Microservices or Monoliths – Fences and Neighbors

Photo of fence separating fields from a road


At the end of my last post, “What Makes a Monolith Monolithic?”, I stated that I didn’t consider the term “monolithic” to be inherently derogatory. It is, rather, a descriptive term relating to the style of organizing an application’s architecture. Depending on the context the system operates within, a monolithic architectural style could lie anywhere on the continuum between perfectly suited and perfectly disastrous. Placing it on that continuum requires a sense of what qualities are most needed or desired and which can be traded off in their stead. Everything comes with a cost, and attempting to ignore that fact merely sets us up for unpleasant future surprises.

After an initial period of unbridled enthusiasm, opinion seemed to gel around the idea that highly distributed application architectures (aka microservice architectures) were not suitable to all contexts. There are prerequisites for jumping into the microservices pool in terms of problem architecture, infrastructure, and organization. Attempting to shoehorn a microservice architecture into an environment that cannot support it will be overly expensive at best and a failure of apocalyptic proportions at worst.

There are many aspects of application design that are commonly recognized as beneficial: modularity, loose-coupling, high cohesion, and separation of concerns. It is critical to realize that these aspects can be found in systems with microservice architectures, monolithic systems, and everything in between. Distributed architectures are not necessary for modularity, nor any of those other aspects. In fact, one could easily create an application with a microservice architecture whose qualities are opposite to these desirable ones.

There are, however, situations where the benefit of a microservice architecture outweighs the costs and complexity. The ability to independently deploy and scale the various parts of an application is a major benefit, in my opinion. A well designed microservice architecture can even allow for the components of an application to be replaced on the fly. These features are not unique to microservice architectures, but are arguably easier to achieve than in other application architectures.

Real design, balancing both costs and benefits, is required. Sticking a bit of network in between the components is insufficient to ensure success. Deliberate design, especially as the boundaries multiply, is critical for an effective system. Identifying and providing for those boundaries at the conceptual level (i.e. before they become physical) is key. Good fences can either make for good neighbors, or they can create a maze of barriers.

What Makes a Monolith Monolithic?

Photo of Stonehenge, 1877


It seems like everybody throws around the term “monolith”, but what do we mean by that?

Sam Newman started the ball rolling yesterday with this tweet:

My first response was a (semi) joke:

I say semi joke because, in truth, semantics (i.e. meaning) is critical. The English language has a horrible tendency to overload terms as it is, and in our line of work we tend to make it even worse. Lack of specificity obscures, rather than enlightens. The problem with the term “monolith” is that, while it’s a powerfully evocative term, it isn’t a simple one to define. My second response was closer to an actual definition:

The purpose of this post is to expand on that a bit.

The “mono” portion of the term is, in my opinion, the crucial part. I believe that quality of oneness is what defines a monolithic system. As I noted in the second tweet, it’s a matter of meta-coupling, whether that coupling exists in the form of deployment, data architecture, or execution style (Jeppe Cramon‘s post “Microservices: It’s not (only) the size that matters, it’s (also) how you use them – part 3” shows how temporal coupling can turn a distributed system into a runtime monolith). The following tweets between Anne Currie and Sam illustrate the amorphous nature of what is and isn’t a monolith:

Modules that can be deployed to run in a single process need not be considered monolithic, if they’re not tightly coupled. Likewise, running distributed isn’t a guarantee against being monolithic if the components are tightly coupled in any way. The emphasis on “in any way” is due to the fact that any of the types of coupling I mentioned above can be a deal killer. If all the “microservices” must be deployed simultaneously for the system to work, it’s a distributed monolith. If the communication is both synchronous and fault intolerant, it’s a distributed monolith. If there’s a single data store backing the entire system, it’s a distributed monolith. It’s not the modularity that defines it (you can have a modular monolith), but the inability to separate the parts without damaging the whole system.

I would also point out that I don’t consider “monolithic” to be derogatory, in and of itself. There is a trade-off involved in terms of coupling and complexity (and cost). While I generally prefer more flexibility, there is always the danger of over-engineering. If we’re hand-carving marble gargoyles to stick on a tool shed, chances are the customer won’t be pleased. The solution should bear at least a passing resemblance to the problem context it’s supposed to address.

