Personal API Experience

Personal API Experience

Daily musings

Sharing my day to day lessons, ideas and contemplations of architecting, developing and maintaining services (API) in the global marketplace we call the internet.

Chain of Trust

APIPosted by Craig Hughes Sun, February 24, 2019 14:00:54

In the traditional two or three-tier world of web development, there is an inherent trust between the client and the server. Each is contextually aware of the other and withholds or shares information depending on the level of trust. The server may generate and persist session information about the client to enforce control or to simplify interaction. The client may do the same for a better user experience. In the world of APIs, this contextual awareness is not guaranteed since APIs can call or orchestrate other components where the context of the client may be different or unknown.

API architecture can be, and often is, layered. Since APIs offer abstraction via their interfaces, the consumer need not know, nor care about, what goes on behind the scenes of the interface. This also means that it should not matter to the consumer what the API does, as long as it sticks to its interface contract. This abstraction allows an API to do any orchestration, composition, translation or enrichment to the original request; be that calling other APIs, interfaces, databases, systems, etc. Basically, the consumer of the API may not know where their request is actually being fulfilled, or how many other components/interfaces are being consumed in the process.

On the counter side of this, components being orchestrated or composed are abstracted from the original request by the API consumer. They may not be contextually aware of the overall intentions of the original consumer, and ideally, need not be; they should only be concerned with responding to their specific request.

This potentially disparate awareness between the original API consumer and the components being manipulated by the API can be a problem for the traditional mindset. Some might struggle with this lack of awareness and see it as a potential risk in security, authorization, authentication or trust. This often becomes the excuse for not adhering to interface-based architecture principles and the insistence in sticking to a two or three-tiered architecture, even with APIs.

The trick here is to implement and understand a chain of trust. Each conversation in an end-to-end request should be based on an established trust between consumer and producer (or requester and responder). The client making the original request should have a trust relationship with the original API. The API, in turn, should have an established trust relationship with each of the components it orchestrates or consumes. And each of these components should have an established trust with their consumed components.

This trust and context can be established using a combination of patterns and approaches. These could include:

· Mutual TLS (or similar) to restrict boundaries to known systems,

· API Gateways to manage the consuming systems

· JWT to manage user authentication and authorization (using OAuth2, SAML, OIDC, etc.)

Plus, many others.

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API Misconceptions

APIPosted by Craig Hughes Fri, February 01, 2019 08:12:00

I’d like to clarify some things about the use of HTTP verbs for RESTful APIs and the apparent misconceptions these can bring about.

Firstly, there are a few people who believe that RESTful APIs are predominantly for CRUD (Create, Read, Update, Delete) operations. I covered how this can be extended to other operations in a previous article; Nouns and Verbs – in the world of APIs.

Secondly, there’s also a large contingency of people who believe the use of HTTP verbs, and their alignment to CRUD operations, implies that RESTful APIs are simply a representation of an underlying database table – or JDBC over HTTP as I like to call it.

In a green-field situation, it’s quite possible to create your database structure based on your API contract, but eventually, these will move apart. It’s far more expensive to change a database structure than an API contract and you cannot really version a database table change and still maintain a previous version (unless you create a new table and inherit all the complexity in your code that that creates). In any other situation, where your database is already defined, exposing your database tables as APIs is exposing your data complexity directly to your consumers – APIs should abstract complexity.

So, if the HTTP verbs of a RESTful imply CRUD operations, but these may not directly correlate with database tables; what does CRUD mean in this context? Quite simple, it means CRUD operations at the API interface level – you still need to implement the validation, transformation and mapping to wherever the data is being processed; be that your underlying database or another API.

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API Fragments - simple flexibility

APIPosted by Craig Hughes Sun, January 27, 2019 14:56:07

Designing APIs often leads to a compromise between producers and/or consumers. Consumers of APIs would like the interface to be as close to their requirements as possible; they want to avoid possible composition or contract navigation. API producers would like their interface to be as simple as possible; they want to support maximum reuse and consumption.

