Best Practices

Building production Elixir systems? This guide teaches industry best practices following the OTP-first principle, ensuring you leverage BEAM's full fault-tolerance and concurrency capabilities.

Why Production Best Practices Matter

Production Elixir differs fundamentally from development environments. The BEAM VM provides powerful OTP primitives for fault tolerance, but wrong patterns create:

Process leaks - Memory growth from unsupervised processes
Message queue overflow - Unbounded mailbox growth causing memory exhaustion
Supervision violations - Child processes outliving supervisors
State corruption - Shared mutable state in supposedly isolated processes
Race conditions - Timing-dependent bugs in concurrent code
Resource exhaustion - Unmanaged database connections, file handles
Deployment disasters - Hot code upgrade failures, configuration errors

These best practices prevent production disasters by establishing OTP patterns that work reliably at scale.

Financial Domain Examples

Examples use Shariah-compliant financial operations:

Zakat calculation - Processing donation percentages for charity
Donation tracking - Managing charitable contribution records
Transaction auditing - Recording all financial state changes

These domains demonstrate production patterns with real business logic.

Supervisor Tree Patterns

Pattern 1: Supervision Strategy Selection

Supervisor strategies determine how process failures affect siblings.

OTP Primitive: Supervisor with :one_for_one strategy.

# Financial system supervision tree
defmodule Finance.Supervisor do
  use Supervisor                                 # => Imports Supervisor behavior
                                                 # => Provides init/1 callback
 
  def start_link(init_arg) do
    Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
                                                 # => Starts supervisor process
                                                 # => Registers with module name
                                                 # => Returns {:ok, pid}
  end
 
  def init(_init_arg) do
    children = [
      {Finance.ZakatCalculator, []},             # => Zakat calculation service
      {Finance.DonationTracker, []},             # => Donation tracking service
      {Finance.AuditLog, []}                     # => Audit logging service
    ]                                            # => List of child specifications
                                                 # => Each tuple: {module, init_args}
 
    Supervisor.init(children, strategy: :one_for_one)
                                                 # => :one_for_one strategy
                                                 # => If child dies, restart only that child
                                                 # => Other children unaffected
                                                 # => Returns {:ok, {supervisor_spec, children}}
  end
end

When to use each strategy:

# :one_for_one - Independent services (DEFAULT)
Supervisor.init(children, strategy: :one_for_one)
                                                 # => Child failures isolated
                                                 # => Use when: Services independent
                                                 # => Example: API endpoints, workers
 
# :one_for_all - Tightly coupled services
Supervisor.init(children, strategy: :one_for_all)
                                                 # => Any child failure restarts ALL
                                                 # => Use when: Services depend on each other
                                                 # => Example: Database + cache + queue
 
# :rest_for_one - Sequential dependencies
Supervisor.init(children, strategy: :rest_for_one)
                                                 # => Child N failure restarts N and all after
                                                 # => Use when: Sequential pipeline
                                                 # => Example: Reader -> Parser -> Writer

Best practice: Start with :one_for_one for independence. Only use :one_for_all when services truly must restart together.

Pattern 2: Nested Supervision Trees

Complex applications require hierarchical supervision.

# Top-level application supervisor
defmodule Finance.Application do
  use Application                                # => Application behavior
                                                 # => Provides start/2 callback
 
  def start(_type, _args) do
    children = [
      Finance.CoreSupervisor,                    # => Core financial services
      Finance.WebSupervisor,                     # => Web API services
      Finance.ReportingSupervisor                # => Reporting and analytics
    ]                                            # => Three major subsystems
                                                 # => Each manages own tree
 
    opts = [strategy: :one_for_one, name: Finance.Supervisor]
                                                 # => Top-level strategy
                                                 # => Subsystem failures isolated
    Supervisor.start_link(children, opts)        # => Returns {:ok, pid}
  end
end
 