Microservices, Monoliths, and Modularity



There are very valid reasons for considering a microservice architecture (MSA) when building/evolving an application. In my opinion, however, forcing modularity isn’t one of those very valid reasons.

Just the other day, I saw tweet from Simon Brown saying this same thing:

I still like his comment from two years back: “I’ll keep saying this … if people can’t build monoliths properly, microservices won’t help”. I believe that if you’re having problems building a monolith properly, trying to use a distributed architecture to force modularity may actually cause harm.

MSAs, like any distributed application architecture, involve increased complexity and costs; table stakes, if you will. Like an iceberg, there’s both a lot more to it than just what’s showing above the waterline and a fair amount of hazard for the unwary. If a development team cannot or will not comply with design guidelines (e.g. modularity requirements), injecting additional complexity is probably not the solution you need.

Distributing an application makes it harder to accidentally entangle different concerns, but it doesn’t make it impossible:

I’d argue that making it harder to accidentally break modularity addresses neither of the groups I mentioned earlier: those that cannot or will not comply. It’s ironic, but those who fail to understand the need for modularity can be very creative in their “solutions”, regardless of the obstacles. Likewise, those who refuse to comply.

In short, distribution as a means of “ensuring” modularity fails the fitness for purpose test.

The situation becomes worse when you factor in the additional complexity inherent in a distributed system. Likewise, there’s the cost of the table stakes (infrastructure, process, staffing, etc.) mentioned above. Of course, having abandoned the principle of cause and effect, one could attempt some “creative” workarounds to avoid having to pay the price (in other words, adding more and more complexity).

When you introduce significant additional complexity (with all its attendant risk) with little chance of the technique actually achieving its goal, you’ve caused harm.

These concerns are not solely limited to the application architecture. Distributing the data architecture has the same limitations in terms of ensuring modularity and introduces additional complexity. Adding boundaries adds the need for governance. A disciplined, monolithic team can maintain modularity in a monolithic data architecture. Multiple separate teams trying to share a monolithic data architecture will either experience a crippling level of governance overhead or a complete breakdown in modularity.

MSAs can be useful when you need independently scalable and replaceable components. When you have multiple teams working on one logical application, they can also be appropriate as well. Using the technique when the cost outweighs the potential payoff, however, is a losing bet.

“Microservices and API Complexity – Inside and Out” on Iasa Global

The signature benefit of a microservice architecture is that its highly granular nature allows for a great deal of flexibility in composing applications. Components are simplified by virtue of a high degree of focus. The ability to replace individual components is enhanced by the modularity inherent in the style.

A very significant drawback to microservice architecture is that its highly granular nature can lead to a great deal of complexity in composing applications. Highly focused components can force service consumers to become more involved in the internals of an interaction than they might otherwise wish. Unwanted options can become more of a source of confusion than useful modularity.

How do you resolve this paradox? See the full post on the Iasa Global Site

Technical Debt and Rolling Re-writes (Who Needs Architects?)

If you think building a system is challenging, try maintaining one.

Tom Cagley‘s recent post “Plan to Throw One Away Re-Read Saturday: The Mythical Man-Month, Part 11”, was a good reminder that while “technical debt” may be something currently on the radar for many, it’s far from a new phenomenon. The concept of instant legacy applications was in place when forty years ago when Frederick Brooks wrote his masterpiece, even if they weren’t called that. As Tom observed in the post:

Rarely is the first attempt useful to the end consumer, and the usefulness of that first attempt is less in the code than in the feedback it generates. Software development is no different. The initial conceptual design and anticipated technical architecture of a large project rarely stands up to the rigors of the discovery process, and those designs should be learned from and then thrown away.

The faulty assumptions and design flaws accumulate not only from sprint to sprint leading up to the initial release, but also from release to release. In spite of the fact that a product can be so seriously flawed, throwing it away and starting over is easier said than done. While sunk costs cannot be recovered, too sanguine an attitude towards them may not enhance your credibility with the customer. Having to pay for the same thing over and over can make them grumpy.

