Tuesday, 15 March 2016

Transaction diagnostics

Most DBMS will report transaction conflicts if concurrent transactions attempt to update the same data (since this is the classic "lost update" fault). Certainly it is a necessary condition for transaction serialisation that such attempts are prevented.
There has recently been some discussion in the community about whether transaction conflicts should also be reported when data that has been read in the transaction has been changed before the transaction commits. In combination with the above condition, this is sufficient to ensure transaction serialisation (so-called view serialisability).
For over ten years, Pyrrho has enforced this very strong requirement (it is the only transaction mode it supports) although for practical reasons the full read set of a transaction is not always available, and Pyrrho will report a transaction conflict in all cases of doubt.
From today, Pyrrho provides some additional information in the standard SQL diagnostics area to identify a reason that a transaction conflict has been reported, provisionally using the keyword WITH. Because of Pyrrho's role-based data models, transactions do not necessarily agree on table names etc, and database objects in such messages are usually identified by their numerical identities.

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