Unable to get genserver to genserver communication working | Elixir Language | Page 1

slow cove Aug 16, 2022, 6:43 AM

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I am trying to get 1 genserver to send a message to another genserver using Genserver.call/3

but even though both genservers are up, I get ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started as an error

I set module names using __MODULE__ and i am passing the module as the first param to call/3 (in this case its BMCServer.PacketHandler)

Any ideas why my stuff isn't working right (i have handle_call() implemented in PacketHandler

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defp handle_packet(data, socket, address, port) do
    # print the message
    IO.puts("Received: #{String.trim data} from #{:inet_parse.ntoa(address)}:#{port}")
    GenServer.call(BMCServer.PacketHandler, {address, port, data})
    {:noreply, socket}
  end

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defmodule BMCServer.PacketHandler do
  use GenServer
  def start_link() do
    IO.puts("#{__MODULE__} has started")
    GenServer.start_link(__MODULE__, name: __MODULE__)
  end

  def init(_opts) do
    {:ok, nil}
  end

  def handle_call({address, port, data}, _from, state) do
    <<packetSize::unsigned-integer-16, rest::binary-size(packetSize)>> = data
    IO.puts("Size #{packetSize}")
    {:reply, {address, port, data, packetSize, rest}, state}
  end
end

glacial crescent Aug 16, 2022, 6:45 AM

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Are both processes started?

slow cove Aug 16, 2022, 6:45 AM

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they should be according to the log output

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Compiling 1 file (.ex)
Elixir.BMCServer.UDPServer started
Elixir.BMCServer.PacketHandler has started
Received: g from 127.0.0.1:54792

23:26:15.510 [error] GenServer BMCServer.UDPServer terminating
** (stop) exited in: GenServer.call(nil, {{127, 0, 0, 1}, 54792, "g\n"}, 5000)
    ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started
    (elixir 1.13.4) lib/gen_server.ex:1019: GenServer.call/3
    (bmcserver 0.1.0) lib/bmcserver/udp_server.ex:49: BMCServer.UDPServer.handle_packet/4
    (stdlib 3.17.2) gen_server.erl:695: :gen_server.try_dispatch/4
    (stdlib 3.17.2) gen_server.erl:771: :gen_server.handle_msg/6
    (stdlib 3.17.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
Last message: {:udp, #Port<0.5>, {127, 0, 0, 1}, 54792, "g\n"}
State: #Port<0.5>
BMCServer.UDPServer started```

glacial crescent Aug 16, 2022, 6:46 AM

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Important thing - calling GenServer functions against process defined in another module is unidiomatic

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Also, snake_case for variable names is more idiomatic

slow cove Aug 16, 2022, 6:46 AM

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then whats the best way to have 2 GenServers communicate?

glacial crescent Aug 16, 2022, 6:47 AM

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Define function in your module that will call the process

slow cove Aug 16, 2022, 6:48 AM

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so something like def blah() do Genserver.call(__MODULE__) end and then have the other genserver call blah() ?

glacial crescent Aug 16, 2022, 6:48 AM

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Indeed

slow cove Aug 16, 2022, 6:49 AM

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seems kind of roundabout, but if thats idiomatic, sure. and sorry about hte variables, coming from Go, so lots of camelcase

magic sundial Aug 16, 2022, 6:49 AM

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slow cove seems kind of roundabout, but if thats idiomatic, sure. and sorry about hte vari...

ehhh i think having client calls scattered around ur codebase is worse

tall coral Aug 16, 2022, 6:49 AM

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GenServer.call(nil, {{127, 0, 0, 1}, 54792, "g\n"}, 5000) it's getting nil for the name/pid, says so in your logs

glacial crescent Aug 16, 2022, 6:49 AM

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And it seems that you call the GenServer.call/2 passing nil as the process name

magic sundial Aug 16, 2022, 6:50 AM

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better to stick to having both client and server api in the same place

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would also reduce repetition in case you make the same function call from different modules

tall coral Aug 16, 2022, 6:52 AM

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slow cove seems kind of roundabout, but if thats idiomatic, sure. and sorry about hte vari...

the reason for this is to shield yourself from the implementation details: BMCServer.PacketHandler.handle_packet(address, port, data) leaves the implementation details opaque. Sure right now it's a GenServer, but maybe you want to refactor it to something else in the future?

slow cove Aug 16, 2022, 6:53 AM

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I acually wrote that exact function, but still getting an error

tall coral Aug 16, 2022, 6:53 AM

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is it still getting nil for some reason?

magic sundial Aug 16, 2022, 6:53 AM

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GenServer.start_link(__MODULE__, name: __MODULE__)

