Related
I have just learnt how to upgrade a module in Erlang and I know that only the function calls that use the fully qualified names (eg. module:function()) gets "relinked" to the current version loaded into the VM, but the function calls that do not specify the module's name do not get "relinked" to the current version, but keep using the older one.
Is there a rule of thumb on when to use a fully qualified function call and when it's OK to call a function just by its name? Is it a bad idea to call all functions using their full name (like module:function())?
Erlang applications normally make use of standard behaviors like gen_server and gen_fsm, which already contain fully qualified function calls within their internal loops and so take care of this issue.
But if for some reason you feel compelled to write your own module with its own recursive message-handling loop and you want that module to be upgradeable at runtime, the loop needs to contain a fully qualified recursive call, and normally you'd place this within a section of code handling a specific upgrade message, similar to the code_change/3 function expected in a callback module used with a standard behavior.
For example, consider the loop below, which is similar to those of the standard behaviors but greatly simplified:
loop(Callbacks, State) ->
{{Next, NState},DoChange} =
receive
{code_change, ChangeData} ->
{Callbacks:handle_code_change(ChangeData, State), true};
{cast,Data} ->
{Callbacks:handle_cast(Data,State), false};
{call,From,Data} ->
Result = Callbacks:handle_call(Data,State),
case Result of
{reply, Reply} ->
From ! Reply;
_ ->
ok
end,
{Reply, false};
Message ->
{Callbacks:handle_info(Message,State), false}
end,
case Next of
stop -> ok;
_ ->
case DoChange of
true -> ?MODULE:loop(Callbacks, NState);
false -> loop(Callbacks, NState)
end
end.
The loop/2 function takes two arguments: Callbacks, the name of a callback module expected to export specific functions invoked for specific messages, and State, which is opaque to the loop but presumably meaningful to the callback module. The loop is tail recursive and it handles several specific messages by calling specific callback functions, and then handles any other messages by calling handle_info/2 in the callback module. (If you've used the standard behaviors you'll find this approach familiar.) The callback functions return a Next value, and a new state to be passed to the next loop. If Next is stop, we exit the loop, otherwise we check the value of DoChange, which is set to true only for code change messages, and if it's true the loop calls itself with a fully qualified call, otherwise it uses just a regular call.
As mentioned earlier, this is all greatly simplified. It's extremely rare that you would need to write your own loops, and if you do there are other important things not shown here like system messages that you need to deal with. You are best off using the standard behaviors.
I am having a hard time wrapping my head around the correct way to make calls against a gen_server instance dynamically created by a supervisor with a simple_one_for_one child strategy. I am attempting to create data access controls as gen_servers. Each entity will have its own supervisor, and that supervisor will create gen_server instances as needed to actually perform CRUD operations on the database. I understand the process for defining the child processes, as well as the process for creating them as needed.
Initially, my plan was to abstract the child creation process into custom functions in the gen_server module that created a child, fired off the requested operation (e.g. find, store, delete) on that child using gen_server:call(), and then returning the operation results back to the calling process. Unless I am mistaken, though, that will block any other processes attempting to use those functions until the call returns. That is definitely not what I have in mind.
I may be stuck in OO mode (my background is Java), but it seems like there should be a clean way of allowing a function in one module to obtain a reference to a child process and then make calls against that process without leaking the internals of that child. In other words, I do not want to have to call the create_child() method on an entity supervisor and then have my application code make gen_server:calls against that child PID (i.e. gen_sever:call(Pid, {find_by_id, Id})). I would instead like to be able to call a function more like Child:find_by_id(Id).
A full answer is highly dependent on your application — for example, one gen_server might suffice, or you might really need a pool of database connections instead. But one thing you should be aware of is that a gen_server can return from a handle_call callback before it actually has a reply ready for the client by returning {noreply, NewState} and then later, once it has a client reply ready, calling gen_server:reply/2 to send it back to the client. This allows the gen_server to service calls from other clients without blocking on the first call. Note though that this requires that the gen_server has a way of sending a request into the database without having to block waiting for a reply; this is often achieved by having the database send a reply that arrives in the gen_server:handle_info/2 callback, passing enough info back that the gen_server can associate the database reply with the correct client request. Note also that gen_server:call/2,3 has a default timeout of 5 seconds, so you'll need to deal with that if you expect the duration of database calls to exceed the default.
when you create, modify or delete a record, you don't need to wait for an answer. You can use a gen_server:cast for this, but you don't need a gen_server for this, as I said in my first comment, a simple call to an interface function executed in the client process will save time.
