Resource Lifecycle

Layer 2: Design Choices

Core Question

When should this resource be created, used, and cleaned up?

Before implementing lifecycle:

• What's the resource's scope?
• Who owns the cleanup responsibility?
• What happens on error?

Lifecycle Pattern → Implementation

Drop

OnceLock

LazyLock

r2d2

deadpool

MutexGuard

Thinking Prompt

Before designing lifecycle:

1. What's the resource cost?

   • Cheap → create per use
   • Expensive → pool or cache
   • Global → lazy singleton

2. What's the scope?

   • Function-local → stack allocation
   • Request-scoped → passed or extracted
   • Application-wide → static or Arc

3. What about errors?

   • Cleanup must happen → Drop
   • Cleanup is optional → explicit close
   • Cleanup can fail → Result from close

Trace Up ↑

To domain constraints (Layer 3):

"How should I manage database connections?"
    ↑ Ask: What's the connection cost?
    ↑ Check: domain-* (latency requirements)
    ↑ Check: Infrastructure (connection limits)

Trace Down ↓

To implementation (Layer 1):

"Need automatic cleanup"
    ↓ m02-resource: Implement Drop
    ↓ m01-ownership: Clear owner for cleanup

"Need lazy initialization"
    ↓ m03-mutability: OnceLock for thread-safe
    ↓ m07-concurrency: LazyLock for sync

"Need connection pool"
    ↓ m07-concurrency: Thread-safe pool
    ↓ m02-resource: Arc for sharing

Quick Reference

Drop

OnceLock

LazyLock

r2d2

deadpool

MutexGuard

Lifecycle Events

new()

Default

OnceLock::get_or_init

&self

&mut self

Drop::drop()

Pattern Templates

RAII Guard

struct FileGuard {
    path: PathBuf,
    _handle: File,
}

impl Drop for FileGuard {
    fn drop(&mut self) {
        // Cleanup: remove temp file
        let _ = std::fs::remove_file(&self.path);
    }
}

Lazy Singleton

use std::sync::OnceLock;

static CONFIG: OnceLock<Config> = OnceLock::new();

fn get_config() -> &'static Config {
    CONFIG.get_or_init(|| {
        Config::load().expect("config required")
    })
}

Common Errors

Option::take()

Anti-Patterns

lazy_static!

std::sync::OnceLock

OnceLock

Related Skills