I have a basic mochiweb polling loop that looks like the following, except it does other things instead of printing to the console and eventually returns:
blah() ->
io:format("Blah")
blah()
loop(Req) ->
PathParts = string:tokens(Req:get(path), "/")
case PathParts of
["poll"] ->
blah()
This works great until the client aborts their request. For instance if the client window is closed, this process keeps running indefinitely.
I would like to know if there is an option in mochiweb's start() or maybe something else I'm overlooking that would have mochiweb automatically terminate this process, or at least send a message on client abort. Any ideas?
Looks like one solution is setting up another process to repeatedly call gen_tcp:recv(Req:get(socket), 0, 1) and looking for the result {error, closed}, then killing the polling process if it is received... Not sure if that's optimal though.
Related
In Erlang, can I call some function f (BIF or not), whose job is to spawn a process, run the function argf I provided, and doesn't "return" until argf has "returned", and do this without using receive clause (the reason for this is that f will be invoked in a gen_server, I don't want pollute the gen_server's mailbox).
A snippet would look like this:
%% some code omitted ...
F = fun() -> blah, blah, timer:sleep(10000) end,
f(F), %% like `spawn(F), but doesn't return until 10 seconds has passed`
%% ...
The only way to communicate between processes is message passing (of course you can consider to poll for a specific key in an ets or a file but I dont like this).
If you use a spawn_monitor function in f/1 to start the F process and then have a receive block only matching the possible system messages from this monitor:
f(F) ->
{_Pid, MonitorRef} = spawn_monitor(F),
receive
{_Tag, MonitorRef, _Type, _Object, _Info} -> ok
end.
you will not mess your gen_server mailbox. The example is the minimum code, you can add a timeout (fixed or parameter), execute some code on normal or error completion...
You will not "pollute" the gen_servers mailbox if you spawn+wait for message before you return from the call or cast. A more serious problem with this maybe that you will block the gen_server while you are waiting for the other process to terminate. A way around this is to not explicitly wait but return from the call/cast and then when the completion message arrives handle it in handle_info/2 and then do what is necessary.
If the spawning is done in a handle_call and you want to return the "result" of that process then you can delay returning the value to the original call from the handle_info handling the process termination message.
Note that however you do it a gen_server:call has a timeout value, either implicit or explicit, and if no reply is returned it generates an error in the calling process.
Main way to communicate with process in Erlang VM space is message passing with erlang:send/2 or erlang:send/3 functions (alias !). But you can "hack" Erlang and use multiple way for communicating over process.
You can use erlang:link/1 to communicate stat of the process, its mainly used in case of your process is dying or is ended or something is wrong (exception or throw).
You can use erlang:monitor/2, this is similar to erlang:link/1 except the message go directly into process mailbox.
You can also hack Erlang, and use some internal way (shared ETS/DETS/Mnesia tables) or use external methods (database or other things like that). This is clearly not recommended and "destroy" Erlang philosophy... But you can do it.
Its seems your problem can be solved with supervisor behavior. supervisor support many strategies to control supervised process:
one_for_one: If one child process terminates and is to be restarted, only that child process is affected. This is the default restart strategy.
one_for_all: If one child process terminates and is to be restarted, all other child processes are terminated and then all child processes are restarted.
rest_for_one: If one child process terminates and is to be restarted, the 'rest' of the child processes (that is, the child processes after the terminated child process in the start order) are terminated. Then the terminated child process and all child processes after it are restarted.
simple_one_for_one: A simplified one_for_one supervisor, where all child processes are dynamically added instances of the same process type, that is, running the same code.
You can also modify or create your own supervisor strategy from scratch or base on supervisor_bridge.
So, to summarize, you need a process who wait for one or more terminating process. This behavior is supported natively with OTP, but you can also create your own model. For doing that, you need to share status of every started process, using cache or database, or when your process is spawned. Something like that:
Fun = fun
MyFun (ParentProcess, {result, Data})
when is_pid(ParentProcess) ->
ParentProcess ! {self(), Data};
MyFun (ParentProcess, MyData)
when is_pid(ParentProcess) ->
% do something
MyFun(ParentProcess, MyData2) end.
spawn(fun() -> Fun(self(), InitData) end).
