[Xapian-discuss] Node.js binding

[Richard Boulton]

On 21 October 2011 00:14, Liam <xapian at networkimprov.net> wrote:
>> Not usually (though for example, a call to an Enquire object will
>> often result in a call to a Database object inside Xapian, so this can
>> be true).
>
> Wouldn't the user know that the Enquire was bound to that Database instance?

Depends on the code structure whether it's easy to know - eg, there
could be a pool of databases, users might write code which gets a
database from the pool and makes an enquire object with it, and then
keeps the enquire object around for a bit after returning the database
to the pool.

I think this comes down to documenting that an Enquire object will
make accesses to the Database object it came from for many methods,
and that such accesses mustn't be performed concurrently.

> Surely you must have many users who allow concurrent connections to xapian
> databases, so there's some standard way of isolating the connections'
> activities?

There's no standard way really, but the usual approach is one of:

 - create all Xapian objects needed for a given request from scratch
each time; don't let any persist, and handle the request in a single
thread.

 - get some Xapian objects from a pool for each request, but be very
careful that all derived objects are destroyed before returnng objects
to the pool.

 - use per thread storage to keep a cache of databases (or whatever
object is needed), and only use these objects in one thread.  This is
my preferred approach for searches, generally; the Database.reopen()
call allows the database version being read to be updated efficiently
before each request.

 - use a queue and a worker in a thread to perform all access to a
database.  This is reasonably appropriate for indexing, since only one
instance of a WritableDatabase for a given path may exist anywhere, so
modifications need to be serialised to a single thread anyway.

I may have missed some approaches.

-- 
Richard