I have a gen_server that mimics a gen_fsm, (don't ask me why...), a gen_server:call will cause this gen_server from the current state to transfer to the next state, if no gen_server:call invoked in a certain time, the gen_server terminates.
To be more specific, the life of the gen_server looks like this:
state_1 -> state_2 -> ... -> state_N -> terminated
when the server is at state_i, if there is no gen_server call invoked on this server, after t_i seconds, the server will go to state terminated, of cause, this is achieved by using {reply, Reply, NewState, t_i} as the return of the handle_call/3.
The problem of the method is that, I cannot retrieve some information from this gen_server, for to do that, I need to invoke gen_server:call on it, which would mess the timeout up.
One possible workaround is to put the last state transfer time stamp into the state, after each retrieval call, reset the new timeout to a appropriate value, a prototype looks like this:
handle_call(get_a, _From, #state{a = 1,
state = state_2,
%% this is the timestamp when the server transfered to state_2
unixtime = T1
}=S) ->
Reply = S#state.a,
NewTimeout = t_2 - (current_unixtime() - T1),
{reply, Reply, S, NewTimeout};
In this way, I can get the effect I want, but it's ugly, is there any better way to to this?
If you want to set timeouts independently of other events like calls it's probably best to use a timed message.
When you set the timeout use erlang:send_after/3:
TimerRef = erlang:send_after(10000, self(), my_timeout).
You can cancel your timeout any time with erlang:cancel_timer/1.
erlang:cancel_timer(TimerRef).
Receive it with handle_info:
handle_info(my_timeout, State) ->
If you need multiple such timeouts, use a different message, or if you have the possibility of some sort of race condition and need further control, you can create unique references with erlang:make_ref/0 and send a message like {Ref, my_timeout}.
Watch out for the edge cases - remember you could cancel a timer and then still receive it unexpectedly (because it's in your message queue when you cancel), and you don't make them unique (as suggested above using a reference) you could be expecting a timeout, and get it early, because it's the previous one that entered your message queue, etc (where as with a reference you can check it is the latest set). These things are easy to deal with, but watch out for them.
Related
As far as I understand, I should use handle_cast for asynchronous requests, by sending messages, then react to response via receive..end on client side.
Here is an example:
Pid ! {auth_user, {User, Password}}.
For synchronous requests, when I want to wait when gen_server sends a response, I must explicitly call the respective function from the module like this:
{ok, Result} = auth_server:auth_user({User, Password}).
Are the statements from above are correct?
If the latter is correct, why then I need a construction like this if I already have an exported function auth_user?
handle_call({authenticate_user, {Login, Password}}, _From, _) ->
{reply, {}, {}}.
You don't interact with a gen_server by sending messages directly like that. You should use gen_server:call or gen_server:cast like this:
Response = gen_server:call(Pid, {auth_user, {User, Password}})
call / cast will handle sending the message to the Pid (and receiving a response for call) itself, along with many edge cases like the process crashing before responding to your request, etc.
If the latter is correct, why then I need a construction like this if I already have an exported function auth_user?
gen_server is useful when you want to keep some state stored between multiple calls. For example, if you want to keep a global counter of the number of pageviews in your app in memory, you should use gen_server, store the counter as the state of the and make a cast or call to it on each request. If you don't have a state to keep, you should use normal functions.
E.g. suppose I have a module that implements gen_server behavior, and it has
handle_call({foo, Foo}, _From, State) ->
{reply, result(Foo), State}
;
I can reach this handler by doing gen_server:call(Server, {foo, Foo}) from some other process (I guess if a gen_server tries to gen_server:call itself, it will deadlock). But gen_server:call blocks on response (or timeout). What if I don't want to block on the response?
Imaginary use-case: Suppose I have 5 of these gen_servers, and a response from any 2 of them is enough for me. What I want to do is something like this:
OnResponse -> fun(Response) ->
% blah
end,
lists:foreach(
fun(S) ->
gen_server:async_call(S, {foo, Foo}, OnResponse)
end,
Servers),
Result = wait_for_two_responses(Timeout),
lol_i_dunno()
I know that gen_server has cast, but cast has no way to provide any response, so I don't think that that's what I want in this case. Also, seems like it should not be the gen_server's concern whether caller wants to handle response synchronously (using gen_server:call) or async (does not seem to exist?).