EDIT: forgot to add an example without send/receive. I use an ETS table to store every result from lambda function. This ETS table is set when we spawn this process. To get result, we can select data from this table. Note, the key of the row is the process id of the process.
spawner(Ets, Fun, Args)
when is_integer(Ets),
is_function(Fun) ->
spawn(fun() -> Fun(Ets, Args) end).
Fun = fun
F(Ets, {result, Data}) ->
ets:insert(Ets, {self(), Data});
F(Ets, Data) ->
% do something here
Data2 = Data,
F(Ets, Data2) end.
To exchange data,it becomes important to link the process first.The following code does the job of linking two processes.
start_link(Name) ->
gen_fsm:start_link(?MODULE, [Name], []).
My Question : which are the two processes being linked here?
In your example, the process that called start_link/1 and the process being started as (?MODULE, Name, Args).
It is a mistake to think that two processes need to be linked to exchange data. Data links the fate of the two processes. If one dies, the other dies, unless a system process is the one that starts the link (a "system process" means one that is trapping exits). This probably isn't what you want. If you are trying to avoid a deadlock or do something other than just timeout during synchronous messaging if the process you are sending a message to dies before responding, consider something like this:
ask(Proc, Request, Data, Timeout) ->
Ref = monitor(process, Proc),
Proc ! {self(), Ref, {ask, Request, Data}},
receive
{Ref, Res} ->
demonitor(Ref, [flush]),
Res;
{'DOWN', Ref, process, Proc, Reason} ->
some_cleanup_action(),
{fail, Reason}
after
Timeout ->
{fail, timeout}
end.
If you are just trying to spawn a worker that needs to give you an answer, you might want to consider using spawn_monitor instead and using its {pid(), reference()} return as the message you're listening for in response.
As I mentioned above, the process starting the link won't die if it is trapping exits, but you really want to avoid trapping exits in most cases. As a basic rule, use process_flag(trap_exit, true) as little as possible. Getting trap_exit happy everywhere will have structural effects you won't intend eventually, and its one of the few things in Erlang that is difficult to refactor away from later.
The link is bidirectional, between the process which is calling the function start_link(Name) and the new process created by gen_fsm:start_link(?MODULE, [Name], []).
A called function is executed in the context of the calling process.
A new process is created by a spawn function. You should find it in the gen_fsm:start_link/3 code.
When a link is created, if one process exit for an other reason than normal, the linked process will die also, except if it has set process_flag(trap_exit, true) in which case it will receive the message {'EXIT',FromPid,Reason} where FromPid is the Pid of the process that came to die, and Reason the reason of termination.
This is an interesting situation, focused on the behavior of erlang ssh modules. I had spent a few hours troubleshooting a problem that turned out to reveal that the Erlang ssh_connection *exec/4* function operates asynchronously.
If you issue the ssh_connection:exec/4 function to run a script that takes several seconds to complete, and then in your erlang program you close the ssh connection, the script execution will terminate. My expectation was that the ssh_connection:exec would be synchronous rather than asynchronous.
Because the time to complete the remote script invoked by ssh_connection:exec is unknown, I chose to not issue the closure ssh:close(). I would like to understand the consequences of that:
Will the gc clear it at some point ?
Will it stay open for good during the whole node existence ?
Is there a way to make the ssh_connection:exec synchronous, as I would believe it should be.
Here is an example of the test erl program that I used to verify this issue. As a script you can run a simple sleep 10 (sleep 10 seconds) to emulate a slow running program.
-module(testssh).
-export([test/5]).
test (ServerName, Port, Command, User, Password) ->
crypto:start(),
ssh:start(),
{ok, SshConnectionRef} = ssh:connect(ServerName, Port, [ {user, User}, {password, Password} , {silently_accept_hosts, true} ], 60000 ),
{ok, SshConnectionChannelRef} = ssh_connection:session_channel(SshConnectionRef, 60000),
Status = ssh_connection:exec(SshConnectionRef, SshConnectionChannelRef, Command, 60000),
ssh:close(SshConnectionRef).
Remote script:
#!/bin/sh
sleep 10
I never had to use the ssh application myself, but you should be reading something wrong, it is clear in the doc that the result will be delivered as messages to the caller:
[...] the result will be several messages according to the following pattern. Note that the last message will be a channel close message, as the exec request is a one time execution that closes the channel when it is done[...]
