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// Copyright 2024 Cloudflare, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//! Cache eviction module
use crate::key::CompactCacheKey;
use async_trait::async_trait;
use pingora_error::Result;
use std::time::SystemTime;
pub mod lru;
pub mod simple_lru;
/// The trait that a cache eviction algorithm needs to implement
///
/// NOTE: these trait methods require &self not &mut self, which means concurrency should
/// be handled the implementations internally.
#[async_trait]
pub trait EvictionManager {
/// Total size of the cache in bytes tracked by this eviction manager
fn total_size(&self) -> usize;
/// Number of assets tracked by this eviction manager
fn total_items(&self) -> usize;
/// Number of bytes that are already evicted
///
/// The accumulated number is returned to play well with Prometheus counter metric type.
fn evicted_size(&self) -> usize;
/// Number of assets that are already evicted
///
/// The accumulated number is returned to play well with Prometheus counter metric type.
fn evicted_items(&self) -> usize;
/// Admit an item
///
/// Return one or more items to evict. The sizes of these items are deducted
/// from the total size already. The caller needs to make sure that these assets are actually
/// removed from the storage.
///
/// If the item is already admitted, A. update its freshness; B. if the new size is larger than the
/// existing one, Some(_) might be returned for the caller to evict.
fn admit(
&self,
item: CompactCacheKey,
size: usize,
fresh_until: SystemTime,
) -> Vec<CompactCacheKey>;
/// Remove an item from the eviction manager.
///
/// The size of the item will be deducted.
fn remove(&self, item: &CompactCacheKey);
/// Access an item that should already be in cache.
///
/// If the item is not tracked by this [EvictionManager], track it but no eviction will happen.
///
/// The call used for asking the eviction manager to track the assets that are already admitted
/// in the cache storage system.
fn access(&self, item: &CompactCacheKey, size: usize, fresh_until: SystemTime) -> bool;
/// Peek into the manager to see if the item is already tracked by the system
///
/// This function should have no side-effect on the asset itself. For example, for LRU, this
/// method shouldn't change the popularity of the asset being peeked.
fn peek(&self, item: &CompactCacheKey) -> bool;
/// Serialize to save the state of this eviction manager to disk
///
/// This function is for preserving the eviction manager's state across server restarts.
///
/// `dir_path` define the directory on disk that the data should use.
// dir_path is &str no AsRef<Path> so that trait objects can be used
async fn save(&self, dir_path: &str) -> Result<()>;
/// The counterpart of [Self::save()].
async fn load(&self, dir_path: &str) -> Result<()>;
}
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