This sets up a dilemma, one that frequently leads to living with technical debt and attempting to incrementally patch it up. There are limits, however, to the number of band-aids that can be applied. This might make it tempting to propose a rewrite, but as Erik Dietrich stated in “The Myth of the Software Rewrite”:

Sure, they know things now that they didn’t know when they started on this code 3 years ago. But won’t the same thing be true in 3 years? Won’t the developers then be looking at the code and saying, “this is a mess — if only we knew in 2015 what we now know in 2018!” And, beyond that, what makes you think that giving the same group of people the same marching orders won’t result in the same kind of code?

The “big rewrite from scratch because this is a mess” is a losing strategy.

Fortunately, there is an alternative. Quoting Tom Cagley again from the same post as above:

If change is both inevitable and good (within limits), then both systems and organizations (a type of system) need to be engineered to support and facilitate change. Architecturally, techniques such as modularization, object-oriented design and other processes that foster simplification and incremental change create an environment in which change isn’t avoided, but rather encouraged.

While we may laugh at the image of changing a tire while the vehicle is in motion, it is an accurate metaphor. Customers expect flexibility and change on the go; waiting equals lost business. The keys to evolving in place are having an intentionally designed, modular architecture and an understanding of where the weaknesses lie. Both of these are concerns that reside squarely on the architect’s plate.

Modularity not only makes an application more easily maintainable via separation of concerns, but it also embraces change by making components replaceable. This is one of the qualities that has made microservices such a hot topic, although it would be a mistake to think that microservices are the only way (or best way in all cases) to achieve modularity.

Modularity brings benefits beyond the purely technical as well. Rewrites of a fraction of an application are more easily sold than big-bang efforts. Demonstrating forethought (while you can’t predict what the change will be, predicting the need for change is more of a sure thing) demonstrates concern for the customer’s welfare, which should make for a better relationship.

Being able to throw a system away a little at a time allows us to keep the car on the road while it changes and adapts to changing conditions.

Who Needs Architects? – Monoliths as Systems of Stuff


In my experience, IT is not a “one size fits all” operation. In both their latest two-speed vision and their older three-speed one, Gartner’s opinion is the same – there is no one process that works for every system across the enterprise (for what it’s worth, I agree with Simon Wardley that Bimodal IT is still too restrictive and three modes comes closer to reflecting the types of systems in use). Process and governance that is appropriate to one system may be too strict for another and too loose for a third. In this light, attempting to find one compromise ensures that all are poorly served. Consequently, more than one mode of governance just makes sense.

The problem is more complex, however, than just picking trimodal or bimodal and dividing applications up according to whether they are systems of record, systems of differentiation, or systems of innovation (or digital versus traditional). Just as “accidental architecture” can result in a “Big Ball of Mud” at the application level, it can also do so in terms of enterprise IT architecture. Monoliths that have grown organically may cross boundaries of the multimodal framework taxonomy, essentially becoming incoherent systems of “stuff”. This complicates their assignment to a process that fits their nature. When the application fits more than one category, do you force it into the more restrictive category or the least restrictive? No matter which way you choose, the answer will be problematic.

Given the fractal nature of IT, it should not be a surprise that design decisions made at the level of individual applications can bubble up to affect the IT architecture of the enterprise as a whole. Separation of concerns (logical) and modularity (physical) remain important from the lowest level to the top. Without a strategic direction, tactical excellence can lead to waste from lack of focus.

Monolithic architectures trade simplicity for modularity at the application architecture level, which may be a valid trade at that level. If, however, a monolith crosses framework category boundaries, then major architectural refactoring may be required to avoid making ugly compromises. Separation of concerns within a monolith can ease the pain of this kind of refactoring, but avoidance of the need for refactoring is even more painless. Paying attention to cohesion across all levels of granularity and designing with extra-application as well as intra-application concerns in mind is necessary to achieve this avoidance.

Knowing the issues and being able to say why you made the choices you did is key.

Microservices – Sharpening the Focus

Motion Blurred London Bus

While it was not the genesis of the architectural style known as microservices, the March 2014 post by James Lewis and Martin Fowler certainly put it on the software development community’s radar. Although the level of interest generated has been considerable, the article was far from an unqualified endorsement:

Despite these positive experiences, however, we aren’t arguing that we are certain that microservices are the future direction for software architectures. While our experiences so far are positive compared to monolithic applications, we’re conscious of the fact that not enough time has passed for us to make a full judgement.

