From: "Berger, Daniel" <Daniel.Berger@...>
Date: 2005-07-30T00:22:46+09:00
Subject: Re: Allow retry to take arguments?

> -----Original Message-----
> From: Jim Weirich [mailto:jim@weirichhouse.org] 
> Sent: Friday, July 29, 2005 9:09 AM
> To: ruby-talk ML
> Subject: Re: Allow retry to take arguments?
> 
> 
> 
> Austin Ziegler said:
> > One problem that I see, Jim. In Daniel's pseudo-code, he can rescue 
> > specific exceptions:
> >
> >     begin
> >       connect_to_something
> >     rescue NetworkBusyException, TimeoutException
> >       retry(3, 30) or raise
> >     rescue NetworkUnavailableException
> >       retry(5, 60) or raise
> >     end
> 
> Gah!  What is the semantics of that?  What if you get a 
> NetworkBusyException, so you want to retry 3 times.  But the 
> next time you get a NetworkUnavailable exception.  Do you 
> leave it at 3 retries or up it to a max of 5 tries.  Or do 
> the tries for each exception count individually, i.e. we 
> could have up to 8 retries, 5 for Unavailable and 3 for 
> Busy/Timeout.  Or is a a max of 11 retries because retries 
> for timeout and busy count together.

That's something that would have to be worked out.  There are two
approaches I would prefer.

The first is that it resets itself whenever the exception changes.  So,
on the first attempt let's say a NetworkBusyException occurs.  On the
next attempt a NetworkUnavailableException occurs.  In that scenario the
number of retries for NetworkBusyException is reset to 3.

This seems logical to me, but also runs a greater risk of infinite
loops.

The second approach is that it's the total number for each exception.
So again, on the first attempt let's say a NetworkBusyException occurs,
and the number of retries is reduced to 2.  On the next attempt a
NetworkUnavailableException occurs.  The total number of retries for
NetworkBusyException remains 2, and the number of retries for
NetworkUnavailableException is now 4.  On the next attempt a
NetworkBusyException happens again, reducing the number of retries to 1,
and so on.

This also seems logical to me, and avoids the risk of infinite loops.
However, I suspect the implementation would be more difficult.
 
> > Yours just allows for one attempt wrapper and doesn't specify what 
> > sort of exceptions can be caught.
> 
> So use ...
> 
>    attempt([NetworkBusyException, TimeoutException] => [3,30],
>             NetworkUnavailableException => [5, 60]) do
>      connect_to_something
>    end
> 
> or something like this ...
> 
>    Attempt = RetryController.new do |retry|
>      retry.on NetworkBusyException, TimeoutException, 
> :trys=>3, :timeout=>30
>      retry.on NetworkUnavailableException, :trys=>5, :timeout=>60
>    end
> 
>    # ... later in code ...
> 
>    Attempt.call { connect_to_something }
> 
> All still doable without changing the language.

True, but it just doesn't look nearly as clean. :)

Regards,

Dan