[erlang-questions] Is a process necessary in front of mnesia?

[Evans, Matthew]

If you mean remove your own gen_server, then yes it is.

One thing that a "private" gen_server does give you is the ability to manage certain operations in a more efficient manner, while keeping a consistent API.

For example I have an application that normally does quick key-based lookups. Sometimes however it is necessary to do select or match_object queries on large data-sets. Having a gen_server "front" these requests allows you to wrap the whole operation in a spawn, while providing a simple to use API for the user.

e.g.

handle_call({really_complex_search,SearchStuff},From,State) ->
	Pattern = #mnesia_table{some_element = SearchStuff, _ = '_'}
   	spawn(fun()->
		case mnesia:dirty_match_object(SearchStuff) of
			[Rec] -> gen_server:reply(From,Rec);
			_ -> gen_server:reply(From,error)
		end
	end)
	{noreply,State};
		

Certainly the calling application could do that too, but in our case the access method needs to be simple.

Another thing I use a gen_server intermediately for is to prioritize certain operations over others. In one case we are supposed to process many writes as a low-priority task that doesn't affect the read rate. Having a gen_server in the way lets us do that.

Matt


-----Original Message-----
From: erlang-questions@REDACTED [mailto:erlang-questions@REDACTED] On Behalf Of Woody Peterson
Sent: Thursday, October 14, 2010 1:41 PM
To: Erlang-Questions Questions
Subject: [erlang-questions] Is a process necessary in front of mnesia?

Hi All,

In the pragmatic screencasts' Erlang series, there is a gen_server in  
charge of writing to mnesia. In refactoring a small erlang system my  
company's building, we still have that boilerplate handling mnesia,  
but thought we should remove the gen_server entirely, as we could just  
use a function instead of a process as per erlang's best practices (http://www.erlang.se/doc/programming_rules.shtml#HDR16 
). Note that this gen_server holds no state, which is probably not a  
rule for when to not use a process, but seems like it could be a  
smell. I figure the gen_server in the screencasts could have just been  
there as a learning tool, so is there a reason beyond education to put  
a serializing process in front of mnesia as a rule? Are we right to  
assume we can have possibly tens of thousands of processes sending  
write messages to mnesia directly as opposed to a gen_server in front  
of it?

Thanks,

-Woody

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