Hi Ertan,

> is there any possible problems to access database at same
> time by different process/threads ? What happens when trying to access a
> database that is already open by another BaseX instance ?

it’s no problem to access the same BaseX database by different
instances, as long as no updates are performed. If an update takes
place, a database is blocked for other read operations, using an
exclusive file lock. You may want to have a look at our Wiki page on
transactions, it gives you some more background information [1].
Please note, however, that the given information refers to the latest
version of BaseX, while Nxdb was last ported from 7.2, so you may need
to browse in the Wiki history, check out an older PDF version of BaseX
[2], or directly talk to Dave Glick, the developer of Nxdb.

Hope this helps,
Christian

[1] http://docs.basex.org/wiki/Transaction_Management
[2] http://docs.basex.org/wiki/Documentation