Microservice Principles, Technical Debt, and Legacy Systems

I see you have a poorly structured monolith. Would you like me to convert it into a poorly structured set of microservices?

— Architect Clippy (@architectclippy) February 24, 2015

Is there a circumstance where the answer to Architect Clippy‘s question is “yes”? In “Microservice Architectures aren’t for Everyone” I used this tweet to underscore the observation that a team that can’t produce a well-modularized monolith is unlikely to be helped by trying to distribute the problem. On the other hand, a team (or teams) tasked with rehabilitating a “Big Ball of Mud” might well find some value in the principles behind microservice architectures.

Some of the relevant principles are cohesion and replaceability. As Dan North noted in “Microservices: software that fits in your head”:

One way to manage the mess is to maximise the likelihood that everyone knows what’s going on in the codebase. This requires two things: consistency and replaceability. Consistency implies you can make reasonable assumptions about unfamiliar parts of the application. Replaceability means you can kill code easily and replace it with something better.

Without achieving separation of concerns, any architectural refactoring effort will be an exercise in chasing fires across the codebase. A divide and conquer strategy that applies the single responsibility principle at a macro level will be more likely to facilitate identification and remediation of lower-level technical debt. Monoliths can benefit from being carved up, not because small is inherently better, but because they reach a point where independence of their components becomes beneficial, even crucial. Components that share fewer dependencies (such as a shared data store) and have independent release cycles offer a great deal of flexibility in structuring an application and the team(s) that develop it.

In “Microservices allow for localized tech debt”, Jim Plush stated: “It’s much easier mentally to tackle $10,000 of debt across 4 credit cards at $2500 each than 1 card at the full $10,000.” Even more to the point, it’s much easier to tackle that debt when you split it with three other people (teams) each working independently.

Re-writes have a well-deserved bad reputation. Shared platforms and shared data stores will often mean that the transition from the legacy system to the re-written one will be a high-risk “big bang” affair. As Edmond Lau observed in “How to Avoid One of the Costliest Mistakes in Software Engineering”, you want to “…get as quickly as possible to a state where you’re again making incremental improvements”. Getting to this state may well happen quicker when the parts are separated.