Understanding Phoenix Architecture

Supervision Tree and OTP Integration

Phoenix applications rely heavily on OTP design principles. Each request, socket, and background process is supervised independently, enabling fault isolation. Misconfigured supervision or memory leaks in long-lived processes can lead to instability or crashes.

LiveView and Channels

LiveView enables server-rendered real-time UI via WebSockets. Each LiveView process holds state. Channel multiplexing allows concurrent real-time communication. Mismanaged state or poor handling of presence tracking can cause runtime desyncs.

Common Symptoms

  • LiveView socket crashes under high load
  • "Protocol.UndefinedError" or "no function clause matching" runtime errors
  • Long query times in Ecto under concurrent access
  • Processes accumulating in memory without garbage collection
  • Hot reload fails or crashes with CompileError or macro conflicts

Root Causes

1. State Leaks in LiveView Processes

Improper state management or recursive updates in handle_info can create zombie processes or memory bloat. Without explicit pruning, long-lived sessions accumulate data.

2. Improper Error Handling in Channels

Channels that pattern-match on messages without a fallback clause raise exceptions. Missing guards or unexpected payload formats often trigger Protocol.UndefinedError.

3. Lack of Ecto Query Indexing or Transaction Isolation

Poorly indexed queries or unbatched inserts lead to table scans and blocking writes. High read/write concurrency surfaces deadlocks or timeout exceptions.

4. OTP Supervisor Misconfiguration

Incorrect restart strategies or children returning unexpected exit statuses cause unanticipated crashes. Processes meant to restart transiently may terminate permanently.

5. Compile-Time Errors from Macros or Code Reload

Macros with compile-time logic (e.g., use blocks) can break hot code reloading if dependencies are recompiled out of order or module state is not reset.

Diagnostics and Monitoring

1. Use :observer for Process Inspection

Launch with :observer.start() in IEx. Inspect the process tree, memory usage, and message queues of LiveView or channel processes.

2. Enable Phoenix and Ecto Telemetry

Attach to telemetry events with Telemetry.attach/4 to track query timings, socket connects, disconnections, and controller execution durations.

3. Capture Logs with Metadata

Use Logger.metadata() to tag requests with correlation IDs. Increase log level for Ecto, Phoenix, and WebSocket events in config.exs.

4. Monitor DB Performance with Repo Metrics

Use Ecto's log: :debug and database-specific EXPLAIN ANALYZE to track query performance. Add indexes on foreign keys and frequently filtered fields.

5. Profile Compilation with mix xref graph

Use mix xref graph --format stats to understand macro dependencies and reload ordering issues during development.

Step-by-Step Fix Strategy

1. Limit LiveView State and Prune Sessions

Keep assigns minimal. Offload large data to JS hooks or separate APIs. Use handle_info(:timeout) to expire inactive sessions.

2. Add Catch-All Clauses in Channel Handlers

Ensure all handle_in/3 and handle_info/2 clauses have safe pattern matches. Log unexpected messages instead of crashing.

3. Tune Ecto Queries and Indexes

Profile with Repo.query/3. Use preloads and avoid N+1 queries. Use transactions only when necessary and keep them short-lived.

4. Review Supervisor Strategies

Set proper restart strategies (:transient, :permanent, etc.) and link long-lived workers correctly. Avoid nesting dynamic children under one-shot supervisors.

5. Disable Hot Reload for Complex Macros

Use config :phoenix, :code_reloader, false in production. Modularize macros to isolate compile-time logic and reload dependencies in correct order.

Best Practices

  • Use Phoenix LiveDashboard in development for real-time introspection
  • Instrument with AppSignal, PromEx, or OpenTelemetry
  • Structure code using Contexts to isolate business logic
  • Limit usage of global assigns or shared ETS tables unless concurrency-safe
  • Wrap critical state transitions in transactions and guard against partial failures

Conclusion

Phoenix combines the reliability of the BEAM VM with real-time web features, but managing stateful processes, macro-based compile flows, and concurrent I/O requires disciplined architecture and monitoring. With structured supervision, telemetry, and memory profiling, developers can resolve issues early and build resilient, scalable applications on Phoenix.

FAQs

1. Why is my LiveView process crashing unexpectedly?

Likely due to pattern match failure or memory overuse. Use Process.info(self()) and monitor assigns for size growth.

2. How do I debug slow Ecto queries?

Enable Ecto query logs, use EXPLAIN in Postgres, and ensure proper indexes are in place for WHERE and JOIN conditions.

3. What causes hot code reload to break with macros?

Macros may recompile dependencies in the wrong order. Use mix xref to inspect module relationships and reload selectively.

4. How can I track socket disconnects and reconnections?

Listen to phx_socket:disconnect and phx_socket:connect telemetry events or hook into terminate/2 in LiveView.

5. Why is memory usage growing in my Phoenix app?

Leaky processes, large assigns, or long-lived ETS entries can accumulate. Use :observer or LiveDashboard to trace and terminate offenders.