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Good write-up! I will add: - make sure to “stream” your transfers in general, instead of keeping whole file blobs in RAM. This to avoid refactoring at the lea

Good write-up! I will add: - make sure to “stream” your transfers in general, instead of keeping whole file blobs in RAM. This to avoid refactoring at the least comfortable moment when a file gets too big in production - it is quite easy to use Mox for isolated testing in general, and it works nicely with ExAws, so make sure to integrate those tests early on

sph 3y ago
Seconding the Mox recommendation, it's pretty good and enforces good separation of concerns in your application. It was created by José himself, so you know it's got a well-designed API. You just need to get used with using behaviours, and using the Application config to tell your code when to use the mock vs the real thing. I use it to separate and test external API calls (i.e. to test the payment layer without making any request), to maintain separation between my code and an external library I might want to swap out at a later date, or even between various components of the application. Then writing unit tests becomes a breeze, and you don't need many integration tests. https://hexdocs.pm/mox/Mox.html https://hexdocs.pm/mox/Mox.html
- sarchertech 3y ago
  I hate Mox with a passion. Because I hate separating out the interface and implementation of something that is only ever going to have one real implementation. The extra layer of indirection just for testing bugs me so much. And I just can’t justify it with “one day I might want to swap out the implementation”. YAGNI and if you do, deal with it then. So much legacy elixir code is an unreadable mess because of all the extra layers of interfaces that only get used for Mox. There’s nothing I dislike more than diving into an unfamiliar section of code and having to keep 15 files open to follow the logic for something that should have been in one module.
  - sitkack 3y ago
    Kinda horribly half serious, but can you use Elixir's Metaprogramming features to demoxify mox? https://www.youtube.com/watch?v=2Bjzml_Hpvk https://www.youtube.com/watch?v=2Bjzml_Hpvk
    - deleted 3y ago
      [deleted]
  - sph 3y ago
    That just means you're putting a boundary when it's not necessary. Mox exists to make tests simpler to write and reason about, if you keep hitting extra layers of indirections then, "you're holding it wrong." Most things are best used sparingly. That said, the vast majority of developers, even senior, have a very limited understanding of testing practices, and tend to create millions of separated, isolated modules, for no reason whatsoever. That's not Mox's fault. A stupid example in practice: a naive blog app does not need Mox at all. If you add SSO sign up, you might want to put a boundary there to unit test without needing to set up a fake auth server, but 99% of your code won't ever touch it, just the authentication tests. Tomorrow you add payments, you might want to put a boundary just before your StripeRestAPI module to test the entire payment flow without contacting Stripe in your unit tests. Again, only the payment module tests ever need to touch Mox, the rest of your code doesn't. You place it before modules in the outer edge that have real-world side effects, not to isolate your functional and mostly pure core. I recommend the DestroyAllSoftware talks, especially https://www.destroyallsoftware.com/talks/boundaries https://www.destroyallsoftware.com/talks/boundaries > And I just can’t justify it with “one day I might want to swap out the implementation”. YAGNI and if you do, deal with it then. I agree with that. Using Mox to swap out implementations is a possible use case, but in practice it's better to hardcode things rather than trying to make it generic and swappable the first time around. Junior devs have a tendency to over-engineer code "because we might need to change it later." My motto is "copy and paste it three times before adding a layer of abstraction"
    - sarchertech 3y ago
      I hope you see the irony in using “you’re holding it wrong” here? If real world use of a library tends to push towards a certain outcome that I dislike, I’m going to tend to dislike that library. You’re also never going to convince me that creating an interface for a single implementation is the best way to facilitate unit testing. Mox is nothing more than a kludge to get around the limitations of the language and I think there are much less invasive kludges available. To be honest when I’m working with devs who are new to elixir I’m embarrassed when I have to explain how Mox works. >but in practice it's better to hardcode things rather than trying to make it generic and swappable the first time around. Junior devs have a tendency to over-engineer code "because we might need to change it later." My motto is "copy and paste it three times before adding a layer of abstraction" This I agree with 100%
  - thibaut_barrere 3y ago
    I hear your pain: if boundaries are not correctly established, it can be problematic, as with any "boundary/interface" system. Finding the right spot is indeed important here!
  - throwawaymaths 3y ago
    > I hate Mox with a passion. Because I hate separating out the interface and implementation of something that is only ever going to have one real implementation Are you injecting mox or substituting it as a dependency at compile time using an attribute?
  - depr 3y ago
    I agree though I wouldn't say I hate Mox, rather I hate working with it (the things it does do it does well). As you say it forces you to do stupid stuff, like creating an interface for something with one implementation, but also the way you have to then add that to your configuration. I've also seen people only add "@callback" to modules, without any behaviour, because then it works with Mox.
  - oxidant 3y ago
    You can write a using macro to automatically create a behaviour from specs in a module with https://hexdocs.pm/elixir/Module.html#spec_to_callback/2 https://hexdocs.pm/elixir/Module.html#spec_to_callback/2 It's been a minute but I've done it. The behaviour/implementation setup reminds me of Java, especially when they're exactly the same. I do recommend only using mox as far down as possible (only around libraries). Hammox is Mox on steroids, check it out too.
  - josevalim 3y ago
    I am Mox author and, honestly, I rarely use it. The most important part is to think strongly about what you are mocking and your interfaces (explicit or implicit). Once I do that, I do my best to use plain anonymous functions, modules, or protocols to test it. If you already have this discipline, then Mox only adds “paperwork”. If you have layers of interfaces, it is worth asking why you need mocking across several layers. YMMV, of course. As long as you are mocking with care, use whatever floats your boat. :)

