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

Stuart Bishop stuart at stuartbishop.net
Thu Jan 24 08:21:29 CET 2008

James Henstridge wrote:

> 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 wouldn't call any of these a point of contention. They where points of
discussion. Attempting to remove the xid() constructor from the spec is
premature when people where just considering if tuples can be used instead.

I don't think omitting transaction ids from tpc_recover() is acceptable.
Doing so means you can't write a transaction manager that plays nicely in a
more complex environment where components may not be under our direct
control, let alone written in Python and using ths API. My use case here is
a reaper script that detects and handles or reports lost transactions.

Here is an edge case with proposal 1. Here, con happens to be a connection
to a MySQL database. Which Xid represents the prepared transaction?

>>> con.tpc_begin('foo')
>>> con.tpc_prepare()
>>> con.tpc_recover()
[<Xid 'foo', '', 1>, <Xid 'foo', '', 0>, <Xid 'foo', 'None', 1>]

You could try fixing this by returning a heterogeneous  list, but I think
this is just making the hole deeper:

>>> con.tpc_begin('foo')
>>> con.tpc_prepare()
>>> con.tpc_recover()
['foo', <Xid 'foo', '', 0>, <Xid 'foo', 'None', 1>]

Proposal 2 seems the better option. I think we need to specify that the
3-tuple cannot contain None values.

I personally feel that an Xid() constructor makes things more readable. It
also means we can have driver specific defaults for the format id rather
than no default.

tpc_begin(Xid('foo', 'bar', 1))		vs.	tpc_begin(('foo', 'bar', 1))
tpc_begin(Xid('foo', 'bar'))		vs.	tpc_begin(('foo', 'bar', 1))
tpc_begin(Xid('foo')) 			vs.	tpc_begin(('foo', '', 1))

Stuart Bishop <stuart at stuartbishop.net>

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