Also, the server is allowed to provide response asynchronously by having handle_call return no_reply and later calling gen_server:reply. So why not also support handling response asynchronously on the other side? Or does that exist, but I'm just failing to find it??
gen_server:call is basically a sequence of
send a message to the server (with identifier)
wait for the response of that particular message
wrapped in a single function.
for your example you can decompose the behavior in 2 steps: a loop that uses gen_server:cast(Server,{Message,UniqueID,self()} with all servers, and then a receive loop that wait for a minimum of 2 answers of the form {UniqueID,Answer}. But you must take care to empty your mail box at some point in time. A better solution should be to delegate this to a separate process which will simply die when it has received the required number of answers:
[edit] make some correction in the code now it should work :o)
get_n_answers(Msg,ServerList,N) when N =< length(ServerList) ->
spawn(?MODULE,get_n_answers,[Msg,ServerList,N,[],self()]).
get_n_answers(_Msg,[],0,Rep,Pid) ->
Pid ! {Pid,Rep};
get_n_answers(_Msg,[],N,Rep,Pid) ->
NewRep = receive
Answ -> [Answ|Rep]
end,
get_n_answers(_Msg,[],N-1,NewRep,Pid);
get_n_answers(Msg,[H|T],N,Rep,Pid) ->
%gen_server:cast(H,{Msg,Pid}),
H ! {Msg,self()},
get_n_answers(Msg,T,N,Rep,Pid).
and you cane use it like this:
ID = get_n_answers(Msg,ServerList,2),
% insert some code here
Answer = receive
{ID,A} -> A % tagged with ID to do not catch another message in the mailbox
end
You can easily implement that by sending each call in a separate process and waiting for responses from as many as required (in essence this is what async is about, isn't? :-)
Have a look at this simple implementation of parallel call which is based on the async_call from rpc library in OTP.
This is how it works in plain English.
You need to make 5 calls so (in the parent process) you spawn 5 child Erlang processes.
Each process sends back to the parent process a tuple containing its PID and the result of the call.
The tuple can be only constructed and send back only when the desired call has been completed.
In the parent process you loop through responses in the receive loop.
You can wait for all responses or just 2 or 3 out of the started 5.
The parent process (which spawns the worker processes) will eventually receive all responses (I mean those you want to ignore). You need a way to discard them if you don't want the message queue to grow infinitely. There are two options:
The parent process itself can be a transient process, created only for the call to spawn the other 5 child processes. Once the desired amount of responses is collected it can send the response back to a caller and die. Messages send to the died process will be discarded.
The parent process can continue receiving messages after it has received the desired amount of responses and simply discard them.
gen_server do not have a concept of async calls on client side. It is not trivial how to implement in consistently because gen_server:call is a combination of monitor for server process, send request message and wait for either answer or monitor down or timeout. If you do something like what you mentioned you will need to deal with DOWN messages from server somehow ... so hypothetical async_call should return some key for yeld and also an internal monitor reference for a case you are processing DONW messages from other processes... and do not want to mix it with yeld errors.
Not that good but possible alternative is to use rpc:async_call(gen_server, call, [....])
But this approach have a limitation in calling process will be a short lived rex child, so if your gen server use caller pid somehow other than send it a reply logic will be broken.
gen_sever:call to the process itself would surely block until timeout. To understand the reason, one should be aware of the fact that gen_server framework actually combine your specific code together into one single module, and gen_server:call would be "translated" as "pid ! Msg" form.
So imagine how this block of code takes effect, the process actually stay in a loop keeping receiving messages, and when the control flow run into a handling function, the receiving process is temporarily interrupted, so if you call gen_server:call to the process itself, since it is a synchronous function, it waits for response, which however would never come in until the handing function returns so that the process can continue to receive messages, so the code is in a deadlock.
The gen_server implementation has this fun little function:
do_send(Dest, Msg) ->
case catch erlang:send(Dest, Msg, [noconnect]) of
noconnect ->
spawn(erlang, send, [Dest,Msg]);
Other ->
Other
end.
The entry for erlang:send/3 says of the noconnect option
If the destination node would have to be auto-connected before doing the send, noconnect is returned instead.
