Efficient Client-Server Communication: An Overview of Protocols and Techniques
Exploring HTTP, Polling, Webhooks, SSE, and WebSockets in the client-server model
Introduction:
In the world of web development and real-time communication, understanding various methods of data exchange is crucial.
Modern web applications require efficient and seamless communication between the client and the server to deliver dynamic content and provide real-time updates.
In this article, we will delve into the diverse protocols and techniques employed in client-server communication, enabling developers to make informed decisions when implementing these interactions.
Table of contents:
Let’s start with the simple HTTP
protocol. It is a widely used protocol in today’s world, and to go further, we need to understand it because all the examples in this article are based on it.
HTTP
The Hypertext Transfer Protocol (HTTP
) is a half-duplex protocol used for communication between web browsers (clients) and web servers. It serves as the foundation for data exchange on the World Wide Web.
HTTP
is a protocol that operates over the TCP
¹ (Transmission Control Protocol) transport layer protocol. By default, it uses port 80
for unencrypted connections and port 443
for encrypted connections (using HTTPS
).
HTTP
— is a classic example of synchronous client-server interaction, when the client initiates a request, waiting for the result, and the server responds with a corresponding response.
Key components:
- Uniform Resource Identifier (URI): The client includes a URI in the request to specify the desired resource. The
URI
consists of aURL
(Uniform Resource Locator) orURN
(Uniform Resource Name). - HTTP Methods:
HTTP
defines several methods (also known as verbs) that specify the desired action to be performed on the server. The most commonly used methods are:
GET
: Retrieves a representation of a resource.POST
: Submits data to be processed by the server, often used for form submissions.PUT
: Updates or replaces a resource with the provided data.DELETE
: Removes a specified resource.
4. Headers:
- The client can include additional headers in the request to provide information to the server. Headers can specify things like the client’s user agent, accepted content types, authentication credentials, and more. Example:
User-Agent: Mozilla/5.0
,Accept: text/html
- The server includes headers in the response to provide information back to the client. Headers can specify things like the content type of the response, caching directives, cookies, and more. Example:
Content-Type: application/json
,Cache-Control: max-age=3600
5. Body: For methods like POST
and PUT
, the client can include a request body that contains additional data to be sent to the server. The format of the request body depends on the data being transmitted (e.g., form data
, JSON
payload). The response body contains the actual data returned by the server, format depends on the requested resource.
Structure of HTTP request:
A request typically includes some form of input or data payload that is needed by the server to perform the requested action.
POST /api/v1/users HTTP/1.1
Host: example.com
Content-Type: application/json
Authorization:Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c
{
"name": "Alice",
"email": "alice@example.com",
"password": "secret123"
}
In this example, the request uses the HTTP POST
method to create a new user resource on the server. The URI
or endpoint is /api/v1/users
, and the request includes several headers, such as Content-Type
and Authorization
. Finally, the request body includes a JSON
object with data for the new user, including their name, email, and password.
The response would look like this:
HTTP/1.1 201 Created
Content-Type: application/json
{
"id": "1234567890",
"name": "Alice",
"email": "alice@example.com",
"createdAt": "2023-05-25T10:15:30Z"
}
There are situations where the client needs to know the status of available data, for example, to display it on a web page. To do this, the client will constantly ask for new data. Such a technique is called polling.
Polling is a method in which the client repeatedly asks the server for new data. There are two types of polling: short polling and long polling.
Short polling
In a short polling
(a.k.a. AJAX
polling), a client requests data from the server at regular intervals, and if it is available, the server returns a response, otherwise it returns an empty response.
In essence, this is a simple HTTP
request, which is called repeatedly.
Process of communication:
- Client request. The client makes a request to the server.
- Server response. The server responds either with the data itself or an empty response.
- Repeat the action. Once the client receives a response from the server, it will wait for the specified interval and repeat the previous actions.
Pros & cons:
Pros of Short Polling:
- Simplicity:
Short polling
is straightforward to implement, requiring minimal server-side setup. It involves making periodicHTTP
requests from the client to the server at regular intervals. - Compatibility:
Short polling
works well with existingHTTP
infrastructure and is compatible with most web servers and browsers. It can be easily implemented using standardAJAX
techniques. - Low server overhead:
Short polling
does not require a persistent connection, which means the server does not need to maintain and manage numerous open connections. This can help reduce server load and resource consumption.
