Architectural Foundation of Phoenix

Concurrency and Supervision Model

Phoenix leverages BEAM's actor-based concurrency. Every request, WebSocket connection, and LiveView session can be spawned as a lightweight process, supervised within a fault-tolerant tree. This power also means misbehaving processes can leak memory or block schedulers if not monitored correctly.

LiveView and Stateful Connections

LiveView enables real-time interactivity over WebSockets. Each LiveView mounts as a stateful process that tracks socket and assigns data. At scale, large numbers of LiveView sessions can cause resource contention if not properly limited or garbage-collected.

Common Production-Level Issues

1. LiveView Crashes or Becomes Unresponsive

This can be caused by unhandled exceptions in mount/update callbacks or inefficient assigns causing deep diffs.

def handle_params(params, _url, socket) do
  case fetch_data(params) do
    {:ok, data} -> {:noreply, assign(socket, :data, data)}
    {:error, _} -> {:noreply, put_flash(socket, :error, "Data fetch failed")}
  end
end

Ensure all data fetching is wrapped in pattern matches and timeouts are configured for API calls in LiveView.

2. WebSocket Connections Dropping Intermittently

Occurs when Cowboy (Phoenix's HTTP/WebSocket handler) hits system or transport limits. Check for exceeded max_connections or dropped TCP packets.

config :phoenix, Endpoint,
  http: [ip: {0, 0, 0, 0}, port: 4000, transport_options: [max_connections: 16384]]

Also monitor file descriptor limits and OS-level TCP backlog queues.

3. Memory Bloat from Long-Lived Processes

Processes holding large assigns or ETS references may never terminate, especially in LiveView dashboards or presence tracking. Use telemetry and `:observer` to trace heap growth.

:telemetry.attach("lv-memory", [:phoenix, :live_view, :mount], fn _event, measurements, _meta ->
  IO.inspect(measurements)
end, nil)

Diagnostics and Tools

:observer and Recon

Use `:observer.start()` in IEx to inspect running processes, memory, and message queues. For headless systems, integrate Recon for runtime diagnostics.

:recon.proc_count(:message_queue_len, 10)
:recon.bin_leak(10)
:recon.long_schedule(100)

Telemetry and Custom Metrics

Leverage Phoenix.Telemetry hooks for custom instrumentation. Surface key metrics to Prometheus or StatsD exporters.

telemetry.attach("channel-duration", [:phoenix, :channel, :join],
  fn _event, %{duration: dur}, meta ->
    Logger.info("Channel #{meta.topic} joined in #{dur} μs")
  end, nil)

Fixes and Long-Term Optimization Strategies

Limit LiveView Process Lifetime

For ephemeral dashboards, use temporary assigns or explicitly terminate processes after inactivity:

Process.send_after(self(), :shutdown, 30_000)
def handle_info(:shutdown, socket), do: {:stop, :normal, socket}

Channel Supervision Hygiene

Ensure long-lived channels clean up after disconnects:

def terminate(_reason, socket) do
  MyApp.Presence.untrack(socket)
  :ok
end

Database and External API Contention

Use `DBConnection.Poolboy` settings and rate limiters for API-bound LiveViews. Prevent overloading with queue-based throttling.

Best Practices for Phoenix at Scale

  • Use Phoenix.PubSub via Redis or PG2 for distributed presence tracking
  • Profile process memory with :observer and avoid storing large blobs in assigns
  • Batch database reads in mount callbacks to reduce socket latency
  • Apply circuit breakers for unstable third-party API calls
  • Instrument WebSocket lifecycle with Phoenix.Telemetry for real-time alerts

Conclusion

Phoenix offers a powerful and scalable foundation for modern back-end applications, especially those requiring real-time interactivity. However, enterprise use demands deliberate process supervision, memory management, and instrumentation. By applying the right diagnostic tools and architectural patterns, engineering teams can sustain high concurrency and stability in production systems.

FAQs

1. Why are LiveViews slowing down over time?

Processes may be holding stale assigns or large diffs. Use temporary_assigns and periodic cleanup to control memory.

2. How can I trace message queue bottlenecks?

Use :observer or recon to list processes with high message queue length and investigate blocked GenServers.

3. My WebSocket connections are dropping—what should I check?

Inspect Cowboy's transport_options, OS-level socket limits, and verify proxy timeouts from Nginx or load balancers.

4. Can I use Phoenix in a multi-node cluster?

Yes. Use Phoenix.PubSub with a distributed backend (e.g., Redis, PG2) and ensure cookie-based BEAM clustering is configured.

5. How do I debug memory leaks in LiveView?

Attach telemetry to track LiveView process growth, and inspect process states with :observer or :recon.bin_leak.