Re: mdb_dbi_open and threads

Muhammed Muneer wrote:

Thanks for clarifying. I did get the impression but there was some part of it where I thought the doc was not clear enough.

It is a fundamental principle of ACID transactions. The "I" in ACID stands for Isolation. That means nothing that changes elsewhere, while a transaction is underway, can affect that particular transaction. Since we already document that LMDB is a full ACID DB engine, none of this should require any further explanation.

Thanks anyway.

On Sat, May 20, 2017 at 10:46 PM, Howard Chu <hyc@symas.com mailto:hyc@symas.com> wrote:

Muhammed Muneer wrote:

    Is the following  valid.

    1) Read transaction 1 is started
    2) Read transaction 2 is started and calls mdb_dbi_open and commits
    3) Read transaction 1 uses the handle from (2)


Please learn how to read. The doc says more than you quoted before, and it
already quite explicitly defines its properties:

http://www.lmdb.tech/doc/group__internal.html#gac08cad5b096925642ca359a6d6f0562a
<http://www.lmdb.tech/doc/group__internal.html#gac08cad5b096925642ca359a6d6f0562a>

        The database handle will be private to the current transaction
until the transaction is successfully committed. If the transaction is
aborted the handle will be closed automatically. After a successful commit
the handle will reside in the shared environment, and may be used by other
transactions.

In your question above, transaction 1 is not after transaction 2 therefore
it cannot use the handle that transaction 2 commits.

The handle does not exist in the shared environment until after its
opening transaction commits. If it doesn't exist in the shared environment
when a transaction begins, then it is not visible or valid in that
particular transaction.

Just follow the recommendation to open all handles at the beginning of the
program.



    On Sat, May 20, 2017 at 6:46 PM, Muhammed Muneer <elendilm@gmail.com
    <mailto:elendilm@gmail.com>
    <mailto:elendilm@gmail.com <mailto:elendilm@gmail.com>>> wrote:

        Thanks.

        On Sat, May 20, 2017 at 6:29 PM, Klaus Malorny
    <Klaus.Malorny@knipp.de <mailto:Klaus.Malorny@knipp.de>
        <mailto:Klaus.Malorny@knipp.de <mailto:Klaus.Malorny@knipp.de>>>
    wrote:

            On 5/20/17 2:02 PM, Muhammed Muneer wrote:

                So when the doc says

                      * The database handle will be private to the current
                transaction until
                      * the transaction is successfully committed.

                "the handle being private" only refers to the first
    mdb_dbi_open.
                Once this transaction is committed, one doesn't have to
    call this
                again in subsequent concurrent transactions and can use
    this handle.
                And then this handle won't be private at all.

                Did I get it right?


            Yes. I do this exactly that way as part of my initial setup of my
            application. Once the transaction is committed, I use the returned
            handles setup from whatever thread that needs to access the
    respective
            database. I never call mdb_dbi_open again after the setup.

            Regards,

            Klaus