Cons of Short Polling:
- Increased network traffic:
Short polling
involves making frequentHTTP
requests even when there is no new data available. This can result in increased network traffic, especially in scenarios with high client concurrency, leading to unnecessary bandwidth usage. - Latency and delay:
Short polling
introduces inherent latency and delay because the client must wait for each request to complete before sending the next one. This can result in a suboptimal real-time experience, as there can be a noticeable delay between data updates. - Server load and scalability: As the number of clients increases,
short polling
can put a significant load on the server due to the frequent requests it generates. Handling a large number of concurrent requests can affect the scalability and performance of the server.
Long polling
With long polling
, the client also repeatedly requests data from the server. But compared to short polling, the server is responsible for the waiting part.
Process of communication:
- Client request. The client makes a request to the server.
- Server response. If the data is available on the server, it responds with the data itself, otherwise the server keeps the connection until the data is available.
- Repeat the action. Once the client receives a response from the server, it makes a new request without waiting.
Pros & cons:
Pros of Long Polling:
- Reduced network traffic:
Long polling
helps minimize unnecessary network traffic compared toshort polling
. Instead of making frequent requests, the server holds the connection open until there is new data to send. This reduces the number of requests made by the client, resulting in lower bandwidth usage. - Near real-time updates:
Long polling
allows for relatively immediate updates as compared to traditional polling methods. The server can hold the connection open until new data is available, and once it is, the response is sent to the client immediately. This enables near real-time updates and a more responsive user experience. - Improved server efficiency:
Long polling
reduces the load on the server by avoiding the need for constant polling. The server only needs to respond when new data is available, leading to improved resource utilization and better server efficiency. - Compatibility:
Long polling
can be implemented using standardHTTP
requests and doesn’t require any specialized protocols or infrastructure. It can work with most web servers and browsers without any additional setup.
Cons of Long Polling:
- Scalability Challenges:
Long polling
can pose scalability challenges for highly concurrent applications. Eachlong-polling
connection consumes server resources, and handling a large number of concurrent connections can impact the server’s ability to scale effectively. - Timeout issues:
Long-polling
requests are typically set with a timeout value (time for request). If new data is not received within the specified timeout period, the server sends a response indicating no updates. This introduces the possibility of timeouts occurring and the need for the client to re-establish the connection, potentially leading to delays in receiving updates. - Complex implementation: Implementing
long polling
requires more complexity than traditional short polling. It involves managing open connections, timeouts, and handling potential edge cases related to network connectivity and server resource management. - Increased server overhead: While
long polling
reduces the number of requests made by the client, it can increase the server’s overhead compared to short polling. Holding connections open for an extended period can consume server resources, especially when there are manylong-polling
clients.
Next, let’s move from request/response exchange to event exchange. Webhook
, Server-Sent Events
, and WebSocket
are primarily designed for event-driven communication.
Instead of constantly requesting information or triggering actions, these techniques allow applications to be notified or listen to specific events.
An event is a message or notification that indicates that something has happened in the system. An event can represent any occurrence that might be of interest to other parts of the system, such as a user action, a sensor reading, or a change in the state of an object.
Webhook
Webhook
is a method of communication that allows one application to automatically send data or trigger events to another application in near real-time. It provides easy integration and allows one system to notify or pass information to another system based on specific events or triggers.
You can think about Webhook
as a user-defined callback over HTTP
.
One important feature of Webhook
is the “reverse API”. You don’t have to constantly poll to see if the new information is ready or not. Instead, you register an HTTP
call to a specific URI
, and when the new information is ready on the server side, it will call your URI
and thus notify you.
Process of communication:
- Configuration: The system that generates the Webhook provides a configuration mechanism to specify the endpoint or
URL
of the receiving system. This endpoint is where theWebhook
data will be sent. - Event triggering: When a predefined event or trigger occurs in the system generating the Webhook, it initiates the Webhook process.
- HTTP request: The system generating the
Webhook
sends anHTTP
request to the specified endpoint (usuallyGET
orPOST
), containing relevant data or payload related to the event. - Receiving and processing: The receiving system, which acts as the Webhook listener or endpoint, receives the
HTTP
request. It extracts the data from the payload and processes it according to its requirements. This may involve updating databases, triggering actions, or any other desired operation.
Usually, by a client, we mean another system, such as a web server, because we can’t expose the URL
in an isolated browser environment.
Examples of Webhook
:
- Payment Notifications: Many payment gateways and platforms utilize Webhooks to provide real-time notifications of payment events. For example, when a successful payment is made, or a refund is issued, the payment gateway can send a
Webhook
to the merchant’s system, allowing them to update their records, trigger order fulfillment processes, or send confirmation emails to customers. - Notifications and Alerting: For example, a messaging or collaboration platform may use Webhooks to deliver notifications to external systems or applications. This could include notifying a chatbot about new messages, updating external dashboards with new metrics, or sending alerts to a monitoring system when certain conditions are met.