One reasonable argument we’ve heard is that you shouldn’t start with a microservices architecture. Instead begin with a monolith, keep it modular, and split it into microservices once the monolith becomes a problem. (Although this advice isn’t ideal, since a good in-process interface is usually not a good service interface.)

So we write this with cautious optimism. So far, we’ve seen enough about the microservice style to feel that it can be a worthwhile road to tread. We can’t say for sure where we’ll end up, but one of the challenges of software development is that you can only make decisions based on the imperfect information that you currently have to hand.

In the course of roughly fourteen months, Fowler’s opinion has gelled around the “reasonable argument”:

So my primary guideline would be don’t even consider microservices unless you have a system that’s too complex to manage as a monolith. The majority of software systems should be built as a single monolithic application. Do pay attention to good modularity within that monolith, but don’t try to separate it into separate services.

This mirrors what Sam Newman stated in “Microservices For Greenfield?”:

I remain convinced that it is much easier to partition an existing, “brownfield” system than to do so up front with a new, greenfield system. You have more to work with. You have code you can examine, you can speak to people who use and maintain the system. You also know what ‘good’ looks like – you have a working system to change, making it easier for you to know when you may have got something wrong or been too aggressive in your decision making process.

You also have a system that is actually running. You understand how it operates, how it behaves in production. Decomposition into microservices can cause some nasty performance issues for example, but with a brownfield system you have a chance to establish a healthy baseline before making potentially performance-impacting changes.

I’m certainly not saying ‘never do microservices for greenfield’, but I am saying that the factors above lead me to conclude that you should be cautious. Only split around those boundaries that are very clear at the beginning, and keep the rest on the more monolithic side. This will also give you time to assess how how mature you are from an operational point of view – if you struggle to manage two services, managing 10 is going to be difficult.

In short, the application architectural style known as microservice architecture (MSA), is unlikely to be an appropriate choice for the early stages of an application. Rather it is a style that is most likely migrated to from a more monolithic beginning. Some subset of applications may benefit from that form of distributed componentization at some point, but distribution, at any degree of granularity, should be based on need. Separation of concerns and modularity does not imply a need for distribution. In fact, poorly planned distribution may actually increase complexity and coupling while destroying encapsulation. Dependencies must be managed whether local or remote.

This is probably a good point to note that there is a great deal of room between a purely monolithic approach and a full-blown MSA. Rather than a binary choice, there is a wide range of options between the two. The fractal nature of the environment we inhabit means that responsibilities can be described as singular and separate without their being required to share the same granularity. Monoliths can be carved up and the resulting component parts still be considered monolithic compared to an extremely fine-grained sub-application microservice and that’s okay. The granularity of the partitioning (and the associated complexity) can be tailored to the desired outcome (such as making components reusable across multiple applications or more easily replaceable).

The moral of the story, at least in my opinion, is that intentional design concentrating on separation of concerns, loose coupling, and high cohesion is beneficial from the very start. Vertical (functional) slices, perhaps combined with layers (what I call “dicing”), can be used to achieve these ends. Regardless of whether the components are to be distributed at first, designing them with that in mind from the start will ease any transition that comes in the future without ill effects for the present. Neglecting these issues, risks hampering, if not outright preventing, breaking them out at a later date without resorting to a re-write.

These same concerns apply higher levels of abstraction as well. Rather than blindly growing a monolith that is all things to all people, adding new features should be treated as an opportunity to evaluate whether that functionality coheres with the existing application or is better suited to being a service from an external provider. Just as the application architecture should aim for modularity, so too should the solution architecture.

A modular design is a flexible design. While we cannot know up front the extent of change an application will undergo over its lifetime, we can be sure that there will be change. Designing with flexibility in mind means that change, when it comes, is less likely to be an existential crisis. As Hayim Makabee noted in his write-up of Rotem Hermon’s talk, “Change Driven Design”: “Change should entail extending the system rather than refactoring.”

A full-blown MSA architecture is one possible outcome for an application. It is, however, not the most likely outcome for most applications. What is important is to avoid unnecessary constraints and retain sufficient flexibility to deal with the needs that arise.

[London Bus Image by E01 via Wikimedia Commons.]