[erlang-questions] Server which spawns an additional server for each call

Joe Armstrong erlang@REDACTED
Tue Apr 19 17:11:36 CEST 2011

It's not entirely clear to me what you want to do. I never understand why
people use gen_servers
for everything, pure Erlang is often easier :-)

I assume you want to delegate the response in the server (ie get some other
process than the
gen server to do the work, since this takes a long time)

One way to do this is like this:

Write a client stub like this:

foo(X) ->
    gen_server:call(?Mod, {foo, X}).

Write a gen_server handle call method like this:

handle_call({foo, X}, From, State) ->
    State1 = func1(X, State),
    State2 = func2(X, State),
    spawn_link(fun() -> do_something(State1, X, From) end),
    {noreply, State2}.

do_something(State, X, From) ->
    Reply = func3(X, State),
    gen_server:reply(From, Reply).

here I've assumed func1 returns State1 and that State1 is some subset of
needed in your computation. State2 (returned by func2) is the continuation
state of the server.
ie the state the server will have after it has received the foo request, but
before the delegated function
has replied. You'll have to write func1 and func2 yourself.

do_something is the delegated function that might take a long time. The four
lines of
code which define handle_call should return quickly, ie don't do much here,
do the work in
do_something. handle call returns {noreply, State2} which means "don't reply
to the client
but continue with state State2.

do_something works in parallel with the server and when it has finished
calls gen_server:reply/2
and at this point the client stub routine  foo/1 will return.

This is only one way of doing this. You could do the complex calculation
inside the client
and request the data you need from the server, the server can spawn a
deligate (as above).
You can use a shared ets table and so on.

if you were to do this in pure erlang it might be easier to see what's going

Define promise and yield thusly:

promise(Fun) ->
      S = self(),                             %% this self() is evaluated
*inside* the current function
      spawn(fun() ->
                     S ! {self(), Fun()}    %% this self() is evaluated
*inside* the spawned function

yield(Promise) ->
          {Promise, Result} -> Result

promise takes a Fun argument and returns a promise. The promise is a promise
compute the value. The promise can be redeemed by calling yield. So we can
write code like this:

P = promise(fun() -> fib(40) end),
.... do some other stuff that takes a while ...
Val = yield(Promise)

So we compute fib(40) which takes a long time in parallel with some other

The gen_server stuff above has just hidden what is essentially a promise and
yield and
a bit of state trickery in a framework module - nothing tricky about about
it at all.

Given promise and yield and a dash of list comprehensions we can write
fun stuff like:

parmap(F, L) ->
     Promises = [promise(fun() -> F(I) end || I <- L],
     [yield(I) || I <- Promises].

which is a parallel mapping function.

Hope this helps


On Tue, Apr 19, 2011 at 10:58 AM, Dave Challis <dsc@REDACTED> wrote:

> Hi,
> I'm trying to work out the structure for a small bit of functionality in
> erlang.  I've got a feeling there's an obvious solution, but I'm not
> experienced enough with the language sure what pattern to best follow...
> So far, I've got a server which implements the gen_server behaviour, a
> supervisor for it, and a module containing an API for interacting with it
> (using gen_server:call mostly).
> The server does a lot of work though, so blocks for a while until the call
> is finished.
> What I'd like to do, is create a new server process each time
> gen_server:call is invoked by a client, with each server terminating when
> it's done processing (but always leaving 1 server running).
> This means that clients using the API will have their request processes
> straight away, without having to wait for all the other calls to the server
> to finish.
> I can't quite figure out where to place the logic for doing this though.
> * Should the API module ask the supervisor to spawn a new child, then send
> the client's call to this?
> * Should API calls go to the supervisor, and have it decide whether to
> spawn new servers or send the call to an existing one?
> * Or should each server take care of telling the supervisor to spawn a new
> child, and pass the call request to the newly spawned one?
> Is this a sensible approach in general, or is there an obvious pattern or
> some functionality I've missed?
> Thanks,
> --
> Dave Challis
> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED
> http://erlang.org/mailman/listinfo/erlang-questions
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