# Core services subtree
defmodule Finance.CoreSupervisor do
  use Supervisor                                 # => Supervisor behavior
                                                 # => Provides init/1 callback
 
  def start_link(init_arg) do
    Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
                                                 # => Starts supervisor process
                                                 # => Registers with module name
                                                 # => Returns {:ok, pid}
  end
 
  def init(_init_arg) do
    children = [
      {Finance.ZakatCalculator, []},             # => Zakat calculation service
                                                 # => First child in tree
      {Finance.DonationTracker, []},             # => Donation tracking service
                                                 # => Second child in tree
      {Finance.TransactionSupervisor, []}        # => Nested: transaction workers pool
                                                 # => Third child manages dynamic workers
    ]                                            # => Core financial services
                                                 # => TransactionSupervisor manages pool
                                                 # => Three children total
 
    Supervisor.init(children, strategy: :one_for_one)
                                                 # => :one_for_one strategy
                                                 # => If child dies, restart only that child
                                                 # => Returns {:ok, {supervisor_spec, children}}
  end
end
 
# Dynamic worker pool subtree
defmodule Finance.TransactionSupervisor do
  use DynamicSupervisor                          # => Dynamic child management
                                                 # => Start/stop children at runtime
                                                 # => Provides start_child/2 interface
 
  def start_link(init_arg) do
    DynamicSupervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
                                                 # => Starts dynamic supervisor process
                                                 # => Registers with module name
                                                 # => Returns {:ok, pid}
  end
 
  def init(_init_arg) do
    DynamicSupervisor.init(strategy: :one_for_one)
                                                 # => Configure supervision strategy
                                                 # => Children started dynamically
                                                 # => Not in init/1
                                                 # => Returns {:ok, state}
  end
 
  def start_transaction(transaction_data) do
    spec = {Finance.TransactionWorker, transaction_data}
                                                 # => Child specification tuple
                                                 # => Module: Finance.TransactionWorker
                                                 # => transaction_data: Worker init args
                                                 # => Format: {module, args}
    DynamicSupervisor.start_child(__MODULE__, spec)
                                                 # => Starts supervised worker
                                                 # => Worker added to supervision tree
                                                 # => Returns {:ok, pid} or {:error, reason}
  end
end

Supervision hierarchy best practices:

Top level: Application supervisor with major subsystems
Middle level: Subsystem supervisors grouping related services
Bottom level: Worker processes or dynamic supervisors for pools

Rule: Keep each supervisor focused. Maximum 5-10 children per supervisor for clarity.

Pattern 3: Restart Strategies

Configure restart behavior for fault tolerance.

defmodule Finance.ZakatCalculator do
  use GenServer
 
  # Client API
  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end
 
  def child_spec(opts) do
    %{
      id: __MODULE__,                            # => Child identifier (must be unique)
      start: {__MODULE__, :start_link, [opts]}, # => Start function: MFA tuple
      restart: :permanent,                       # => Restart strategy
      shutdown: 5000,                            # => Shutdown timeout (milliseconds)
      type: :worker                              # => Process type
    }                                            # => Child specification map
                                                 # => Used by Supervisor
  end
 
  # Server callbacks
  def init(_opts) do
    {:ok, %{}}                                   # => Initial state: empty map
  end
end

Restart strategy options:

# :permanent - Always restart (DEFAULT for critical services)
restart: :permanent                              # => Supervisor always restarts child
                                                 # => Use for: Core services
                                                 # => Example: Database, API server
 
# :temporary - Never restart (fire-and-forget tasks)
restart: :temporary                              # => Supervisor never restarts child
                                                 # => Use for: One-off tasks
                                                 # => Example: Email send, log write
 
# :transient - Restart only on abnormal exit (recommended for workers)
restart: :transient                              # => Restart if exit not :normal
                                                 # => Use for: Batch jobs
                                                 # => Example: Report generation

Best practice: Use :permanent for long-lived services, :transient for workers, :temporary for fire-and-forget tasks.

Process Registry Patterns

Pattern 4: Named vs Registry-Based Processes

Choose process naming strategy based on cardinality.

