What Causes Message Latency Spikes in Mobile In-App Chat Systems?
Blog post from Stream
A chat message's journey involves numerous technical processes, including DNS resolution, TLS negotiation, and WebSocket framing, any of which can introduce latency spikes that degrade user experience. The main challenge in maintaining a seamless chat system lies in minimizing tail latency, especially at the 99th percentile, where delays can cluster unpredictably. Factors contributing to latency include network transitions, such as switching from WiFi to cellular networks, which can invalidate TCP connections due to IP changes, and the inherent latency of cellular radios. Protocols like QUIC offer improvements by using connection IDs instead of IP tuples, allowing connections to survive network changes. Additionally, reconnection processes, message ordering issues, and reconnection storms exacerbate latency, requiring strategic use of exponential backoff and server-assigned sequence numbers. Server-side challenges, such as message fan-out, can lead to nonlinear scaling of latency, while presence and typing indicators can consume significant bandwidth. Edge infrastructure is crucial in reducing latency by shortening the mobile connection to the nearest server and utilizing reliable backbone connections. Language and runtime choices impact the latency floor by affecting garbage collection behavior, with systems like Go and ScyllaDB offering advantages. Ultimately, the irreducible latency floor for mobile chat is influenced by physical limitations, TLS reconnection overhead, and database latency, with engineering strategies focusing on edge termination, off-main-thread processing, and optimistic UI design to optimize performance.
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