sph 3y ago

Seconding the Mox recommendation, it's pretty good and enforces good separation of concerns in your application. It was created by José himself, so you know it's got a well-designed API. You just need to get used with using behaviours, and using the Application config to tell your code when to use the mock vs the real thing. I use it to separate and test external API calls (i.e. to test the payment layer without making any request), to maintain separation between my code and an external library I might want to swap out at a later date, or even between various components of the application. Then writing unit tests becomes a breeze, and you don't need many integration tests. https://hexdocs.pm/mox/Mox.html https://hexdocs.pm/mox/Mox.html

sarchertech 3y ago
I hate Mox with a passion. Because I hate separating out the interface and implementation of something that is only ever going to have one real implementation. The extra layer of indirection just for testing bugs me so much. And I just can’t justify it with “one day I might want to swap out the implementation”. YAGNI and if you do, deal with it then. So much legacy elixir code is an unreadable mess because of all the extra layers of interfaces that only get used for Mox. There’s nothing I dislike more than diving into an unfamiliar section of code and having to keep 15 files open to follow the logic for something that should have been in one module.
- sitkack 3y ago
  Kinda horribly half serious, but can you use Elixir's Metaprogramming features to demoxify mox? https://www.youtube.com/watch?v=2Bjzml_Hpvk https://www.youtube.com/watch?v=2Bjzml_Hpvk
  - deleted 3y ago
    [deleted]
- sph 3y ago
  That just means you're putting a boundary when it's not necessary. Mox exists to make tests simpler to write and reason about, if you keep hitting extra layers of indirections then, "you're holding it wrong." Most things are best used sparingly. That said, the vast majority of developers, even senior, have a very limited understanding of testing practices, and tend to create millions of separated, isolated modules, for no reason whatsoever. That's not Mox's fault. A stupid example in practice: a naive blog app does not need Mox at all. If you add SSO sign up, you might want to put a boundary there to unit test without needing to set up a fake auth server, but 99% of your code won't ever touch it, just the authentication tests. Tomorrow you add payments, you might want to put a boundary just before your StripeRestAPI module to test the entire payment flow without contacting Stripe in your unit tests. Again, only the payment module tests ever need to touch Mox, the rest of your code doesn't. You place it before modules in the outer edge that have real-world side effects, not to isolate your functional and mostly pure core. I recommend the DestroyAllSoftware talks, especially https://www.destroyallsoftware.com/talks/boundaries https://www.destroyallsoftware.com/talks/boundaries > And I just can’t justify it with “one day I might want to swap out the implementation”. YAGNI and if you do, deal with it then. I agree with that. Using Mox to swap out implementations is a possible use case, but in practice it's better to hardcode things rather than trying to make it generic and swappable the first time around. Junior devs have a tendency to over-engineer code "because we might need to change it later." My motto is "copy and paste it three times before adding a layer of abstraction"
  - sarchertech 3y ago
    I hope you see the irony in using “you’re holding it wrong” here? If real world use of a library tends to push towards a certain outcome that I dislike, I’m going to tend to dislike that library. You’re also never going to convince me that creating an interface for a single implementation is the best way to facilitate unit testing. Mox is nothing more than a kludge to get around the limitations of the language and I think there are much less invasive kludges available. To be honest when I’m working with devs who are new to elixir I’m embarrassed when I have to explain how Mox works. >but in practice it's better to hardcode things rather than trying to make it generic and swappable the first time around. Junior devs have a tendency to over-engineer code "because we might need to change it later." My motto is "copy and paste it three times before adding a layer of abstraction" This I agree with 100%
- thibaut_barrere 3y ago
  I hear your pain: if boundaries are not correctly established, it can be problematic, as with any "boundary/interface" system. Finding the right spot is indeed important here!
- throwawaymaths 3y ago
  > I hate Mox with a passion. Because I hate separating out the interface and implementation of something that is only ever going to have one real implementation Are you injecting mox or substituting it as a dependency at compile time using an attribute?
- depr 3y ago
  I agree though I wouldn't say I hate Mox, rather I hate working with it (the things it does do it does well). As you say it forces you to do stupid stuff, like creating an interface for something with one implementation, but also the way you have to then add that to your configuration. I've also seen people only add "@callback" to modules, without any behaviour, because then it works with Mox.
- oxidant 3y ago
  You can write a using macro to automatically create a behaviour from specs in a module with https://hexdocs.pm/elixir/Module.html#spec_to_callback/2 https://hexdocs.pm/elixir/Module.html#spec_to_callback/2 It's been a minute but I've done it. The behaviour/implementation setup reminds me of Java, especially when they're exactly the same. I do recommend only using mox as far down as possible (only around libraries). Hammox is Mox on steroids, check it out too.
- josevalim 3y ago
  I am Mox author and, honestly, I rarely use it. The most important part is to think strongly about what you are mocking and your interfaces (explicit or implicit). Once I do that, I do my best to use plain anonymous functions, modules, or protocols to test it. If you already have this discipline, then Mox only adds “paperwork”. If you have layers of interfaces, it is worth asking why you need mocking across several layers. YMMV, of course. As long as you are mocking with care, use whatever floats your boat. :)