Trait L2CacheBackend 

pub trait L2CacheBackend: CacheBackend {
    // Required method
    fn get_with_ttl<'life0, 'life1, 'async_trait>(
        &'life0 self,
        key: &'life1 str,
    ) -> Pin<Box<dyn Future<Output = Option<(Value, Option<Duration>)>> + Send + 'async_trait>>
       where Self: 'async_trait,
             'life0: 'async_trait,
             'life1: 'async_trait;
}

Extended trait for L2 cache backends with TTL introspection

This trait extends CacheBackend with the ability to retrieve both a value and its remaining TTL. This is essential for implementing efficient L2-to-L1 promotion with accurate TTL propagation.

Use Cases

• L2-to-L1 promotion with same TTL
• TTL-based cache warming strategies
• Monitoring and analytics

Example

use multi_tier_cache::{CacheBackend, L2CacheBackend, async_trait};
use std::time::Duration;
use anyhow::Result;

struct MyDistributedCache;

#[async_trait]
impl CacheBackend for MyDistributedCache {
    async fn get(&self, _key: &str) -> Option<serde_json::Value> { None }
    async fn set_with_ttl(&self, _k: &str, _v: serde_json::Value, _t: Duration) -> Result<()> { Ok(()) }
    async fn remove(&self, _k: &str) -> Result<()> { Ok(()) }
    async fn health_check(&self) -> bool { true }
}

#[async_trait]
impl L2CacheBackend for MyDistributedCache {
    async fn get_with_ttl(&self, key: &str) -> Option<(serde_json::Value, Option<Duration>)> {
        // Retrieve value and calculate remaining TTL
        None
    }
}

Required Methods

fn get_with_ttl<'life0, 'life1, 'async_trait>( &'life0 self, key: &'life1 str, ) -> Pin<Box<dyn Future<Output = Option<(Value, Option<Duration>)>> + Send + 'async_trait>>

where Self: 'async_trait, 'life0: 'async_trait, 'life1: 'async_trait,

Get value with its remaining TTL from L2 cache

This method retrieves both the value and its remaining time-to-live. This is used by the cache manager to promote entries from L2 to L1 with the correct TTL.

Arguments

• key - The cache key to retrieve

Returns

• Some((value, Some(ttl))) - Value found with remaining TTL
• Some((value, None)) - Value found but no expiration set (never expires)
• None - Key not found or expired

TTL Semantics

• TTL represents the remaining time until expiration
• None TTL means the key has no expiration
• Implementations should use backend-specific TTL commands (e.g., Redis TTL)

Implementors

impl L2CacheBackend for RedisCache

Implement L2CacheBackend trait for RedisCache

This extends CacheBackend with TTL introspection capabilities needed for L2->L1 promotion.