All posts in Domain Driven Design

What is Domain-Driven Design?

Over the last decade or two, the philosophy of Domain-Driven Design, or DDD for short, has developed as an undercurrent in the object community. The premise of DDD is two-fold:

  • For most software projects, the primary focus should be on the domain and domain logic
  • Complex domain designs should be based on a model.
Domain-driven design is not a technology or a methodology. It is a way of thinking and a set of priorities, aimed at accelerating software projects that have to deal with complicated domains.

To accomplish that goal, teams need an extensive set of design practices, techniques and principles. Refining and applying these techniques will be the subject of discussion for this site, generally starting from the language of patterns laid out in Domain-Driven Design, by Eric Evans.

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SOA goals and realities

SOA has been long been the holy grail of Increased Business Agility and Cost Reduction which should be achieved through Increased Usability, Improved Maintainability and Redundancy Reduction.

Curious about how SOA and Microservices relate to each other? Read more on our blog.

Common SOA Realities

You have spent a considerable amount of money on technical infrastructure (e.g. Enterprise Service Bus)  trying to become more aligned to your business, but you have most likely arrived at the following realities:

  • Systems are more fragile
  • Development and Maintenance costs are higher
  • Your services are not being reused
  • You thought SOA would solve your integration problems, but they’ve gotten worse
  • No one want to build or maintain services
  • System performance is worse

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Part 1 – Microservices: It’s not (only) the size that matters, it’s (also) how you use them
Part 2 – Microservices: It’s not (only) the size that matters, it’s (also) how you use them
Part 3 – Microservices: It’s not (only) the size that matters, it’s (also) how you use them
Part 4 – Microservices: It’s not (only) the size that matters, it’s (also) how you use them
Part 5 – Microservices: It’s not (only) the size that matters, it’s (also) how you use them

As I explained in Microservices: It’s not (only) the size that matters, it’s (also) how you use them – part 5, to me a service is a logical boundary that is the technical responsible for a given business capability.
This means that the service owns all data, logic and UI for this capability everywhere it is used.

What does it mean that a service is a logical boundary?

As explained in Philippe Kruchten’s 4+1 view of software architecture, we should not automatically force the logical view to be the same as the physical implementation or for that matter the deployment view.
This often means that a service isn’t a single implementation or deployment artifact.

Examples of business capabilities are: Sales, Shipping, Marketing, Billing, Policy Management, etc.
These capabilities are pretty broad in their scope. You’ve probably read that a microservice should follow the Single Responsibility Principle (SRP) – it should do one thing and do it well.
But if a microservice should cover an entire business capability it would most like be fairly big, which goes against many of the qualities we like about microservices, such as:

  • Small (easy to comprehend)
  • Replaceable (discard the old and write a new in 2 weeks)
  • Upgradable (upgrade just the parts you want without interrupting other parts)
  • Fast startup/shutdown
  • Individually deployable

A large service is still individual deployable, but from a scaling point it’s typically all or nothing: either you scale the entire deployable unit or you don’t.
What if it is only certain use-cases that needed scaling? This is often harder with too big a deployable unit (what some people refer to a monolith) due to individual components inside the unit being too tightly coupled, like a tangled ball of yarn.

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Part 1 – Microservices: It’s not (only) the size that matters, it’s (also) how you use them
Part 2 – Microservices: It’s not (only) the size that matters, it’s (also) how you use them
Part 3 – Microservices: It’s not (only) the size that matters, it’s (also) how you use them
Part 4 – Microservices: It’s not (only) the size that matters, it’s (also) how you use them
Part 6 – Service vs Components vs Microservices

First of all, sorry to those who’ve been waiting for part 5. My schedule has been too busy to find focused time to write part 5 before now 😳 .

In part 4 we looked at building our services around functional areas or business capabilities/bounded-contexts. We discussed that the business data and logic pertaining to a business capability must be encapsulated inside a service to ensure single source of truth for the data. This also means that other services aren’t allowed to own the same data that another service owns (we want to avoid multi master services). Since we want our service to be autonomous (i.e. be able to make a decision without having to communicate synchronously with other services), we also looked how to avoid 2 way communication (RPC, REST or Request/Response) between services. The options we looked at were Composite UI’s and Data duplication over Events. We also briefly discussed a 3rd option which involves a different view on services, where they’re not autonomous. Instead services expose intentional interfaces and coordinate updates/reads between several System of Records (SoR) that them selves are autonomous. I believe that organizations with many large legacy systems (and most likely multi master systems) should look into the possibilities of the 3rd option as I believe it may create less friction than trying to develop new autonomous services that are well aligned with business capabilities.

In part 5 I will continue discussing SOA and Microservices in the light of  autonomous services.

Business Capabilities and Services

In part 4 I suggested building our services around functional areas or business capabilities/bounded-contexts.
I would like to tighten up that statement and rephrase to: We should align our services with business capabilities.
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Last week we enjoyed the company of several other SOA and microservice interested developers and architects at the µService Conference in London –

We did a talk on Thursday called  “Microservices – SOA reminded of what it was supposed to deliver?” – the video and slides are available on line.
If you’re new to SOA, Microservices and/or DDD – we highly recommend watching Udi Dahans keynote before watching the video of our talk.

Enjoy 🙂