Modern software is built over the network with systems hooked-up either privately within the internal enterprise eco-system or with a trusted partner or via a public channel
Traditional system integration (ESB) engineers working within project directives would have delivered interfaces for system to system or partner to system integration while working closely with the consuming teams
This would have created, despite our best effort, very “leaky abstraction” of the provider system’s service implementation in the form of internal ids, system namespaces, validation logic etc in our API to the consumer
Impact of a leaky abstraction
APIs are a means to decouple the implementation by providing a good abstraction (we can never have a perfect abstraction) and our past experience with system integration has show that thinking only in a single context (EAI) in a project sense leads to a leaky abstraction (and tight coupling agnostic of technology used). Thus not reasoning for all contexts early on and thinking of your APIs as Products for Digital, B2B etc can lead to an improper abstraction
Impact of a leaky abstraction are
– Tighter coupling between service provider and consumer:
– Security: Attacker can guess internal implementation and launch a data hack by manipulating requests which would look valid to the server
Where should we focus?
– Service naming: API URI leaks something about the system serving it and not about the domain product, its business context and resource
– Resource Identifier: Does your Create Object return an internal database row ID?
– Date Time: Internal server datetime representation vs a standard causing usual datetime issues and logic to be implemented by consumers to handle your server issues
– Language implementation details in data: Ever see a java.util.List come back in a payload?
– Server headers: HTTP response headers from internal server often do not get the full treatment. We either skip them or block them all. When sending back backend server headers, try to reason why it is necessary or obsfucate (hash) so that someone cannot guess a sequence etc
APIs are the abstractions over technical services. Good APIs mirror strategic thinking in an organisation and lead to better customer experience by enabling high-degree of connectivity via secure mechanisms
Too much focus is on writing protocols & semantics with the desire to design good APIs and too little on business objectives. Not enough questions are asked early on and the focus is always on system-system integration. I believe thinking about what a business does and aligning services to leads us to product centric thinking with reuseable services
History As an ardent student of software design and engineering principles, I have been keen on Domain Driven Design (DDD) and had the opportunity to apply these principles in the enterprise business context in building reusable and decoupled microservices. I believe the best way to share this experience is through a metaphor and I use a “Shopping mall” metaphor with “Shops” to represent a large enterprise with multiple lines of businesses and teams
Like all metaphors – mine breaks beyond a point but it helps reason about domains, bounded contexts, APIs, events and microservices. This post does not provide a dogmatic point-of-view or a “how to guide”; rather it aims to help you identify key considerations when designing solutions for an enterprise and is applicable upfront or during projects
I have been designing APIs and microservices in Health and Insurance domains across multiple lines of business, across varying contexts over the past 5-8 years. Through this period, I have seen architects (especially those without Integration domain knowledge) struggle to deliver strategic, product centric, business friendly APIs. The solutions handed to us always dealt with an “enterprise integration” context with little to no consideration for future “digital contexts” leading to brittle, coupled services and frustration from business teams around cost of doing integration ( reckon this is why IT transformation is hard )
This realisation led me to asking questions around some of our solution architecture practices and support them through better understanding and application of domain modeling and DDD (especially strategic DDD ). Thought this practice, I was able to design and deliver platforms for our client which were reusable and yet not coupled
In one implementation, my team delivered around 400 APIs and after 2 years the client has been able to make continuous changes & add new features withoutcompromising the overall integrity of the connected systems or their data
Though my journey with DDD in the Enterprise, I discovered some fundamental rules about applying these software design principles in a broader enterprise context but first we had to step in to our customer’s shoes and ask some fundamental questions about their business and they way they function
The objective is to key aspects of the API ecosystem you are designing for, below are some of the questions you need to answer through your domain queries
What are your top-level resources leading to a product centric design?
When do you decide what they are? Way up front or in a project scrum?
What are the interactions between these domain services?
How is the quality and integrity of your data impacted through our design choices?
How do you measure all of this “Integration entropy” – the complexity introduced by our integration choices between systems?
