Re: mdb_txn_abort() thread

Sam Dave wrote:

...

Read-write txns don't use TLS. TLS is only used for read-only txns.

Why then is it important that a read-write transaction stays on the same thread (including the final abort or commit)? (I just assumed it was because it needed access to TLS, looks like I was wrong)

Write txns are serialized by a mutex. The mutex is taken when a write txn begins and is released when the txn ends. It is illegal for any thread other than the original locking thread to release the mutex.

If you don't want these semantics, just use MDB_NOLOCK and do your own concurrency management.

Jun 7, 2023, 17:23 by hyc@symas.com:

Sam Dave wrote:

    Alternatively, could a more stateful API be provided (as an alternative to API relying on TLS)?

    Roughly what I have in mind is, mdb_txn_create() returns a state pointer, which should be passed into the next mdb_put(), which returns a state pointer,
    which
    should be passed into the next mdb_put(), and so on, until the final state is passed into mdb_commit() or mdb_abort().


A txn pointer is a state pointer.


    Jun 7, 2023, 10:26 by samdave@tuta.io:

    Thanks, I was just hoping abort was a (currently undocumented) exception to this :)

    Would it be possible/feasible in the future to bring MDB_NOTLS support to read-write transactions? (or at least some partial NOTLS support for only e.g.
    abort)


Read-write txns don't use TLS. TLS is only used for read-only txns.


    Motivation: Ability to abort "later" (e.g. at garbage collection in some languages).. as this "later" stuff does not necessarily happen on the original
    thread.

    - Samuel



    Jun 6, 2023, 18:44 by hyc@symas.com:

    Sam Dave wrote:

    Hi,

    The documentation for mdb_txn_begin() says "A transaction and its cursors must only be used by a single thread, and a thread may only have a single
    transaction
    at a time.".

    Does this also hold for mdb_txn_abort() for read-write transactions? I.e. does the abort also have to happen on the same OS thread as all the
    previous things
    (begin, put, etc.)?


    Calling mdb_txn_abort() requires passing the txn as a parameter, which looks pretty suspiciously to me like "using" it. Is this really a serious question?