[erlang-questions] process hierarchy and passing references

Ladislav Lenart <>
Fri Aug 15 16:04:13 CEST 2014


Hello.

All very good points. I like this approach:

> Put B and A under the same supervisor. Make sure B is spawned first in
> a rest_for_one. B can ask for A to be spawned later on.

Ladislav Lenart


On 15.8.2014 14:40, Fred Hebert wrote:
> On 08/14, Schneider wrote:
>> Besides the domain specific functionality, every program has a RPC middleman
>> and an XML-RPC interface, each implemented as gen_servers. In the current
>> setup, the main gen_server process starts the RPC middleman, passing it its
>> Pid, and the RPC gen_server starts the XML-RPC server which is passed the
>> RPC middleman's Pid.
> 
> What's the 'main gen_server process'?
> 
>> Using the passed in Pid's, the XML-RPC server can do a
>> gen_server:call()/cast() back to the RPC server which on its turn can call
>> the main server. Works ok, but I would like to have the RPC middleman and
>> XML-RPC controller under the control of a supervisor. The main supervisor
>> should start the main gen_server process plus a supervisor that on its turn
>> should start the RPC midleman and the XML-RPC handler.
> 
> Is there a reason why you can't use a named process and pass that name
> instead of having Pids returned? Is it because you risk having to many
> middlemen or XML-RPC servers?
> 
> You've told us the kind of design you want to do, but not why you want
> to do it. That hides a lot of information if the objective is to get a
> good architecture going.
> 
> For your specific case, you can very well have the supervisor pass its
> pid to the server, and have the server call the supervisor back (outside
> of its init function, or asynchronously) to start the child you want to
> know about.
> 
> Or you could go an entirely different way and have an XML-RPC
> application that spawns a server ready to listen and can just be called
> from anywhere on the node, but that depends on what you plan on doing.
> Then the 'main gen_server', whatever it's doing, does not need to start
> the XML-RPC server -- it's another application on which it can depend,
> like stdlib or kernel.
> 
>> The thing I can't figure out is how the XML-RPC process should find the RPC
>> middleman process and how that process should locate the main server when
>> using supervision. Since there can be many RPC middlemen and XMP-RPC servers
>> around, each belonging to different main servers, naming and registering
>> seems a little hard.
>> So how to pass references around?
>>
> 
> There is a general theme here: if process A needs to know about process
> B, then process B is a dependency of A, and therefore B should be
> spawned before A.
> 
> This yields any of the following approaches:
> 
> - Give B a name, let A know the name. This works best if you have only one B
>   needed for the entire node.
> - Put B and A under the same supervisor. Make sure B is spawned first in
>   a rest_for_one. B can ask for A to be spawned later on. If there are
>   many As, B can first spawn a supervisor S as its next sibling, and
>   boot As under that supervisor. This works well if you have many pairs
>   of processes like that in a node. They can even have dynamic names
>   (say using gproc), but because they have a direct dependency, I only
>   want the names to be used by outsiders. Between themselves, both
>   processes should use their pids.
> 
> This thread had the suggestion to keep asking the supervisor for who
> your siblings are. I hate this design, because it's confused and doesn't
> trace a clear path to dependencies. If you have B depending on A and A
> depending on B, this may be the only 'obvious' option, but I'd rather
> have an orchestrator process C that spawns both and lets them both know
> about each other as part of a negotiation sequence.
> 
> Regards,
> Fred.
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