Twisted - succes (or failure) callback for LineReceiver sendLine

Question

I'm still trying to master Twisted while in the midst of finishing an application that uses it.

My question is:

My application uses LineReceiver.sendLine to send messages from a Twisted TCP server. I would like to know if the sendLine succeeded.

I gather that I need to somehow add a success (and error?) callback to sendLine but I don't know how to do this.

Thanks for any pointers / examples

Answer 1

You need to define "succeeded" in order to come up with an answer to this.

All sendLine does immediately (probably) is add some bytes to a send buffer. In some sense, as long as it doesn't raise an exception (eg, MemoryError because your line is too long or TypeError because your line was the number 3 instead of an actual line) it has succeeded.

That's not a very useful kind of success, though. Unfortunately, the useful kind of success is more like "the bytes were added to the send buffer, the send buffer was flushed to the socket, the peer received the bytes, and the receiving application acted on the data in a persistent way".

Nothing in LineReceiver can tell you that all those things happened. The standard solution is to add some kind of acknowledgement to your protocol: when the receiving application has acted on the data, it sends back some bytes that tell the original sender the message has been handled.

You won't get LineReceiver.sendLine to help you much here because all it really knows how to do is send some bytes in a particular format. You need a more complex protocol to handle acknowledgements.

Fortunately, Twisted comes with a few. twisted.protocols.amp is one: it offers remote method calls (complete with responses) as a basic feature. I find that AMP is suitable for a wide range of applications so it's often safe to recommend for new development. It largely supersedes the older twisted.spread (aka "PB") which also provides both remote method calls and remote object references (and is therefore more complex - in my experience, more complex than most applications need). There are also some options that are a bit more standard: for example, Twisted Web includes an HTTP implementation (HTTP, as you may know, is good at request/response style interaction).