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.
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.
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
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.
APIPosted by Craig Hughes Sun, January 27, 2019 14:56:07
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.
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
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.
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.
is the entire model of a car:
It’s a fairly
simple model, but I’ll use it to explain the approach.
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.
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).
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.
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:
Is there another
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.
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:
in question is still the car, but the consumer is asking for additional sub-resources
of the car known by “engine” and “interior”.
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
Photo by Lukas from Pexels
APIPosted by Craig Hughes Tue, January 15, 2019 16:58:24
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.
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
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. http://api.sample.com/burgers/cheeseBurger
· 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. http://api.sample.com/burgers?type=Cheese%20Burger
· 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.
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…
Photo by Fancycrave.com from Pexels
APIPosted by Craig Hughes Fri, January 04, 2019 14:34:58
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.
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.
all done. The show is over, the money spent and everyone goes home.
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.Understand
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.Collaborate
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
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
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. Manage
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.
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
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.
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.
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.
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.
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.
events are the ideal contenders for using PubSub since these are business
activities that others may care about.
through an example – an online payment.
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.
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.).
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.
rawpixel.com from Pexels
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.
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
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
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:
- gets a list of customers
- creates a new customer based on the representation in the body of the request
- replaces the customer identified by “123” with the representation in the body
of the request
PATCH http://www.api.com/customers/123 - updates
the customer identified by “123” with the representation in the body of the
- 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?
driven approach to URL design is often used: http://www.api.com/authenticateCustomer
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.
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
consider: POST http://www.api.com/customers/123/authenticate - “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.
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).
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
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.
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.
APIPosted by Craig Hughes Fri, November 30, 2018 08:56:01
A good friend of mine was out walking his dog in the country side the other day. After a while, his dog noticed some turkeys in the adjacent field, and being a dog, decided to investigate. This involved some barking and general running up and down along the fence. I suppose, in the dog’s mind, he was telling the turkeys that he was defending his master and they should not come any closer. The turkeys responded with a cacophony of gobbling and rapid gurgling. The dog got scared and now has a phobia for turkeys…
This often happens when we want to share a message to a wider audience. We have something we want to say. We’ve identified the audience. We start to speak and the response is completely different to what we expected. The dog, barking at the turkeys, expects them to run away. The turkeys do the opposite, running back towards the dog and shouting “Hey! New Guy” (in turkey talk). Leaving the dog scared and confused.
To give you an example from my daily work with APIs; I was involved in a meeting recently where I ended up in a divergent debate with another architect on what the profile of a customer meant. Turns out, the opposing opinions where brought about by lack of context; I was assuming a retail customer (person) while he was assuming a corporate customer (organization). Context in any communication is crucial.
Does this mean we should start every conversation by giving some context? Yes, probably a good idea, but I believe it may be more relevant to start by assuming your audience does not have the same context as you. If you begin with that context (pun intended), you will probably begin by providing context to your audience before diving into the details.
This is also the approach we should take with APIs. During verbal conversation, we have the opportunity to change tack or adjust our speech to provide explanation or context. With APIs, we don’t have this luxury. REST APIs are based on resources. Resource sometimes need context to have more meaning. Encapsulating this resource within some context could offer meaning, but might create a confusing data contract. Ideally, we should provide context via the URL.
Let’s look at an example. Assuming I need to consume an API that presents transactions on my account. The URL https://api.somedomain.com/transactions could return this list of transactions, but they have no context – they could be for anyone’s accounts. We could provide context to these transactions by including the account these transactions belong to. For example: https://somedomain.com/accounts/12345/transactions makes the statement that these transactions belong to the account identified by “12345”. It’s a simple and seemingly innocuous approach, it can be very powerful, but often forgotten or missed.