design a REST API

First, What is a REST API?

REST stands for Representational State Transfer. It is a software architectural style created by Roy Fielding in 2000 to guide the design of architecture for the web.

Any API (Application Programming Interface) that follows the REST design principle is said to be RESTful.

Simply put, a REST API is a medium for two computers to communicate over HTTP (Hypertext Transfer Protocol), in the same way clients and servers communicate.

1. Identify the resources – Object Modeling

The very first step in designing a REST API-based application is – identifying the objects which will be presented as resources.

For a network-based application, object modeling is pretty much more straightforward. There can be many things such as devices, managed entities, routers, modems, etc. For simplicity sake, we will consider only two resources i.e.

Devices
Configurations

Here configuration may be a sub-resource of a device. A device can have many configuration options.

Note that both objects/resources in our above model will have a unique identifier, which is the integer id property.

2. Create Model URIs

Now when the object model is ready, it’s time to decide the resource URIs. At this step, while designing the resource URIs – focus on the relationship between resources and their sub-resources. These resource URIs are endpoints for APIs.

In our application, a device is a top-level resource. And configuration is a sub-resource under the device. Let’s write down the URIs.

/devices

/devices/{id}

/configurations

/configurations/{id}

/devices/{id}/configurations

/devices/{id}/configurations/{id}

Notice that these URIs do not use any verb or operation. It’s crucial not to include any verb in URIs. URIs should all be nouns only.

3. Determine Resource Representations

Now when resource URIs have been decided, let’s work on their representations. Most representations are defined in either XML or JSON format. We will see XML examples as it is more expressive on how data is composed.

3.1. Collection Resource of Devices

When returning a collection resource, include only the most important information about that resource. This will keep the size of the response payload small, and so will improve the performance of the API.

<deviceFamily>apple-es</deviceFamily>

<name>apple-srx_200</name>

<status>active</status>

</device>

<deviceFamily>apple-es</deviceFamily>

<name>apple-srx_200</name>

<status>active</status>

</device>

</devices>

3.2. Single Device Resource

Opposite to collection URI, a single resource URI includes complete information of a particular device. It also includes a list of links to sub-resources and other supported operations. This will make your REST API HATEOAS driven.

<deviceFamily>apple-es</deviceFamily>

<name>apple-srx_100_lehar</name>

<hostName>apple-srx_100_lehar</hostName>

<status>active</status>

</configuration>

</configuration>

</configurations>

</device>

3.3. Collection Resource of Configurations

Similar to device collection representation, create configuration collection representation with only minimal information.

</configuration>

</configuration>

…

</configurations>

Please note that configurations collection representation inside device is similar to top-level configurations URI.

The only difference is that configurations for a device are only two, so only two configuration items are listed as subresources under the device.

3.4. Single Configuration Resource

Now, single configuration resource representation must have all possible information about this resource – including relevant links.

<status>active</status>

</configuration>

3.5. Collection Resource of Configuration under a Single Device

This sub-collection of configurations will be a subset of the primary collection of configurations and will be specific to a device only.

As it is the subset of primary collection, DO NOT create a different representation data fields than primary collection. Use the same presentation fields as the primary collection.

</configuration>

</configuration>

</configurations>

Notice that this sub-resource collection has two links. One for its direct representation inside sub-collection i.e. /devices/12345/configurations/333443 and other pointing to its location in primary collection i.e. /configurations/333443.

Having two links is essential as you can provide access to a device-specific configuration in a more unique manner, and you will have the ability to mask some fields (if the design requires it), which shall not be visible in a secondary collection.

3.6. Single Configuration Resource under a Single Device

This representation should have either exactly similar representation as of Configuration representation from the primary collection, OR you may mask few fields.

This subresource representation will also have an additional link to its primary presentation.

<status>active</status>

</configuration>

Now, before moving forward to the next section, let’s note down a few observations, so you don’t miss them.

Resource URIs are all nouns.

URIs are usually in two forms – collection of resources and singular resource.

Collection may be in two forms primary collection and secondary collection. Secondary collection is sub-collection from a primary collection only.

Each resource/collection contain at least one link i.e. to itself.

Collections contain only most important information about resources.

To get complete information about a resource, you need to access through its specific resource URI only.

Representations can have extra links (i.e. methods in single device). Here method represent a POST method. You can have more attributes or form links in altogether new way also.

We have not talked about operations on these resources yet.

4. Assigning HTTP Methods

So our resource URIs and their representation are fixed now. Let’s decide all the possible operations in the application and map those operations to the resource URIs.

For example, a user of our network application can perform browse, create, update, or delete devices from the network as well as create/deploy/remove the device configurations. So let’s assign these operations to respective resources.

4.1. Browse all devices or configurations [Primary Collection]

HTTP GET /devices

HTTP GET /configurations

If the collection size is large, you can apply paging and filtering as well. e.g., the below requests will fetch the first 20 records from the collection.

HTTP GET /devices?startIndex=0&size=20

HTTP GET /configurations?startIndex=0&size=20

4.2. Browse all configurations under a device [Secondary Collection]

HTTP GET /devices/{id}/configurations

It will be mostly a small-size collection – so no need to enable filtering or soring here.

4.3. Browse a single device or configuration

To get the complete detail of a device or configuration, use GET operation on singular resource URIs.

HTTP GET /devices/{id}

HTTP GET /configurations/{id}

4.4. Browse single configuration under a device

HTTP GET /devices/{id}/configurations/{id}

Subresource representation will be either same as or a subset of primary presentation.

4.5. Create a device or configuration

CREATE is not an idempotent operation and in HTTP protocol – POST is also not idempotent. So use POST.

HTTP POST /devices

HTTP POST /configurations

Please note that the request payload will not contain any id attribute, as the server is responsible for deciding it. The response to CREATE request will look like this:

HTTP/1.1 201 Created

Content-Type: application/xml

Location: http://example.com/network-app/configurations/678678

<status>active</status>

</configuration>

4.6. Update a device or configuration

The update operation is an idempotent operation and HTTP PUT is also is an idempotent method. So we can use the PUT method for update operations.

HTTP PUT /devices/{id}

HTTP PUT /configurations/{id}

PUT response may look like this.

HTTP/1.1 200 OK

Content-Type: application/xml

<status>active</status>

</configuration>

4.7. Remove a device or configuration

Removing is always a DELETE operation.

HTTP DELETE /devices/{id}

HTTP DELETE /configurations/{id}

A successful response SHOULD be 202 (Accepted) if resource has been queued for deletion (async operation), or 200 (OK) / 204 (No Content) if the resource has been deleted permanently (sync operation).

In the case of async operation, the application shall return a task id that can be tracked for success/failure status.

Please note that you should put enough analysis in deciding the behavior when a subresource is deleted from the system. Usually, you may want to SOFT DELETE a resource in these requests – in other words, set their status INACTIVE.

By following this approach, you will not need to find and remove its references from other places as well.

4.8. Applying or Removing a configuration on/from a device

In a real application, you will need to apply the configuration on the device – OR you may want to remove the configuration from the device (not from the primary collection). You shall use PUT and DELETE methods in this case, because of its idempotent nature.

//Apply Configuration on a device

HTTP PUT /devices/{id}/configurations

//Remove Configuration on a device

HTTP DELETE /devices/{id}/configurations/{id}

5. More Actions

So far, we have designed only object models, URIs and then decided HTTP methods or operations on them.

#viastudy