How Do Servers Handle Requests? A Comprehensive Guide

Have you ever wondered what happens behind the scenes when you visit a website or send a request to a server? The answer to that is in the intricate process of request handling, where a server receives, processes, and responds to client requests efficiently.

Understanding how servers handle requests provides insight into the efficiency, scalability, and performance of modern web applications.

This article teaches you everything about how servers handle requests and how it is done. So, without further ado, let us get started!

How do Servers Handle Requests?

When a server handles a request, it undergoes several stages, from accepting the incoming connection to processing the request and sending the response.

Each step involves components of the server like the operating system and the network stack.

When a server receives a request (or multiple requests), the process of handling and responding to them depends on several key factors such as the server’s architecture, threading model, and load-balancing mechanisms. 

1. Request Acceptance

Once the TCP connection is established between the server and a client, the server starts accepting the requests.

• Servers typically have a request queue (accept queue) where incoming requests are temporarily held until they can be processed.
• The operating system handles this queue and passes each request to the server’s application when the server is ready to accept it. If the server is overwhelmed or the queue fills up, the server may start rejecting requests or timing out connections.

2. Thread or Event Dispatching

After the server accepts the request, it decides how to allocate resources (threads or event handlers) to process the requests.

2.1. Thread-Per-Request (Blocking I/O)

• Some older servers (e.g. Apache in older configurations) create a new thread for each incoming request.
• The server receives the request, assigns it to the newly created thread, and everything from processing the request to sending the response is handled by it.

Advantages:

• Simple to implement.
• Handles requests in a straightforward, sequential manner.

Disadvantages:

• This approach can cause inefficiency if there are too many requests because creating, managing, and destroying threads requires significant system resources (CPU, memory).
• There is also an overhead from frequent context switching between threads.

2.2. Thread Pooling (Most Common in Modern Servers)

• Instead of creating a new thread for each request, most modern servers use thread pools.
• The server maintains a fixed number of threads in a pool. When a request is received, it is placed in a task queue, and an available worker thread from the pool picks it up for processing.

Advantages:

• Limits the number of threads, reducing resource consumption.
• Threads are reused, so thread creation and destruction overhead is minimized.
• Easy to scale better under heavy loads.

Disadvantage: If all threads are busy, incoming requests must wait in the queue until a thread becomes available.

Example: Most Java-based web servers (e.g., Apache Tomcat) use thread pools to handle HTTP requests efficiently.

2.3. Event-Driven Model (Non-Blocking I/O)

• In this model, the server uses an event loop to handle multiple connections without dedicating one thread per connection.
• When a request comes in, the server registers the request for processing and moves on to handle other tasks without waiting for the request to finish (i.e.non-blocking).
• Once the required I/O (e.g. reading a file or accessing a database) is ready, the server processes the event and sends the response to the client.

Advantages:

• Extremely efficient in handling thousands of concurrent requests.
• Ideal for I/O-bound applications where there are long waiting times (e.g. waiting for a database query).

Disadvantages:

• Requires more complex coding, especially when dealing with state and concurrency.
• Not ideal for CPU-bound tasks unless coupled with worker threads.

Example: Node.js, NGINX use event-driven models.

3. Request Processing

Once a thread (or event handler) picks up the request, the next phase is processing. This can involve several steps, depending on the application.

3.1. Parsing the Request

The server needs to parse the incoming request, which includes:

• Reading the HTTP headers (like Content-Type, Authorization).
• Parsing the request method (e.g. GET, POST, PUT).
• Extracting request parameters (e.g. query strings, body data).
• In a typical web server, this involves decoding HTTP messages, which might include reading JSON payloads or multipart form data.

3.2. Routing the Request

• After parsing the request, the server routes it to the appropriate handler based on the request’s URL and HTTP method.
• The routing logic determines which function or controller to invoke to handle the specific request.
• Example: In a RESTful API, a GET /user request might be routed to a function that retrieves a user’s data from the database.

3.3. Business Logic Execution

This phase involves executing the business logic for the request. In most cases, this is the stage where the server performs CPU or I/O-intensive tasks, such as:

• Accessing databases.
• Performing complex calculations.
• Communicating with external services (APIs, microservices).

4. Generating the Response

Once the request has been processed and the result is ready (e.g. database query results or the rendered HTML page), the server prepares the response to send back to the client.

4.1. Generating the Response Data

This involves formatting the response based on the requested resource and the output format:

• For web servers, this could mean rendering an HTML page, returning JSON data, or sending a file.
• For RESTful APIs, it usually involves serializing data to JSON or XML format.

4.2. Adding HTTP Headers

• The server also prepares HTTP response headers, such as:
  • Content-Type: The type of content (e.g. application/json, text/html).
  • Status Code: Indicates the status of the response.
  • Content-Length: The size of the response body.
• Additional headers like caching directives (Cache-Control), cookies, or security headers (e.g. CORS, HSTS) may also be added.

5. Sending the Response to the Client

Once the response is fully constructed, the server sends the response back to the client

• The response is typically sent over the same TCP connection established during the request.
• In some cases, the server might compress the response data using gzip or another algorithm to reduce bandwidth usage, especially for large payloads like HTML or JSON.

After sending the response:

• In a blocking model (thread-per-request), the thread finishes its work and terminates (or goes back to the pool in the case of thread pooling).
• In an event-driven model, the server registers the completion of the event and continues to handle other requests.

6. Handling Multiple Concurrent Requests

When a server needs to handle multiple requests concurrently, it utilizes the following mechanisms:

6.1. Thread Pooling

• A fixed number of threads (or worker processes) handle multiple requests in parallel.
• Each thread or process picks up a task (request) from a queue, processes it, and then moves to the next one.
• The size of the thread pool can often be dynamically adjusted based on the server load.
• This model ensures that the server can handle many concurrent connections without creating excessive threads, which would otherwise overwhelm the system.

6.2. Non-blocking I/O

• Servers using an event-driven model can handle thousands of connections simultaneously with just a few threads by using non-blocking I/O. Instead of waiting for each request to finish, the server handles other tasks while waiting for I/O.
• This approach is extremely efficient for I/O-bound applications, where most of the time is spent waiting for external data.

6.3. Load Balancing

• In large-scale applications, a load balancer distributes incoming requests across multiple servers. Each server processes a portion of the total requests, allowing the application to scale horizontally.
• If one server is overloaded or fails, the load balancer redirects traffic to other available servers.

7. Closing the Connection

After the response is sent, the server typically:

• Closes the connection: For most HTTP/1.0 and older HTTP/1.1 configurations, the server closed the connection after sending the response.
• Keeps the connection alive: Modern servers often use persistent connections (via the Connection: keep-alive header). This allows the same connection to handle multiple requests from the same client, reducing the overhead of establishing a new TCP connection for each request.

Conclusion

In conclusion, Servers play a crucial role in ensuring seamless communication between clients and web applications by efficiently managing incoming requests.

From accepting connections and dispatching threads to processing data and sending responses, each step in the request-handling pipeline contributes to overall system performance.

A well-optimized server not only improves user experience but also ensures reliability and efficiency in handling concurrent requests in real-world applications.