How to structure microservices?

When structuring microservices we have to consider;

  • how fine grain should our components be?

  • how distributed should they be?

I believe it’s important to treat these as separate concerns, even to the extent of ensuring your business logic components are not dependant on the choice of transport, or even whether you have a transport at all.

How do we right size microservices?

The business component in your system are;

  • the smallest deployable unit of code and data.

  • functionality which could makes sense to a business user based on your requirements.

The right size for your components can depend on why you chose microservices.

  • is it to scale the development effort, allowing each developer to work as independently as possible. (Developer efficiency)

  • is it to scale the utilisation of the hardware, allowing each CPU or server to work as independently as possible. (Computer efficiency)

Often we want some of each where the number of components under development is likely to be a function of the number of developers and the number of deployed instances of those components is likely to be a function of the number of CPU or servers.

Unless you know performance is a key concern, I would make sure that you prioritise developer efficiency. I would expect for most projects to have around one to three business components per developer.

How do we deploy our components?

A microservice is a business component with the option of a transport. I believe having an optional transport is an important consideration because the business component shouldn’t be influenced too heavily by the choice of transport and not having a transport can be helpful at different stages of the project.

When you are writing unit tests and debugging your business component, you shouldn’t need to have a transport involved. If you want to see how two components interact you should be able to create a simple program which creates two or more compoenents, run from your IDE and see how they work together in the same thread.

Levels of distribution

There is different levels of distribution you want to consider. Initially you might want no transport at all, provided you ensure these component can be distributed. Later you need either fine grain or course grained distribution, but over time as the component matures you don’t need the overhead of managing lots of services and you want the ability to consolidate them.

  1. Instances are in same logical task and are wired together with no layer between them.

  2. Instances are in separate logical tasks, but share the same thread pool [1].

  3. Instances are in separate thread pool, but in the same process.

  4. Instances are in separate OSes, but in the same physical machine.

  5. Instances are in the same data centre, but different machines.

  6. Instances are in different data centres.

For performance, I favour #1, #2 & #3, for testing and development I favour #1 and #3 and for High Availability you need either #5 and #6.

If you don’t need to distriibute instances across machines for HA, you want your instances to be no more seperate than needed. The degree of separation desirable can change over time. When a component is being added to production initially, it can be useful to be able to deploy, upgrade and restart that component without impacting the rest of the system. However as the component stablises and is not changing so much you may want to reduce the overhead of managing this service and include it in a process which has multiple services. You might find the service uses a trivial amount of CPU and you can combine it into the same thread pool as other services. If it turns out it’s interaction with another instance is highly coupled you might wire them together as a single component.

Migrating to Microservices

Say you have a monolith, how do you migrate to using microservices? Do you have to re-engineer your entire solution? NO. Migrating is easier than it sounds as the main advantage for microservices is in the early stages of development when you are putting a service into production. Anything which is running well in your monolith, leave it there. Anything you need to add or make major changes to, move it into it’s own service while it’s under development and maturing. You can imagine this as one monolith, planet sized microservice and a number of smaller satelite micro-services surrounding it.

Flexable Design

You want your developers to work efficiently and your system to make good utilisation of your servers. You don’t want the choices you made at the start to bind you into a solution you will regret later. If you use a flexable design, you can maximise productivity, and ensure you use microservices to the extend they are helping you, but without the disadvantages of over using this technique.

Further reading

To read more see my other posts about Microservices

1. a thread pool might have one thread
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