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Faking it with TextTest
How to avoid running your whole system for real

Introduction
TextTest is about testing at the system level, and it is expected that you are trying to run your system in as similar a way as possible to how it is used in practice. It is, however, more important to have maintainable, repeatable tests that are free of global side effects. Sometimes it is necessary to disable some part of the system, or fake some behaviour that cannot easily be triggered for real in a repeatable way. This is referred to as “mocking out” a subsystem for testing purposes.
(All the following mechanisms involve the test data mechanism : it's probably a good idea to read that document first if you aren't familiar with how it works)
The problem can here be broken down into 4 sub-categories, of which TextTest offers direct help with 3:
  1. System calls Somewhere your program makes a system call to a “third-party” command-line program. (You might have written it too, but it is external to what you're trying to test). For whatever reason the behaviour of this program is hard to control in a repeatable way. One a example would be a build script which updated the current source from your version control system. Clearly you don't want to do this for real every time you run the test. There are two methods available here: one of which is based on writing your own version of the program to do something predictable and another is based on TextTest observing and reproducing what the program actually does once, and then re-creating those conditions for you.
  2. Built-in modules. Somewhere your program uses a standard utility whose behaviour is hard to control or simulatae. For example, say your program should print a warning if the GUI library has too old a version. You don't want to have to install an old version of in order to test this. If you've written your program in Python or Java help is at hand: you can simply write your own version of the module (Python) or class (Java)
  3. Databases. These are good at global side effects and changes in them are hard to reverse. The recommended approach here is to find and use a file-based database which can quickly be started from a file and shut down when the test is done. In this way you can avoid having to try and shut things down by hand. Recommended are Firebird and MySQL for example, while Oracle and PostgreSQL are less good at this.Once you have the file defining the data based you can simply define it as editable test data and TextTest doesn't need to know that a database is present.
  4. Distributed systems with plain-text messaging. If you have a distributed system with many components, it can be impractical to start and stop the whole thing every time you run the tests. It can be useful to have a way to define component tests from system tests: i.e. to create tests running the whole system that can nevertheless be run with the other parts of the system mocked out.
Executable and importable test data
When TextTest creates the temporary write directory on running the test, and populates it with test data, it also makes sure to insert that directory at the start of the PATH, PYTHONPATH and CLASSPATH variables. That makes it possible to provide executable programs that will be run instead of their real versions via the normal test data mechanism (“link_test_path”). Likewise, you can provide a Python module or a Java class, which will then be imported instead of the real version at the appropriate moment. Then you just need to place such executable/importable test data in the appropriate place in the permanent test structure, just as you would with more passive test data.
Intercepting and replaying command line programs
The above mechanism for executable programs is powerful but can be overkill. It's also not very easy to steer in the case where a program is called many times and needs to behave differently on each occasion. It can be better and easier to simply ask TextTest to take over and “record” what the program does. This is known as the command-line traffic interception mechanism.
To enable this, add the name of the program concerned to the config file list entry “collect_traffic”. For each test for which you want to mock out this program, go to the Advanced tab under “Running” and check the “(Re-) record command-line traffic” box. TextTest will then create its own fake version of the program and place it in the temporary write directory as if it were test data, as above.
When called, this program will send the command line it was given back to TextTest via a socket. TextTest will then execute it, and record to a new definition file called “traffic.<app>” what the command line given was, what was returned on standard output and standard error and what the exit status was.
When the test is next run without the above box checked, this traffic file will be used instead of the real program. As before, the command line is captured and sent to TextTest via a socket. This time, however, it will look up the given command in the traffic file, and for the closest matching command line recorded, will return the standard output, standard error and exit status via the socket, which in turn will be relayed back to your system as if the real program had run.
It's then easy to fake certain conditions by simply editing this traffic file by hand, if desired
Providing environment variables for intercepted programs
Sometimes environment variables need to be provided for such programs. As they are run directly from TextTest they don't automatically inherit any environment your program may have set up. In this case you should set the “collect_traffic_environment” dictionary config setting, with the keys being the program names as provided for “collect_traffic” and the values being the names of the environment variables. In this case these environment variables with their values will also be sent to TextTest and will be part of the information recorded.
Intercepting and replaying plain-text network messages
The “traffic interception” mechanism can also be used for this purpose. Here it is less a matter of configuring TextTest and more of configuring your own application. As above, TextTest considers itself to be recording such traffic when the “(Re-) record traffic” switch is enabled, and replaying such traffic when a “traffic.<app>” file already exists. In these circumstances it sets up its own “traffic server” and sets the environment variable TEXTTEST_MIM_SERVER (MIM stands for “Man in the Middle”) to <host:port> for this place.
On testing a client-server system you probably need to write a wrapper script that can for example start server, start client connecing to server, close client, shutdown server. To use this traffic-recording mechanism you should modify this script such that the client will instead connect to the host and port given by TEXTTEST_MIM_SERVER, instead of that given by its own server. You should also modify it so that on discovering the host and port where the real server is running, this information should be sent to TextTest's MIM server in the format <some_message> <host:port>. TextTest will then parse this and know where the server is.
When your client sends a request it will go to TextTest instead. TextTest will record it as a client request in the traffic 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.
You can then replay this either as client or server. When running the server with a traffic file already in place, TextTest will read the file in order. It will itself send the client traffic in the order it is written down, and each time a server reply is present it will suspend and wait for such a reply. Replaying the client is much the same, except that TextTest suspends initially and waits for contact from the client before doing anything.
This may be easier to understand by example. There is a nice little fake client-server system tested in this way as part of the self-tests, under TestSelf/TrafficInterception/ClientServer/TargetApp.


Last updated: 05 October 2012