The Shopping Mall example
Imagine being asked to implement the IT system for a large shopping complex or shopping mall. This complex has a lot of shops which want to use the system for showing product information, selling them, shipping them etc
There are functions that are common to all the shops but with nuanced difference in the information they capture – for example, the Coffee Shop does “Customer Management” function with their staff, while the big clothes retail store needs to sell its own rewards point and store the customer’s clothing preferences and the electronics retail does its customer management function through its own points system
You have to design the core domains for the mall’s IT system to provide services they can use (and reuse) for their shops and do so while being able to change aspects of a shop/business without impacting other businesses
Asking Domain and Context questions
What are your top-level “domains” so that your can build APIs to link the Point-of-Sale (POS), CRM, Shipping and other systems?
Where do you draw the line? Is a service shared by all businesses or to businesses of a certain type or not shared at all?
Bounded contexts? What contexts do you see as they businesses do their business?
APIs or Events? How do you share information across the networked systems to achieve optimal flow of information while providing the best customer experience? Do you in the networked systems pick consistency or availability?
Though my journey with DDD in the Enterprise, I discovered some fundamental rules about applying these software design principles in a broader enterprise context. I found it useful to apply the Shopping Mall metaphor to a Business Enterprise when designing system integrations
It is important to understand the core business lines, capabilities (current and target state), business products, business teams, terminologies then do analysis on any polysemy across domains and within domain contexts leading to building domains, contexts and interactions
We then use this analysis to design our solution with APIs, events and microservices to maximise reuse and reduce crippling coupling
Microservices holding state while performing some longer-than-normal execution time type tasks. They have the following characteristics
They have an API to start a new instance and an API to read the current state of a given instance
They orchestrate a bunch of actions that may be part of a single end-to-end transaction. It is not necessary to have these steps as a single transaction
They have tasks which wrap callouts to external APIs, DBs, messaging systems etc.
Their Tasks can define error handling and rollback conditions
They store their current state and details about completed tasks
Stateless microservice requests are generally optimised for short-lived request-response type applications. There are scenarios where long-running one-way request handling is required along with the ability to provide the client with the status of the request and the ability to perform distributed transaction handling and rollback (because XA sucked!)
So you need stateful because
there are a group of tasks that need to be done together as a step that is asynchronous with no guaranteed response-time or asynchronous one-way with a response notification due later
or there are a group of tasks where each step individually may have a short response time but aggregated response-time is large
or there are a group of tasks which are part of a single distributed transaction if one fails you need to rollback all
Stateful microservice API
Microservices implementing this pattern generating provide two endpoints
An endpoint to initiate: for example, HTTP POST which responds with a status code of “Created” or “Accepted” (depending on what you do with the request) and responds back with a location
An endpoint to query request state: for example, HTTP GET using the process id from the initiate process response. The response is then the current state of the process with information about the past states
Sample use case: User Signup
The process of signing-up or registering a new user requires multiple steps and interaction looks like this [Command]
The client can then check the status of the registration periodically [Query]
While the pattern is simple, I have seen the implementation vary with some key anti-patterns. These anti-patterns make the end solution brittle over time leading to issues with stateful microservice implementation and management
Enterprise business process orchestration: Makes it complex, couples various contexts. Keep it simple!
Hand rolling your own orchestration solution: Unlike regular services, operating long-running services requires additional tools for end-to-end observability and handling errors
Implementing via a stateless service platform and bootstrapping a database: The database can become the bottleneck and prevent your stateful services from scaling. Use available services/products as they optimised their datastores to make them highly scalable and consistent
Leaking internal process id: Your end consumer should see some mapped id not the internal id of the stateful microservice. This abstraction is necessary for security (malicious user cannot guess different ids and query them) and dependency management
Picking a state machine product without “rollback”: Given that distributed transaction rollback and error-handling are two big things we are going need to implement this pattern, it is important to pick a product that lets you do this. A lightweight BPM engine is great for this otherwise you may need to hack around to achieve this in other tools
Using stateful process microservices for everything: Just don’t! Use the stateless pattern as they are optimal for the short-lived request/responses use cases. I have, for example, implemented request/response services with a BPEL engine (holds state) and lived to regret it
Orchestrate when Choreography is needed: If the steps do not make sense within a single context, do not require a common transaction boundary/rollback or the steps have no specific ordering with action rules in other microservices then use event-driven choreography
Stateful microservices are a thing! Welcome to my world. They let you orchestrate long-running or a bunch of short-running tasks and provide an abstraction over the process to allow clients to fire-and-forget and then come back to ask for status
Like everything, it is easy to fall into common traps when implementing this pattern and the best-practice is to look for a common boundary where orchestration makes sense