Backend Communication Design Patterns: Request-Response Pattern

Introduction

Building software applications requires seamless communication between different systems components and services. In software engineering, this harmony is often achieved through various messaging patterns, and one of the classic patterns used is the "Request-Response" pattern. Software components use this pattern to request things and get swift, meaningful responses in return.

In this article, we're about to embark on an interesting adventure into the heart of the Request-Response messaging pattern. We will peel back the layers, uncover its superpowers, and see how it makes real-world software tick. Whether you're a seasoned developer or just starting your coding career, grasping this pattern is like wielding a magic wand to craft responsive and dependable software.

So, get ready to join us as we explore the fascinating world of this classic messaging pattern!

What is a request-response pattern?

This is one of the simplest methods computers use in communicating with each other over a network. In this pattern, one entity typically called the "client" sends a request to another entity, known as the "server", and the server processes the request and sends back a response.
Simply put, in a request/response pattern, a request is sent to the backend, the backend will process the request and it responds with data. This pattern is widely used in various applications like remote procedure calls (RPCs), secure shell (SSH), web services and client-server interactions.

Real-world Analogy

Imagine you are at a fast food and want to place an order for a quick meal. You, being the customer (client), approach the waiter (protocol) and request the dish you want from the chef (server). The waiter takes your order and goes to the kitchen. The chef then goes in and processes your order. After a while, the waiter returns with your meal, which is the response to your request. This process is similar to the request-response pattern in computing:

Client: you, the customer, start the communication by making a specific request (ordering the dish).
Server: the chef, acting as a server, receives your request, processes it (prepares the order), and responds by bringing your order through the waiter.

In this analogy, the client explicitly asks for something, and the server provides a tailored response, just as in the request-response messaging pattern, where a client sends a request, and a server sends back a response based on that request.

What happens in a request-response pattern?

In this section, we will talk about the key components of the request-response pattern, the step-by-step process of a request-response pattern, and the role of synchronous communication in the request-response pattern.

Key Components in a request-response pattern:

Sender: The sender, often referred to as the client, is the component that initiates the communication by sending a request. In the context of web applications, for example, a user's browser can be the sender when it requests data from a server.
Receiver: The receiver, usually called the server, is the component that receives the request, processes it, and sends back a response. In web applications, the server handles the client request and provides the necessary data or performs the requested action.
Request: The request is a message or packet of data sent by the sender to the receiver. A request contains information about the action or data the sender is requesting from the receiver. Requests can take various forms such as function calls in remote procedure calls (RPC), SQL queries in database interactions or HTTP requests in web applications.
Response: The response is the message or data sent by the receiver back to the sender in reply to the request. It contains the requested information or indicates the outcome of the requested action. Responses can be in the form of database queries, RPC responses or HTTP responses.

Step-by-Step Process of a Request-Response Pattern:

Sender sends a request: the sender (client) initiates the interaction by sending a request to the receiver (server). The client needs to define the request and the server needs to understand the request too. The request specifies the action or data it needs.
The receiver receives the request: the receiver (server) receives the request and processes it. It interprets the request to determine what action to take or what data to receive.
The receiver parses the request: here, the receiver (server) tries to understand the request that has been sent.
The receiver processes the request: here, the receiver (server) executes the request. This may be confusing but there is a difference between knowing the request is a GET request and executing the request to fetch data from an API or query the database is another thing.
The receiver generates a response: after processing, the receiver (server) generates a response based on the request. The response contains the requested data or indicates the result of the action.
The receiver sends a response: the receiver (server) sends the response back to the sender (client).
The sender receives the response: the sender (client) receives the response and can now use the data or information provided in the response for further actions or display to the users.

t0: The client finished writing the request. The network takes time to transfer the request.
t4: The server receives the request and understands it.
t4-t50: The server processes the request.
t50: the server starts writing a response.
t54: The client receives the response.

The Role of Synchronous Communication in the Request-Response Pattern

Synchronous communication in the request-response pattern means that the sender (client) typically waits for the response from the receiver (server) before proceeding with other actions. It's like making a phone call where you pause and listen for the other person to respond before continuing the conversation.

