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Notes / Docker Sandboxes / Zero-Latency Feature Flag Engine: Server-Sent Events (SSE) & Local Memory
Docker Sandboxes · Tier 3: Reliability & Messaging · Lab 68

Zero-Latency Feature Flag Engine: Server-Sent Events (SSE) & Local Memory

Eliminate network latency from feature flag checks. Build a real-time feature flag engine that pushes rule updates over Server-Sent Events (SSE) into local process memory, prove zero network calls during flag evaluation, inject network partitions with Toxiproxy to observe kill switch misses, and recover with heartbeat half-open socket detection.

Time30 minutes
StackSSE StreamingPorts 18350, 18480, 18481
MemoryAbout 90 MB
Before thisLab 12Timeouts, Retries, Exponential Bac
Download lab (.zip) View steps
Verified runWindows, Mac, LinuxBash and PowerShell
  1. 1PrepareWhat you will learn
  2. 2PredictGuess before you run
  3. 3RunStep by step
  4. 4ObserveWhat you should see
  5. 5Break itChange one thing
  6. 6Clean upLeave nothing running
On this page
  1. 00 / What You Will Learn
  2. 01 / Why It Matters
  3. 02 / What We Are Building
  4. 03 / Words You Need
  5. 04 / Predict First
  6. 05 / Step by Step
  7. 06 / What You Should See
  8. 07 / Look Inside
  9. 08 / Break It, Fix It
  10. 09 / In Real Projects
  11. 10 / Interview Corner
  12. 11 / Clean Up

00 /What You Will Learn

By the end you will be able to
  • Explain why production feature flag evaluations must run 100 percent in-memory.
  • Stream flag delta mutations in real time using Server-Sent Events (SSE).
  • Measure and assert that zero network calls occur during flag evaluations.
  • Simulate a network partition with Toxiproxy to observe why kill switches miss in partitioned nodes.
  • Implement heartbeat watchdog timers to detect half-open TCP connections and trigger full resynchronization.

Before you start

Complete Lab 12 (Timeouts, Retries, and Toxiproxy) first. You should know how Toxiproxy injects latency and network partitions. Docker Desktop must be running.