If you want to read, 2 cases:
you can do something else while waiting the answer, then a gen_server call is ok, but a simple spawned process waiting for the answer and sending it back to the client will provide the same service.
you cannot do anything before getting the answer, then there is no blocking issue, and I think that it is really preferable to use as less code as possible so again a simple function call will be enough.
gen_server is meant to be persistent and react to messages. I don't see in your example the need to be persistent.
-module(access).
-export([add/2,get/1]).
-record(foo, {bar, baz}).
add(A,B) ->
F = fun() ->
mnesia:write(#foo{bar=A,baz=B})
end,
spawn(mnesia,activity,[transaction, F]). %% the function return immediately,
%% but you will not know if the transaction failed
get(Bar) ->
F = fun() ->
case mnesia:read({foo, Bar}) of
[#foo{baz=Baz}] -> Baz;
[] -> undefined
end
end,
Pid = self(),
Ref = make_ref(),
Get = fun() ->
R = mnesia:activity(transaction, F),
Pid ! {Ref,baz,R}
end,
spawn(Get),
Ref. %% the function return immediately a ref, and will send later the message {Ref,baz,Baz}.
If the problem you see is that you are leaking that the internal implementation of your db-process is a gen_server, you could implement the api such that it takes the pid as argument as well.
-module(user).
-behaviour(gen_server).
-export([find_by_id/2]).
find_by_id(Pid, Id) ->
gen_server:call(Pid, {find_by_id, Id}).
%% Lots of code omitted
handle_call({find_by_id, Id}, From, State) ->
ok.
%% Lots more code omitted.
This way you don't tell clients that the implementation is in fact a gen_server (although someone could use gen_server:call as well).
I'm writing an event manager that will take a lot of different event handlers. This event manager will be notified with a lot of different events. Each handler only handle certain events, and ignore the rest. Each handler can also trigger certain other events based on situation.
For example, first handler to handle Event1
-module (first_handler).
-behavior (gen_event).
...
handle_event(Event1, State) -> {ok, State};
handle_event(_, State) -> {ok, State}.
Second handler to handle Event2
-module (second_handler).
-behavior (gen_event).
...
handle_event(Event2, State) ->
gen_event:notify(self(), Event1),
{ok, State};
handle_event(_, State) -> {ok, State}.
The event triggering can be done by calling gen_event:notify(self(), NewEvent) within a handle_event of the handler, but I would rather abstract and export that out so that it can be called from the event manager.
Since pattern matching and ignoring events and triggering events are common to all the handlers, is there anyway I can extend gen_event behavior to provide those as built-ins?
I'll start with the default way to create a custom behavior:
-module (gen_new_event).
-behaviour (gen_event).
behaviour_info(Type) -> gen_event:behaviour_info(Type).
I'm not sure what to do next.
What are you trying to do exactly? I could not understand from the examples you provided. In second_handler's handle_event/2, Event1 is unbound. Also, does using self() work? Shouldn't that be the registered name of the manager. Not sure whether handle_event/2 gets executed by the manager or each handler process (but the latter makes more sense).
By implementing your gen_new_event module, you are implementing a handler (i.e. a callback module), and not an event manager. The fact that you have -behaviour(gen_event) means that you're asking the compiler to check that gen_new_event actually implements all the functions listed by gen_event:behaviour_info(callbacks), thereby making gen_new_event an eligible handler which you could add to an event manager via gen_event:add_handler(manager_registered_name, gen_new_event, []).
Now, if you take away -behaviour (gen_event), gen_new_event no longer has to implement the following functions:
35> gen_event:behaviour_info(callbacks).
