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Comment by vouwfietsman | Hacker News Reader

I get that.

The point is that its hard to prevent asymmetry in message versions if you are working with many communicating systems. Lets say four services inter-communicate with some protocol, it is extremely annoying to impose a deployment order where the producer of a message type is the last to upgrade the message schema, as this causes unnecessary dependencies between the release trains of these services. At the same time, one cannot simply say: "I don't know this message version, I will disregard it" because in live systems this will mean the systems go out of sync, data is lost, stuff breaks, etc.

There's probably more issues I haven't mentioned, but long story short: in live, interconnected systems, it becomes important to have intelligent message versioning, i.e: a version number is not enough.

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> Lets say four services inter-communicate with some protocol, it is extremely annoying to impose a deployment order where the producer of a message type is the last to upgrade the message schema

i don't know how you arrived at this conclusion

the protocol is the unifying substrate, it is the source of truth, the services are subservient to the protocol, it's not the other way around

also it's not just like each service has a single version, each instance of each service can have separate versions as well!

what you're describing as "annoying" is really just "reality", you can't hand-wave away the problems that reality presents

I think I see what you’re getting at? My mental model is client and server, but you’re implying a more complex topology where no one service is uniquely a server or a client. You’d like to insert a new version at an arbitrary position in the graph without worrying about dependencies or the operational complexity of doing a phased deployment. The result is that you try to maintain a principled, constructive ambiguity around the message schema, hence asymmetrical fields? I guess I’m still unconvinced and I may have started the argument wrong, but I can see a reasonable person doing it that way.

Yes thats a big part, but even bigger is just the alignment of teams.

Imagine team A building feature XYZ Team B is building TUV

one of those features in each team deals with messages, the others are unrelated. At some point in time, both teams have to deploy.

If you have to sync them up just to get the protocol to work, thats an extra complexity in the already complex work of the teams.

If you can ignore this, great!

It becomes even more complex with rolling updates though: not all deployments of a service will have the new code immediately, because you want multiple to be online to scale on demand. This creates an immediate necessary ambiguity in the qeustion: "which version does this service accept?" because its not about the service anymore, but about the deployments.

Ah, I see. Team A would like to deploy a new version of a service. It used to accept messages with schema S, but the new version accepts only S’ and not S. So the only thing you can do is define S’ so that it is ambiguous with S. Team B uses Team A’s service but doesn’t want to have to coordinate deployments with Team A.

I think the key source of my confusion was Team A not being able to continue supporting schema S once the new version is released. That certainly makes the problem harder.

Exactly!

> one cannot simply say: "I don't know this message version, I will disregard it" because in live systems this will mean the systems go out of sync, data is lost, stuff breaks, etc.

You already need to deal with lost messages, rejected messages, so just treat this case the same. If you have versions surely you have code to deal with mismatches and e.g. fail back to the older version.

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