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| Guest |
 Posted: Tue Sep 18, 2007 6:31 am |
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Guest
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erlang-questions-request@erlang.org wrote:
> Why use 'noreply'? I can only really think of one reason---you have a
> relatively long-running request but don't want to block the gen_server
> while it runs. Maybe there's also some case where you can avoid a
> deadlock, but I haven't thought about that.
>
>
I use noreply quite often. One reason is that I do not want my server to
do blocking receives as that will spoil the soft upgrade for that process.
I almost always use active_once on a socket to collect arbitary number
of bytes in my server
and then returning the response to the client with gen_server:reply/2
when I got the needed amount. (Could also be gen_fsm:reply/2).
> In the case of the long-running request, what I do instead is
>
> {reply, {pending, Ref}, State}
>
> where Ref is a reference which is then included in a later message
> sent the old-fashioned way. This amounts to more or less the same as
> 'noreply', though it's one message more expensive and has the benefit
> of making it easier to reason about timeouts, at least to my mind.
>
I do not agree. That changes the semantics for the client. I think a
client that
does a call shall hang until the reply is delivered (or is timed out)
that is the semantics of call otherwise you might as well use cast. Of
course for a long running request you do not want the server to hang
for the duration of the call and that is one of the reasons why
gen_server:reply/2 exists. If it is an independent calculation the best
option is to spawn a new process that does the calculation and then
calls gen:server_reply/2 in the new process. Otherwise you
save "From" in the state and send the reply sometime later when you got
enough information from
elsewhere to send the reply. Timeout can always be handled with the help
of erlang:send_after.
Regards Ingela - OTP team
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| Guest |
| Posted: Tue Sep 18, 2007 11:11 am |
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Guest
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Hi again!
Humm ... I might have been a bit quick in my last response. I agree
that letting the gen_server:call
return a reference and later sending a response including that
reference is
a good strategy when you have the situation that you want some job done
and you want to
know when it is ready, but until then you can keep on doing other
things. But if you
need to wait for the job to be done I do not think that you should get
an answer from gen_server:call/2 and then have do an explicit do receive
on the return value from gen_srever:call/2 .
Regards Ingela - OTP team
erlang-questions-request@erlang.org wrote:
>> In the case of the long-running request, what I do instead is
>>
>> {reply, {pending, Ref}, State}
>>
>> where Ref is a reference which is then included in a later message
>> sent the old-fashioned way. This amounts to more or less the same as
>> 'noreply', though it's one message more expensive and has the benefit
>> of making it easier to reason about timeouts, at least to my mind.
>>
>>
> I do not agree. That changes the semantics for the client. I think a
> client that
> does a call shall hang until the reply is delivered (or is timed out)
> that is the semantics of call otherwise you might as well use cast. Of
> course for a long running request you do not want the server to hang
> for the duration of the call and that is one of the reasons why
> gen_server:reply/2 exists. If it is an independent calculation the best
> option is to spawn a new process that does the calculation and then
> calls gen:server_reply/2 in the new process. Otherwise you
> save "From" in the state and send the reply sometime later when you got
> enough information from
> elsewhere to send the reply. Timeout can always be handled with the help
> of erlang:send_after.
>
> Regards Ingela - OTP team
>
>
>
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| yeti |
| Posted: Sat Sep 01, 2012 5:57 am |
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Joined: 01 Sep 2012
Posts: 17
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