[{init,1},
{handle_event,2},
{handle_call,2},
{handle_info,2},
{terminate,2},
{code_change,3}]
You could make gen_new_event a behaviour (i.e. an interface) by adding more functions which you will be requiring any module which uses -behaviour(gen_new_event) to implement:
-module (gen_new_event).
-export([behaviour_info/1]).
behaviour_info(callbacks) ->
[{some_fun, 2}, {some_other_fun, 3} | gen_event:behaviour_info(callbacks)].
Now, if in some module, for e.g. -module(example), you add the attribute -behaviour(gen_new_event), then the module example will have to implement all the gen_event callback functions + some_fun/2 and some_other_fun/3.
I doubt that's what you were looking for, but your last example seemed to suggest that you wanted to implement a behaviour. Note that, all you're doing by implementing a behaviour is requiring other modules to implement certain functions should they use -behaviour(your_behaviour).
(Also, if I understood you correctly, if you want to extend gen_event then you could always simply copy the code in gen_event.erl and extend it ... I guess, but is this really necessary for what you're trying to do?).
Edit
Objective: extract common code out of gen_event implementations. So for e.g. there's a handle_event/2 clause which you want in every one of your gen_events.
One way of going about it: You could use a parameterized module. This module would implement the gen_event behaviour, but, only the common behaviour which all your gen_event callback modules should have. Anything which is not "common" can be delegated to the module's parameter (which you'd bind to a module name containing the "custom" implementation of the gen_event callback.
E.g.
-module(abstract_gen_event, [SpecificGenEvent]).
-behaviour(gen_event).
-export(... all gen_event functions).
....
handle_event({info, Info}, State) ->
%% Do something which you want all your gen_events to do.
handle_event(Event, State) ->
%% Ok, now let the particular gen_event take over:
SpecificGenEvent:handle_event(Event, State).
%% Same sort of thing for other callback functions
....
Then you'd implement one or more gen_event modules which you'll be plugging into abstract_gen_event. Lets say one of them is a_gen_event.
Then you should be able to do:
AGenEvent = abstract_gen_event:new(a_gen_event). %% Note: the function new/x is auto-generated and will have arity according to how many parameters a parameterized module has.
Then, I guess you could pass AGenEvent to gen_event:add_handler(some_ref, AGenEvent, []) and it should work but note that I have never tried this out.
Perhaps you could also get around this using macros or (but this is a bit overkill) do some playing around at compilation time using parse_transform/2. Just a thought though. See how this parameterized solution goes first.
2nd Edit
(Note: not sure whether I should delete everything prior to what is in this section. Please let me know or just delete it if you know what you're doing).
Ok, so I tried it out myself and yes, the return value of a parameterized module will crash when feeding it to gen_event:add_handler/3's second argument... too bad :(
I can't think of any other way of going about this then other than a) using macros b) using parse_transform/2.
a)
-module(ge).
-behaviour(gen_event).
-define(handle_event,
handle_event({info, Info}, State) ->
io:format("Info: ~p~n", [Info]),
{ok, State}).
?handle_event;
handle_event(Event, State) ->
io:format("got event: ~p~n", [Event]),
{ok, State}.
So basically you would have all the callback function clauses for the common functionality defined in macro definitions in a header file which you include in every gen_event which uses this common functionality. Then you ?X before/after each callback function which uses the common functionality... I know it's not that clean and I'm generally weary of using macros myself but hey... if the problem is really nagging you that's one way to go about it.
b) Google around for some info on using parse_transform/2 in Erlang. You could implement a parse_transform which looks for the callback functions in you gen_event modules which have the specific cases for the callbacks but do not have the generic cases (i.e. clauses like the ({info, Info}, State) in the macro above). Then you would simply add the forms which make up the generic cases.
I would suggest doing something like this (add exports):
-module(tmp).
parse_transform(Forms, Options) ->
io:format("~p~n", [Forms]),
Forms.
-module(generic).
gen(Event, State) ->
io:format("Event is: ~p~n", [Event]),
{ok, State}.
Now you can compile with:
c(tmp).
c(generic, {parse_transform, tmp}).
