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In web development, your application constantly handles incoming requests from clients and sends back appropriate responses. This lesson teaches you how to read data from a client's request and construct different types of responses, like HTML and JSON, using Clojure's Ring.

When a request hits your Clojure application, Ring transforms it into a standard Clojure map, often called the request map. This map contains all details about the incoming request, such as:

• :uri: The path of the request (e.g., "/users")
• :request-method: The HTTP method (e.g., :get, :post)
• :headers: A map of all HTTP headers
• :query-params: A map of URL query parameters
• :body: The request body, if any

Clients often send data via URL query parameters (e.g., /search?q=clojure) or HTML form parameters. Ring provides these parsed in the :query-params and :form-params keys of the request map, respectively. You can access them like any other map entry.

Try running this example to see how to extract a query parameter. We'll simulate a request map for a handler function.

When clients send data in the request body (e.g., for POST or PUT requests), this data is available in the :body key of the request map. For common formats like JSON, you'll typically use a Ring middleware to parse the raw body into a Clojure map automatically. Without middleware, :body contains an InputStream.

After processing a request, your handler function must return a response map. This map tells Ring how to construct the HTTP response sent back to the client. A minimal response map needs:

• :status: The HTTP status code (e.g., 200 for OK, 404 for Not Found)
• :headers: A map of HTTP headers (e.g., {"Content-Type" "text/html"})
• :body: The actual content to send back (e.g., HTML string, JSON string)

To send an HTML page, your response map's :body should be an HTML string, and the "Content-Type" header must be set to "text/html". Ring's response helper function is a great starting point, and header helps add headers.

Here's how to create a simple handler that responds with an HTML page. Note the use of header to set the correct content type.

For API endpoints, you'll often send data back as JSON. This requires setting the "Content-Type" header to "application/json" and ensuring your :body contains a valid JSON string. You'll typically use a library like clojure.data.json or cheshire to convert Clojure data structures into JSON strings.

This example shows how to send a JSON response. We use clojure.data.json to convert a Clojure map to a JSON string.

You're building an API and need to send a successful response with a JSON object {"message": "Item created"}. Which of the following correctly represents the Ring response map?

You've learned how to handle the core of web interactions in Clojure! We covered:

• The structure of the Ring request map and how to access its data.
• Extracting query and form parameters from requests.
• Understanding how request bodies (like JSON) are handled.
• Constructing Ring response maps with :status, :headers, and :body.
• Sending back both HTML and JSON content by setting the appropriate Content-Type header.

These are fundamental skills for building any web application with Ring and Compojure!