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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





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