str * table, str * first, str * second,
          db_key_t* _k, db_val_t* _v, int _n)

   6. int (* ul_db_delete_t) (str * table, str * first, str * second,
          db_key_t* _k, db_op_t* _o, db_val_t* _v, int _n)

   7. int (* ul_db_query_t) (str * table, str * first, str * second,
          db_con_t *** _r_h, db_key_t* _k, db_op_t* _op, db_val_t* _v,
          db_key_t* _c, int _n, int _nc, db_key_t _o, db_res_t** _r);

   8. int (* ul_db_free_result_t)(db_con_t ** dbh, db_res_t * res);

   These are the primary functions that are used to perform the SQL
   queries.

1. load_ul_db_api(ul_db_api_t * api)

   Import the dbd API, setup the master database connection.

   Meaning of the parameters is as follows:
     * api - Pointer to distributed database API structure

2. int (* ul_db_insert_t) (str * table, str * first, str * second, db_key_t*
_k, db_val_t* _v, int _n)

   Lookup the first and if needed the second key, and insert the given
   values into the choosen databases.

   Meaning of the parameters is as follows:
     * table - Pointer to the table name.
     * first - Pointer to the first key.
     * second - Pointer to the second key.
     * _k - Pointer to the inserted db keys.
     * _v - Pointer to the inserted db values.
     * _n - Number of key-value pairs in _k and _v parameters.

3. int (* ul_db_update_t) (str * table, str * first, str * second, db_key_t*
_k, db_op_t * _op, db_val_t* _v, db_key_t* _uk, db_val_t* _uv, int _n, int
_un);

   Lookup the first and if needed the second key, and update the given
   values in the choosen databases.

   Meaning of the parameters is as follows:
     * table - Pointer to the table name.
     * first - Pointer to the first key.
     * second - Pointer to the second key.
     * _k - Pointer to the db keys that will be matched.
     * _op - Pointer to the db options for this operation.
     * _v - Pointer to the db values that will be matched.
     * _uk - Pointer to the updated db keys.
     * _uv - Pointer to the updated db values.
     * _n - Number of key-value pairs in _k and _v parameters.
     * _un - Number of key-value pairs in _uk and _uv parameters.

4. int (* ul_db_insert_update_t) (str * table, str * first, str * second,
db_key_t* _k, db_val_t* _v, int _n)

   Lookup the first and if needed the second key, and insert on duplicate
   key update the given values in the choosen databases. This is like an
   insert, but update if the key already exist.

   Meaning of the parameters is as follows:
     * table - Pointer to the table name.
     * first - Pointer to the first key.
     * second - Pointer to the second key.
     * _k - Pointer to the inserted or updated db keys.
     * _v - Pointer to the inserted or updated db values.
     * _n - Number of key-value pairs in _k and _v parameters.

5. int (* ul_db_replace_t) (str * table, str * first, str * second, db_key_t*
_k, db_val_t* _v, int _n)

   Lookup the first and if needed the second key, and replace the given
   values in the choosen databases.

   Meaning of the parameters is as follows:
     * table - Pointer to the table name.
     * first - Pointer to the first key.
     * second - Pointer to the second key.
     * _k - Pointer to the replaced db keys.
     * _v - Pointer to the replaced db values.
     * _n - Number of key-value pairs in _k and _v parameters.

6. int (* ul_db_delete_t) (str * table, str * first, str * second, db_key_t*
_k, db_op_t* _o, db_val_t* _v, int _n)

   Lookup the first and if needed the second key, and delete the given
   values into the choosen databases.

   Meaning of the parameters is as follows:
     * table - Pointer to the table name.
     * first - Pointer to the first key.
     * second - Pointer to the second key.
     * _k - Pointer to the deleted db keys.
     * _op - Pointer to the db options for this operation.
     * _v - Pointer to the deleted db values.
     * _n - Number of key-value pairs in _k and _v parameters.

7. int (* ul_db_query_t) (str * table, str * first, str * second, db_con_t
*** _r_h, db_key_t* _k, db_op_t* _op, db_val_t* _v, db_key_t* _c, int _n, int
_nc, db_key_t _o, db_res_t** _r);

   Lookup the first and if needed the second key, and performs a query in
   one of the choosen databases. The returned handle _r_h must be used to
   free the result set after the usage of the returned database entries,
   otherwise a memory leak will occur.You must call ul_db_free_result
   before you can call ul_db_query again!

   Meaning of the parameters is as follows:
     * table - Pointer to the table name.
     * first - Pointer to the first key.
     * second - Pointer to the second key.
     * _r_h - Pointer to the result handle, to free the result.
     * _op - Pointer to the db options for this operation.
     * _k - Pointer to the queried db keys.
     * _v - Pointer to the queried db values.
     * _c - Pointer to the db keys that should be returned.
     * _n - Number of key-value pairs in _k and _v parameters.
     * _nc - Number of key-value pairs in _c parameter.
     * _o - Order by options for the query.
     * _nc - Pointer to the result set.

8. int (* ul_db_free_result_t)(db_con_t ** dbh, db_res_t * res);

   Frees the given result set, .

   Meaning of the parameters is as follows:
     * dbh - Pointer to the result handle.
     * res - Pointer to the result.