Single instance - Named registration:

defmodule Finance.AuditLog do
  use GenServer
 
  def start_link(_opts) do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
                                                 # => Registers globally with module name
                                                 # => Only ONE instance allowed
                                                 # => Returns {:ok, pid}
  end
 
  def log_transaction(transaction) do
    GenServer.cast(__MODULE__, {:log, transaction})
                                                 # => Async message to named process
                                                 # => No reply expected
  end
 
  def init(:ok) do
    {:ok, []}                                    # => Initial state: empty list
  end
 
  def handle_cast({:log, transaction}, state) do
    new_state = [transaction | state]            # => Prepend transaction
    {:noreply, new_state}                        # => Update state, no reply
  end
end

Multiple instances - Registry-based lookup:

defmodule Finance.DonationTracker do
  use GenServer                                  # => GenServer behavior
                                                 # => Provides init/1, handle_call/3 callbacks
 
  # Client API
  def start_link(user_id) do
    GenServer.start_link(__MODULE__, user_id, name: via_tuple(user_id))
                                                 # => Starts GenServer process
                                                 # => Registers with Registry via via_tuple
                                                 # => Multiple instances (one per user)
                                                 # => Returns {:ok, pid}
  end
 
  defp via_tuple(user_id) do
    {:via, Registry, {Finance.Registry, {__MODULE__, user_id}}}
                                                 # => Registry-based name tuple
                                                 # => Format: {:via, Registry, {registry_name, key}}
                                                 # => {module, user_id} as unique key
                                                 # => Allows multiple instances per user
                                                 # => Unique per user
  end
 
  def track_donation(user_id, amount) do
    case Registry.lookup(Finance.Registry, {__MODULE__, user_id}) do
      [{pid, _}] ->                              # => Process found in registry
                                                 # => pid: Process identifier
                                                 # => _: Process value (unused)
        GenServer.call(pid, {:donate, amount})   # => Send synchronous message to process
                                                 # => Returns updated total
      [] ->                                      # => Process not found
                                                 # => Empty list from lookup
        {:error, :not_found}                     # => Return not_found error
                                                 # => User has no tracker process
    end
  end
 
  # Server callbacks
  def init(user_id) do
    state = %{user_id: user_id, total: 0}        # => Initial state: zero donations
                                                 # => user_id: User identifier
                                                 # => total: Accumulated donation amount
    {:ok, state}                                 # => Return initial state tuple
  end
 
  def handle_call({:donate, amount}, _from, state) do
    new_total = state.total + amount             # => Add donation amount to total
                                                 # => Accumulates user's donations
    new_state = %{state | total: new_total}      # => Update state with new total
                                                 # => Map update syntax
    {:reply, new_total, new_state}               # => Reply with new total
                                                 # => Update process state
                                                 # => Format: {:reply, response, new_state}
  end
end
 
# Registry setup in application supervisor
def start(_type, _args) do
  children = [
    {Registry, keys: :unique, name: Finance.Registry},
                                                 # => Registry for process lookup
                                                 # => keys: :unique - One value per key
                                                 # => name: Finance.Registry - Registry identifier
                                                 # => Used by via_tuple for process registration
                                                 # => Enables multiple named processes
    # ... other children
  ]
  Supervisor.start_link(children, strategy: :one_for_one)
                                                 # => Starts application supervisor
                                                 # => Registry started as first child
                                                 # => Returns {:ok, pid}
end

When to use each approach:

Pattern	Use When	Example
Named	Single instance, global service	AuditLog, ConfigStore
Registry (via)	Multiple instances by key	UserSession, OrderBot
DynamicSupervisor	Pools of workers	JobWorker, TaskRunner

Pattern 5: Process Lifecycle Management

Properly initialize and cleanup process resources.

defmodule Finance.DatabaseConnection do
  use GenServer
 
  # Client API
  def start_link(config) do
    GenServer.start_link(__MODULE__, config, name: __MODULE__)
  end
 
  # Server callbacks
  def init(config) do
    # SYNCHRONOUS initialization in init/1
    case establish_connection(config) do
      {:ok, conn} ->                             # => Connection successful
        state = %{conn: conn, config: config}    # => Store connection
        {:ok, state}                             # => Return initial state
      {:error, reason} ->                        # => Connection failed
        {:stop, reason}                          # => Stop process immediately
    end                                          # => Supervisor will retry
  end
 
  def handle_info(:timeout, state) do
    # Cleanup on timeout
    cleanup_connection(state.conn)               # => Close database connection
    {:stop, :normal, state}                      # => Stop process normally
  end
 
  def terminate(reason, state) do
    # ALWAYS cleanup in terminate/2
    cleanup_connection(state.conn)               # => Release connection
    :ok                                          # => Return value ignored
  end                                            # => Called before process exits
 
