[DB-SIG] Two-phase commit API proposal (was Re: Any standard for two phase commit APIs?)

M.-A. Lemburg mal at egenix.com
Thu Jan 24 15:41:09 CET 2008

On 2008-01-24 15:36, M.-A. Lemburg wrote:
> On 2008-01-24 02:44, James Henstridge wrote:
>> I've had a bit more time to think about this, and have two proposals
>> on how to handle transaction IDs.  I think they offer equivalent
>> functionality, so the choice comes down to what we want the API to
>> look like.
>> Proposal 1:
>> * Plain string IDs should work fine as transaction identifiers for
>>   applications built from scratch with that assumption: they would
>>   need to identify the global and branch parts in their own way.
>> * A plain string can be stuffed inside an XA style transaction
>>   identifier, even if it isn't making use of all the different
>>   components.
>> * Therefore, all methods accepting transaction IDs should accept
>>   strings.
>> * As some transaction IDs in the database might not match this simple
>>   form, there are two options for the recover() method:
>>     1. return a special object that represents the transaction, which
>>        will be accepted by commit()/rollback().  How string-like must
>>        these objects be?
>>     2. omit such transaction IDs from the result.
>> * For databases that support more structured transaction IDs (such as
>>   those used by XA), the 2PC methods may accept objects other than
>>   strings.
>> Proposal 2:
>> * Many databases follow the XA specification, so it makes sense to use
>>   transaction identifiers structured in the same way.
>> * For databases that do not use XA-style transaction IDs, it is
>>   usually possible to serialise such an ID into a form that it can
>>   work with.
>> * Therefore, all methods accepting transaction IDs should accept
>>   3-sequences of the form (formatID, gtrid, bqual).
>> * For databases using non-XA transaction IDs, it is possible that some
>>   transaction IDs might exist that do not match the serialised form.
>>   The recover() method has two options:
>>     1. return a special object representing the ID that will be
>>        accepted by commit()/rollback().  Such an object should act
>>        like a 3-sequence.
>>     2. omit such transaction IDs from the result.
>> * For databases not using XA-style transactions, the 2PC methods may
>>   accept objects other than 3-sequences as transaction IDs.
>> Both of these proposals seem to get rid of the main points of contention:
>> * removes the xid() constructor from the spec.
>> * allow use of simple objects (strings or tuples) as transaction IDs
>> * provides an obvious way to expose database-specific transaction IDs.
> I'm coming to agree with Stuart that the conn.xid() might actually
> help us with this.
> So I'd be in favor of proposal 2 and an .xid() constructor that
> returns an object which provides a 3-sequence interface, e.g.
> # Wrap the IDs for use by the database module
> xid = conn.xid(fid, gid, bid)
> # Use the xid
> conn.tpc_begin(xid)
> conn.tpc_prepare(xid)
> ...
> # Unwrap the IDs:
> fid, gid, bid = xid

Plus require that all three components are strings to avoid the
None issue.

Marc-Andre Lemburg

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