Consuming an API that connects via TCP connection

adam

I’m looking at a potential project where an entry system for a premises has a “push notifications API” for people entering an establishment.

The system can be enabled to POST the entry “event” to a third party system upon its completion. The documentation is vague at present, but, it does involve the system connecting to the third party via a “TCP Socket”. The data sent includes a binary of a jpeg of the person entering / using the system. From the docs.

" When pushing a notification, a terminal establishes a socket connection with the subscriber by using the IP address and port, and then sends records. After sending, the terminal closes the connection if short connection is used, and keeps keep-alive connection."

I have zero experience with an API sending data over a TCP socket, and my Google Fu is eluding me on this one.

Does anyone have experience of the best way to approach this in a Phoenix/Elixir app?

Thanks in advance.

6 comments

/phoenix #tcp

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2020-07-23 13:23:43 UTC

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shanesveller

Usually there should be a documented line protocol that defines what kinds of bits and bytes you send across the wire, and in what order, to indicate various intents. It might be a custom binary format, something like Protobufs, line-separated JSON, etc. There are all kinds of answers there and they all get handled a little differently. I think only the vendor you’re trying to integrate with can answer the initial part of this question for you.

Post #1

adam

thanks @shanesveller, I’ll definitely be seeking out more documentation.

I guess I also need to open a TCP port on the app to listen to the incoming api calls?

Post #2

ityonemo

:gen_tcp erlang module is useful for this. I’ve written several tcp protocols using it, it’s not terrible.

Post #3

ityonemo

Haha just rereading this, I was low on sleep when I wrote this due to having gone out to view the comet through a telescope. It’s not just “not terrible”, but writing TCP socket logic in erlang/elixir is the most reasonable in any language that I’ve done it for (to include C, C++, and python).

Post #4

adam

Thanks @ityonemo, anything in particular I should be looking out for?

Post #5

ityonemo

For a long time. I didn’t know about “connection”, which is a fantastic library for writing clients with a specialized genserver-like behaviour. If you’ll eventually need tcp and tls, I recommend “transport”, which is a library I wrote that abstracts the difference between tcp and tls and makes it so you can seamlessly transition between one and the other.

This is the place to start to learn: Buckets of Sockets | Learn You Some Erlang for Great Good!

You’ll have to switch between erlang and elixir (a good exercise in and of itself) and I recommend writing a generic udp client/server pair and then a tcp client/server pair on localhost, then you can graduate to your application.

I would also recommend first learning by using active: true sockets (packets get turned into messages) and if you want, later learn active: false sockets. I think active: false sockets are better but active: true sockets are simpler.

Post #6

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