Parallel execution of scenarios

[mitchgrout]

I'm using cucumber to provide BDD testing within C/C++ libraries that I'm working on; this typically involves creating a REPL executable around my library, and using aruba to send various queries to perform my tests.

The main issue I have is that my test executable, and cucumber, are both single-threaded, and so I often have a significant portion of CPU time left unspent. Similarly, some test executables tend to be IO-bound, leading to large waits between steps.

I am aware that parallel_cucumber can assist here by spawning a process per feature, but this has a few downsides:

1. Most reporters, including the pretty and html formatters, are unusable since each process attempts to write content to the same resource (stdout, or the report file) simultaneously
2. Since parallelism is performed per feature, there is no benefit when running a single feature in isolation

Looking at the source of cucumber and cucumber-core, it seems feasible that cucumber itself could invoke scenarios in parallel, capturing the events and passing them to the underlying reporters in the correct fashion. C#'s SpecFlow also appears to have the same type of parallelism capabilities.

I'd be happy to work on implementing this for Ruby.

[mpkorstanje]

I reckon you would be best of creating a feature request at https://github.com/cucumber/cucumber-ruby.