I’ve touched on this in a previous article, Should APIs be Pre-made or Deli style?, where I offered different options for consumer flexibility or producer simplicity.

Pre-made. Defining API interfaces based on specific consumer needs - simple for each consumer, but not necessarily reusable.

Deli-style. Defining an API interface that allows consumers to select the attributes they need via arguments in the request - maximum flexibility for the consumer, but at a cost for the producer.

Providing the entire model as one interface. Allowing the consumer to pick the elements they want from the response and discard the rest - simple for the producer, requires contract navigation for the consumer.

For this discussion, let’s look at how we can make the third option a little more flexible for the consumer while keeping it simple for the producer.

Assume this is the entire model of a car:

It’s a fairly simple model, but I’ll use it to explain the approach.

A producer could expose the entire model via GET: /cars/1234abcd. This would force the consumers to download the entire model and find the values they need. It’s the simplest for the producer, but will probably get challenged by consumers if the model is large.

An alternative is to expose each sub-object as a separate URI – giving consumers the basic information about the car and links to allow them to discover more about the model through hyperlinks (HATEOUS).

This reduces the payload of the response, and allows consumers to retrieve the additional data they might need independently. From a producer’s perspective; each object can be managed in isolation, it’s still simple and supports maximum reuse. For the producer it offers flexibility and reduced payload.

However, even this simple approach leads to another consumer argument: “What if I need more than one object? I will have to make repeated calls to compose all the data I need!”. By example, assuming a consumer was interested in a car’s engine and body, this would result in the consumer having to make three API calls:

GET: /cars/1234abcd
GET: /cars/1234abcd/engine
GET: /cars/1234abcd/interior

Is there another alternative here?

What about using the HTTP URL Fragment? By definition (RFC 3986): “The fragment identifier component of a URI allows indirect identification of a secondary resource …”. This is donated in a URL as the “#” (hash) symbol and allows users to navigate to a specific portion of an HTML document.

We could use the HTTP URL fragment to identify secondary objects within a model, and allow the API consumer to define which objects in the model they are interested in. Since we already have these objects exposed via end-points, we can offer a short-circuit URI using the fragment, as follows:

GET: /cars/1234abcd#engine,interior

The resource in question is still the car, but the consumer is asking for additional sub-resources of the car known by “engine” and “interior”.

Obviously, only requesting one object via a fragment is redundant, but I believe this does offer a lot more flexibility for the consumer without introducing too much complexity for the producer.

Let me know your thoughts.

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API: Arrays or named elements?

APIPosted by Craig Hughes Tue, January 15, 2019 16:58:24

The humble restaurant menu follows a common pattern where menu items are generally separated into a logical hierarchy. At the top level, we may have breakfast items, dinner items or drinks items. Each of these could be grouped further into categories like meat dishes, fish dishes or pasta dishes (dinner items), and again into burgers and steaks (meat dishes). Each item is then described so that we (as humans) can make a choice on what to eat.

Now, consider making that same menu available as an API (for machine consumption). Think of the challenge that becomes immediately apparent; how do we describe objects that contain similar properties? How should we model burgers for example?

The natural human tendency might be to describe each burger as an object with defined properties. We may describe the “cheeseBurger”, the “baconBurger”, the “hawaiianBurger” – this follows the menu approach (the intended business purpose). Alternatively, we could define the “burgers” as an array and identify each burger as an element with that array based on its property values.

There are a number of pros and cons to consider for each approach.

The “Named Object” approach


· The model is easy to understand (from a human perspective – developers are sometimes human)

· Consumers can navigate directly to the object needed; e.g. json.burgers.baconBurger

· Each object (burger) can evolve in isolation

· Each object can be managed via individual API end-points; e.g.


· Renaming or removing objects will lead to contract-breaking changes – forcing a new API version

· Filtering is usually done on attribute values. Doing so on objects is not intuitive.

