Re: [lmdb] MDB_BAD_DBI error upon mdb_txn

Thanks. I also have a search in the previous discussions and the following is probably the solution to my question.

https://www.openldap.org/lists/openldap-technical/201409/msg00077.html

Quoting from the above -

> Before starting any other threads:
> Create the environment.
> Open a transaction.
> Open all DBI handles the app will need.
> Commit the transaction.

May I confirm my understanding that, after the above steps, each thread created for accessing the database in the same process should :

1. Open a transaction in the same environment

2. Directly use the persisted DBI handles (saved before creating the threads) for read / write without calling mdb_dbi_open

3. Commit / abort the transaction,

regardless of whether the DBI handle is used for read / write?

Also, in respect of the initial transaction for opening the DBI handles, is a read-only transaction sufficient even if the database it opened would be written to in subsequent transactions, since the initial transaction itself would not be writing to the database?

Thanks a lot.

- H Law

2015-04-03 2:54 GMT+08:00 Howard Chu <hyc@symas.com>:
>
> hlaw wrote:
>>
>> Thank you!
>>
>> It appears that mdb_dbi_open() must be called before any read / write
>> access to a database in a transaction,
>
>
> True.
>
>> and all read / write have to be
>> done in a transaction?
>
>
> True.
>
>> If this is the case, is it correct that due to
>> the concurrency restriction above, there is essentially no concurrent
>> access (including read and write) to the same database in a
>> multi-threaded process,
>
>
> False.
>
>> since a database is available to one transaction
>> in a time?
>
>
> False. Reread the mdb_dbi_open doc. http://symas.com/mdb/doc/group__mdb.html#gac08cad5b096925642ca359a6d6f0562a
>
>>
>> 2015-03-30 19:52 GMT+08:00 Hallvard Breien Furuseth
>> <h.b.furuseth@usit.uio.no <mailto:h.b.furuseth@usit.uio.no>>:
>>
>> You're abusing mdb_dbi_open(). Its doc says:
>>
>> "This function must not be called from multiple concurrent
>> transactions in the same process. A transaction that uses this
>> function must finish (either commit or abort) before any other
>> transaction in the process may use this function."
>>
>> That text could have stood out a bit better in the doc:-(
>>
>> The error is simple for lmdb to try to catch, though without
>> locking it cannot guarantee to catch it. Try this patch:
>> <http://folk.uio.no/hbf/__OpenLDAP/mdb.serial-open.diff
>> <http://folk.uio.no/hbf/OpenLDAP/mdb.serial-open.diff>>
>>
>> (Howard, this is branch "mdb/serial-open" in my UiO repo.)
>> Not sure if it should do more than this. E.g. attack mdb_drop
>> too. Maybe set MDB_TXN_ERROR since a program with broken
>> threading can break the DB.
>>
>> --
>> Hallvard
>>
>>
>
>
> --
> -- Howard Chu
> CTO, Symas Corp. http://www.symas.com
> Director, Highland Sun http://highlandsun.com/hyc/
> Chief Architect, OpenLDAP http://www.openldap.org/project/

Re: [lmdb] MDB_BAD_DBI error upon mdb_txn_commit (newbie question)