Understanding Erlang Architecture

BEAM VM and Process Isolation

Erlang runs on the BEAM virtual machine, which supports lightweight, isolated processes. Errors in one process should not affect others—unless there is incorrect supervision strategy or linked processes.

OTP Framework and Supervision Trees

OTP provides building blocks for designing fault-tolerant systems through behaviors like gen_server, gen_statem, and supervisors. Failures often trace back to misconfigured restart strategies or missing callbacks.

Common Erlang Issues

1. Process Crashes Without Restart

Caused by missing supervision or improper supervisor strategy (e.g., using one_for_one when rest_for_one is needed).

2. Mailbox Overflows or Latency

Processes with large mailboxes due to message backlog can cause latency or crash due to memory exhaustion.

3. Node-to-Node Communication Fails

Occurs when Erlang nodes can't connect due to mismatched cookies, firewalls, or network splits in a cluster.

4. Hot Code Upgrade Breaks State

When code_change/3 is not properly implemented in a gen_server, state transition during upgrade leads to invalid state errors.

5. Memory or CPU Spikes in Production

Usually due to runaway processes, memory leaks in ETS tables, or recursive functions without tail-call optimization.

Diagnostics and Debugging Techniques

Use observer and etop

Start the graphical observer tool:

observer:start().

Or use etop for CLI profiling:

etop:start().

Trace Messages with dbg

Enable function and message tracing:

dbg:tracer().
dbg:p(all, [c]).
dbg:tpl(Module, Function, Arity, []).

Inspect Process Info

Use:

erlang:process_info(Pid)

to view mailbox size, memory usage, and status of individual processes.

Check Node Connectivity

Ping other nodes and inspect cookies:

net_adm:ping('node@host').
erlang:get_cookie().

Review Logs and Crash Reports

Look into erl_crash.dump or log/ directories for detailed error stacks.

Step-by-Step Resolution Guide

1. Restore Fault-Tolerant Supervision

Review supervisor strategy:

{ok, Pid} = supervisor:start_link({local, my_sup}, MySup, []).

Ensure child specs include restart: permanent or transient as needed.

2. Reduce Mailbox Backlogs

Use flow control or drop stale messages:

receive
  Msg when erlang:process_info(self(), message_queue_len) < 1000 -> handle(Msg)
after 1000 -> ok
end.

3. Reconnect Distributed Nodes

Ensure both nodes have the same cookie and ports are open (default is 4369 + dynamic range):

erl -name node1@host -setcookie secret

4. Handle Hot Code Upgrades Safely

Implement code_change/3 correctly in gen_server:

code_change(_OldVsn, State, _Extra) -> {ok, NewState}.

5. Fix Resource Exhaustion

Use fprof or eprof to analyze performance. Clean up large ETS tables and ensure recursive functions are TCO-compliant.

Best Practices for Erlang Applications

  • Use short-lived processes and avoid state bloat.
  • Segment responsibilities using OTP behaviors (e.g., separate servers for cache, state, compute).
  • Configure supervisor trees thoughtfully with cascading strategies.
  • Monitor with observer or telemetry in production environments.
  • Always test code_change logic before deploying upgrades.

Conclusion

Erlang's actor-based model and robust OTP framework are powerful tools for building resilient systems, but require careful process management, supervision planning, and state control. With effective use of debugging tools, distributed tracing, and well-structured code upgrades, developers can maintain high-availability Erlang systems even under demanding production loads.

FAQs

1. Why isn't my process restarting after a crash?

Check if it's linked to a supervisor and if the restart strategy is correctly defined (e.g., permanent or transient).

2. How do I debug a mailbox growing too large?

Use process_info(Pid, message_queue_len). Apply flow control and ensure messages are handled quickly or dropped.

3. What causes distributed Erlang nodes to disconnect?

Common causes include different cookies, blocked ports, or name mismatches. Use net_adm:ping/1 to test connectivity.

4. Why did my hot code upgrade crash the process?

If code_change/3 is not implemented properly in a gen_server, the state conversion may fail during upgrade.

5. How can I profile CPU/memory usage in Erlang?

Use tools like etop, observer, or eprof. For detailed analysis, try fprof to trace function calls.