Implementing Backpressure Handling
1. Understanding Backpressure
Backpressure occurs when a producer outpaces a consumer's drain rate, causing buffers to grow and memory pressure to rise. In WebSockets the symptom is bufferedAmount climbing toward unbounded.
Layer Buffer App outbox Your queue Browser send ws.bufferedAmountTCP send Kernel socket buffer Server receive ws library buffer
2. Monitoring bufferedAmount Property
Example: Drain detection
function whenDrained ( ws ) { return new Promise ( res => { const tick = () => ws.bufferedAmount === 0 ? res () : setTimeout (tick, 50 ); tick (); }); }
Property Semantics bufferedAmountBytes queued in UA, not yet on wire Polling No native drain event — poll or use timer
3. Implementing Flow Control
Mechanism Detail Credit-based Server grants N tokens; client decrements Window ack Sender pauses until ack of last N Pause/resume msg Receiver tells sender to stop
4. Using High Water Mark
Example: HWM gating
const HWM = 256 * 1024 ; // 256 KB function canSend () { return ws.bufferedAmount < HWM ; }
Threshold Action Low water Resume producer High water Pause producer Critical Drop / close
5. Dropping Messages
Strategy Best For Drop newest Order matters more than freshness Drop oldest Latest data (cursor, prices) Sample (1/N) Telemetry Coalesce Idempotent updates (state replace)
6. Implementing Message Prioritization
Example: Priority lanes
const lanes = { hi: [], mid: [], lo: [] }; function flush () { for ( const lane of [ "hi" , "mid" , "lo" ]) { while (lanes[lane]. length && canSend ()) ws. send (lanes[lane]. shift ()); } }
Priority Example Hi Auth, control, errors Mid User actions Lo Telemetry, analytics
7. Handling Slow Clients
Server Action Detail Per-client buffer cap Drop / close when exceeded Timeout slow drains Close 1013 Demote subscription Lower update rate Snapshot + delta Skip back-to-snapshot
8. Implementing Client-Side Buffering
Bound Reason Size limit Cap memory Age limit Avoid stale Count limit Predictable drain time
9. Detecting Backpressure Conditions
Signal Threshold bufferedAmount risingΔ > 0 over N seconds Pong RTT spike 2× baseline Queue depth App outbox > N
10. Testing Backpressure Scenarios
Test How Throttle network DevTools Slow 3G Pause consumer Suspend message handler Burst producer Send 10k msgs / sec Memory probe performance.memory