The synchronous nature ensures that the sender has access to the most up-to-date information or the result of an action, making it suitable for scenarios where immediate and specific feedback is required. For example, in a web application, when you click a link to view your bank account balance, you expect the page to wait for the response (balance data) before displaying it to you.

Synchronous communication ensures that the sender and receiver are in sync and that the sender can act on the response once it's received. However, it can also introduce potential delays if the processing on the server side takes a significant amount of time, which is why asynchronous communication patterns might be preferred in some cases where responsiveness and scalability are crucial.

The Anatomy of an HTTP Request-Response

When working with Web APIs, the communication between our clients and our API will be done using HTTP requests.

Generally, an HTTP request is divided into 3 parts:

A request line.
A set of HTTP headers or header fields.
A message body, if needed.

Request line

This contains the HTTP method used which indicates the type of action the client wants to perform, the URI or path of the request and the version of HTTP being used.

An example of a request line is:

POST /auth/login HTTP/1.1

The HTTP method here is POST meaning the client wants to send some data to the server, the URI or path is /auth/login, this helps in identifying the resource on the server and the HTTP version number is HTTP/1.1. By the way, this request line may contain additional items like a query string. A query string provides some information that the resource can use for some purpose.

HTTP headers

Headers are metadata written on a message to provide the recipient with information about the request being sent, the sender and how the sender (client) wants to communicate with the recipient (server). Each HTTP header is made up of a name and a value.

An example of an HTTP header:

Accept-Language: fr, en-US
Content-Type: application/json

The Accept-Language header indicates that the client only wants to read the requested document in a specified language. In this case, the header is Accept-Language and the value is fr, en-US (French or English respectively) while the header Content-Type header indicates the type of data that is sent and in this case, the client is sending a JSON to the server. A single request may have multiple headers.

An empty line also known as the Carriage Return Line Feed (CRLF) is used after a series of headers to divide the headers from the message body.

Message body

Simply put, this is where we put additional information that we are going to send to the server. This can contain anything we want to send to the server. For example, a username and password or a file of some sort.

An example of a message body:

{
    "email": "someone@example.com",
    "password": "Password@1."
}

An example of an HTTP request with its 3 components:

POST /auth/login HTTP/1.1
Host: someapi.com
Content-Type: application/json

{
    "email": "someone@example.com",
    "password": "Password@1."
}

The request we have above is a POST request towards /auth/login. We have 2 headers: Host and Content-Type. Finally, we have the message body of the request. Take note of the blank line before the body. That is the CRLF.

HTTP response

Knowing that the client sends an HTTP request, the server has to respond with an HTTP response. The HTTP response has a structure and it looks like the structure of an HTTP request. A typical HTTP response contains these parts:

A status line
A series of HTTP headers, or header fields.
A message body. This is usually needed.

Status line

This is the first line in the HTTP response and it consists of three items:

The HTTP version number.
A status code. This is often a three-digit number used to indicate the result of the request.
A status text. This is also known as a reason phrase and it is a human-readable text that tells us in a summary, the meaning of the status code.

An example of a status line is:

HTTP/1.1 200 OK

In this example, the HTTP version is HTTP/1.1, the status code is 200 and the status text is OK.

HTTP headers

The HTTP headers are a set of headers just like the HTTP request headers and the server can send us as many headers as it wants.

An empty line also known as the Carriage Return Line Feed (CRLF) is used after a series of headers to divide the headers from the message body.

Message body

A message body or response body is used when responding to a client's request. Sometimes, the server doesn't respond with a response body. For example, when a request is made using the HEAD method (this asks for just the header but not the body of the response), and where the server is using certain status codes, e.g. 204.

For a response to an unsuccessful request, the message body may provide further information about the cause of the error, or about some things the clients need to take to complete the request successfully. For a response to a successful request, the message body may contain either the data requested by the client or some information about the status of the action.