· The object name provides identity within the interface schema. This name may need to be translated into an appropriate UI label and also makes localization challenging. The obvious solution would be to include a “label” attribute – but wouldn’t that be redundant?

The “Array” approach


· We can add, rename or remove objects (burgers) without effecting the interface model (no potential version changes).

· We can still retrieve a defined object via an API end-point; e.g.


· Consumers may need to loop through the array to find the required object by value.

· Objects have to maintain the same structure and cannot evolve in isolation.

The choice of which approach to take depends on the domain you are modeling. However, as the domain expert, the decision is yours – just understand the consequences of your choice.

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DDD – what isle is your attribute in?

APIPosted by Craig Hughes Wed, January 09, 2019 18:41:59

During a Domain Driven Design (DDD) workshop this week, one of my colleagues very cleverly used grocery store aisles as an analogy to explain why attributes should be grouped into entities. He explained that grocery stores group logical produce together to make it easier for customers to find what they are looking for. Thereby suggesting that grouping attributes together in entities would help API consumers find relevant data. I really liked this analogy since it highlights some of the challenges of Domain Driven Design.

Have a look next time you go to your local grocery store. The teas, coffees and other hot drinks will be in one aisle. Condiments may be in another, so too will dry goods. Each isle can be seen as a group of items that logically go together. But, and there is always the exception, there are some items that don’t clearly fit into one category; like sugar for example. Should sugar be with the hot drinks, the dry goods or the baking goods? The stores around me can’t seem to agree, but at least they’ve each made a decision and mostly keep to it. As regular shoppers, we begin to know where to find our favorite items and understand the store's layout.

DDD can be a very time-consuming and on-going exercise. Not everyone is going to agree with the models you define, but you can’t go on ad infinitum. Eventually, at some point, you have to make a decision. That decision will be based on the best available knowledge your team has at that point in time. Yes, it may not be 100% right, but it is a decision none-the-less.

Many of these decisions will be around which entities attributes logically belong to. Once all the decisions have been made, you are going to publish your model as an API. Consumers may initially be confused by the placement of one or more attributes and will either accept or challenge them.

Being challenged on a model is not a big deal. The beauty of APIs is that they can be versioned. This allows us a little flexibility in making a decision on our models. A change to the model (that breaks the current contract of your API) can be published as a new API version. That’s not to say we are free to change the model whenever the whim takes us, but, we don’t have to wait for the absolutely perfect model before releasing our APIs. We can start with a minimum viable product (MVP) and evolve it as our knowledge and experience in the domain grows.

The alternative is, of course, to not model your domain at all, and simply publish an API with a long list of root attributes. Imagine trying to shop in a store that randomly puts stuff on shelves without any thought…

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Fireworks and APIs - “meh”

APIPosted by Craig Hughes Fri, January 04, 2019 14:34:58

Every New Year’s Eve we are wowed by extravagant fireworks from around the world - we anticipate them and we expect them. This New Year’s Eve was no different, except for me – this year I was in Copenhagen city center; on the balcony of a friend’s apartment, watching.

In Denmark anyone, over a certain age, can purchase and set off fireworks during New Year’s Eve. It’s quite spectacular – bangs, fizzes, pops and smoke for a number of hours; explosions are constant and everywhere.

Some of the fireworks are spectacular and clearly cost a small fortune, others are unexceptional, but the result of all of them is the same. Firstly, there’s great expectation followed by excitement in lighting the fuse. Then there’s anticipation; waiting for them to go off, tailed by wonder as the explosion and accompanying light show is released. In the end, there’s the “meh” moment – either because it was the same as the previous one, not as good as the next one, or a little lack-luster when compared to others.

Finally, it’s all done. The show is over, the money spent and everyone goes home.

Since this is a new year, and we generally try to aspire to greater things, let’s try with our APIs. Let’s not simply build excitement and anticipation only to release an API that’s “meh” and soon forgotten. Let’s try to do them right this time.


