Polyglot programming is a bad goal 

People have extended the goal of building micro service systems such that we aren’t locked into a language to a new goal of every repository being able to use any language.

It’s phrased as ‘shouldn’t we use the best language for the job.‘ But the implication is that we should be able to pick a language for this new project independent of the rest of our existing software stack.

The problem is that it’s not a free choice. Even if you’re in an ideal microservices environment where everything is API based, logging and metrics are handled by the platform and shared libraries are banned there are costs to having more languages in play.

The most basic cost is expertise splintering. If you only use one language and everyone knows it. Then everyone can read and extend all the code. But as you start throwing more languages and paradigms in that slowly becomes impossible.

You quickly reach a point where transferring people to a new team is expensive. Transferring from one Java project to another in the same stack is easy. Switching from Java to async JavaScript is trickier. Switching from Java to Haskell takes awhile.

Polyglot makes away team work more difficult. And it does it slowly so you won’t really notice until it’s too late. 

Shared libraries are another area polyglot sucks. You can’t always make a library an API. 

Whether it’s because you can’t externalize metrics and logging or you have high performance business logic you need to run locally. Often times you will need to duplicate shared libraries across languages.

Polyglot is nice in a lot of ways. I’d rather get to use Pandas for data science if I had the opportunity. We don’t really need to use only Java for everything. But when it comes to application code, I’d rather everyone used the same language, framework and RPC schemas. It doesn’t make any sense to write certain microservices in Java, others in Haskell, some in Python and a few in Typescript. If they are RPC or event based services you are just using five languages to do the same job instead of one language to do one job. 

Then go ahead and write sidecars in Go or Rust, frontends in Typescript and data science stuff in Python. Those are all different use cases with different trade offs.