Monolith architecture has been very successful in shaping the software world the way we see it today. However, the last few years have seen a sharp decline in its adoption, especially with the advent of Microservices. The popularity of microservices was caused by the need of scalability and changeability which in turn is caused by the penetration of IT in almost every entity, animate or inanimate. Modern applications see no boundary when it comes to scale and these applications are fond of change and this is where Monolith doesn’t fit at all.
Microservices, at least in theory, is “The Silver Bullet” that will solve all the problems and will serve humankind till eternity, but it doesn’t happen. Microservices bring lots of challenges which were nonexistent earlier. Still, what it does, it does it beautifully and efficiently and most importantly serves the purpose.
The popular idea is to have very fine grained services where each service is responsible for a single task. Practitioners give a contempt look to coarse-grained services as it is deemed against the philosophy of Microservices and this is where Monolith is left for slow death. Is monolith really that bad and if so then how it was one of the most successful architecture for years?
Fine grained microservices have their own challenges e.g. transactions or latency. To make matters worse, the management overhead is overwhelming and agreeing to the fine-ness is no easy job. Fine grained microservices are preferred because there is no single point of failure, possibility to scale independently, ability to change and deploy often and list goes on. However, if you look at these goodies carefully, all of this can be achieved only by complex design patterns and development discipline. On the flip side, there are challenges like unavoidable network latency which in turn results in degraded performance (unless it uses additional complex systems like caching), huge management overhead, complex transactions and many more.
A monolith by definition is a system that consists of every part of a system, but for ages organizations have been building monoliths that at least have different processes for UI and backend and these parts integrate via interfaces. This segregated model is referred to here.
Monolith systems have the edge when it comes to simplicity. If development process can somehow can avoid turning it into a big ball of mud and if a monolith system (as defined above) can be broken into sub-systems such that each of these sub-systems is a complete unit in itself, and if these subsystems can be developed in a microservices style, we can get best of both worlds. This sub-system is nothing but a “Coarse Grained Service”, a self-contained unit of system.
A coarse-grained service can be a single point of failure. By definition, it consists of significant sub-parts of a system and so its failure is highly undesirable. If a part of this coarse grained service fails (which otherwise would have been a fine-grained service itself), it should take the necessary steps to mask the failure, recover from it and report it. However, the trouble begins when this coarse-grained service fails as a whole. Still, it is not the deal breaker and if the right mechanism is in place for high availability (containerized, multi-zone, multi-region, stateless), there will be very bleak chances for it. On the flip side, it takes away the complexity of failure management for sub-parts like needing to employ circuit breakers. There is a trade-off but it is worth evaluating.
Scaling a coarse grained service is not very different from fine grained one. If its boundaries are defined carefully and if it is developed as a stateless, scaling these services is trivial. They can run inside containers and can even be deployed in serverless manner on cloud (e.g. AWS Fargates).
One of the challenges a coarse grained service faces is to react to change. In the past, during the Monolith era, the journey from code to production was manual and tedious. However, with modern methodologies it can be automated easily. A coarse grained service is not very fine, still it is not supposed to be of the scale of a Monolith and so reacting to this change is not that challenging as it seems. There can still be some trade offs but it is worth considering them.
A coarse grained service is often a complete unit in itself and so it can take advantage of running in a single process, which means network calls can be replaced with method calls which not only improves performance but also simplifies management of components.
Quite evidently, there is a need for an amalgam between a Microservice and Monolith. In fact, microservices is not really about building very small services but it is an architecture style to build software as a service app that can be built independently and that interact via interfaces. All of this can be weaved into a monolith or a coarse grained service which allows to reap benefits of both the architecture style.
And so, Monolith is not dead but it just incarnated into a different form to serve for time to come.