Welcome to this lesson on sending data with POST requests using Scala 3 and Requests-Scala. As you continue your exploration of interacting with RESTful APIs, you'll learn how to send data to a server using the POST method. POST requests are essential when you want to create new resources or submit data, such as filling out a web form or adding a new entry to a database. Unlike GET requests, which you have already encountered, POST requests do not solely retrieve information; they actually transmit data to an API.

Understanding these differences is crucial as you expand your skill set in HTTP methods. Let's dive deeper into utilizing POST requests to comprehend how they stand apart from GET requests.

Before diving into POST requests, let's briefly compare them to GET requests:

• GET Requests:

  • Purpose: Retrieve data from a server.
  • Data Location: Data is sent in the URL as path or query parameters.
  • Success Status: Expect a 200 status code for successful data retrieval.

• POST Requests:

  • Purpose: Send data to a server to create or update a resource, such as submitting a form, uploading a file, or adding a new item to a database.
  • Data Location: Data is sent in the request body.
  • Success Status: Expect a 201 status code for successful resource creation.

These differences clarify when to use each method. POST requests, in particular, require careful handling of the request body.

For POST requests, the request body is crucial as it holds the data you want to send to the server. This data is usually structured in formats like JSON, XML, or form data, with JSON being a common choice due to its readability and compatibility.

Here's an example of a JSON request body:

JSON
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1{
2  "title": "Learn Scala requests library",
3  "description": "Complete a course on Scala API calls.",
4  "done": false
5}

This represents a new todo item, including a title, completion status, and description. Noticeably, we are not sending an id, as this is typically managed by the server upon resource creation.

With a solid understanding of GET vs. POST and the role of the request body, you're ready to explore how to utilize Requests-Scala to handle POST operations.

To work with POST requests, we will use Scala 3 and the Requests-Scala library. First, make sure you have included the Requests-Scala library in your project. Then, set your base URL, for example, http://localhost:8000, which will serve as the foundation for making API requests.

Here's a basic setup:

Scala
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1// Import the Requests-Scala library and Try
2import requests.*
3import scala.util.{Try, Success, Failure}
4
5// Define the base URL for the API
6val baseUrl: String = "http://localhost:8000"

This sets up the foundation for us to explore POST requests further.

Let's walk through an example of how to craft a POST request to add a new todo item to our API using Scala.

First, we need to prepare the data we wish to send. We'll be adding a new todo item with a specific title, a completion status, and a description:

Scala
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1// New todo data in JSON format
2val newTodo: ujson.Obj = ujson.Obj(
3  "title" -> "Learn Scala requests library",
4  "description" -> "Complete a course on Scala API calls.",
5  "done" -> false
6)
7
8// Send the POST request wrapped in Try
9val response: Try[requests.Response] = Try(
10  requests.post(s"$baseUrl/todos", data = ujson.write(newTodo))
11)

Here, s"$baseUrl/todos" forms the complete endpoint URL, and data = ujson.write(newTodo) sends the JSON data in the request body.

Interpreting the response from a POST request is an integral part of the process. After sending the request, the server provides a response indicating whether the operation was successful. A 201 status code signifies successful resource creation. Typically, POST requests return the newly created resource in the response body. This allows the client to immediately access and utilize details of the new resource, such as server-generated fields like IDs.

Scala
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1// Handle the response using pattern matching
2response match
3  case Success(resp) if resp.statusCode == 201 =>
4    println("New todo added successfully!")
5    println(resp.text())
6  case Success(resp) =>
7    println(s"Failed to add a new todo")
8    println(s"Status Code: ${resp.statusCode}")
9    println(s"Error Details: ${resp.text()}")
10  case Failure(exception) =>
11    println(s"Request failed: ${exception.getMessage}")

For example, if the operation is successful, the output will be:

Plain text
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1New todo added successfully!
2{"description":"Complete a course on Scala API calls.","done":false,"id":5,"title":"Learn Scala requests library"}

A POST request may fail if required fields are missing. For example, omitting a "title" might lead to an error response:

Scala
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1// New todo data with missing title
2val newTodo: ujson.Obj = ujson.Obj(
3  // "title" -> "Learn Scala requests library",  // Title is missing
4  "done" -> false,
5  "description" -> "Complete a course on Scala API calls."
6)
7
8// Send POST request wrapped in Try
9val response: Try[requests.Response] = Try(
10  requests.post(s"$baseUrl/todos", data = ujson.write(newTodo))
11)
12
13// Handle the response using pattern matching
14response match
15  case Success(resp) if resp.statusCode == 201 =>
16    println("New todo added successfully!")
17    println(resp.text())
18  case Success(resp) =>
19    println(s"Failed to add a new todo")
20    println(s"Status Code: ${resp.statusCode}")
21    println(s"Error Details: ${resp.text()}")
22  case Failure(exception) =>
23    println(s"Request failed: ${exception.getMessage}")

Running this code would produce:

Plain text
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1Failed to add a new todo
2Status Code: 400
3Error Details: {"error":"Invalid request. 'title' is required."}

The 400 status code signifies a bad request, which is often due to missing or incorrect data in the request body. In this case, the error message specifies that the "title" field is required, making it clear what needs to be corrected for the POST request to succeed. This feedback allows you to quickly identify and amend the missing or erroneous part of the data before resending the request.

In this lesson, you have learned how to send data to an API using POST requests. We explored how POST differs from GET in terms of its function of creating new resources. With Requests-Scala in Scala, you are equipped to craft and send POST requests, handle responses using Try, and manage errors to ensure robust API interactions.

As you proceed to the practice exercises, you will have the opportunity to apply everything you have learned here. Practice creating POST requests to reinforce your understanding and discover firsthand the nuances of sending data to an API. Keep up the excellent work, as each lesson brings you closer to mastering API interactions with Scala 3!