OTP

Choose processes, supervision strategies, and storage for runtime concurrency and fault recovery.

The Iron Law

GENSERVER IS A BOTTLENECK BY DESIGN

A GenServer processes ONE message at a time. Before creating one, ask:

1. Do I actually need serialized access?
2. Will this become a throughput bottleneck?
3. Can reads bypass the GenServer via ETS?

The ETS pattern: GenServer owns ETS table, writes serialize through GenServer, reads bypass it entirely with :read_concurrency.

No exceptions: Don't wrap stateless functions in GenServer. Don't create GenServer "for organization".

GenServer Patterns

Function	Use For
call/3	Synchronous requests expecting replies
cast/2	Fire-and-forget messages

When in doubt, use call to ensure back-pressure. Set appropriate timeouts for call/3.

Use handle_continue/2 for post-init work—keeps init/1 fast and non-blocking.

Task.Supervisor, Not Task.async

Task.async spawns a linked process—if task crashes, caller crashes too.

Pattern	On task crash
Task.async/1	Caller crashes (linked, unsupervised)
Task.Supervisor.async/2	Caller crashes (linked, supervised)
Task.Supervisor.async_nolink/2	Caller survives, can handle error

Use Task.Supervisor for: Production code, graceful shutdown, observability, async_nolink. Use Task.async for: Quick experiments, scripts, when crash-together is acceptable.

DynamicSupervisor + Registry = Named Dynamic Processes

DynamicSupervisor only supports :one_for_one (dynamic children have no ordering). Use Registry for names—never create atoms dynamically:

defp via_tuple(id), do: {:via, Registry, {MyApp.Registry, id}}

PartitionSupervisor scales DynamicSupervisor for millions of children.

:pg for Distributed, Registry for Local

Tool	Scope	Use Case
Registry	Single node	Named dynamic processes
:pg	Cluster-wide	Process groups, pub/sub

:pg replaced deprecated :pg2. Horde provides distributed supervisor/registry with CRDTs.

Broadway vs Oban: Different Problems

Tool	Use For
Broadway	External queues (SQS, Kafka, RabbitMQ) — data ingestion with batching
Oban	Background jobs with database persistence

Broadway is NOT a job queue.

Broadway Gotchas

Processors are for runtime, not code organization. Dispatch to modules in handle_message, don't add processors for different message types.

one_for_all is for Broadway bugs, not your code. Your handle_message errors are caught and result in failed messages, not supervisor restarts.

Handle expected failures in the producer (connection loss, rate limits). Reserve max_restarts for unexpected bugs.

Supervision Strategies Encode Dependencies

Strategy	Children Relationship
:one_for_one	Independent
:one_for_all	Interdependent (all restart)
:rest_for_one	Sequential dependency

Use :max_restarts and :max_seconds to prevent restart loops.

Think about failure cascades BEFORE coding.

Abstraction Decision Tree

Need state?
├── No → Plain function
└── Yes → Complex behavior?
    ├── No → Agent
    └── Yes → Supervision?
        ├── No → spawn_link
        └── Yes → Request/response?
            ├── No → Task.Supervisor
            └── Yes → Explicit states?
                ├── No → GenServer
                └── Yes → GenStateMachine

Storage Options

Need	Use
Memory cache	ETS (:read_concurrency for reads)
Static config	:persistent_term (faster than ETS)
Disk persistence	DETS (2GB limit)
Transactions/Distribution	Mnesia

:sys Debugs ANY OTP Process

:sys.get_state(pid)        # Current state
:sys.trace(pid, true)      # Trace events (TURN OFF when done!)

Telemetry Is Built Into Everything

Phoenix, Ecto, and most libraries emit telemetry events. Attach handlers:

:telemetry.attach("my-handler", [:phoenix, :endpoint, :stop], &handle/4, nil)

Use Telemetry.Metrics + reporters (StatsD, Prometheus, LiveDashboard).

Red Flags - STOP and Reconsider

• GenServer wrapping stateless computation
• Task.async in production when you need error handling
• Creating atoms dynamically for process names
• Single GenServer becoming throughput bottleneck
• Using Broadway for background jobs (use Oban)
• Using Oban for external queue consumption (use Broadway)
• No supervision strategy reasoning

Any of these? Re-read The Iron Law and use the Abstraction Decision Tree.