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that cant work

slow cove Aug 16, 2022, 6:54 AM

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BMCServer.PacketHandler.parse_packet(address, port, data)```
```elixir
def parse_packet(address, port, data) do
    GenServer.call(__MODULE__, {address, port, data})
  end

❯ mix run --no-halt
Elixir.BMCServer.UDPServer has started
Elixir.BMCServer.PacketHandler has started
Received: s from 127.0.0.1:54792

23:51:40.966 [error] GenServer BMCServer.UDPServer terminating
** (stop) exited in: GenServer.call(BMCServer.PacketHandler, {{127, 0, 0, 1}, 54792, "s\n"}, 5000)
    ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started
    (elixir 1.13.4) lib/gen_server.ex:1019: GenServer.call/3
    (bmcserver 0.1.0) lib/bmcserver/udp_server.ex:48: BMCServer.UDPServer.handle_packet/4
    (stdlib 3.17.2) gen_server.erl:695: :gen_server.try_dispatch/4
    (stdlib 3.17.2) gen_server.erl:771: :gen_server.handle_msg/6
    (stdlib 3.17.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
Last message: {:udp, #Port<0.5>, {127, 0, 0, 1}, 54792, "s\n"}
State: #Port<0.5>
Elixir.BMCServer.UDPServer has started

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no, im not getting nil anymore, that was an old log

magic sundial Aug 16, 2022, 6:54 AM

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GenServer.start_link(__MODULE__, [], name: __MODULE__)

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is what u meant

tall coral Aug 16, 2022, 6:55 AM

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yeah, you're missing the initial state

slow cove Aug 16, 2022, 6:55 AM

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well that did it. I read a bad guide then, because they had a bunch of start_link calls without that middle part.

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getting a different error, but its related to the binary pattern matching

tall coral Aug 16, 2022, 6:56 AM

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so, GenServer.start_link(__MODULE__, name: __MODULE__) is actually starting your genserver with a random pid and setting it's initial state to [name: __MODULE__]

magic sundial Aug 16, 2022, 6:56 AM

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the docs are solid for genserver

tall coral Aug 16, 2022, 6:56 AM

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can you link us to the bad guide? curious...

slow cove Aug 16, 2022, 6:56 AM

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i dunno which one it was, i looked through a bunch trying to figure stuff out at first.

tall coral Aug 16, 2022, 6:57 AM

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oh, maybe it got mixed up with Agent: https://hexdocs.pm/elixir/1.12/Agent.html#start_link/2

slow cove Aug 16, 2022, 6:57 AM

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mostly trying to get syntax right

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possible

magic sundial Aug 16, 2022, 6:57 AM

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tall coral oh, maybe it got mixed up with `Agent`: <https://hexdocs.pm/elixir/1.12/Agent.ht...

naaaahh agent needs a start function

slow cove Aug 16, 2022, 6:58 AM

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shoudln't binary pattern matching work on an arbitrary data, even if its being treated as a string?

tall coral Aug 16, 2022, 6:58 AM

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well, I meant in that it's a /2 and the second arg takes the process opts

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yes, strings are binaries, so binary pattern matching works on them

slow cove Aug 16, 2022, 7:00 AM

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<<packetSize::unsigned-integer-16, rest::binary-size(packetSize)>> = data does that look correct?

magic sundial Aug 16, 2022, 7:00 AM

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yes

slow cove Aug 16, 2022, 7:01 AM

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so I am assuming i am getting ** (MatchError) no match of right hand side value: "s\n" because the incoming data is not match the pattern?

magic sundial Aug 16, 2022, 7:01 AM

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You can have another handle_call for the same message and inspects it

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to see what youre receiving

magic sundial Aug 16, 2022, 7:01 AM

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slow cove so I am assuming i am getting ```** (MatchError) no match of right hand side val...

yes

tall coral Aug 16, 2022, 7:02 AM

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iex(3)> <<packet_size::unsigned-integer-16, rest::binary-size(packet_size)>> = <<1::16, 2>>
<<0, 1, 2>>
iex(4)> packet_size
1
iex(5)> rest
<<2>>

magic sundial Aug 16, 2022, 7:02 AM

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def handle_call({address, port, data}, _from, state) do

should have some identifier

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not just the tuple size

slow cove Aug 16, 2022, 7:02 AM

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ok. thats fine, looks like I need to construct some custom data to send its way

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you mean like handle_call({:parse, address, port, data}, _from, state)

magic sundial Aug 16, 2022, 7:03 AM

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yes

slow cove Aug 16, 2022, 7:03 AM

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sounds good.

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coming from a past history with Akka and other Actor implementations in other languages, idiomatic elixir is definitely different. Need to get in the right mindset

tall coral Aug 16, 2022, 7:05 AM

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I might also suggest: {message_name, value}. so: {:parse, {address, port, data}}. i.e.: standardize your message type to a 2-tuple

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but I dunno, might just be my opinion 😅

slow cove Aug 16, 2022, 7:06 AM

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thats fair.

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its probably good to get hte foundational stuff right, so that when i get into the business logic and db stuff, its more robust

glacial crescent Aug 16, 2022, 7:24 AM

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The same error?

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Error is in lib/bmcserver/udp_server.ex at line 49

#Unable to get genserver to genserver communication working