How to do RESTful web services right

Kerry Buckley & Paul Moser

BT DevCon2

9 March 2010

How to do RESTful web services right

One way

How to do RESTful web services right

(for some value of ‘right’)

One way

What is REST?

REST is an architectural style, not a protocol or a standard. Hence many values of “right” (and much disagreement).

‘A style of software architecture for

distributed hypermedia systems such as the World Wide Web’

Not necessarily HTTP, although in practice it generally is.

Not SOAP

Seems to be a common (but wrong) definition.

Constraints

• Client-server

• Stateless

• Cacheable

• Layered system

• Uniform interface

Principles

• Identification of resources

• Manipulation of resources through these representations

• Self-descriptive messages

• Hypermedia as the engine of application state

Goals

• Scalability of component interactions

• Generality of interfaces

• Independent deployment of components

• Intermediary components to reduce latency, enforce security and encapsulate legacy systems

Shop example

• List products

• View a product

• Create an order for a number of products

• Cancel order

• Pay for order

XML-RPC(URI-tunnelling)

GET /api?action=list_products

GET /api?action=view_product&product_id=123

GET /api?action=create_order&product_id=123&product_id=456…

GET/api?action=pay_order&order_id=42&card_number=1234…

Sometimes only a single ‘endpoint’ URI

GET /list_products

GET /view_product?product_id=123

GET /create_order?product_id=123&product_id=456…

GET/pay_order?order_id=42&card_number=1234…

Or one endpoint per method

GET /list_products

GET /view_product?product_id=123

POST /create_order?product_id=123&product_id=456…

POST /pay_order?order_id=42&card_number=1234…

Marginally better without unsafe GETs

POST /pay_order?order_id=42&card_number=1234…

def pay_order(order_id, card_number) { …}

Request maps onto method call.

✓Easy to understand

✓Works well for simple procedure calls

✓Simple to implement

✓Interoperable

✗Brittle

✗Tightly coupled

✗Failure cases require manual handling

✗No metadata

Need to know all the URIs, methods and parameters in advance (out-of-band documentation)

✗ Identification of resources

✗ Manipulation of resources through these representations

✗ Self-descriptive messages

✗ Hypermedia as the engine of application state

But is it REST?

POX(plain old XML)

Request

POST /create_order

<create_order_request> <line> <product_id>123</product_line> <quantity>1</quantity> </line> …</create_order_request>

Response

200 OK

<create_order_response> <status>OK</status> <order_id>42</order_id></create_order_response>

Both request and response have XML bodies.

✓Simple to implement

✓Interoperable

✓Allows complex data structures

✗Tightly coupled

✗No metadata

✗Doesn’t use web for robustness

✗Doesn’t use SOAP for robustness either

✗ Identification of resources

✗ Manipulation of resources through these representations

✗ Self-descriptive messages

✗ Hypermedia as the engine of application state

But is it REST?

CRUD

GET /products

GET /products/123

POST /orders

PUT /orders/42

DELETE /orders/42

POST /orders/42/payment

Representations• Hypermedia

- XHTML

- Atom

- Custom XML schema

• Non-hypermedia

- Generic XML

- YAML

- JSON

- CSV

- etc

✓Makes good use of HTTP

✓Uniform interface

✓Good for database-style applications

Because each resource has a URI you can use caching etc. Uniform interface: verbs are GET, POST, PUT and DELETE.

✗ Ignores hypermedia

✗Tight coupling through URI templates

✗Not self-describing

✓ Identification of resources

✓Manipulation of resources through these representations

✗ Self-descriptive messages

✗ Hypermedia as the engine of application state

But is it REST?

REST

API rootRequest

GET /

API rootResponse

200 OKContent-Type: application/vnd.rest-example.store+xml

<store> <link method="get" rel="products" href="http://rest-demo.local/products"/> <link method="get" rel="orders" href="http://rest-demo.local/orders"/></store>

Links contain information on what we can do.