An example of an HTTP response with its 3 components:

HTTP/1.1 200 OK
Server: nginx/1.18.0 (Ubuntu)
Date: Fri, 22 Sep 2023 13:19:02 GMT
X-Powered-By: Express
Access-Control-Allow-Origin: https://<somedomain>.com
Vary: Origin
Content-Type: application/json; charset=utf-8
Content-Length: 1082

We have the status line in the first line, the status code was 200 OK. Then we have other headers from the response.

Benefits of Using Request-Response Pattern

Explicit Communication: Request-response provides a clear means of communication between different components in a software system. The sender explicitly requests specific data or actions, and the receiver provides a direct response, enhancing the clarity of communication.
Reliability: Request-response interactions work well in scenarios where reliability is crucial. In a nutshell, the sender knows when to expect a response, and the synchronous nature of the communication ensures that the response is received and processed predictably.
Data consistency: The sender can wait for confirmation (response) that an action has been successfully executed before proceeding. This reduces the risk of data inconsistencies. Request-response can help ensure that actions are performed in a synchronized manner.
Error Handling: Request-response patterns facilitate error handling by providing a clear mechanism for reporting errors or exceptions. The response can include status codes or error messages that convey information about the success or failure of the requested operation, allowing the sender to take appropriate action. For instance, in web applications, HTTP status codes (e.g., 404 for "Not Found" or 500 for "Internal Server Error") are part of the request-response mechanism and help with error handling.
Security: Request-response can be used to implement secure communication patterns. For instance, authentication and authorization checks can be performed before processing requests, ensuring that only authorized clients can access certain services or data.
Fault Tolerance: Request-response interactions allow for easier fault detection and handling. If the sender does not receive a response within a reasonable timeframe, it can implement retry mechanisms or handle the error gracefully, ensuring that the system remains robust even in the face of temporary failures.

Where is the request-response pattern used?

Web Applications: In a web application, when a user submits a form, the server uses a request-response pattern to process the form data and provide a response, such as displaying a confirmation message or showing validation errors.
Remote Procedure Calls (RPC): RPC systems often use request-response to invoke methods on remote servers or services and receive results. For instance, a client application may call a remote procedure to perform a calculation and receive the computed result.
Database Queries: When querying a database, request-response ensures that the client receives the requested data or an error message if the query is invalid or encounters issues.
Messaging Services: In messaging systems, request-response can be used to request specific information from message queues or topics, ensuring that the sender gets the desired data in response to its request.
IoT Devices: Internet of Things (IoT) devices often use request-response to communicate with cloud services or other devices. For instance, a smart thermostat may request weather information from a cloud server to optimize heating and cooling.

Problems with request-response pattern

Synchronous Nature: The synchronous nature of request-response can lead to potential bottlenecks if not managed properly. Clients might wait indefinitely for a response, causing performance issues.
Timeouts: Setting appropriate timeout values is crucial. If timeouts are too short, legitimate responses may be prematurely considered failures. If timeouts are too long, it can lead to a slow system response.
Reliability on the Receiver: If the receiver (server) becomes unavailable or experiences a high load, it can result in delayed or failed responses, affecting system reliability.
Network Latency: Variability in network latency can impact response times. Long network delays can make synchronous request-response interactions slow and less predictable.
Error Handling: Handling errors in a request-response system requires careful consideration. Errors in the request may not always result in clear responses and error codes or messages must be properly defined and communicated.

Best Practices for designing robust request-response systems

Use Asynchronous When Appropriate: Consider using asynchronous patterns when immediate responses are not critical. This can improve system scalability and responsiveness.
Set Realistic Timeouts: Define appropriate timeout values based on expected response times and the importance of timely responses. Implement mechanisms to handle timeouts gracefully.
Implement Retry Mechanisms: Incorporate retry logic on the client side to account for transient failures. Ensure that retries are performed with back-off strategies to avoid overwhelming the server.
Load Balancing: Distribute incoming requests among multiple server instances using load balancers to ensure even load distribution and fault tolerance.
Throttling and Rate Limiting: Implement throttling and rate limiting mechanisms to prevent overloading the server with too many requests from a single client or source.
Circuit Breaker Pattern: Implement the circuit breaker pattern to temporarily stop making requests to a service that is consistently failing, allowing it time to recover.
Monitoring and Logging: Use comprehensive monitoring and logging to track request-response interactions, identify performance bottlenecks, and troubleshoot issues quickly.

