alt Hacker NewsThen every microservice network in existence is a distributed monolith so long as they communicate with one another.
If you communicate with one another you are serializing and deserializing a shared type. That shared type will break at the communication channels if you do not simultaneously deploy the two services. The irony is to prevent this you have to deploy simultaneously and treat it as a distributed monolith.
This is the fundamental problem of micro services. Under a monorepo it is somewhat more mitigated because now you can have type checking and integration tests across multiple repos.
Make no mistake the world isn’t just library dependencies. There are communication dependencies that flow through communication channels. A microservice architecture by definition has all its services depend on each other through this communication channels. The logical outcome of this is virtually identical to a distributed monolith. In fact shared libraries don’t do much damage at all if the versions are off. It is only shared types in the communication channels that break.
There is no way around this unless you have a mechanism for simultaneous merging code and deploying code across different repos which breaks the definition of what it is to be a microservice. Microservices always and I mean always share dependencies with everything they communicate with. All the problems that come from shared libraries are intrinsic to microservices EVEN when you remove shared libraries.
People debate me on this but it’s an invariant.
Replies
> If you communicate with one another you are serializing and deserializing a shared type.
Yes, this is absolutely correct. The objects you send over the wire are part of an API which forms a contract the server implementing the API is expected to provide. If the API changes in a way which is backwards compatible, this will break things.
> That shared type will break at the communication channels if you do not simultaneously deploy the two services.
This is only true if you change the shared type in a way which is not backwards compatible. One of the major tenets of services is that you must not introduce backwards incompatible changes. If you want to make a fundamental change, the process isn't "change APIv1 to APIv2", it's "deploy APIv2 alongside APIv1, mark APIv1 as deprecated, migrate clients to APIv2, remove APIv1 when there's no usage."
This may seem arduous, but the reality is that most monoliths already deal with this limitation! Don't believe me? Think about a typical n-tier architecture with a backend that talks to a database; how do you do a naive, simple rename of a database column in e.g. MySQL in a zero-downtime manner? You can't. You need to have some strategy for dealing with the backwards incompatibility which exists when your code and your database do not match. The strategy might be a simple add new column->migrate code->remove old column, including some thought on how to deal with data added in the interim. It might be to use views. It might be some insane strategy of duplicating the full stack, using change data capture to catch changes and flipping a switch.[0] It doesn't really matter, the point is that even within a monolith, you have two separate services, a database and a backend server, and you cannot deploy them truly simultaneously, so you need to have some strategy for dealing with that; or more generally, you need to be conscious of breaking API changes, in exactly the same way you would with independent services.
> The logical outcome of this is virtually identical to a distributed monolith.
Having seen the logical outcome of this at AWS, Hootsuite, Splunk, among others: no this isn't true at all really. e.g. The RDS team operated services independently of the EC2 team, despite calling out to EC2 in the backend; in no way was it a distributed monolith.
[0] I have seen this done. It was as crazy as it sounds.
> That shared type will break at the communication channels if you do not simultaneously deploy the two services.
No. Your shared type is too brittle to be used in microservices. Tools like the venerable protobuf has solved this problem decades ago. You have a foundational wire format that does not change. Then you have a schema layer that could change in backwards compatible ways. Every new addition is optional.
Here’s an analogy. Forget microservices. Suppose you have a monolithic app and a SQL database. The situation is just like when you change the schema of the SQL database: of course you have application code that correctly deals with both the previous schema and the new schema during the ALTER TABLE. And the foundational wire format that you use to talk to the SQL database does not change. It’s at a layer below the schema.
This is entirely a solved problem. If you think this is a fundamental problem of microservices, then you do not grok microservices. If you think having microservices means simultaneous deployments, you also do not grok microservices.
Bingo. Couldn't agree more. The other posters in this comment chain seem to view things from a dogmatic approach vs a pragmatic approach. It's important to do both, but individuals should call out when they are discussing something that is practiced vs preached.
I believe in the original amazon service architecture, that grew into AWS (see “Bezos API mandate” from 2002), backwards compatibility is expected for all service APIs. You treat internal services as if they were external.
That means consumers can keep using old API versions (and their types) with a very long deprecation window. This results in loose coupling. Most companies doing microservices do not operate like this, which leads to these lockstep issues.