Tuesday, July 08, 2014

The Slim protocol

I wrote earlier about Fitnesse and Slim style integration testing.

There are only a few commands associated with Slim tables, but the precise meaning of the command is left to the Slim server. For example, according to the Slim documentation, the Import instruction:

causes the <path> to be added to the search path for fixtures. In java <path> gets added to the CLASSPATH. In .NET, the <path> would name a dll. 

This explains why import tests are always green. You can pass them any random string and it's just added to the path. In WaferSlim, the concept is extended slightly: if the string has a slash or backslash in it, it's considered a path. Otherwise, it's considered a file and is added to the list of files searched for classes. Let's take an example. Create a file called fixtures.py with the following code:

class SomeDecisionTable:
def setInput(self, x):
self.x = str(x)

def getOutput(self):
return int(self.x) + 1

There's an odd Unicode issue that I think requires context variables to be saved as strings. I don't know if Fitnesse, Slim or Waferslim is responsible for that. At any rate, it's simple enough: set the input to an integer and the output will be the integer plus one.

Save that file. Now the Import table will take two lines, one with the path, one with the file:

!|import |

Now WaferSlim knows where to find our code, so let's take a look at another table.

Here's what the Slim protocol tells us: it can send Make instructions and Call instructions. A Make instruction tells the server to create an object; a Call instruction tells it to call a method on that object. I'd hoped to go through the Fitnesse code to determine exactly how that works, but I didn't. We'll take it on faith that the first line of a table sends a Make instruction and further lines send Call instructions. So, in order to make calls into our object, we write:

|input|get output?|
|1    |2          |
|10   |11         |

From the first line of the table, Fitnesse sends a Make: SomeDecisionTable command to the Slim server. Because of our Import statements, the server searches the /path/to/fixtures directory for the fixtures.py file, finds the SomeDecisionTable class, and instantiates it. 

The second line of the table tells Fitnesse what Call statements to make. It will call setInput and then getOutput. Each further line of the table gives arguments for the input and output. So the sequence of events from the Slim server's perspective is as follows:

Make: SomeDecisionTable
Call: setInput(1)
Call: getOutput()
Call: setInput(10)
Call: getOutput()

Click the test button now and everything should run green.