[sr-dev] [kamailio/kamailio] ims_usrloc_pcscf: implementation of db_mode DB_ONLY (value 3) (PR #3279)

[petermarianF]

In order to support a redundant PCSCF configuration - i.e. a logical PCSCF consists of 2 physical nodes (node1 and node2) some enhancements were introduced for handling of contacts. Redundancy means that SIP messages for a client are normally handled by node1 but in case node1 is not reachable SIP messages are redirected to node2. Of course the DB_ONLY mode must work also for single PCSCF node configuration. Important aspects of this implementation are database integrity - i.e. avoid invalid table entries (for example data which are expired long time ago or have invalid states) - and keeping PCSCF cache in sync with database tables. A wrapper was built for method get_pcontact which tries to find the pcontact in cache and if search is not successful tries to download and insert from db location table - also some effort is added here to find the pcontact if it exists in cache. The contact expiry handler was modified to sync contact expiry in cache with db location entry and in case of real contact expiry sends PUBLISH to SCSCF to let NOTIFY finally delete the contact. An audit for older expired pcontacts was introduced which cares for getting rid of these contacts.
So db queries for this db mode are only supported for Mysql.
Some code was introduced to help registering callbacks for contacts which are inserted into cache when being downloaded from database - for example ims_qos callback as at the time of insertion the message that triggered the original callback registering is long gone.

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#### Type Of Change
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#### Description
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You can view, comment on, or merge this pull request online at:

  https://github.com/kamailio/kamailio/pull/3279

-- Commit Summary --

  * ims_usrloc_pcscf:   implementation of db_mode DB_ONLY (value 3)

-- File Changes --

    M src/modules/ims_usrloc_pcscf/README (11)
    M src/modules/ims_usrloc_pcscf/ims_usrloc_pcscf_mod.c (22)
    M src/modules/ims_usrloc_pcscf/pcontact.c (85)
    M src/modules/ims_usrloc_pcscf/udomain.c (511)
    M src/modules/ims_usrloc_pcscf/udomain.h (5)
    M src/modules/ims_usrloc_pcscf/ul_callback.c (52)
    M src/modules/ims_usrloc_pcscf/ul_callback.h (10)
    M src/modules/ims_usrloc_pcscf/ul_rpc.c (156)
    M src/modules/ims_usrloc_pcscf/usrloc.c (7)
    M src/modules/ims_usrloc_pcscf/usrloc.h (12)
    M src/modules/ims_usrloc_pcscf/usrloc_db.c (50)
    M src/modules/ims_usrloc_pcscf/usrloc_db.h (2)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/3279.patch
https://github.com/kamailio/kamailio/pull/3279.diff

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