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Thread: Attunity MS-SQL CDC – Transactional features of transient storage – How?

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    MS-SQL CDC – Transactional features of transient storage – How?

    Attunity MS-SQL CDC – Transactional features of transient storage – How?

    SQLCDCLGR data written into transient storage is subjected to a basic transactional protocol.
    Practically - in terms of sequential writing – this means that the ‘writing front’ is tracked in a way that lets applying a ‘rollback’ to the most recent persistent safe point.

    As LGR service allocates a new CDC file, it also creates an associated TOC file companion.
    Once the CDC file is filled up, LGR ‘seals’ this couple by appending the latest LSN value as the TOC file name suffix.
    This operation is denoted as ‘Sealing’ and is logically equivalent to ‘COMMIT’-ing the CDC contents.
    As previously explained – the TOC name - consisting of LSN boundaries couple - is also used for fast stream positioning.

    In case of ‘soft’ exceptions – LGR is trapped and conducted via a run down sequence where ‘Sealing’ is carried out neatly.
    This is not necessarily the case where ‘hard’ exception occurs. A power failure for example can leave a CDC file in an ‘unsealed’ state.

    As the service starts it may detect that a previous run has terminated abnormally.
    The indication for this situation is reflected by the status of the last transient storage unit which is not properly ‘sealed’ – namely – UNCOMITTED.
    The service responds to this situation by ‘ROLLBACK-ing’ the transient storage to the last committed unit. Actually – an UNCOMMITED unit is merely deleted.
    Processing now starts from the last committed persistent context held in the transient storage.

    Following LOG shows an instance of this sequence:
    <<20060608-120727>> Module:SQLCDC_TRNSTOR/Line:465 Attending LGR abnormal end situation:
    Last LGR run terminated abnormally.
    Following unsealed TOC entry (and related CDC data) are 'ROLLBACK-ed':
    d:\dev\connect\prod\def\sqlcdctmp\0000005e_0000022 2_0002.toc
    Last edited by Admin_attu; 09-21-2006 at 03:47 PM.
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