HAL format overview in Rest API - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this HAL JSON snippet?

Given this HAL JSON snippet, what is the value of the 'self' link href?

Rest API

{
  "_links": {
    "self": { "href": "/orders/123" },
    "next": { "href": "/orders/124" },
    "find": { "href": "/orders{?id}", "templated": true }
  },
  "total": 1,
  "_embedded": {
    "orders": [
      { "id": 123, "status": "shipped" }
    ]
  }
}

A "/orders{?id}"

B "/orders/123"

C "/orders/124"

D "/orders/125"

💡 Hint

Look inside the _links object for the 'self' key.

The 'self' link in HAL points to the current resource URL, which is '/orders/123' here.

🧠 Conceptual

intermediate

What is the purpose of the _embedded section in HAL?

In HAL format, what does the _embedded section do?

A It contains related resources embedded inside the current resource.

B It lists all available HTTP methods.

C It specifies the API version used.

D It defines HTTP headers for the response.

💡 Hint

Think about how HAL helps clients get related data in one response.

The _embedded section holds related resources so clients can access them without extra requests.

🔧 Debug

advanced

What error does this HAL JSON cause?

This HAL JSON snippet is invalid. What error will it cause when parsed?

Rest API

{
  "_links": {
    "self": { "href": "/users/1" },
    "friends": [
      { "href": "/users/2" },
      { "href": "/users/3" }
    ]
  },
  "name": "Alice",
  "age": 30
}

A No error, valid HAL JSON

B TypeError because 'friends' should not be an array

C KeyError because '_embedded' is missing

D SyntaxError due to missing comma between 'name' and 'age'

💡 Hint

Check JSON syntax carefully between properties.

There is a missing comma after the 'name' property, causing a JSON syntax error.

📝 Syntax

advanced

Which option correctly represents a templated link in HAL?

Choose the correct HAL syntax for a templated link to search orders by id.

A "search": { "href": "/orders/{id}", "templated": true }

B "search": { "href": "/orders?id", "templated": false }

C "search": { "href": "/orders{?id}", "templated": true }

D "search": { "href": "/orders?id", "templated": true }

💡 Hint

Templated links use curly braces with question mark for optional query params.

Option C correctly uses the URI template syntax with {?id} and sets templated to true.

🚀 Application

expert

How many items are in the embedded orders list?

Given this HAL JSON, how many orders are embedded inside the _embedded section? { "_links": { "self": { "href": "/orders" } }, "_embedded": { "orders": [ { "id": 101, "status": "pending" }, { "id": 102, "status": "shipped" }, { "id": 103, "status": "delivered" } ] }, "count": 3 }

A 3

B 4

C 0

D 1

💡 Hint

Count the number of objects inside the orders array in _embedded.

There are three order objects inside the orders array in the _embedded section.

Practice

1. What is the main purpose of the _links section in a HAL formatted REST API response?

easy

A. To define the API version number

B. To store user authentication tokens

C. To list all data fields in the response

D. To provide URLs to related resources for easy navigation

Solution

1. Step 1: Understand the role of _links in HAL

   The _links section contains URLs pointing to related resources, helping clients navigate the API easily.

2. Step 2: Compare options with HAL purpose

   Options B, C, and D do not describe navigation or linking related resources, which is the key feature of _links.

3. Final Answer:

   To provide URLs to related resources for easy navigation -> Option D

4. Quick Check:

   HAL _links = URLs for related resources [OK]

Hint: Remember: _links always holds related resource URLs [OK]

Common Mistakes:

• Confusing _links with data fields
• Thinking _links stores authentication info
• Assuming _links defines API version

2. Which of the following is the correct way to represent a link to a 'self' resource in HAL JSON?

easy

A. "_links": { "self": { "href": "/orders/123" } }

B. "links": { "self": "/orders/123" }

C. "_links": { "self": "/orders/123" }

D. "_link": { "href": "/orders/123" }

Solution

1. Step 1: Recall HAL syntax for links

   HAL requires a _links object with named links, each having an href property.

2. Step 2: Check each option's structure

   "_links": { "self": { "href": "/orders/123" } } correctly uses _links with self containing an object with href. Others miss underscores, use wrong keys, or omit href.

