A short presentation on what is and how to design a RESTFul API. Comes with an introduction to Micro Services and how to adopt REST for it.

1. RESTFul API Design
Principles, Guidelines and Best Practices

2. Who am I
Christian Günther
Solution Architect
SAP / Non-SAP
Cloud Computing
NoSQL and Open
Source evangelist
Twitter: @siliconchris

3. The storyline behind
The company Microdolls Inc.
produces miniature action figures.
The company runs a website with
a web-shop, a doll-designer and
discussion boards.
The website is quite old and they
would like to redesign it.
They would also like to create a
mobile version of the shop and
the doll designer.
microdolls.com
Where imagination meets action

4. Current situation
The website and all offerings (such as the shopping cart, the doll designer
and the discussion board) is written in PHP
The backend is a MySQL database
Website and DB are directly coupled using embedded sql-statements in
the php-code
The designer consists of php-code that executes scripted ImageMagick
functions on the OS level to manipulate images and return the new image
to the browser
All in all the technical implementation is quite old, but it is not unlike many
other sites with a shop one can find on the internet

5. Current interaction model
Customers of microdolls.com open their browser and enter the shop’s url
The web server takes the browsers request (this of course being GET
www.microdolls.com/shop/index.php), possibly executes some php code and
returns plain html
The user’s browser then interpreted and renders the html code and presents the
visual representation of the site
Any element, which is not directly included in the html payload (such as images),
are retrieved through subsequent GET-requests with a URL pointing to the image
location on the server.

6. Issues with the current setup
There is no real separation (be it layered or otherwise) between the different technical
elements such as presentation, application and persistence - everything is very tightly
coupled.
This programming paradigm and the underlying architecture makes it extremely
difficult and time consuming to introduce new functionality.
Reuse of code is mainly done by duplicating needed functionality from one page in
another - that is to say, it is not really reused, but rather used again.
Changes to any element (be it shop, board, designer or database) need to be
thoroughly tested throughout the whole website and all areas.
To enter the mobile market, the company would need to re-create the whole website
with all functionality and would also need to implement new abstraction layers.

7. The conclusion
Instead of simply trying to
recreate the website on
mobile devices, the
company decides to
redesign their infrastructure
and programming model.
In essence, tear down the
monolithic php site and
establish a micro services
architecture

9. SOAP? REST? THRIFT?
Like our fictional company microdolls.com, a lot of companies have designed their
applications in a very monolithic way (in the past, one has to say).
A monolith is an application which provides all of its services through a giant, singular
application. Due to the drawbacks of such an approach, the modern API development
architecture has been steadily moving towards micro services.
Unlike monolithic systems, Microservices deliver their functionality split between multiple,
separate applications and processes, and provide their functionality across multiple
servers when needed.
As Microservice development has grown, so has the need for more diverse and
extensible architecture designs.
There are basically two approaches (technologies) on how to create an API for a micro
services based architecture and one that is rather new: SOAP, REST and Apache THRIFT

10. SOAP - Simple Object Access Protocol
SOAP is an architectural concept that was created in 1998 by Dave Winer, Don
Box, and Bob Atkinson, in collaboration with Microsoft.
Designed explicitly for the purpose of making communication between web
services easier, SOAP has four main characteristics that made it so popular in
its infancy:
Extensibility through user generated extensions tying into the base
structure;
Neutrality by operating over any transport protocol;
Independence by allowing variable programming paradigms and structures;
Large Data Handling through asynchron calls to conversions, calculations,
etc.

11. REST - Representational State Transfer
REST was created in 2000 by Roy Fielding in UC, Irvine; later versions of
this architecture were created in collaboration with the W3C Technical
Architecture Group.
While SOAP aimed to be a complete system, REST was designed to be
more lightweight for building the following right into the design;
Scalability by using cached data from the client and intermediate
nodes built into HTTP to self-define;
Portability by tying the transfer of data to the program code during
transfer;
Extensibility by allowing individual elements of the greater network to
develop independent of one another, using uniform interfaces.

12. Apache THRIFT
Thrift, in 2007 designed by Facebook, was soon released as an open source project under
the Apache Software Foundation label.
The architecture, designed specifically to compete with SOAP and REST while delivering data
quickly in a number of formats, includes a complete stack for the manufacture, maintenance,
and expansion of clients and servers.
The system was designed specifically to include:
Scalability by supporting cross-language services seamlessly between C#, C++,
Cappuccino, Cocoa, Haskell, and more
Simplicity by eschewing the frameworks of REST and the XML of SOAP in favor of a
simple library
Speed by utilizing binary serialization to handle data
Evolution by allowing for soft versioning, supporting third party teams to develop RPC
calls as needed

13. Text
REST in Peace SOAP
REST vs SOAP: Simplicity wins, again

14. REST vs SOAP
REST is not always better than SOAP
There are circumstances, where a SOAP-based approach could
be preferable over REST
Those situations, however, are scarce to find
Eg, if you need highly coupled applications, build by your own or
your co-team, you could use SOAP, but
in general, if you build an API that is to be consumed by clients
outside your company or application domain, NEVER use SOAP

15. What is REST?
REST = Representational State Transfer is an architectural
style and programming paradigm for distributed systems,
especially (but not necessarily only) those found in
applications running on the world wide web
REST gives a coordinated set of constraints to the design of
components in a distributed hypermedia system that can lead
to a higher-performing and more maintainable architecture
To the extent that systems conform to the constraints of
REST they can be called RESTful.

