[crdrost]

I agree that we need to at least name nanoservices -- microservices that are "too small". Like surely we can all agree that if your current microservices were 100x smaller, so that each handled exactly one property of whatever they're meant to track, it'd be a nightmare. So there must be a lower limit, "we want to go this small and no smaller."

I think we also need to name something about coupling. "Coupling" is a really fluid term as used in the microservice world. "Our microservices are not strongly coupled." "Really, so could I take this one, roll it back by 6 months in response to an emergency, and that one would still work?" "err... no, that one is a frontend, it consumes an API provided by the former, if you roll back the API by 6 months the frontend will break." Well, I am sorry, my definition of "strong coupling" is "can I make a change over here without something over there breaking", for example rolling back something by 6 months. (Maybe we found out that this service's codebase had unauthorized entries from some developer 5 months ago and we want to step through every single damn thing that developer wrote, one by one, to make sure it's not leaking everyone's data. IDK. Make up your own scenario.)

[dinvlad]

I'm just surprised no one mentioned https://martinfowler.com/bliki/MonolithFirst.html yet :-)

Nano actually did (and does) make sense from access control perspective - if a service has permissions to do one thing and one thing only, it is much harder to escalate from. But I'm not sure if these benefits outweigh the potential complexity.

[MobiusHorizons]

Your definition of coupling seems a bit too strong to be useful. By that definition, just about nobody has an uncoupled API, because if anyone uses it (even outside your company) then you can’t really just revert six months of changes without creating a lot of problems for your dependents. If those changes just happen to be not user facing (eg an internal migration or optimizations) then you might be ok, but that is a characteristic of the changes, not of the coupling of a service and it’s dependents.

IMO it’s more valuable to have strong contracts that allow for changes and backwards compatible usage, so that services that take a dependency can incrementally adopt new features.

[crdrost]

That definition of strong coupling is in fact standard. Like if you ask people why they don't want strong coupling they tell you exactly that when you change a strongly coupled thing you induce bugs far away from the thing you changed, and that sucks.

Now you might want this strong coupling between front-end and back-end and that's OK—just version them together! (Always version strongly coupled things together. You should not be guessing about what versions are compatible with what other versions based on some sort of timestamp, instead just have a hash and invest a half-week of DevOps work to detecting whether you need to deploy it or not. Indeed, the idea of versioning a front-end separate from a back-end is somewhat of an abdication of domain-driven design, you are splitting one bounded context into two parts over what programming language they are written in—literally an implementation detail rather than a domain concern.)

Other patterns which give flexibility in this sense include:

- Subscription to events. An event is carefully defined as saying “This happened over here,” and receivers have to decide what that means to them. There's no reason the front-end can't send these to a back-end component, indeed that was the MVC definition of a controller.

- Netflix's “I’ll take anything you got” requests. The key here is saying, “I will display whatever I can display, but I'm not expecting I can display everything.”

- HATEOAS, which can function as a sort of dynamic capability discovery. “Tell me how to query you” and when the back-end downgrades the front-end automatically stops asking for the new functionality because it can see it no longer knows how.

- HTTP headers. I think people will probably think that I am being facetious here, what do HTTP headers have to do with anything. But actually the core of this protocol that we use, the true heart and soul of it, was always about content negotiation. Comsumers are always supposed to be stating upfront their capabilities, they are allowed a preflight OPTIONS request to interrogate the server’s capabilities before they reveal their own, servers always try to respond with something within those capabilities or else there are standard error codes to indicate that they can't. We literally live on top of a content negotiation protocol and most folks don't do content negotiation with it. But you can.

The key to most of these is encapsulation, the actual API request, whatever it is, it does not change its form over that 6 month period. In 12 months we will still be requesting all of these messages from these pub/sub topics, in 12 months’ time our HATEOAS entry point will still be such-and-so. Kind of any agreed starting point can work as a basis, the problem is purely that folks want to be able to revise the protocol with each backend release, which is fine but it forces coupling.

There's nothing wrong with strong coupling, if you are honest about it and version the things together so that you can test them together, and understand that if you are going to split the responsibilities between different teams than they will need to have regular meetings to communicate. That's fine, it's a valid choice. I don't see why people who are committing to microservices think that making these choices is okay, as long as you lie about what they are. That's not me saying that the choices are not okay, it's me saying that the self-deception is not okay.

[dinvlad]

I think strong versioning in event-driven arch is a must, to avoid strong coupling. Otherwise, it becomes even worse than "normal" call-driven service arch, because it's already plenty hard to find all of the receivers, and if they don't use strong versioning then it's so easy to break them all with one change in the sender.

[crdrost]

Yeah I would tend to agree! I think there is freedom to do something like semver where you have breaking.nonbreaking.bugfix, which might be less “strong”... But in a world with time travel you tend to get surprised by rollbacks which is why they are always my go-to example. “I only added a field, that's non-breaking” well maybe, but the time reversed thing is deleting a field, are you going to make sure that's safe too?

And I think there's a case to be made for cleaning up handlers for old versions after a certain time in prod of course.

[CasualSuperman]

I'm not sure how rolling back 6 months of updates could ever be possible unless you never ship new features.

[crdrost]

As fuel for thought, consider that when python was being upgraded from 2 to 3, shipping lots and lots and lots of new features, and breaking several old ones, there were many libraries which supported both versions.

Some of them may have functioned by only programming in the mutually agreed upon the subset, but given that that subset did not really include strings, that was clearly the exception rather than the norm. Instead people must have found a way to use the new features if they were available, but fall back to old ways of doing things if they weren't.

Some of those mechanisms we're internal to the language, “from __future__ import with_statement”. So how can my JSON-over-HTTP API, within that JSON, tell you what else is possible with the data you fetched?

[crdrost]

Ugh sorry for weird autocorrect errors, I had to abandon for a mealtime haha... I was going to add that there are also mechanisms outside the language, for instance that you might just detect if you can do something by catching the exception if it doesn't work... If it's syntax like JS arrow functions this might require eval() or so. The API version is just to expect the failure codes that would have been present before you added the new feature, usually 404, and handle them elegantly. If it's a follow-up request that RFC-“must” be done for consistency, maybe your app needs a big shared event bucket where those can always go, so that when “POST /foo-service/foos/:fooId/bars/:barId/fix" fails, you can fall back to posting to the deferred request buffer something like

    {
      eventName: "foo-service.fix-bar-requested", 
      version: 1,
      params: {fooId: 5, barId: 7}
    }

Just catch the exception, record the problem for later resolution, use a very generic template for storage that doesn't have to change every month...

Think creatively and you can come up with lots of ways to build robust software which has new features. Remember the time reversal of adding a feature is removing it, so you have to fix this anyway if you want to be able to clean up your codebase and remove old API endpoints.