pyhton API: where does "42" prefix come from?

[RomanBrodetski]

bots/python/etherdeltaclientservice.py

Line 362 in 3bc9411

tosend = '42["getMarket",{"token":"' + self.token + '","user":"' + self.userAccount + '"}]'

Here it seems like all the method invocations contain a leading "42" number. I didn't find this described in the docs. Where does it come from?
I'm trying to replicate the python client behaviour using scala and akka-http without much luck. I keep ending up with the following response:

0{"sid":"ndWNgwRDSzvcRkioAABg","upgrades":[],"pingInterval":25000,"pingTimeout":60000}
40

cc @tomvanbraeckel

[RomanBrodetski]

I figured out that the API provided is not plain websockets API. It's socket.io API with only websocket transport enabled (It would be nice to edit the API description to reflect this fact).

These "42" numbers is what socket.io uses for namespacing.

[mshenfield]

Linking some resources for future users:

• First number is the packet type - used to determine the type of protocol request being made (4 is a message)
• Second number is the message object type - the type of the message. "2" is a socket.io Event (the type of thing you on(event, callback)).