Stacking with Debug

This tutorial uses the same comm.Client, comm.Server classes from the previous one, so continuing on all it needs is the comm.Debug class, so import it:

>>> from bacpypes.comm import Debug

Because there could be lots of Debug instances, it could be confusing if you didn’t know which instance was generating the output. So you can initialize an instance with a lobel:

>>> d = Debug("middle")

As you can guess, this is going to go into the middle of a stack of objects. The top of the stack is a client, then bottom of a stack is a server. When messages are flowing from clients to servers they are called downstream messages, and when they go from server to the client they go upstream.

The comm.bind() function takes an arbitrary number of objects, but it assumes that the first one will always be a client, the last one is a server, and that the objects in the middle are both a kind of server that can be bound with the client to its left in the parameter list, and a client that can be bound to a server to its right:

>>> bind(c, d, s)

Now when the client generates a request, rather than the message being sent to the MyServer instance, it is sent to the debugging instance. That is acting as a server, so it prints out that it received an indication:

>>> c.request('hi')
Debug(middle).indication
    - args[0]: hi

Now it acts as a client and forwards it down to the server in the stack. That generates a print statement and responds with the string uppercase:

working on hi

Upstream from the server is the debugging instance again, this time as a confirmation:

Debug(middle).confirmation
    - args[0]: HI

Now it acts as a server and continues the response up the stack, which is printed out by the client:

thanks for the HI

With clearly defined “envelopes” of protocol data, matching the combination of clients and servers into layers can provide a clear separation of functionality in a protocol stack.