Discussions monolith vs microservice are hotter than ever. Usually, a monolith is synonym for “big ball of mud” in these discussions. It of course needn’t be so. A modular monolith is perfectly possible. Also, microservices isn’t an entirely new idea either. As some says, it’s SOA done right.
The usual argument in favor of microservices is that autonomy is a good thing: teams can pick the best appropriate tools, develop in parallel without friction, and scale services independently of each other. The main drawback is an increased complexity of the overall system, primary on the operations side but also on the tools side.
The usual argument in favor of a modular monolith is that it’s simple: the code base can be modularised to enable parallel development, the tech stack is standardized for everyone which reduces complexity. The main drawback is that the release cycle is the same for everyone which implies some coordination and possibly reduces the release cadence. The risk of inadvertent coupling is also higher since modularisation boundaries are internal and not external as with microservices.
The distinction microservices vs monolith is a continuum though. You can for instance have microservices with a standardized tech stack or a distributed monolith with the ability to scale some parts independently.
It’s up to you to decide which levels of autonomy you want.
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The mindset that lead to large monoliths is a mindset rooted in economy of scale. Development, testing, database and operations work is organised in silos. The idea is that the effort is reduced if the product is large and infrequently released. You do things once, at large scale, with specialists.
With microservices, the effort for a microservice is small enough that one cross functional team can undertake development, testing, database and operations work all by itself. There is less economy of scale but also less coordination needed.
“Because you can doesn’t mean you should.” Deviations from established practices or technologies can have attractive payoffs, but also come with some risk. Teams with lots of autonomy should be aware of the long term consequences of their choice and balance them against short-term benefits.
Services need complete teams when they are actively developed. With the time, some modules will stabilize and their maintenance concentrated to fewer teams. Inversely, services might grow and require splitting in multiple teams. In either case, teams ownership might change over time. If the technologies are very heterogeneous this might be more challenging.
Ultimately how much autonomy you want to give to the team is an organizational choice, not a technical choice. If you trust your organisation to be able to work with autonomous teams yet converge toward shared goals, microservices might work for you. If the organization maturity isn’t there, don’t go for microservices: you’ve translated your technical issues into people issues, which are even harder to solve.