See http://www.erlang.org/doc/man/ssh_connection.html#exec-4
So after you call ssh_connection:exec/4 , test with a loop like this:
wait_for_response(ConnectionRef) ->
receive
{ssh_cm, ConnectionRef, Msg} ->
case Msg of
{closed, _ChannelId} ->
io:format("Done");
_ ->
io:format("Got: ~p", [Msg]),
wait_for_response(ConnectionRef)
end
end.
You should receive the command output, and other ssh messages, and finally a 'closed' message that is your signal that the ssh command has properly finished.
I'm currently writing a piece of software in erlang, which is now based on gen_server behaviour. This gen_server should export a function (let's call it update/1) which should connect using ssl to another service online and send to it the value passed as argument to the function.
Currently update/1 is like this:
update(Value) ->
gen_server:call(?SERVER, {update, Value}).
So once it is called, there is a call to ?SERVER which is handled as:
handle_call({update, Value}, _From, State) ->
{ok, Socket} = ssl:connect("remoteserver.com", 5555, [], 3000).
Reply = ssl:send(Socket, Value).
{ok, Reply, State}.
Once the packet is sent to the remote server, the peer should severe the connection.
Now, this works fine with my tests in shell, but what happens if we have to call 1000 times mymod:update(Value) and ssl:connect/4 is not working well (i.e. is reaching its timeout)?
At this point, my gen_server will have a very large amount of values and they can be processed only one by one, leading to the point that the 1000th update will be done only 1000*3000 milliseconds after its value was updated using update/1.
Using a cast instead of a call would leave to the same problem. How can I solve this problem? Should I use a normal function and not a gen_server call?
From personal experience I can say that 1000 messages per gen_server process wont be a problem unless you are queuing big messages.
If from your testing it seems that your gen_server is not able to handle this much load, then you must create multiple instances of your gen_server preferably under a supervisor process at the boot time (or run-time) of your application.
Besides that, I really don't understand the requirement of making a new connection for each update!! you should consider some optimization like cached connections/ pre-connections to the server..no?
It seems that an erlang process will stay alive until the 5 sec default timeout even if it has finished it's work.
I have gen_server call that issues a command to the window CLI which can be completed in less than 1 sec but the process waits 5 sec before I see the result of the operation. What's going on? is it soemthing to do with the timeout, or might it be something else.
EDIT This call doesn't do anything for 5 seconds (the default timeout!)
handle_call({create_app, Path, Name, Args}, _From, State) ->
case filelib:ensure_dir(Path) of
{error, Reason} ->
{reply, Reason, State};
_ ->
file:set_cwd(Path),
Response = os:cmd(string:join(["Rails", Name, Args], " ")),
{reply, Response, State}
end;
I'm guessing the os:cmd is taking that long to return the results. It's possible that maybe the os:cmd is having trouble telling when the rails command is completed and doesn't return till the process triggers the timeout. But from your code I'd say the most likely culprit is the os:cmd call.
Does the return contain everything you expect it to?
You still have not added any information on what the problem is. But I see some other things I'd like to comment on.
Current working dir
You are using file:set_cwd(Path) so the started command will inherit that path. The cwd of the file server is global. You should probably not be using it at all in application code. It's useful for setting the cwd to where you want erlang crash dumps to be written etc.
Your desire to let rail execute with the cwd according to Path is better served with something like this:
_ ->
Response = os:cmd(string:join(["cd", Path, "&&", "Rails", Name, Args], " ")),
{reply, Response, State}
That is, start a shell to parse the command line, have the shell change cwd and the start Rails.
Blocking a gen_server
The gen_server is there to serialize processing. That is, it processes one message after the other. It doesn't handle them all concurrently. It is its reason for existence to not handle them concurrently.
You are (in relation to other costs) doing some very costly computation in the gen_server: starting an external process that runs this rails application. Is it your intention to have at most one rails application running at any one time? (I've heard about ruby on rails requiring tons of memory per process, so it might be a sensible decision).
If you dont need to update the State with any values from a costly call, as in your example code, then you can use an explicit gen_server:reply/2 call.
_ ->
spawn_link(fun () -> rails_cmd(From, Path, Name, Args) end),
{no_reply, State}
And then you have
rails_cmd(From, Path, Name, Args) ->
Response = os:cmd(string:join(["cd", Path, "&&", "Rails", Name, Args], " ")),
gen_server:reply(From, Response).