  # Helper functions
  defp establish_connection(config) do
    # Simulate connection establishment
    {:ok, :connection_handle}                    # => Placeholder connection
  end
 
  defp cleanup_connection(conn) do
    # Release connection resources
    :ok
  end
end

Critical lifecycle rules:

init/1 must be fast - Heavy initialization blocks supervisor
Use terminate/2 for cleanup - Always release resources
Handle :timeout - Detect and cleanup hung connections
Return {:stop, reason} - Signal initialization failure to supervisor

Pattern 6: Async Initialization

Defer expensive initialization to prevent supervisor blocking.

defmodule Finance.ReportGenerator do
  use GenServer
 
  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end
 
  def init(opts) do
    # Fast initialization - return immediately
    state = %{opts: opts, data: nil, status: :initializing}
    {:ok, state, {:continue, :load_data}}        # => Return with :continue tuple
                                                 # => Triggers handle_continue/2
                                                 # => Non-blocking for supervisor
  end
 
  def handle_continue(:load_data, state) do
    # Expensive initialization happens HERE (async)
    data = load_large_dataset()                  # => Expensive operation
    new_state = %{state | data: data, status: :ready}
    {:noreply, new_state}                        # => Update state when ready
  end
 
  def handle_call(:generate_report, _from, %{status: :initializing} = state) do
    {:reply, {:error, :not_ready}, state}        # => Reject if not initialized
  end
 
  def handle_call(:generate_report, _from, %{status: :ready} = state) do
    report = generate_from_data(state.data)      # => Process data
    {:reply, {:ok, report}, state}               # => Return report
  end
 
  defp load_large_dataset do
    # Simulate expensive operation
    :timer.sleep(5000)                           # => 5 second delay
    [:data1, :data2, :data3]                     # => Return dataset
  end
 
  defp generate_from_data(data) do
    "Report based on #{inspect(data)}"           # => Generate report string
  end
end

Async initialization benefits:

Supervisor doesn't block - Children start immediately
Application boots faster - Services become ready incrementally
Graceful degradation - Service handles calls during initialization

Pattern: Return {:ok, state, {:continue, :init_task}} from init/1, perform expensive work in handle_continue/2.

GenServer State Management

Pattern 7: Immutable State Updates

GenServer state must be immutable - return new state, never mutate.

defmodule Finance.ZakatTracker do
  use GenServer
 
  # State structure: %{user_id => total_zakat}
 
  def start_link(_opts) do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end
 
  def add_zakat(user_id, amount) do
    GenServer.call(__MODULE__, {:add, user_id, amount})
  end
 
  def init(:ok) do
    {:ok, %{}}                                   # => Initial state: empty map
  end
 
  def handle_call({:add, user_id, amount}, _from, state) do
    # ❌ WRONG: Mutating state
    # state[user_id] = (state[user_id] || 0) + amount
    # This doesn't work - maps immutable!
 
    # ✅ CORRECT: Return new state
    current = Map.get(state, user_id, 0)         # => Get current zakat
    new_total = current + amount                 # => Calculate new total
    new_state = Map.put(state, user_id, new_total)
                                                 # => Create new map
                                                 # => Old state unchanged
    {:reply, new_total, new_state}               # => Return new state
  end
end

State update patterns:

# Map update with Map.put
new_state = Map.put(state, key, value)           # => Returns new map
                                                 # => Original state unchanged
 
# Map update with map syntax (kernel special form)
new_state = %{state | key: new_value}            # => Updates existing key
                                                 # => Raises if key missing
 
# Map update with default
new_state = Map.update(state, key, default, fn old -> old + 1 end)
                                                 # => Updates if exists
                                                 # => Uses default if missing
 
# Nested map update
new_state = put_in(state, [:user, :profile, :name], "Ahmad")
                                                 # => Updates nested path
                                                 # => Returns new map

Pattern 8: State Validation

Validate state consistency on updates.

