Capture-Replay Mocking with CaptureMock
Intercepting plain-text network messages
CaptureMock can also be used for this purpose. It does however currently rely on messages being plain-text and synchronous: the client opening a socket, sending a message, reading a reply and then closing the socket. This functionality has not been used in many contexts yet and should be regarded as more experimental than the rest of CaptureMock.
Recording and replaying works in a similar way to the Python or system command variants, see there for more details. Run CaptureMock as for command-line interceptions: it will start its server and set the environment variable CAPTUREMOCK_SERVER to <host:port> for this place.
An example: The String-Length Server
Suppose we have a Python server which listens for input and says how long the strings it receives were.
## server.py
from SocketServer import TCPServer, StreamRequestHandler
import socket

class MyRequestHandler(StreamRequestHandler):
    def handle(self):
        clientData = self.rfile.read()
        self.wfile.write("Length was " + str(len(clientData)))

server = TCPServer((socket.gethostname(), 0), MyRequestHandler)
host, port = server.socket.getsockname()
address = host + ":" + str(port)
message = "Started string-length server at " + address
print message

server.serve_forever()
Then naturally, we have a client that we can use to send stuff to it, that takes the server address as an argument:
## client.py
import sys, socket

def runQuery(serverAddress, toSend):
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect(serverAddress)
    sock.sendall(toSend)
    sock.shutdown(1)
    response = sock.makefile().read()
    print "Sent to server:", toSend
    print "Got reply:", response
    sock.close()

servAddr = sys.argv[1]
host, port = servAddr.split(":")
serverAddress = (host, int(port))
runQuery(serverAddress, "Here is a string")
runQuery(serverAddress, "Here is a longer string")
We can then run them like this:
$ ./server.py &
Started string-length server at 10.67.20.109:41541
$ ./client.py 10.67.20.109:41541
Sent to server: Here is a string
Got reply: Length was 16
Sent to server: Here is a longer string
Got reply: Length was 23
To test them automatically, we would of course write a wrapper script to do something like this:
## runsystem.py
import subprocess

serverProc = subprocess.Popen("server.py", stdout=subprocess.PIPE)
addressLine = serverProc.stdout.readline()
serverAddress = addressLine.strip().split()[-1]
subprocess.call([ "client.py", serverAddress ])
serverProc.kill()
With all that in place, we can now use CaptureMock to intercept and record the traffic, allowing us to in future test either the server or the client alone.
We need to modify our wrapper script (note: not the client or the server) such that the client will connect to the host and port given by the environment variable 'CAPTUREMOCK_SERVER', instead of connecting directly to its own server.
It is currently assumed that the location of the real server is determined dynamically and hence cannot be provided to CaptureMock on startup. The wrapper script should therefore send the location of the real server to the CaptureMock server, when it knows what it is. So here is our modified wrapper script:
## runsystem.py, modified to work with CaptureMock
import subprocess, socket, os

def sendAddress(cpMockAddress, serverAddress):
    host, port = cpMockAddress.split(":")
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((host, int(port)))
    sock.sendall("SUT_SERVER:Real server started on " + serverAddress + "\n")
    sock.close()

serverProc = subprocess.Popen("server.py", stdout=subprocess.PIPE)
addressLine = serverProc.stdout.readline()
serverAddress = addressLine.strip().split()[-1]
cpMockAddress = os.getenv("CAPTUREMOCK_SERVER")
if cpMockAddress:
    sendAddress(cpMockAddress, serverAddress)
    subprocess.call([ "client.py", cpMockAddress ])
else:
    subprocess.call([ "client.py", serverAddress ])
serverProc.kill()
This information should be in the format SUT_SERVER:<some_message> <host:port>. CaptureMock will then parse this and know where the "real" server is.
We can now record the traffic using CaptureMock, using the command
$ capturemock --record clientserver.mock runsystem.py
which will produce the following in clientserver.mock:
<-SRV:Real server started on 10.67.20.109:35620
->CLI:Here is a string
<-SRV:Length was 16
->CLI:Here is a longer string
<-SRV:Length was 23
When the client sends a request it will now go to CaptureMock instead. CaptureMock will record it as a client request in the mock file, and forward it to the server. The server will then reply, which will be recorded as a server reply, and forwarded back to the client. In this way a complete log of the communication can be built up.
The format is similar to that used for command-line and python interception, with "CLI" referring to client requests and "SRV" to server responses.
Replay: testing the client or server in isolation
You can then use this recorded file to test either the client or the server in isolation. The initial server notification is still crucial when testing the server, of course, so in that case the "runsystem.py" program can just be modified to not start the client. The initial notification will cause the first "client" message ("Here is a string"), to be sent to the real server, and its response will trigger the next message and so on.
To test the client, we need to change the recorded mock file around a bit currently. The main things are that the directions need to change, so that CaptureMock knows which things to expect from outside and which it should produce.
<-CLI:Here is a string
->SRV:Length was 16
<-CLI:Here is a longer string
->SRV:Length was 23
We can then run with
$ capturemock --replay clientserver.mock sh -c 'client.py $CAPTUREMOCK_SERVER'
In future it will hopefully be possible to replay either server or client from the same recorded mock file.


Last updated: 05 October 2012