[erlang-questions] Looking for practical testing advice

Brujo Benavides elbrujohalcon@REDACTED
Wed Mar 28 16:10:35 CEST 2018

Not so long ago I would’ve gone with #3 immediately, but lately my co-worker Marcelo Gornstein <https://github.com/marcelog> is slowly persuading me of using something like #2.
I think he’s about to write a blog post exactly about that.

Brujo Benavides <http://about.me/elbrujohalcon>

> On 28 Mar 2018, at 10:18, Sölvi Páll Ásgeirsson <solvip@REDACTED> wrote:
> Hi all
> I'm wondering about various different approaches to writing testable
> Erlang and I'd be super interested in hearing about how you approach
> this.
> As a concrete example, let's assume that we're building software that
> talks to financial exchanges.
> When submitting orders, you might need to map symbols to/from order
> book ids, so you run a named gen_server that:
> - Knows how to map an order book id to symbol (i.e., 1234 -> ERIC)
> - Knows how to map symbol to id (ERIC -> 1234)
> - Knows how to refresh it's map periodically from an external service,
> such as a database or whatever
> Then you want to test your protocol handler, which depends on this id
> mapping server, but you don't want to make an expensive call to an
> external service for every test you run, so you want to provide fake
> data.
> I see a few different possible approaches to orchestrate testing of
> this, but I'm having a hard time figuring out which of these I should
> follow in general:
> 1. You can start the protocol handler with a parameter, IdMapMod ::
> module(), and implement a stub_idmap module that knows how to map
> to/from certain ids to use in your tests.
> The downside of this is that all calls from the protocol handler to
> the id map must extract a dynamic module name from the state, and then
> you lose dialyzer, xref, ...
> Another downside(or upside?) is that you won't actually perform any
> actual calls to your id mapping server.
> 2. You can start the id mapping server with a parameter,
> ExternalServiceMod :: module(), and fake out the external service call
> only.
> Then your tests of the protocol handler would actually exercise the id
> mapping server; but it's calls to an external service would be
> answered with a fake stub.
> This way you get the benefit of dialyzer + xref as your protocol
> handler simply calls id_map:symbol_to_id/1 or whatever.
> 3. You can use a mocking library such as meck to do something very
> similar to either of the two above things.
> In past lives using other languages, I've generally developed a
> distaste for mocking, as I've felt that it can create brittle tests.
> Generally, I think I'd prefer to be able to test things in terms of
> interfaces, but I might be swayed otherwise.
> Is there anything else I'm missing?  How do you write testable Erlang?
> Is there a general approach you follow?
> Thanks & regards
> Sölvi Páll Á
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