16. What is REST?
RESTful systems typically, but not always, communicate over
HTTP with the same HTTP verbs (this being GET, POST, PUT
and DELETE) used by a web browser that access a web page
REST interfaces with external systems using resources
identified by a URI which can be operated upon using the
named standard verbs
REST demands that the response to a resource-request must
always return the same content regardless how many times
the URI was accessed

17. The idea behind REST
With REST the idea is that, rather than using complex
mechanisms such as CORBA, RPC or SOAP to connect
between clients and servers, simple HTTP queries are
used.
RESTful applications use HTTP requests to POST data
(create), PUT data (update), GET data (make queries),
and DELETE, to delete data on the server.
Thus, REST uses HTTP for all four CRUD (Create/Read/
Update/Delete) operations

18. The idea behind REST
For example, if a client application wants to get information on the action figure
Neo, it sends our API server an HTTP GET with the URL:
Using jQuery we can implement this AJAX request in our client application simply
like this:
The HTTP reply our server sends back to the client is the raw result data — not
embedded inside an HTML page, not XML-encoded, just the data you need in a
way you can immediately use – a String or a JSON object you can use directly.
$.getJSON("http://microdolls.com/dolls/neo",
function(data) {
console.log("The Action figure neo fancies "+data );
});
http://microdolls.com/dolls/neo

19. The idea behind REST
REST can easily handle more complex requests, including multiple
parameters. In most cases, you’ll just use HTTP GET parameters in
the URL.
For example, if a specific doll can only uniquely specified by both
template and sex, our API might accept a request like this:
As a convention (although I’d rather see this as a strict guideline),
HTTP GET requests should be for read-only queries; GET should not
change the state of the server and its data. For creation, updating,
and deleting data, use POST, PUT and DELETE requests.
http://microdolls/dolls/?template=matrix&sex=male

20. WHAT is a RESTFul API?
Short Answer: „A RESTFul API is one that exposes it’s
services in a way that complies to the principles of
REST“

21. WHAT is a RESTFul API?
The long answer is this: A RESTFul API is one, that can be
consumed by a system, in the same intuitive way a user
consumes and uses a website.
A user does not need to know the structure of a website prior
using it. He/She will learn about the site’s structure along the
way, simply by following presented navigation links (URLs)
A RESTFul API provides the same smooth consumption
pattern. A system connecting to your API the first time, will learn
about the internal structure of the API by navigating the API’s
response

22. RESTFul API
Guidelines on RESTFul API Design

23. RESTFul API Design
HTTP is your driver
GET never changes an objects state
Use nouns instead of verbs
Always use plural
Use sub-locations for relations
Use grouping, if applicable
HTTP headers contain the serialization format
Use HATEOAS
Provide operations for collections
Version your API
Transport Errors as HTTP Return Codes
Allow overriding of HTTP methods

24. HTTP is your driver
Although this should be rather obvious:
A RESTFul API relies entirely on HTTP
All operations on exposed objects are done through well defined
methods
GET - retrieves an object, a collection or displays specific data
POST - creates an object
PUT - updates an object (or a predefined collection of objects)
DELETE - hm? any guess?

25. GET never changes state
More precisely GET method and query parameters should not
alter the state of an object or data
Use PUT, POST and DELETE methods instead of the GET
method to alter the state.
Do not use GET for state changes:
GET /users/711?activate
or
GET /users/711/activate Nooooooooh!!!

26. Use nouns instead of verbs
For easy readability nouns are always preferable over verbs.
After all, your API will expose an object as first citizen.
The methods to alter or work with your object are of second concern - the are
implemented using standard HTTP methods.
Therefore never use anything like these:
/getAllUser
but always use the simple
/users
along with a GET/POST/PUT/DELETE HTTP Method

27. Use plural nouns
As well for easy readability always use plurals
Do not mix up singular and plural nouns. Keep it simple and
use only plural nouns for all resources.
/cars instead of /car
/users instead of /user
/products instead of /product
/settings instead of /setting

28. Use sub-locations for relations
If a resource is related to another resource use sub-
resources.
GET /cars/711/drivers/
Returns a list of drivers for car 711
GET /cars/711/drivers/4
Returns driver #4 for car 711

29. Use grouping, if applicable
Identify common or shared properties:
Is there something you can group in a domain specific
context? Something that is shared by most or at least
a majority of objects/services?
Create an abstract class (if we talk of objects) or a
group (if we talk of a service) and apply the shared
property to it - then derive your class from the abstract
class or put your services in that group, respectively.