Before you begin, make sure you understand your business. Make sure you understand how your API will be used in the context of your business. Make sure your API will be used to grow or enhance your business. And finally, make sure the purpose of your API is clearly understood.


Work with your colleagues, your business, your partners and your consumers. Understand their needs, work through their ideas, grasp their constraints – work with the requirements. Agree the contractual interface (payload, URLs, operations, etc.) and simulate them (using SwaggerHub or similar). Let everyone play and plan ahead with the simulations – there is no need for consumers to wait for the final product. Get feedback early.

Simple and reusable

Keep your APIs simple and try make them useable by as many parties as possible (avoid single consumer builds). Make them intuitive and keep them constrained to a single functional responsibility. If you have to document your APIs too much, they are probably too complicated. Mocking your APIs will help you understand if you’re on the right path.

Loose Coupling

Don’t place assumptions on consumers; don’t assume they understand the complexity of the system behind your APIs – it’s not their responsibility. Your API interface should abstract them from your complexity. After all, managing complexity is part of the service that an API offers.


Finally, manage you APIs through their lifecycle. Give them an explicit version (using Symantec versioning) and let your consumers know when new versions are available. Of course, if you are already collaborating with your consumers, they should be ready for your API changes.

So, fireworks offer all the excitement, anticipation and wonder; followed by the “meh” moment. Your API’s can offer the same, but try not to aspire to that – try to maintain the feel-good excitement and wonder by delivering something lasting and worthwhile to all.

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What event are you waiting for?

APIPosted by Craig Hughes Fri, December 14, 2018 15:58:22

In one of my previous articles, the Hollywood Principle, I introduced the concept of an event-driven architecture. This is a popular distributed asynchronous architecture pattern used to produce highly scalable applications. It is underpinned by asynchronous messaging and often implemented using the Publish and Subscribe (PubSub) pattern. The premise of this pattern is that an event triggers messages for distribution to multiple subscribers. However, in order to implement an effective event-driven architecture, it is important to understand what an event is in context.

Let’s consider the first type of Event. Application developers, and more specifically UI developers are all too familiar with concept of an event. A user clicking a button or moving a mouse is considered an event. Each event has an associated event handler to perform the specific action anticipated by the user. We’ll call these UI events and exclude them from this discussion since they are generally handled within the UI container.

Secondly, during some form of business process, an artefact may be produced (a file for example) which needs to be handed over to another system. The system creating the artefact needs to let the other system know that that artefact is ready for further processing. This can be considered an event. Likewise, a UI event handler may need to trigger a process in another system. These can also be considered events. We’ll call these process events.

Finally, we have the scenario where a business domain or entity is updated or created, via some process or another. The update of this entity can be considered an event since this update may be relevant or of importance to other systems. A good example of these could include a customer update, a payment or an account update. We’ll call these business events.

Process events nearly always involve only two parties; the producer of the event and the consumer of the event message. The producer of the event needs to be aware of the consumer of the event message. To support asynchronous behavior these events should be transported via standard messaging. Using PubSub is overkill in this context and should not be advocated as it clutters the PubSub topic range.

Business events are the ideal contenders for using PubSub since these are business activities that others may care about.

Let’s walk through an example – an online payment.

Using the UI channel, the user clicks the submit payment button. This “click” (UI event) might be handled by a function that validates all the required fields are populated. All these activities are encapsulated with the UI. Assuming successful validation, the payment request might be submitted to a payments engine asynchronously (a process event) since the payment may only be cleared later.

Within the payments engine, the payment request is processed. This process might involve using several external systems, including funds check, AML and fraud checks. Each of these systems might raise an event that other systems might care about (analytics, alerts, etc.). These systems might publish these events via PubSub using a topic relevant to the event: “overdrawn”, suspect persons” and “suspicious behavior”. Finally, the payment is cleared – this will trigger the final event – and publish the transaction via PubSub for other systems to react to (account balance update, balance SMS, analytics, etc.).

