a good example of this that I've used in production at my current $dayjob: dynamic PDF generation. user makes request from our website, request data is used to fill out a pdf template context which is then sent over to our PDFgen microservice which does its thing and streams a response back to the user.
All of that and much more needs to be replicated for each microservice, right?
Why not just have a module in your monolithic app that does it. The logic will still be separate. In most languages/frameworks you can spawn pdf generation task. Any changes are easier to introduce as well. There's no artificially materialised interface. Updates are naturally introduced. All auth logic is there already, you don't need to worry about deploying yet another service, same with logging etc.
the template has values that are related to database models. the main app (still mostly monolithic) fills out the template context. the context itself is what's passed to the microservice. the microservice does not connect to a database at all.
> Does it keep connection pool of let's say 5 connection always open (as libraries like to do)?
no. the service probably handles a few hundred requests per day, it is not in constant use. communication is over HTTPS. it opens a new connection on each request. this does impact throughput, but its a low throughput use case, and pdf rendering itself is much slower and that time totally dominates the overhead of opening and closing connections anyway.
> Does it have authentication?
yes, it auths with a bearer token that is borne only by our own internal server. this is backend technology so we don't have to auth an arbitrary user. we know in advance which users are authorized.
> Is it public or private API?
private
> Who is managing security?
we are, with a lot of assistance from the built-in security model of AWS.
> Is it running behind it's own nginx or other proxy?
the main app is behind nginx. the microservice is running in a docker container that exposes itself over a dedicated port. there's no proxy for the microservice, again, because of the low throughput/low load on the service. no need to have a load balancer for this so the most obvious benefit of a proxy wasn't applicable.
> Does it have DoS protection (PDF generation can be CPU intense)?
yes, it's an internal service and our entire infrastructure is deployed behind a gatekeeper server and firewall. the service is inaccessible by outside requests. the internal requests are queue'd up and processed 1 at a time.
> What about the schema for request?
request payload validation handled on both ends. the user input is validated by the main app to form a valid template context. the pdf generator validates the template context before attempting to generate one also. its possible to have a valid schema that has data that can't be handled correctly though. errors are just returned as a 500 response though. happens infrequently.
> They need to be deployed together with changes in other services, right?
nope. the microservice is fully stand alone.
> What about changes to database schema - you need to remember to update that service as well and redeploy it at the right time as well - just after successful db migrations - which live in another project.
the microservice doesn't interact with a database at all. schema changes in the main app database could potentially influence the pdf template context generation, but there are unit tests for that, so if it does happen we'll get visibility in a test failure and update the template context generation code as needed. none of this impacts the microservice itself though. it is fully stand alone. that's the point.
> All of that and much more needs to be replicated for each microservice, right?
in principle yes, and these are good guidelines for determining what is or is not suitable to be a microservice. if it would need to auth an arbitrary user, or have direct database access, or be exposed to public requests, it might not be a good candidate for a microservice. things that can stand alone and have limited functional dependencies are much better candidates.
> Why not just have a module in your monolithic app that does it.
because the monolithic app is Python/django and the PDF generation tool is Java. one of the main advantages of microservices architecture is much greater flexibility in technology selection. A previous solution used Python subprocesses to call out to PDF generation software. It's actually easier and cleaner for us to use a microservice instead.