[{attribute,1,file,{"../src/generic.erl",1}},
{attribute,4,module,generic},
{attribute,14,compile,export_all},
{function,19,gen,2,
[{clause,19,
[{var,19,'Event'},{var,19,'State'}],
[],
[{call,20,
{remote,20,{atom,20,io},{atom,20,format}},
[{string,20,"Event is: ~p~n"},
{cons,20,{var,20,'Event'},{nil,20}}]},
{tuple,21,[{atom,21,ok},{var,21,'State'}]}]}]},
{eof,28}]
{ok,generic}
That way you can copy-paste the forms you'll be injecting. You would copy them into a proper parse_transform/2 which, rather than just printing, would actually go through your source's code and inject the code you want where you want it.
As a side note, you could include the attribute -compile({parse_transform, tmp}) to every gen_event module of yours which needs to be parse_transformed in this way to add the generic functionality (i.e. and avoid having to pass this to the compiler yourself). Just make sure tmp or whichever module contains your parse_transform is loaded or compiled in a dir on the path.
b) seems like a lot of work I know...
Your installed handlers are already running in the context of the event manager which you start and then install handlers into. So if their handle-event function throws out data, they already do what you want.
You don't need to extend the event behaviour. What you do is:
handle_event(Event, State) ->
generic:handle_event(Event, State).
and then let the generic module handle the generic parts. Note that you could supply generic a way to callback to this handler module for specialized handler behaviour should you need it. For example:
generic:handle_event(fun ?MODULE:callback/2, Event, State)...
and so on.
One of the things that attracted me to Erlang in the first place is the Actor model; the idea that different processes run concurrently and interact via asynchronous messaging.
I'm just starting to get my teeth into OTP and in particular looking at gen_server. All the examples I've seen - and granted they are tutorial type examples - use handle_call() rather than handle_cast() to implement module behaviour.
I find that a little confusing. As far as I can tell, handle_call is a synchronous operation: the caller is blocked until the callee completes and returns. Which seems to run counter to the async message passing philosophy.
I'm about to start a new OTP application. This seems like a fundamental architectural decision so I want to be sure I understand before embarking.
My questions are:
In real practice do people tend to use handle_call rather than handle_cast?
If so, what's the scalability impact when multiple clients can call the same process/module?
Depends on your situation.
If you want to get a result, handle_call is really common. If you're not interested in the result of the call, use handle_cast. When handle_call is used, the caller will block, yes. This is most of time okay. Let's take a look at an example.
If you have a web server, that returns contents of files to clients, you'll be able to handle multiple clients. Each client have to wait for the contents of files to be read, so using handle_call in such a scenario would be perfectly fine (stupid example aside).
When you really need the behavior of sending a request, doing some other processing and then getting the reply later, typically two calls are used (for example, one cast and the one call to get the result) or normal message passing. But this is a fairly rare case.
Using handle_call will block the process for the duration of the call. This will lead to clients queuing up to get their replies and thus the whole thing will run in sequence.
If you want parallel code, you have to write parallel code. The only way to do that is to run multiple processes.
So, to summarize:
Using handle_call will block the caller and occupy the process called for the duration of the call.
If you want parallel activities to go on, you have to parallelize. The only way to do that is by starting more processes, and suddenly call vs cast is not such a big issue any more (in fact, it's more comfortable with call).
Adam's answer is great, but I have one point to add
Using handle_call will block the process for the duration of the call.
This is always true for the client who made the handle_call call. This took me a while to wrap my head around but this doesn't necessarily mean the gen_server also has to block when answering the handle_call.
In my case, I encountered this when I created a database handling gen_server and deliberately wrote a query that executed SELECT pg_sleep(10), which is PostgreSQL-speak for "sleep for 10 seconds", and was my way of testing for very expensive queries. My challenge: I don't want the database gen_server to sit there waiting for the database to finish!
My solution was to use gen_server:reply/2:
This function can be used by a gen_server to explicitly send a reply to a client that called call/2,3 or multi_call/2,3,4, when the reply cannot be defined in the return value of Module:handle_call/3.