defmodule Finance.DonationPool do
  use GenServer
 
  # State: %{total: integer, donations: [%{user_id, amount, timestamp}]}
 
  def start_link(_opts) do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end
 
  def add_donation(user_id, amount) when amount > 0 do
    GenServer.call(__MODULE__, {:add, user_id, amount})
  end
 
  def init(:ok) do
    state = %{total: 0, donations: []}           # => Initial state
    {:ok, state}
  end
 
  def handle_call({:add, user_id, amount}, _from, state) do
    donation = %{
      user_id: user_id,
      amount: amount,
      timestamp: DateTime.utc_now()              # => Record donation time
    }
 
    new_donations = [donation | state.donations] # => Prepend donation
    new_total = state.total + amount             # => Update total
    new_state = %{state | total: new_total, donations: new_donations}
 
    # Validate state consistency
    case validate_state(new_state) do
      :ok ->                                     # => State valid
        {:reply, {:ok, new_total}, new_state}    # => Accept update
      {:error, reason} ->                        # => State invalid
        {:reply, {:error, reason}, state}        # => Reject update, keep old state
    end
  end
 
  defp validate_state(state) do
    calculated_total = Enum.sum(Enum.map(state.donations, & &1.amount))
                                                 # => Sum all donation amounts
    if calculated_total == state.total do
      :ok                                        # => Totals match
    else
      {:error, :total_mismatch}                  # => Inconsistent state
    end
  end
end

Validation best practices:

Validate invariants - Check state consistency on updates
Reject invalid updates - Return old state on validation failure
Use guard clauses - Validate inputs in function heads
Log validation failures - Track consistency violations

Error Handling Strategies

Pattern 9: Let It Crash (Supervised Processes)

Embrace failures - let supervisors handle recovery.

defmodule Finance.TransactionProcessor do
  use GenServer
 
  def start_link(_opts) do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end
 
  def process_transaction(transaction) do
    GenServer.call(__MODULE__, {:process, transaction})
  end
 
  def init(:ok) do
    {:ok, %{}}
  end
 
  def handle_call({:process, transaction}, _from, state) do
    # ❌ DEFENSIVE: Try/catch everything
    # try do
    #   result = validate_and_process(transaction)
    #   {:reply, {:ok, result}, state}
    # rescue
    #   e -> {:reply, {:error, e}, state}
    # end
 
    # ✅ LET IT CRASH: Trust supervisor to restart
    result = validate_and_process!(transaction)  # => Raises on invalid transaction
                                                 # => Process crashes
                                                 # => Supervisor restarts process
                                                 # => State reset to init/1
    {:reply, {:ok, result}, state}               # => Only reached if success
  end
 
  defp validate_and_process!(transaction) do
    # Business logic that may crash
    unless transaction.amount > 0 do
      raise ArgumentError, "amount must be positive"
                                                 # => Raises exception
    end
    # Process transaction...
    {:processed, transaction}
  end
end

When to let it crash:

✅ Crash for:

Invalid input that violates contracts
Unexpected internal errors
Corrupted state requiring reset
Errors where recovery is complex

❌ Don't crash for:

Expected business errors (user not found, insufficient balance)
External service failures (network timeout, API error)
User validation failures

Pattern 10: Explicit Error Handling (Expected Failures)

Use tagged tuples for expected errors.

defmodule Finance.WithdrawalService do
  use GenServer
 
  def start_link(_opts) do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end
 
  def withdraw(user_id, amount) do
    GenServer.call(__MODULE__, {:withdraw, user_id, amount})
  end
 
  def init(:ok) do
    state = %{balances: %{}}                     # => Initial state: empty balances
    {:ok, state}
  end
 
  def handle_call({:withdraw, user_id, amount}, _from, state) do
    current_balance = Map.get(state.balances, user_id, 0)
 
    # EXPLICIT error handling for business logic
    cond do
      amount <= 0 ->                             # => Invalid amount
        {:reply, {:error, :invalid_amount}, state}
 
      current_balance < amount ->                # => Insufficient funds
        {:reply, {:error, :insufficient_funds}, state}
 
      true ->                                    # => Success case
        new_balance = current_balance - amount   # => Deduct amount
        new_balances = Map.put(state.balances, user_id, new_balance)
        new_state = %{state | balances: new_balances}
        {:reply, {:ok, new_balance}, new_state}  # => Return new balance
    end
  end
end

Error handling pattern:

