Re: [Evolution-hackers] Introduction and Questions

[Jeffrey Stedfast <fejj novell com>]

On Sun, 2007-06-10 at 15:55 -0700, Ross Boylan wrote:
> On Fri, 2007-06-08 at 20:22 -0400, Jeffrey Stedfast wrote:
> > > 
> > > Second question: even if it creates a folder, does it need to stick
> > > around for the folder creation to finish?  I think I remember seeing
> > > that camel was single-threaded
> > 
> > not true...

I should have been clearer here:

yes, it needs to wait for the folder object instantiation to finish (the
function returns a pointer to the instantiated object, afterall).

not true that camel is single-threaded - it has a few of its own threads
going - most notably is the thread for filtering incoming mail for
providers like imap and maildir (they share the same thread).

> > 
> > > , relying on the client app to do
> > > threading.  Would there be a way to multi-thread this somewhere
> > (either
> > > in camel or in the app)?  Obviously doing so would complicate
> > things,
> > > because at some point one might need to block (e.g., if I move a
> > message
> > > from folder A to B and then switch the UI to look at B).
> > 
> > okay, I think you need to familiarize yourself with Camel's API before
> > we discuss anything further :)
> > 
> http://www.go-evolution.org/Mail_Threading begins
> "The Camel API is blocking and synchronous"
> although it then goes on to qualify this:
> "Custom Threads
> Two tasks inside Camel use threads internally in order to implement a
> cancellable api ontop of a non-cancellable one."
> 
> Further
> "Mail-OPS
> mail-ops.h contains asynchronous versions of varous Camel api's"
> It sounds as if mail-ops is outside of Camel, however.

it is external to camel, yes.

> 
> So it sounds as if Camel could (in principle) respond to a move request
> by issuing the appropriate IMAP command and then, starting a thread to
> do the other activities (indexing the target folder and deleting the the
> message from the  source folder) and return.

it could, yes, but it'd need a way to report an error unless it waited
for the operations to finish before returning. For moving mail, you
typically want to know that it succeeded :)

all of the current camel APIs are designed such that the caller expects
that when the function returns, the operation has either succeeded or
failed (where the failure would be set on the CamelException argument).

>   It would then block on
> operations that attempted to access the target folder until the other
> operations completed.

yes, this is true... well, the way folders are implemented at this time
anyway...

> 
> I think this could be called a syncronous API, though perhaps that's a
> stretch.

it is indeed a synchronous API :)

> 
> On the other hand, http://www.go-evolution.org/Camel.Operation does not
> sound like a bocking syncronous API at all, so maybe the statement
> quoted at the top is just obsolete?

Camel code is written under the assumption that it is likely it is being
used in a thread, so it has CamelOperation as a means to:

1. report the progress
2. cancel operations (typically i/o)

what happens is Evolution has a thread-pool with a CamelOperation object
for each of the threads the mailer creates so that it can cancel
operations and/or get progress updates.

> 
> So, first of all I'm confused about the nature of Camel's API and
> operation as far as threading and syncronicity.  
> 
> Second, I don't have a sense of whether its features are historical
> accidents (camel was implemented in a simple way and evo then used it as
> it was)

evolution mailer was originally not multi-threaded if my recollection
serves correctly, although around the time of the first 0.0 release
threading was added (about 7 years ago).

camel and evolution were developed together, so their "designs" evolved
together.

>  or the result of some deliberate design decisions.  Blocking
> syncronous operations are simpler to implement,

yep

>  to use,

yep

>  to debug,

yep, altho with threads thrown in and gdb multi-thread support sucking,
this was still a nightmare...

>  and to
> understand, so they clearly have some advantages.   But it seems that
> the entire application (evolution) does not have that character, so the
> benefits of that simplicity end up lost anyway.

I'm not sure I understand what you mean here...

I guess what you are saying is that evolution code isn't easy to
understand. I think that's true for all large/complex projects. I think
that it also feels that way when you are new to the project because it
is so large and overwhelming - I mean, I know that's how I'd feel if I
were just getting into Evolution sources...

I think that it probably doesn't help that things aren't super well
documented either (kinda hard to do when things change so much so
frequently). From what I can tell, most software (proprietary included)
suffers greatly from this problem :(

I think it's obvious that most of evolution's design is evolutionary
(hah, bad joke, eh?). Despite that, I think it actually turned out quite
good, but some APIs are certainly clumsy/awkward.

Anyways... it's always easier to criticize than it is to create.

Hopefully I've cleared things up

Jeff