In code:
-module(database_server).
-behaviour(gen_server).
-define(DB_TIMEOUT, 30000).
<snip>
get_very_expensive_document(DocumentId) ->
gen_server:call(?MODULE, {get_very_expensive_document, DocumentId}, ?DB_TIMEOUT).
<snip>
handle_call({get_very_expensive_document, DocumentId}, From, State) ->
%% Spawn a new process to perform the query. Give it From,
%% which is the PID of the caller.
proc_lib:spawn_link(?MODULE, query_get_very_expensive_document, [From, DocumentId]),
%% This gen_server process couldn't care less about the query
%% any more! It's up to the spawned process now.
{noreply, State};
<snip>
query_get_very_expensive_document(From, DocumentId) ->
%% Reference: http://www.erlang.org/doc/man/proc_lib.html#init_ack-1
proc_lib:init_ack(ok),
Result = query(pgsql_pool, "SELECT pg_sleep(10);", []),
gen_server:reply(From, {return_query, ok, Result}).
IMO, in concurrent world handle_call is generally a bad idea. Say we have process A (gen_server) receiving some event (user pressed a button), and then casting message to process B (gen_server) requesting heavy processing of this pressed button. Process B can spawn sub-process C, which in turn cast message back to A when ready (of to B which cast message to A then). During processing time both A and B are ready to accept new requests. When A receives cast message from C (or B) it e.g. displays result to the user. Of course, it is possible that second button will be processed before first, so A should probably accumulate results in proper order. Blocking A and B through handle_call will make this system single-threaded (though will solve ordering problem)
In fact, spawning C is similar to handle_call, the difference is that C is highly specialized, process just "one message" and exits after that. B is supposed to have other functionality (e.g. limit number of workers, control timeouts), otherwise C could be spawned from A.
Edit: C is asynchronous also, so spawning C it is not similar to handle_call (B is not blocked).
There are two ways to go with this. One is to change to using an event management approach. The one I am using is to use cast as shown...
submit(ResourceId,Query) ->
%%
%% non blocking query submission
%%
Ref = make_ref(),
From = {self(),Ref},
gen_server:cast(ResourceId,{submit,From,Query}),
{ok,Ref}.
And the cast/submit code is...
handle_cast({submit,{Pid,Ref},Query},State) ->
Result = process_query(Query,State),
gen_server:cast(Pid,{query_result,Ref,Result});
The reference is used to track the query asynchronously.
I noticed that messages sent to the pid of a gen_fsm process are matched in the state callbacks as events. Is this just accidental or can I rely on this feature?
Normally I would expect general messages sent to a gen_fsm to show up in the handle_info/3 callback and thought I would have to re-send it using gen_fsm:send_event.
Does gen_fsm try to match the message first to the state callback and then allways with the handle_info/3 callback? Or only if it doesn't match a state callback clause?
However when I try it my message seems to be handled twice according to debug output.
So basically the question can also be stated like: how to correctly handle received messages as events in gen_fsm state functions?
Clarification: that some of the events are occurring by getting messages passed should be considered given for this question.
I'm aware that in many cases its cleaner to make the protocol visible by using function calls into the fsm only.
I'm not so sure if this would improve the current framework where the mentioned gen_fsm has to fit in: Diverse protocol stacks where each layer calls a connect() function to attach (and sometimes start) the lower layer. Packets are sent to lower layers ba calling a function (send) and received by receiveing a message. Much like gen_tcp.
By looking at the code for gen_fsm I already figured out that general messages are only passed to handle_info, so only the question remains wether to call the state function directly from the handle_info/3 callback or resent using gen_fsm:send_event.
General messages are handled by handle_info callback, unless you have something like this in your code:
handle_info(Info, StateName, StateData) ->
?MODULE:StateName(Info, StateData).
Which avoids resending, but I do not recommend neither that, nor resending.
Delivering events exclusively by means of API calls encapsulating send_event/sync_send_event/send_all_state_event/sync_send_all_state_event makes protocol explicit. Which is a right thing, as it is easier to understand, maintain and document with edoc.