Redland transactions API (was Re: [redland-dev] Perl/BDB question: atomicity and transactions question)

[Dave Beckett]

i found my earlier thoughts on transactions, tidied them up.

Dave
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Transactions ideas and API for redland storages

For Sleepycat / BerkleyDB alone

All redland storages have options which can be used by any of the
store implementations.  It would be quite easy to add a
bdb-transactions='yes' option which could be used by the hashes storage
(rdf_hashes_storage.c) when it tries to open a hash.  This calls
librdf_hash_open (rdf_hash.c) with the same options passed to the
hash implementations such as the BDB hash (rdf_hash_bdb.c) where the
actual BDB open is done, in librdf_hash_bdb_open and there a small
patch would allow the DB_TXN or whatever to be added to the flags for
bdb->open / bdb->set_flags.

The other way, passing down an (existing) BDB DB_TXN transaction
handle, that would mean modifying the APIs.  Unless you did it at
open time using the bdb-transactions='0x234567' with some C pointer.
Which is pretty ugly.


More general API

I was thinking about transaction support, and however you do it
internally, it is a good idea to allow the users to explicitly
request it from the system.  I've got a sketch of the API calls that
would be needed at the user level, so here goes, in pseudo code:

class model {
  ...
  int transaction_start()
  int transaction_start_with_handle(void* handle)
  int transaction_commit()
  int transaction_abort()
  int transaction_rollback()
  void* transaction_get_handle()
  ...
}

So what is a handle?  A store-specific transaction pointer that you
can use if you know what you are doing.

Here's an example of when you know the store provides Sleepycat
DB_TXN, and you'd do

  storage=new hashes_storage (hash-type='bdb', transactions='yes')
  model=new model_storage(storage)

then

  model.transaction_start()
  DB_TXN tx=(DB_TXN)model.transaction_get_handle()
  ... commit other stuff using tx ...
  model.transaction_commit()

OR
  DB_TXN tx;
  ... init tx ...
  model.transaction_start_with_handle((void*)tx)
  ... commit other stuff with tx ...
  model.transaction_commit()
  ... delete TX ...


I'm not sure if you'd really want to pass it in at redland
transaction start time, but it seems plausible.


For a SQL-backed store, it'd be something similar, however you'd
likely not get the internal handle directly but just use the
transaction_start/commit/abort/rollback.