Some Alternatives and when not to use request-response pattern

Publish-Subscribe (Pub-Sub) pattern: In the publish-subscribe pattern, multiple entities (subscribers) express interest in specific topics or events. Publishers broadcast messages to all interested subscribers without direct requests from the subscribers. Subscribers receive messages based on their stated interests.
Message Queues: Message queues involve sending messages to a queue, where they are stored until they are consumed by one or more consumers. This pattern decouples the sender from the receiver, allowing for asynchronous communication. Common message queue technologies include RabbitMQ, Apache Kafka, and Amazon SQS.

When to use alternative messaging patterns

Publish-Subscribe (Pub-Sub):
- Scalability and Real-time Updates: Pub-Sub is suitable when you need to broadcast real-time updates to multiple subscribers. It's often used in chat applications, news feeds, or systems where many clients want to receive the same information simultaneously.
- Loose Coupling: When you want to achieve loose coupling between publishers and subscribers, enabling them to evolve independently. Subscribers don't need to know about or explicitly request updates from publishers.
Message Queues:
- Asynchronous Processing: When you need asynchronous processing of tasks or messages. Message queues are great for handling background jobs, task queues, and event-driven architecture where different parts of the system can operate independently and at their own pace.
- Load Leveling: In scenarios where you want to balance the load between producers and consumers, message queues can help buffer requests and prevent overloading downstream services.

When Request-Response May Not Be the Best Choice

High Concurrency and Scalability Requirements: In situations where you require high concurrency and scalability, request-response may not be the most efficient choice. Asynchronous patterns like publish-subscribe and message queues can better handle a large number of simultaneous clients or tasks.
Real-time Streaming: When you need real-time streaming of data to multiple clients, such as live updates in online gaming or financial trading, the publish-subscribe pattern is often more appropriate as it doesn't involve explicit requests and allows for immediate broadcast.
Loose Coupling and Decoupling: In cases where you want to minimize dependencies between components and promote loose coupling, request-response may not be the best choice. Publish-subscribe and message queues facilitate decoupling, allowing components to evolve independently without knowledge of one another.
Batch Processing and Background Jobs: For scenarios involving batch processing, periodic tasks, or background jobs, asynchronous message queue systems are generally preferred. These patterns allow for efficient handling of tasks without blocking the main application flow.
Fire-and-Forget Communication: When the sender doesn't require a direct response or immediate feedback, using a publish-subscribe or message queue pattern can simplify communication by eliminating the need for a synchronous request and response.

Conclusion

In the ever-evolving landscape of software development, effective communication between different components and services is the backbone of success. Among the array of messaging patterns at our disposal, the "Request-Response" pattern shines as a classic and reliable choice. In this journey through the intricacies of the Request-Response pattern, we've delved into its inner workings, its real-world applications, and the immense value it brings to the development process.

From its key components to step-by-step processes, the request-response pattern plays a vital role in ensuring seamless communication. It provides a structured means of interaction, enhancing clarity and reliability. While it excels in various scenarios, we've also explored its limitations and learned when alternative patterns like publish-subscribe and message queues may be more suitable. By embracing best practices, such as setting realistic timeouts and implementing retry mechanisms, you can ensure that request-response interactions remain dependable, even in challenging situations.

In the ever-evolving landscape of software development, the Request-Response pattern remains a steadfast ally. By understanding when and how to leverage this classic pattern, you'll be well-equipped to navigate the intricate world of software development, making it a valuable tool in your arsenal for building robust and efficient software systems. So, armed with this knowledge, venture forth into the world of software development, and let the Request-Response pattern be your trusty guide to success.

Thank you :D