# Pattern match on result
case Finance.WithdrawalService.withdraw(user_id, 100) do
  {:ok, new_balance} ->                          # => Success
    IO.puts("Withdrawal successful. New balance: #{new_balance}")
  {:error, :insufficient_funds} ->               # => Expected error
    IO.puts("Insufficient funds")
  {:error, :invalid_amount} ->                   # => Expected error
    IO.puts("Amount must be positive")
end

Testing OTP Applications

Pattern 11: Testing GenServers

Test GenServers through public API, not internals.

defmodule Finance.ZakatCalculatorTest do
  use ExUnit.Case, async: false                  # => async: false for stateful tests
                                                 # => Prevents parallel execution
 
  alias Finance.ZakatCalculator
 
  setup do
    # Start supervised process for each test
    start_supervised!(ZakatCalculator)           # => Starts process
                                                 # => Automatically stopped after test
    :ok                                          # => Return :ok (no context needed)
  end
 
  test "calculates zakat correctly" do
    # Test through public API
    wealth = 10_000                              # => Total wealth: 10,000
    result = ZakatCalculator.calculate_zakat(wealth)
                                                 # => Call public function
    expected = 250                               # => 2.5% of 10,000
    assert result == {:ok, expected}             # => Verify result
  end
 
  test "rejects negative wealth" do
    result = ZakatCalculator.calculate_zakat(-1000)
    assert result == {:error, :invalid_wealth}   # => Verify error handling
  end
 
  test "maintains state across calls" do
    # Test state persistence
    assert {:ok, _} = ZakatCalculator.set_nisab(5_000)
                                                 # => Set minimum threshold
    assert {:ok, 5_000} = ZakatCalculator.get_nisab()
                                                 # => Verify persistence
  end
end

Testing best practices:

Test public API only - Don't access internal state
Use start_supervised! - Automatic cleanup
Set async: false for stateful tests - Prevent race conditions
Test error cases - Verify error handling

Pattern 12: Testing Supervision Trees

Test supervision behavior with process monitoring.

defmodule Finance.SupervisorTest do
  use ExUnit.Case, async: false
 
  test "supervisor restarts crashed children" do
    # Start supervisor
    {:ok, supervisor_pid} = Finance.Supervisor.start_link([])
                                                 # => Start supervision tree
 
    # Find child process
    children = Supervisor.which_children(supervisor_pid)
                                                 # => List all children
                                                 # => Returns [{id, pid, type, modules}]
    {_id, child_pid, _type, _modules} = List.first(children)
                                                 # => Get first child
 
    # Monitor child to detect restart
    ref = Process.monitor(child_pid)             # => Monitor child process
 
    # Kill child
    Process.exit(child_pid, :kill)               # => Force crash child
 
    # Verify DOWN message received
    assert_receive {:DOWN, ^ref, :process, ^child_pid, :killed}
                                                 # => Child died
 
    # Verify supervisor restarted child
    :timer.sleep(100)                            # => Wait for restart
    new_children = Supervisor.which_children(supervisor_pid)
    {_id, new_pid, _type, _modules} = List.first(new_children)
    assert new_pid != child_pid                  # => New PID - process restarted
  end
end

Supervision testing patterns:

Monitor processes - Use Process.monitor/1 to detect crashes
Verify restart - Check new PID after crash
Test strategies - Verify :one_for_one, :one_for_all behavior
Test restart intensity - Verify max_restarts enforcement

Production Checklist

Before deploying Elixir applications:

Trade-Offs: Raw GenServer vs Abstractions

Aspect	Raw GenServer	Higher Abstractions (Agent, Task)
Control	Full control over callbacks	Limited, simplified API
Learning curve	Steeper (init, handle_call, etc.)	Gentler (get, update)
Use case	Complex state machines, lifecycle	Simple state, fire-and-forget
Debugging	More callbacks to trace	Simpler, fewer moving parts
Performance	Optimal (no indirection)	Slight overhead

Recommendation: Use GenServer for stateful services requiring lifecycle control. Use Agent for simple state wrappers. Use Task for async operations without state.

Next Steps

Anti Patterns - Learn common mistakes to avoid
Processes and Message Passing - Deep dive into BEAM process model
Supervisor Trees - Advanced supervision patterns
Testing Strategies - Comprehensive testing approaches

Best Practices

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