Very nice, I missed MDB_NOLOCK completely. This might work for me.

However, one thing got my curious about MDB_NOLOCK docs: "For proper operation [of MDB_NOLOCK] the caller must ... ensure that no readers are using old transactions while a writer is active."

Is this also true without MDB_NOLOCK, i.e. does the built-in locking mechanism also "ensure that no readers are using old transactions while a writer is active."? (I guess not because it sounds like it would break "readers don't block writers.")

I was hoping I could do my own concurrency management only for write transactions without worrying about read transaction - I thought that's what "multiversioning" was all about.

Jun 7, 2023, 19:17 by hyc@symas.com:

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?

--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/