Thoughts about microservices
Everybody is talking about micro-services as a preferred approach to structure complex informational systems. Here are some popular reasons:
- Our monolithic app became too big and complex, we gonna split it into services
- We’ve reached performance limit for our monolith, by splitting into services we hope to boost it
- Our system will be huge, so we are starting from micro-services architecture to scale in the future
- Successful companies are reporting micro-services, we just want to use industry best practices.
From my point of view this motivations do not have direct connection to this architectural pattern and may bring you into the real troubles or waste. Decision to use micro-services architectural style should be conscious and with understanding all consequences and trade-offs.
Let’s list some serious penalties you will pay for microservices:
Consistency and dependency management
When you split your system into independently evolving services, this parts still have logical dependencies between each other and the system as a whole should be consistent. In monolithic app this deps are hard-coded explicitly in your code, but with splitting it into pieces, dependencies became “implicit” and you need additional efforts (versions, contracts, tests etc) to make sure they are satisfied. Your system becomes more fragile and unpredictable. This in some ways similar to choice between ACID and BASE options. In case of ACID in every point of time you have only one confident version of the world, in BASE world — many parallel galaxies could coexist and you have to manually handle consistency. In worst case you could end up with well known problem — dependency hell. So without solid “dependency management” skills and tools you are on a dangerous road.
The most challenging concern in micro-services approach (in programming at all :)— how to decompose the whole into pieces? As many matured architects know — initial decomposition almost always incorrect. Doing this “right” means you are able to predict the future. Instead of refer you to theoretical hints, let’s discuss consequences of “wrong” decomposition.
Cross-cutting feature: it’s easy to get new feature request, which require to make changes in many services and then consistently ship it. Sometimes this feature would take minutes in monolith and hours/days in microservices environment. Same about cross-cutting bugs.
Re-composition — if you’ve found better way to structure your system, then you have to re-compose your architecture. In monolith — it’s just well known “refactoring”, which could be done relatively safely. With microservices such transformation in many cases is not a trivial task.
It’s relatively known payment — every new component brings operational overhead in a non-linear manner. Distributed systems require qualitatively more complicated infrastructure, tools and experts to make it work reliably.
Growing your company and system right is not easy. You have to manage two interrelated entities — organization and system. Most important argument for microservices, for me, is align both in agreement with Conway’s law. (Micro)Service make possible to feet service to cross-functional team boundaries — to make team independent as much as possible. The critical constraint, you should not miss, the team should be able to design, develop and deliver “real value” to end user with minimal overlapping and conflicts with other teams. “Scale your system and organization” is worth goal to pay microservices price. The simple formula sounds like:
- one team → many services →wrong (overheads)
- many teams → one service →wrong (conflicts)
- one team→one service → right
Under this conditions the size of the service should not be “too micro”, but defined only by the size and skills of your teams. As we know, team could scale in very limited interval — 2–7 people, but skills can vary in couple of orders.
Another valid argument for me is “technological switch”, situation when your current technology could not provider required technical characteristics, for example performance, and you have to “write in c” some critical portion of your system.
This challenges and it’s derivatives will eat most of your time at the start, significantly slow you down in critical section. That’s why “micro-services first” approach for me sounds very suspicious. Most of successful “microservices” applications were done as re-work of monoliths in production into pieces. In that cases engineers had deep understanding system internals and problems.
In most of cases, I vote for “Monolitish Fist”.