The function here avoids the delay in setting up a connection between nodes by forcing a spawned process to do the waiting. Clever!
There's another option to erlang:send/3, nosuspend:
If the sender would have to be suspended to do the send, nosuspend is returned instead.
Per, erlang:send_nosuspend/2 the sender will be suspended if the connection is overloaded. Why would not gen_server wish to pull the same trick to avoid suspension of the sending process?
It does this when Dest is on another erlang node. It first tries to send the message without forcing a connection to be set-up if the nodes aren't connected, the [noconnect] option. If this can be done then erlang:send/3 sends the message. If this can't be done then we spawn a process which does a send which waits for the connection to be set up. Setting up a connection between two nodes can take time. This is, of course, so we don't sit and wait unnecessarily for the send.
EDIT:
The gen_server doesn't handle the nosuspend case at all, it just worries about the case where sending a message to a remote process could take time because of the need to wait for a connection to be set up. In which case a process is spawned so we can go on. This does not change the semantics. The nosuspend does a more complex handling of eventual network problems which would probably need more complex handling than should be provided in a standard API.
What is the most recommended way in Erlang to ensure that some process exists before sending a message / event to it? In my scenario I am starting process upon first occurrence of the message and then it stays alive. While keep passing further messages, I first try to start the process with the same name to ensure it is started, something like this (using gen_fsm and simple-one-for-one restart scenario):
%% DeviceId - process name
heartbeat(ApplicationKey, DeviceId, Timeout) ->
ensure_session_started(ApplicationKey, DeviceId, Timeout),
gen_fsm:send_event(DeviceId, {heartbeat, Timeout}).
ensure_session_started(ApplicationKey, DeviceId, Timeout) ->
case session_server_sup:start_child(ApplicationKey, DeviceId, Timeout) of
{ok, _Pid} -> {ok, running};
{error, {already_started, _}} -> {ok, running};
{error, Error} -> erlang:throw({error, {failed_to_start_session, Error}})
end.
I believe this solution is not perfect and probably has some overhead but still believe it is less race condition prone than using erlang:is_process_alive. Am I right? Any ideas how to improve it?
You are right, erlang:is_process_alive/1 approach is useless in this scenario because of race condition.
Your example is workable and I saw it in the wild few times. Note that it does not guarantee that message will be processed. To be sure of that you need to monitor your receiver and get confirmation from it. This is how it is done in gen_server:call/2.
With gen_fsm:sync_send_event/2,3 you can send events to your FSM and wait for a response. So you can tell your calling process that its message has been received.
This could be a very basic question but is Erlang capable of calling a method on another prcoess and wait for it to repond back with some object type without sleeping threads?
Well, if you're waiting for an answer, the calling process will have to sleep eventually… But that's no big deal.
While processes are stuck on the receive loop, other processes can work. In fact, it's not uncommon to have thousands of processes just waiting for messages. And since Erlang processes are not true OS threads, they're very lightweight so the performance loss is minimal.
In fact, the way sleep is implemented looks like:
sleep(Milliseconds) ->
receive
% Intentionally left empty
after Milliseconds -> ok
end.
Yes, it is possible to peek into the mailbox if that is what you mean. Say we have sent a message to another process and now we want to see if the other process has sent something back to us. But we don't want to block on the receive:
receive
Pattern -> Body;
Pattern2 -> Body2
after 0 ->
AfterBody
end
will try to match against the Pattern and Pattern2 in the mailbox. If none matches, it will immediately time out and go to AfterBody. This allows you to implement a non-blocking peek into the mailbox.
If the process is a gen_server the same thing can be had by playing with the internal state and the Timeout setting when a callback returns to the gen_server's control. You can set a Timeout of 0 to achieve this.
What am getting from the question is that we are talking of Synchronous Message Passing. YES ! Erlang can do this perfectly well, its the most basic way of handling concurrency in Erlang. Consider this below:
rpc(Request, To)->
MySelf = self(),
To ! {MySelf,Request},
receive
{To,Reply} -> Reply
after timer:seconds(5) -> erlang:exit({error,timedout})
end.
The code above shows that a processes sends a message to another and immediately goes into waiting (for a reply) without having to sleep. If it does not get a reply within 5 seconds, it will exit.