In Webhook
, we solve the problem of additional resources used in polling by allowing the event provider to notify the client side (reverse API). But that may not be enough when it comes to real-time communication. SSE
and Websocket
fix this problem.
Server-sent events
Server-Sent Events (SSE): SSE
is a one-way, server-to-client communication technique where the server sends continuous streams of data to the client over a single HTTP
connection.
SSE
is commonly used in real-time updates for web applications such as social media feeds and stock market tickers.
SSE
follows an event-driven model, where the server sends a stream of events to the client as they occur. This allows asynchronous communication.
The server initiates the SSE
connection by responding to the client’s initial request with a specific content type text/event-stream
indicating that it will be sending a continuous stream of events.
Structure of event:
Here’s an example of a complete SSE
structure of an event with event type, event data, event ID, and event retry fields:
event: eventType
data: This is the event data
id: eventID12345
retry: 5000
Description of fields:
- event: The event type provides additional context. Default values are
message, open, error
. The server may specify a custom type of event. - data: The data field for the message. It can be
plain text
,JSON
, or any other string type. - id: Event identifier, allowing clients to keep track of the order and uniqueness of events.
- retry: Reconnection time interval in milliseconds used in case of connection loss. The browser will take and wait for the specified time before trying to reconnect.
All other fields will be ignored.
Process of communication:
- Client-Side EventSource: The client establishes an
EventSource
object in JavaScript, which serves as the interface for receiving server-sent events. The client typically creates this object by specifying theURL
of theSSE
endpoint. - SSE Endpoint: On the server side, a dedicated endpoint is set up to handle
SSE
requests. When the client establishes a connection to this endpoint, the server keeps the connection open and begins sending events as they occur. - Event Stream: The server sends events to the client as text-based
UTF-8
encoded messages within theSSE
stream. Each event is encapsulated in a specific format, consisting of one or more lines. The minimum requirement for an event is to include a “data” field, but additional fields like “event” (for specifying the event type) and “id” (for assigning an event identifier) can also be included. - Client Event Handling: As events are received by the client, the
EventSource
object triggers the appropriate event handlers in JavaScript, allowing the application to process the received data. The client can react to events, update the user interface, or perform any desired actions based on the received information. - Closing the connection: If the connection is closed, the client will automatically reconnect.
- If the server wants the browser to stop reconnecting, it should respond with
HTTP
status204
. - If the browser wants to close the connection, it needs to call the
.close()
method in theEventSource
instance. - In addition, there will be no reconnection if the response contains a
Content-Type
different fromtext/event-stream
SSE
offers several advantages over other real-time communication techniques.
- It utilizes a standard
HTTP
connection, making it widely compatible with existing infrastructure and browsers. - It provides a lightweight and efficient approach, as the connection remains open for as long as necessary, eliminating the need for continuous request-response cycles.
SSE
is particularly suitable for scenarios where the client only consumes a continuous stream of updates, such as stock tickers, social media feeds, or real-time monitoring data.
Although SSE
can be used for most requirements, its main drawback is one-way communication. Next, we will look at a protocol that allows two-way communication in real-time.
WebSocket
WebSocket is a protocol that enables real-time, two-way, full-duplex communication between client and server over a single, long-lived connection.
It utilizes the HTTP
protocol to establish a connection. WebSocket
can be used for various real-time applications, such as chat applications, real-time gaming, and live streaming.
WebSocket
, like SSE
, provides a persistent connection. But in contrast to SSE
, it enables full-duplex communication, which means that both client and server can communicate over the same channel. This allows you to implement synchronous and asynchronous communication in real-time.
Structure of message:
In the WebSocket
protocol, the information transmitted is called a message.
Once the WebSocket
connection is established, the communication thereafter operates using WebSocket
frames. WebSocket frames contain the actual data being transmitted and have their own structure, separate from the initial human-readable handshake.
WebSocket
frames include control frames (e.g., connection close
, ping
, pong
) and data frames (e.g., text
or binary
messages).
