Runtime verification in Erlang by using contracts

About this paper

Runtime verification in Erlang by using contracts, L.-A. Fredlund et al, presented at WFLP 2018.


Spoiler alert, but the conclusion to my book OOP the Easy Way is that we should have independently-running objects, like we do in Erlang. We should also document the contract between an object and its collaborators, like we do in Eiffel. The concurrency is there because processors are getting wider, not faster, and applications are becoming more distributed across IOT, “edge compute”, and servers. The contracts are there because we want to tame that complexity, tell people how each of the bits of our system can be composed, and what to expect if they try that. And we want to verify that documentation automatically, keeping ourselves honest.

As the authors here note, Erlang already has good facilities for recovering from errors, but that’s only really suitable for errors that are “things that might go wrong”. Programmer mistakes – call them bugs, logic errors, whatever – are things that shouldn’t go wrong, and Erlang doesn’t help there. Eiffel has really good tools for dealing with things that shouldn’t go wrong.

This paper introduces Erlang Design by Contract, which adds the relevant Eiffel bits to Erlang. At its core is a contract specification that’s richer than that found in Eiffel, allowing developers to assert that particular functions are pure, that they complete within given time, or that a recursive algorithm is trending towards completion.

The bit that achieves our aim, of combining the parallelism of Erlang with the safety of Eiffel, is the cpre/3 function, for implementing concurrent preconditions. The server inspects a message, the sender (from), and its internal state, and decides either to handle the message or queue it for later, changing the server state as appropriate.

In this way, preconditions act not as assertions that trigger failure, but as wait conditions that say when an object is ready to handle a message. The Concurrency Made Easy project found this too, when they built SCOOP, which works the other way and adds concurrency to Eiffel.

EDBC and SCOOP are two very different approaches that start from different places and aim for the middle. It’s really interesting to see that there is common ground: that “precondition as wait” arises whether you add concurrency to contracts, or contracts to concurrency.

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