Why does Facebook Messenger use long polling over WebSockets?

Facebook Messenger does not explicitly state why it might prefer long polling over WebSockets, but based on the characteristics of both technologies, we can infer some reasons:

Long Polling vs WebSockets

Advantages of Long Polling

1. Compatibility: Long polling is built on top of XMLHttpRequest, making it more compatible with a broader range of web browsers and network configurations15. This could be beneficial for ensuring that Messenger works across various devices and networks.

2. Network Adaptability: Long polling can handle network changes more gracefully than WebSockets. When users switch between networks (e.g., from LTE to WiFi), WebSockets require re-establishing the connection, which can be complex. Long polling automatically re-establishes connections after a timeout, making it more suitable for environments with unstable or changing network conditions1.

3. Low-Frequency Updates: Long polling is beneficial for applications with low-frequency real-time updates. It reduces the overhead of maintaining persistent connections, which might be unnecessary if updates are infrequent1.

Disadvantages of Long Polling

1. Latency and Resource Intensity: Long polling can introduce higher latency and is more resource-intensive on the server side compared to WebSockets35. However, for applications where real-time updates are not as frequent, this might be a trade-off for better compatibility and network adaptability.

2. Half-Duplex Communication: Long polling supports half-duplex communication, meaning it requires a new request-response cycle for each update. This can be less efficient than WebSockets for full-duplex communication3.

Why Facebook Might Prefer Long Polling

• Scalability and Compatibility: Facebook's massive user base requires solutions that work across a wide range of devices and networks. Long polling's compatibility and adaptability might be more beneficial for ensuring that Messenger functions consistently for all users.
• Low-Frequency Updates: While Messenger does require real-time updates, the frequency of these updates might not necessitate the full-duplex capabilities of WebSockets. Long polling could be sufficient for handling less frequent updates efficiently.
• Infrastructure and Resource Management: Facebook's infrastructure is highly scalable and optimized. They might have specific strategies for managing server resources that make long polling more efficient for their use case.

However, without explicit confirmation from Facebook, these are speculative reasons based on the characteristics of long polling and WebSockets.