3. Final Answer:

   "_links": { "self": { "href": "/orders/123" } } -> Option A

4. Quick Check:

   HAL link = _links + href [OK]

Hint: HAL links always use _links and href keys [OK]

Common Mistakes:

• Omitting underscore in _links
• Using string instead of object for link value
• Missing href property inside link

3. Given the following HAL JSON snippet, what is the URL to access the customer's details?

{
  "_links": {
    "self": { "href": "/orders/123" },
    "customer": { "href": "/customers/456" }
  },
  "orderNumber": "123"
}

medium

A. /orders/123

B. /customers/456

C. /orders/456

D. /customers/123

Solution

1. Step 1: Identify the customer link in _links

   The customer key has an href value of "/customers/456" which points to the customer's details URL.

2. Step 2: Confirm other options

   self points to the order URL "/orders/123". Other options mix order and customer IDs incorrectly.

3. Final Answer:

   /customers/456 -> Option B

4. Quick Check:

   Customer link = _links.customer.href = /customers/456 [OK]

Hint: Look inside _links for the named resource URL [OK]

Common Mistakes:

• Confusing self with customer link
• Mixing order and customer IDs
• Ignoring the href property

4. Identify the error in this HAL JSON snippet:

{
  "_links": {
    "self": "/orders/123",
    "items": [{ "href": "/items/1" }, { "href": "/items/2" }]
  }
}

medium

A. The JSON is correct as is

B. "items" should be a single object, not an array

C. "self" link should be an object with an "href" property

D. "_links" key should be "links" without underscore

Solution

1. Step 1: Check the structure of the "self" link

   In HAL, each link must be an object with an href property. Here, "self" is a string, which is incorrect.

2. Step 2: Verify other parts

   "items" is correctly an array of link objects. The _links key must have an underscore. So only "self" link is wrong.

3. Final Answer:

   "self" link should be an object with an "href" property -> Option C

4. Quick Check:

   HAL links = objects with href [OK]

Hint: All HAL links must be objects with href keys [OK]

Common Mistakes:

• Using string instead of object for single links
• Removing underscore from _links
• Thinking arrays are not allowed for multiple links

5. You want to design a HAL response for a blog post that includes links to the author and comments. Which JSON structure correctly follows HAL format?

hard

A. {
"title": "My Post",
"_links": {
"self": { "href": "/posts/1" },
"author": { "href": "/users/42" },
"comments": [
{ "href": "/comments/100" },
{ "href": "/comments/101" }
]
}
}

B. {
"title": "My Post",
"links": {
"self": "/posts/1",
"author": "/users/42",
"comments": ["/comments/100", "/comments/101"]
}
}

C. {
"title": "My Post",
"_links": {
"self": "/posts/1",
"author": "/users/42",
"comments": "/comments/100,/comments/101"
}
}

D. {
"title": "My Post",
"_link": {
"self": { "href": "/posts/1" },
"author": { "href": "/users/42" },
"comments": [
{ "href": "/comments/100" },
{ "href": "/comments/101" }
]
}
}

Solution

1. Step 1: Verify the _links key and link objects

   { "title": "My Post", "_links": { "self": { "href": "/posts/1" }, "author": { "href": "/users/42" }, "comments": [ { "href": "/comments/100" }, { "href": "/comments/101" } ] } } correctly uses _links with each link as an object containing href. Multiple comments are an array of link objects.

2. Step 2: Check other options for errors

   { "title": "My Post", "links": { "self": "/posts/1", "author": "/users/42", "comments": ["/comments/100", "/comments/101"] } } uses "links" without underscore and strings instead of objects. { "title": "My Post", "_links": { "self": "/posts/1", "author": "/users/42", "comments": "/comments/100,/comments/101" } } uses strings instead of objects for links. { "title": "My Post", "_link": { "self": { "href": "/posts/1" }, "author": { "href": "/users/42" }, "comments": [ { "href": "/comments/100" }, { "href": "/comments/101" } ] } } uses incorrect key "_link" instead of "_links".

3. Final Answer:

   Option A JSON structure correctly follows HAL format -> Option A

4. Quick Check:

   HAL requires _links with objects having href [OK]

Hint: Use _links with objects and href for all links [OK]

Common Mistakes:

• Missing underscore in _links
• Using strings instead of objects for links
• Wrong key name like _link