Reacting to events can be a very powerful architectural pattern – it promotes real-time responses and reduces “overnight” processing. But, it is important to know which messaging pattern to implement.

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Nouns and Verbs – in the world of APIs

APIPosted by Craig Hughes Fri, December 07, 2018 16:13:30

Please note that this is a little more technical than my previous articles – sometimes I need to let my inner-geek out.

Simply because RESTful APIs are based on resources and use the HTTP verbs (GET, POST, PUT, DELETE, PATCH), does not mean they should only support CRUD (Create, Read, Update, Delete) operations. RESTful APIs can also be used for performing other actions on resources.

I was challenge with this recently when explaining the concepts of a layered API architecture (see my previous article for more information) during a Domain Driven Design workshop. Core APIs, which expose core domains objects, support pure CRUD operations and make full use of the HTTP verbs. These are well within the perceived CRUD definitions of RESTful services. Process and experience APIs, which orchestrate or compose core APIs to create a defined process, tend to move away from this understanding; and are sometimes a cause for confusion. What HTTP verb should be used for an operation outside of the CRUD set and how should we graft the URL?

Let’s start with the basics. The naming convention for resources in RESTful APIs should be noun based; we work with “accounts”, “customers”, “products”, etc. The basic CRUD operations we perform on these resources are defined by the HTTP verbs. So, for example:

GET - gets a list of customers

POST - creates a new customer based on the representation in the body of the request

PUT - replaces the customer identified by “123” with the representation in the body of the request

PATCH - updates the customer identified by “123” with the representation in the body of the request

DELETE - deletes a customer identified by “123”.

The PUT, PATCH and DELETE HTTP verbs imply very specific Update and Delete actions in the realm of HTTP; they will not work for activity-based APIs. A GET request, only consists of the URL to fulfill the HTTP request; you cannot provide content in the request body (we’ll ignore the use of HTTP Headers for now – they serve a completely different purpose). A POST request allows you to provide content within the request body, so is best placed for an activity-based API.

Now that we’ve identified that the HTTP POST verb is the appropriate choice, how then do we graft the URL to identify what activity (“action”) we want to perform on the resource?

A method driven approach to URL design is often used: or However, while this seems to make sense, in my view, it moves away from the resource orientation of RESTful URLS. You may struggle to consider one resource in isolation, but rather have to be aware of multiple related or interacting resources and actions. This can lead to a plethora of inconsistent URL patterns and approaches – making it difficult for developers to learn how to use your APIs.

Maintaining a resource-based URL representation is surprisingly simple. I use a verb-based terms to identify the action I want to perform on the resource in question. The URL is grafted by appending the verb to the resource-based URL. Using the POST method, I can provide the details required for the activity in the request body.

For example, consider: POST - “authenticate” is the action I want to execute on the customer identified by “123”. The request body contains a representation of the customer required for the authentication.

The important principle to be aware of here is that resources MUST be noun-based (describe the resource) and actions MUST be verb-based (describe the action).

By definition and for clarity, REST (Representational State Transfer) is a software architecture style that defines a set of constraints to be used when creating web services. These constraints include:

1. A client-server architecture which supports the separation of concerns principle – allowing components to evolve independently.

2. A stateless client-server communication – where each request from the client contains all the data required to service the request. No client context is stored on the server.

3. Responses must, implicitly or explicitly, define themselves as cacheable or not to prevent clients from getting stale or inappropriate data in response to further requests.

4. A layered system where the client cannot ordinarily tell whether it is connected directly to the end server, or to an intermediary along the way.

5. A uniform interface is fundamental to the design of any RESTful system. It simplifies and decouples the architecture, which enables each part to evolve independently.

Being resource based is inherent in RESTful APIs and the standard set of HTTP verbs tend to imply CRUD operations only; so, I do understand were the misconception comes from.

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