The actual WebSocket frame looks like this:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-------+-+-------------+-------------------------------+
|F|R|R|R| opcode|M| Payload len | Extended payload length |
|I|S|S|S| (4) |A| (7) | (16/64) |
|N|V|V|V| |S| | (if payload len==126/127) |
| |1|2|3| |K| | |
+-+-+-+-+-------+-+-------------+ - - - - - - - - - - - - - - - +
| Extended payload length continued, if payload len == 127 |
+ - - - - - - - - - - - - - - - +-------------------------------+
| |Masking-key, if MASK set to 1 |
+-------------------------------+-------------------------------+
| Masking-key (continued) | Payload Data |
+-------------------------------- - - - - - - - - - - - - - - - +
: Payload Data continued ... :
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
| Payload Data continued ... |
+---------------------------------------------------------------+
Process of communication:
1. Handshake: The WebSocket
communication begins with a handshake process.
1.1 The client sends an initial HTTP
request to the server, specifying the WebSocket
protocol by upgrading the connection. This request includes a special header called “Upgrade” with the value “websocket” (Upgrade: websocket
) and a “Connection” header set to “Upgrade” (Connection: Upgrade
).
1.2 The server supporting ws
protocol responds with the same headers and HTTP 101
status code, indicating a successful protocol upgrade.
Once the handshake is complete, the WebSocket connection is established. The WebSocket
protocol utilizes a specific URL
format, typically starting with ws://
for unencrypted connections or wss://
for encrypted (secured) connections, similar to HTTP
and HTTPS
protocols.
2. Data Exchange: After the connection is established, the client and the server can exchange data in a full-duplex manner. Each party can send messages to the other party at any time without the need for explicit request-response cycles. Messages can be of various types, including text
, binary data
, or even structured data like JSON
.
3. Connection Termination: The WebSocket
connection can be terminated explicitly by either the client or the server, or it can be closed unexpectedly due to network issues or server-side errors. To close the connection, either party sends a close frame, indicating the intention to terminate the connection. The other party responds with a close frame as well, acknowledging the closure. Both parties then perform cleanup operations and release associated resources.
When dealing with complex applications built on the WebSocket protocol, it is common to utilize an additional protocol layered on top of it. This additional protocol allows for the definition of event structures, such as a routing system, metadata, and data format.
Unlike HTTP
, defining a route in a URI
of a path is not a good idea, because WebSocket
has a persistent connection, and creating another one will increase the use of system resources and just take away all benefits of this protocol (for example if we only open and close connections for one request).
Instead it is better to use the routing mechanism in the event itself. This way we keep one connection and can access to listen and send notifications for different events. By default, WebSocket
doesn’t have a routing mechanism and only support open
, message
, error
, close
events.
For example, we can define the following structure:
{
"route": "/messages/create",
"payload": {
"sender": "John",
"content": "Hello, everyone!"
}
}
Where the “route” field specifies the destination or purpose of the event and the “payload” field contains the actual data associated with the event. And on the server side we can parse new input messages to determine how to handle the event.
But there are also ready-made solutions. One of them is the Socket.IO protocol.
Conclusion
In this article we took an overview of popular techniques and protocols used in client-server communication. Understanding these methods of data exchange is essential for creating efficient and interactive client-server communication systems.
We began by delving into the foundation of web communication, the HTTP
protocol, which governs the exchange of requests and responses between clients and servers. We learned about the structure of HTTP
messages, including headers, methods, and the transmission of data.
Next, we examined different types of polling, namely short polling
and long polling
. Short polling
involves frequent requests from clients, while long polling enables near real-time updates by maintaining connections until new data becomes available. We discussed the advantages and considerations associated with each approach.
Moving on, we explored the concept of Webhooks
, which allows servers to notify other systems through HTTP
calls. Webhooks
enable seamless integration and event-driven communication, making them invaluable in various scenarios such as automated notifications and triggering actions.
We also covered Server-Sent Events
(SSE), which provides a unidirectional channel for real-time data streaming from servers to clients. SSE
facilitates the delivery of continuous updates, making it an effective solution for applications requiring live data feeds or dynamic content.
Lastly, we examined WebSocket
, a bidirectional communication protocol that enables full-duplex, real-time interaction between clients and servers. WebSocket
is great for scenarios where instant data exchange, push notifications, and collaboration features are required.
By understanding these protocols and techniques, developers can choose the most suitable method for their specific requirements. Whether it’s the simplicity of HTTP
, the responsiveness of long polling
, the event-driven nature of Webhooks
, the continuous updates of SSE
, or the bidirectional communication of WebSockets
, each approach offers unique benefits and use cases.
Notes
HTTP/3
does not use aTCP
connection, but instead,QUIC
overUDP
is used.
Thank you for taking the time to read this article. I appreciate your feedback and would love to hear your thoughts in the comments below.