View productsRequest

Get /products

Following the link with a relation of ‘products’ (link rel="products").

View productsResponse

200 OKContent-Type: application/vnd.rest-example.products+xml

<products> <link method="get" rel="latest" href="http://rest-demo.local/products"/> <product> <link method="get" rel="view" href="http://rest-demo.local/products/1"/> <code>A-001</code> <description>Tartan Paint</description> <price>4.95</price> </product> …</products>

Note link to retrieve the latest version of this resource (eg if you had it cached).

View ordersRequest

Get /orders

View ordersResponse

200 OKContent-Type: application/vnd.rest-example.orders+xml

<orders> <link method="get" rel="latest" href="http://rest-demo.local/orders"/> <link type="application/vnd.rest-example.order+xml" method="post" rel="new" href="http://rest-demo.local/orders"/></orders>

No orders yet, but note link to create a new one.

Place orderRequest

POST /ordersContent-Type: application/vnd.rest-example.order+xml

<order> <line> <product>http://rest-demo.local/products/1</product> <quantity>2</quantity> </line></order>

Again following a link, this time posting data of the specified type.

Place orderResponse

201 CreatedContent-Type: application/vnd.rest-example.order+xml

<order> <link method="get" rel="latest" href="http://rest-demo.local/orders/9"/> <link method="delete" rel="cancel" href="http://rest-demo.local/orders/9"/> <link type="application/vnd.rest-example.payment-details+xml" method="post" rel="pay" href="http://rest-demo.local/orders/9/pay"/> <line> <product>http://rest-demo.local/products/1</product> <quantity>2</quantity> <cost>9.90</cost> </line> <total>9.90</total></order>

Note links to cancel or pay for the order – HATEOAS! The links included would depend on the allowable state transitions (eg no cancel link for a despatched order).

Clients only need know

• The root URI (‘home page’) of the API

• Definitions for the media types used

- eg schemas or example documents

• Everything else is just HTTP and links

✓Makes full use of HTTP

✓Self-describing

✓Loosely coupled

Only need to know entry URI and media types – API can be explored using links. If link URIs change, well-behaved clients should not break.

✗More complex to implement

✗Transition links can lead back to RPC-style

Frameworks help. Tempting to just add a bunch of actions as links, using overloaded post.

✓ Identification of resources

✓ Manipulation of resources through these representations

✓ Self-descriptive messages

✓ Hypermedia as the engine of application state

But is it REST?

This is not the One True Way

As mentioned before, REST is just an architectural style. There are other approaches to creating RESTful APIs.

An alternative:‘Web site is your API’

• No separation between web site and API

• Representations are HTML

• Data submitted using HTML forms

• Semantics via microformats

Using HTTP features

Response codes• 200 OK

• 201 Created

• 202 Accepted

• 204 No content

• 301 Moved permanently

• 400 Bad request

• 403 Forbidden

• 404 Not found

• 405 Method not allowed

• 409 Conflict

• 410 Gone

• etc

Restricting transitions

Request Response

OPTIONS /orders/123(an open order)

200 OKAllow: GET, PUT, DELETE

OPTIONS /orders/42(a despatched order)

200 OKAllow: GET

Restricting transitions

Request Response

DELETE /orders/123Expect: 100-Continue

100 Continue

DELETE /orders/42Expect: 100-Continue

417 Expectation Failed

Prevent race conditions

Request

PUT /orders/123If-Unmodified-Since: Tue, 9 Mar 2010 11:00:00 GMT

Response

200 OK

or

412 Precondition Failed

Prevent race conditions

Request

GET /orders/123

Response (partial)

200 OKETag: 686897696a7c876b7e

Request

PUT /orders/123If-Match: 686897696a7c876b7e

Security

• HTTP basic

• HTTP digest

• Shared secret (OAuth etc)

• SSL

• Selective encryption

More

• Jim Webber tutorial

http://tinyurl.com/rest-tutorial

• Restfulie framework (Rails & Java):

http://tinyurl.com/restfulie