There are many ways to implement message push, such as:

• Short Polling
• Long Polling
• WebSocket
• Server-Sent Events (SSE)
• HTTP/2 Server Push
• MQTT Protocol
• Third-Party Push Services

This article will discuss when to choose which solution so that you won’t feel overwhelmed by too many options.

Short Polling

• Network Resources: Short polling generates a large number of network requests, especially when the client’s polling interval is very short. This can lead to significant network overhead.
• Server Processing: The server needs to process each polling request and send a response, even if there is no new data. This frequent request processing can increase CPU and memory usage.
• Summary: If updates are infrequent but clients continue to send requests frequently, short polling can waste resources because most responses may simply indicate "no new data."

Long Polling

The client sends a request to the server, and if the server does not have new data ready, it keeps the connection open until new data is available. Once the data is sent, the client processes it and immediately sends a new request, repeating this cycle.

Long polling is commonly used for real-time or near-real-time notifications and updates, such as event notifications.

• Network Resources: Compared to short polling, long polling reduces unnecessary network requests. The server only responds when new data is available, reducing network traffic.
• Server Processing: Long polling requires the server to maintain more open connections because it keeps each client request open until new data arrives or times out. This can increase memory usage and may reach the server's concurrent connection limits.
• Summary: Long polling can provide more efficient resource usage in some scenarios, especially when updates are infrequent but need to be quickly delivered to clients. However, if many clients are using long polling simultaneously, the server may need to handle a large number of concurrent open connections.

WebSocket

WebSocket provides a full-duplex communication channel, allowing both the server and client to send data to each other at any time. It is a highly real-time and efficient solution, ideal for real-time chat.

• Network Resources: WebSocket requires only a single handshake to establish a connection, after which data can be transmitted bidirectionally without the need for new request-response cycles for each message. This significantly reduces network overhead.
• Server Processing: Once a WebSocket connection is established, it remains open until either the client or the server decides to close it. This means the server must maintain all active WebSocket connections, which may consume memory and other resources.
• Summary: WebSocket is highly effective in scenarios where data updates frequently and needs to be delivered to the client in real-time. Although it requires maintaining persistent connections, it is usually more efficient due to reduced network overhead.

Server-Sent Events (SSE)

Server-Sent Events (SSE) provide a simple way for the server to send new data to the client. It is simpler than WebSocket but only allows one-way communication from the server to the client. It is suitable for event notifications and alerts.

• Network Resources: Similar to WebSocket, SSE requires only a single handshake to establish a persistent connection. Once established, the server can continuously push messages to the client.
• Server Processing: SSE requires maintaining a persistent connection to send data, but unlike WebSocket, it is unidirectional. This means the server does not need to handle messages from the client.
• Summary: SSE is an efficient technology for scenarios where only the server needs to push data to the client, such as real-time notifications.

HTTP/2 Server Push

The HTTP/2 protocol supports server push, allowing the server to proactively send data before the client requests it. This can reduce latency but is usually used to send associated resources, such as CSS or JavaScript files, rather than for general message pushing.

• Primarily used to pre-send associated resources like CSS and JavaScript files to reduce load times and improve web performance.
• Can improve page load speed and user experience.
• Not suitable for general message pushing and requires HTTP/2 support, which may need specific server configurations.

MQTT Protocol

MQTT (Message Queue Telemetry Transport) is a lightweight communication protocol based on the publish/subscribe model. It allows clients to subscribe to relevant topics to receive messages and is a standard transport protocol for the Internet of Things (IoT).

This protocol decouples message publishers from subscribers, enabling reliable messaging services for remotely connected devices even in unreliable network environments. Its usage is somewhat similar to traditional message queues (MQ).

• The TCP protocol operates at the transport layer, while MQTT operates at the application layer. MQTT is built on top of TCP/IP, meaning it can be used wherever TCP/IP is supported.

Why Use MQTT for IoT?

Why is MQTT widely favored in IoT instead of other protocols like the more familiar HTTP?

• HTTP is a synchronous protocol, where the client must wait for a response after making a request. In IoT environments, devices are often affected by bandwidth constraints, high latency, and unstable network connections, making an asynchronous messaging protocol more suitable.
• HTTP is one-way, meaning clients must initiate a connection to receive messages. However, in IoT applications, devices and sensors often act as clients, meaning they cannot passively receive commands from the network.
• Often, a single command or message needs to be sent to all devices in a network. Achieving this with HTTP is not only difficult but also costly.

Third-Party Push Services

Using third-party push services such as Firebase Cloud Messaging (FCM) or Apple Push Notification Service (APNs) to handle message pushing.

Comparison

WebSocket and Server-Sent Events provide low latency and high real-time performance but may require more server resources. Long polling may introduce higher latency and may not be the most efficient solution. HTTP/2 Server Push and third-party push services may be better suited for applications that do not require high real-time performance. Message queues and the publish/subscribe model offer a way to decouple the server and client but may increase system complexity.

When choosing an implementation method, consider the specific application needs, including real-time requirements, server resources, network conditions, and development and maintenance complexity. It is also possible to combine multiple methods to meet different needs.

• If there are many clients and data updates are infrequent, long polling may be more efficient than short polling because it reduces unnecessary network requests.

• If the server has connection limits or limited resources, a large number of long polling requests may exhaust resources, making the server unstable.

• If data updates are very frequent, short polling may be more suitable because it can handle frequent requests more simply.

• WebSocket is usually more efficient and resource-friendly for applications requiring real-time communication. It reduces network overhead and provides continuous, low-latency bidirectional communication.

• Short polling and long polling may be more suitable for scenarios where persistent connections are unnecessary or when WebSocket is unavailable or inappropriate.

• WebSocket: Provides bidirectional communication, suitable for applications requiring real-time interaction, such as online chat. Since it is full-duplex, it may require more resources to handle bidirectional message transmission.

• SSE: Provides unidirectional communication, suitable for applications where only the server needs to push data, such as stock market updates. SSE is usually lighter than WebSocket because it only handles one-way communication.

• Short polling: Can generate significant network overhead, especially in scenarios with frequent data updates.

• Long polling: Reduces network overhead but requires the server to maintain many open connections until new data is available or the request times out.

From a Resource Consumption Perspective:

• WebSocket and SSE require maintaining persistent connections but are generally more efficient than short and long polling because they reduce network overhead.
• SSE may be lighter than WebSocket because it is one-way.
• Short polling is the most resource-intensive, especially when requests are frequent and data updates are infrequent.
• Long polling can be more efficient than short polling in some cases but is still less efficient than WebSocket or SSE.

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