30. HTTP headers contain the
serialization format
Both, client and server, need to know which format is
used for the communication.
The format has to be specified in the HTTP-Header.
Content-Type defines the request format.
Accept defines a list of acceptable response formats.

31. Use HATEOAS
HATEOAS = Hypermedia As The Engine Of Application
State is a principle that hypertext links should be used
to create a better navigation through the API.
In essence that means, in an API designed after the
HATEOAS principle, a client will be able to navigate
through the API by following URLs and links provided
by the server - this is much the same, as a user
navigating through a website by following links on that
site.
As this is a bit of a complicated topic, a more in deep description is provided within this presentation

32. Provide operations for
collections
Filtering - filter a return list
Sorting - sort a collection
Field Selection - select fields
Paging - display a limited set of resources per query

33. Filtering
Use a unique query parameter for all fields or a query
language for filtering.
To return a list of alien action figures
GET /dolls?family=alien
To return a list of action figures with a maximum of 2
heads
GET /dolls?heads<=2

34. Sorting
Allow ascending and descending sorting over multiple
fields.
To return a list of action figures sorted by descending
manufacturers and ascending models.
GET /dolls?sort=-manufactorer,+model

35. Field Selection
Mobile clients display just a few attributes in a list.
They don’t need all attributes of a resource.
Give the API consumer the ability to choose returned fields.
This will also reduce the network traffic and speed up the
usage of the API.
GET /dolls?
fields=manufacturer,model,id,family

36. Paging
Use limit and offset. It is flexible for the user and common in leading databases. The
default should be limit=20 and offset=0
GET /dolls?offset=10&limit=5
To send the total entries back to the user use the custom HTTP header: X-Total-
Count.
Links to the next or previous page should be provided in the HTTP header link as well.
It is important to follow this link header values instead of constructing your own URLs.
<https://blog.microdolls.com/catalog/api/v1/dolls?offset=15&limit=5>; rel=„next",
<https://blog.microdolls.com/catalog/api/v1/dolls?offset=50&limit=3>; rel=„last",
<https://blog.microdolls.com/catalog/api/v1/dolls?offset=0&limit=5>; rel=„first",
<https://blog.microdolls.com/catalog/api/v1/dolls?offset=5&limit=5>; rel="prev",

37. Version your API
Make the API Version mandatory and do not release an unversioned API.
Decide on your API versioning scheme - simple ordinals or year plus
month or the like. It doesn’t really matter, but stick to it for all your APIs
within a domain and/or application system
If using numbers, use simple ordinal number and avoid dot notation such
as 2.5.
Finally it is common sense to prefix the url for the API versioning with the
letter „v“
/catalog/api/v1

38. Transport errors as HTTP
status codes
It is hard to work with an API that ignores error
handling.
Pure returning of a HTTP 500 with a stacktrace is not
very helpful.
Also, inventing new meanings to established http status
codes makes using your API a nightmare on elm street
Instead use standard HTTP status codes

39. Use HTTP status codes
The HTTP standard provides over 70 status codes to describe
the return values.
Use these 4 codes for successful requests
200 – OK – Here is your result
201 – OK – New resource was created / resource was updated
204 – OK – The resource was successfully deleted
304 – Not Modified – The client can use cached data

40. HTTP error status codes
Use these 5 codes for failed requests
400 – Bad Request – The request was invalid or cannot be served. The exact
error should be explained in the error payload. E.g. „The JSON is not valid“
401 – Unauthorized – The request requires an user authentication
403 – Forbidden – The server understood the request, but is refusing it or the
access is not allowed.
404 – Not found – There is no resource behind the URI.
422 – Unprocessable Entity – Should be used if the server cannot process the
enitity, e.g. if an image cannot be formatted or mandatory fields are missing in
the payload.

41. HTTP 500
API developers should avoid this error in normal
situation and use specific error codes (as shown in the
previous slide).
If an error occurs in the global catch blog (bad luck),
the stracktrace should be logged and not returned as
response.
500 – Internal Server Error

42. Use error payloads
All exceptions should be mapped in an error payload.
Here is an example how a JSON payload should look
like.
04 "userMessage": "Sorry, the requested resource
does not exist“,
05 "internalMessage": "No doll found in the
database",

43. Allow overriding HTTP
methods
Some proxies support only POST and GET methods.
To support a RESTful API with these limitations, the API
needs a way to override the HTTP method.
Use the custom HTTP Header X-HTTP-Method-
Override to override the POST Method.