Add ping command for PubSub connections #274
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This is gonna be a useful addition. However, few notes:
- We need to handle
:pingcalls when the gen_statem is in thedisconnectedstate. - We need tests.
- Instead of sending a
PINGcommand with a message, could we just keep a queue of pendingPINGcommands, and for everyPONGthat the gen_statem receives we send it to the next caller waiting in the queue? Redis is single-threaded so it will reply toPINGs in order anyway.
lib/redix/pubsub/connection.ex
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| {:ok, task_pid} = | ||
| Task.start(fn -> | ||
| res = | ||
| receive do | ||
| {:pong, ^random_string} -> :ok | ||
| after | ||
| 5000 -> :error | ||
| end | ||
|
|
||
| :ok = :gen_statem.reply(from, res) | ||
| end) |
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We won't need the task here if we go with a queue of pending clients.
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Thank you for the feedback.
Would the caller get an :error after 5000 ms without the task?
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The flow to implement here is using gen_statem timeouts and processe aliases, see
Timeout specifies how long to wait for a response. If no response is received within the specified time, this function returns timeout. Assuming that the server executes on a node supporting aliases (introduced in OTP 24) the request will also be abandoned. That is, no response will be received after a time-out. Otherwise, a stray response might be received at a later time.
from the gen_statem docs. Basically you don't care about timeouts, you just reply to every pong and BEAM throws away the responses for procs that timed out already
This allows for periodic checking of the underlying TCP connection. According to the Redis documentation, PING is one of very few commands that can be issued on a pubsub connection: "Once the client enters the subscribed state it is not supposed to issue any other commands, except for additional SUBSCRIBE, SSUBSCRIBE, PSUBSCRIBE, UNSUBSCRIBE, SUNSUBSCRIBE, PUNSUBSCRIBE, PING, RESET and QUIT commands"
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I've added some tests, and actually discovered that the reply format is different if you're subscribed to any channels or not. I've also removed the task, and replaced it with a callback that is sent after 5000 ms. EDIT: |
This allows for periodic checking of the underlying TCP connection.
According to the Redis documentation, PING is one of very few commands that can be issued on a pubsub connection:
"Once the client enters the subscribed state it is not supposed to issue any other commands, except for additional SUBSCRIBE, SSUBSCRIBE, PSUBSCRIBE, UNSUBSCRIBE, SUNSUBSCRIBE, PUNSUBSCRIBE, PING, RESET and QUIT commands"
I was not sure if the chosen flow is the best one, ie. sending a serialized PID as part of the payload and using that for sending the reply back to the waiting task.