Simulation Mode

The API has a simulation mode, it uses hard coded data in responses, but tries to mimic some conditions.

Watch on YouTube: "How to use simulation mode"

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The simulator is stateless and does not track your usage, making it deterministic for multiple users. Which means:

  • Entities created do not show in the 'entities' call, but can be retrieved by a 'GET'
  • Entities deleted do not show in the 'entities' and respond to a 404, but the delete for them will return a 200... you can only delete 'specific' entities, other entities will respond with a forbidden request.
  • There are 'inconsistencies' but they are logical based on the needs of a stateless simulator. Use the actual API that underpins the challenges or the Simple API if you want a 'real' API.

How to Use

Work through the requests in sequence to achieve a fairly logical interaction.

  • try different tooling, the only difference then will be the tool because the API is fairly forgiving and no-one else can interfere with your practice. Use it to learn the tools.
  • try different automated execution approaches. The API is simple, there are only a few requests and sequences, so use it to learn a new automated execution tool. It won't change as you are automating, if something goes wrong then it is most likely some nuance of the tool.

This simulator is designed to make starting with API testing as simple as possible.

Suggested Request Sequence

Try the verbs and payloads listed below as a way of making sure your tooling is setup and you understand the absolute basics about API usage and Testing.

Step 1 - GET all the Entities

Use your API Client to issue a GET request on the /sim/entities end point.

  • GET verb will retrieve information from the server.
  • The endpoint is a top level endpoint for a domain entity.
  • Most REST APIs use a plural endpoint like entities or todos

You should see a 200 status code, which means that the request was a success.

In the response you should see a JSON payload with 10 items.

  • Entities 1-10
  • Get all the entities in the simulator

Step 2 - GET a single Entity

Issue a GET request to /sim/entities/1

  • by adding /1 to the endpoint we are requesting the entity with id equal to 1
  • The response should have a status code of 200, meaning OK, the API found the item
  • The response should Return the details of entity number 1

For extra practice try and GET other entities (2-8) listed in the /sim/entities response

NOTE: Entities 1-8 are suitable for getting with a 200 response. 9 and 10 are reserved for later simulated scenarios so if you GET them then you may not get the response you are expecting

Step 3 - Try to GET an Entity that does not exist

A 404 status code is returned when the API cannot find the item requested.

There was no entity with id equal to 13 so if we try to GET that we will receive a 404 status code in the response.

  • Entity does not exist, receive a 404 response
  • Try other values for the id and see that you receive 404 responses

Step 4 - Use POST to create an Entity

A GET request is used to retrieve information. We can use POST to create new items or amend existing items.

  • We need to issue a POST request to the /sim/entities end point.
  • We use the top level endpoint without an id because we want to create a new entity (if we add an id then we are trying to amend a specific entity)
  • We also need to add a body with the details of the entity we want to create

With body:

    {"name": "bob"}

NOTE: we've tried to make the simulator easy to use so it doesn't actually matter what you add as the payload, you'll still see the response below.

In response you will see that a new entity has been created with id 11:

  "id": 11,
  "name": "bob",
  "description": ""

The response will also have a 201 status code, which indicates that a new entity was created.

If you look at the headers in the response you will also see a Location header containing the endpoint where the created item can be found.

HTTP/1.1 201 Created
Content-Type: application/json
Location: /sim/entities/11

You can now issue a GET request on /sim/entities/11 and you'll see the entity returned again.


NOTE: entity with id 11 does not show in the entities list because no state is stored on the server.

Step 5 - Use POST to Amend an Entity

With this API you can also use POST to amend an entity. We'll amend entity id 10

Entity 10 is listed in the /sim/entities response as:

  "name":"entity number 10",

Issuing a POST on endpoint /sim/entities/10 means that we want to amend a specific entity.


Add a payload which contains the information you want to update:

    {"name": "eris"}

You will receive a 200 status code.

  • Amend an entity...note we assume you are amending to the payload above, because that is what we return.
  • Will amend Entity with ID 10, to have the name eris

Once you amend you can GET this item and check it has amended

  "id": 10,
  "name": "eris",
  "description": ""

NOTE: because this is a simulator and nothing has changed on the server if you issue a GET on /sim/entities it will show the original name "entity number 10"

Step 6 - Use PUT to Amend an Entity

You can also use PUT with this API to amend the entity.

Let's assume that someone has amended the name back to the original "entity number 10", which we can see if we GET


Entity 10 is listed in the /sim/entities response as:

  "name":"entity number 10",

So we will issue a PUT request to amend entity with id 10 to have the name eris


With the payload in the body as:

    {"name": "eris"}

PUT and POST are not always interchangeable with APIs. For each API you use, read the documentation to learn how the API works.

Step 7 - DELETE an Entity

We can use the DELETE HTTP verb to delete a specific item.

The only entity we can delete is id equal to 9


This should respond with a status code of 204 meaning OK successfully completed, but no additional information to show in the body of the response.

To check that something is deleted, try to GET it and make sure that you receive a 404 response.

  • if you GET id 9 then you will find it 404's (because it was deleted)

Additional thing to try with DELETE:

  • DELETE an entity listed in the /sim/entities/id call is forbidden when id < 9
  • Try to DELETE all the entities by using DELETE on /sim/entities you should see a status 405 meaning that the HTTP verb (method) is not allowed
  • Try to DELETE an entity that does not exist e.g. /sim/entities/56 you should see a 404 status with an error message

STEP 8 - Find out what Verbs are allowed with OPTIONS

APIs will often report errors if you use an HTTP Verb on an endpoint that you are not allowed to.

To find out what Verbs, or Methods, we are allowed to use, issue an OPTIONS request on the endpoint.


By looking at the Accept header in the response we can see that we are allowed to GET, POST, PUT, HEAD, OPTIONS

If we tried to DELETE or PATCH then we should receive an appropriate status code of 405

Try and see:


DELETE correctly returns a 405 response as we expected.



Patch returns a 501 status, which indicates a problem with the API. No API should return a 500 range status code as it indicates a server problem:

  • 1xx - Information response
  • 2xx - OK
  • 3xx - Redirect response (check the Location header)
  • 4xx - Invalid request - you've sent an incorrect message
  • 5xx - Server error - something went seriously wrong with the API

STEP 9 - HEAD request

From the OPTIONS request we can see that we are allowed to issue HEAD requests.

A HEAD request is like a GET but only the headers are returned. The headers should be the same as those returned from a GET request.

Try this by issuing a HEAD request.


And compare the response with a GET request:


The headers should be the same.

Other Things to Try

If you followed all the Simulator steps then you've managed to use your HTTP API Client tool to issue a range of different request types.

If you want to explore the tool more then you could try the experiments below, or move on directly to the Simple API

  • Try changing the Accept header on GET requests to application/xml and application/json
  • POST/PUT an entity listed in the /sim/entities/id call is forbidden for id < 10
  • PATCH and TRACE should be 501 for all end endpoints
  • any other /sim/* endpoints should respond with a 404

Swagger OpenAPI File

You can download a simple Swagger [OpenAPI File for simulation mode](/sim/docs/swaggercd ..).

Simulation Mode Walkthrough - Insomnia

Watch on YouTube: "How to use Insomnia with simulation mode as example api"

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Simulation Mode Walkthrough - Postman

Watch on YouTube: "How to use Postman with simulation mode as example api"

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