threading

[Marko Rauhamaa]

"Frank Millman" <frank at chagford.com>:

> I understand that, if one uses threading, each thread *can* block
> without affecting other threads, whereas if one uses the async
> approach, a request handler must *not* block, otherwise it will hold
> up the entire process and not allow other requests to be handled.

Yes.

> How does one distinguish betwen 'blocking' and 'non-blocking'? Is it 
> either/or, or is it some arbitrary timeout - if a handler returns within 
> that time it is non-blocking, but if it exceeds it it is blocking?

Old-school I/O primitives are blocking by default. Nonblocking I/O is
enabled with the setblocking() method.

In the new asyncio package, I/O is nonblocking by default (I'm sure, but
didn't verify).

> In my environment, most requests involve a database lookup. I
> endeavour to ensure that a response is returned quickly (however one
> defines quickly) but I cannot guarantee it if the database server is
> under stress. Is this a good candidate for async, or not?

Database libraries are notoriously bad for nonblocking I/O. It's nothing
fundamental; it's only that the library writers couldn't appreciate the
worth of async communication. For that, asyncio provides special
support:

   <URL: https://docs.python.org/3.4/library/asyncio-dev.html#
   handle-blocking-functions-correctly>



Marko