カスタムOTPビヘイビア

You've learned about OTP behaviors like GenServer and GenStatem. They provide a standard way to build robust, fault-tolerant components.

But what if you have a recurring pattern that isn't perfectly covered by existing behaviors? This is where custom OTP behaviors come in handy!

They let you define your own generic component structure.

Creating custom behaviors offers several key advantages:

• Code Reuse: Encapsulate common logic once and reuse it across many modules.
• Consistency: Ensure all components following your behavior adhere to a specific interface and structure.
• Abstraction: Hide complex internal details, exposing a simpler API to users.
• Maintainability: Changes to the core logic only need to happen in one place.

An OTP behavior typically consists of two main parts:

• The Behavior Module: This module defines the public interface (functions users call) and often provides helper functions for the callback module. It uses the -behaviour(gen_server) or similar attribute to link to a generic server.
• The Callback Module: This is where the actual logic lives. It implements the callback functions (like init/1, handle_call/3) required by the behavior module.

Think of it as a contract between the two.

The "behavior module" is what other modules -behaviour(...) against. For custom behaviors, you'll often define a module that wraps an existing generic behavior (like gen_server) but adds your specific API.

It acts as the client-side interface for users of your custom behavior.

Key aspects:

• Defines the public functions (e.g., start_link/0, my_action/1).
• These functions typically call gen_server:start_link/3 or gen_server:call/2 internally.
• It specifies the callback module using the -callback attribute.

The "callback module" is where the core functionality of your custom behavior resides. It's the module that actually implements the required functions defined by the underlying generic behavior (like gen_server or gen_statem).

• It must implement functions like init/1, handle_call/3, handle_cast/2, etc.
• These functions manage the state and respond to messages.
• This module is what the behavior module (e.g., gen_server) calls directly.

Let's create a simple custom counter behavior. We'll wrap a gen_server to manage an integer count.

First, define the behavior module, which acts as the client API and starts the underlying gen_server.

Now, let's add the public functions to interact with our counter (increment, decrement, get_count) and implement their corresponding handle_call logic.

These public functions will use gen_server:call/2 to send requests to the actual counter process.

With our my_counter behavior defined, we can now easily use it from an Erlang shell or another module. Notice how simple the client-side code is!

You don't need to know the gen_server details; you just use the custom behavior's API.

Custom OTP behaviors are powerful, but not every component needs one. Consider creating a custom behavior when:

• You find yourself writing similar gen_server or gen_statem boilerplate repeatedly.
• You want to enforce a specific pattern or interface across multiple components.
• You need to provide a simpler, higher-level API for a complex underlying process.
• You are building a reusable library or framework component.

You've explored the world of custom OTP behaviors!

• We saw that custom behaviors allow you to encapsulate common patterns.
• They typically consist of a **behavior module** (public API) and a **callback module** (logic).
• By wrapping existing behaviors like gen_server, you can create powerful, reusable components.

Mastering custom behaviors empowers you to build highly modular and consistent Erlang applications.