Least Recently Used (LRU) Caches and expirable LRU caches

A Least Recently Used (LRU) Cache organizes items in order of use. It drops items not used for the longest amount of time in favor of caching new items. There are also expirable LRU caches that drop expired records and may prolong record lifetime on usage.

To implement those caches userver provides a base component class cache::LruCacheComponent that manages an instance of cache::ExpirableLruCache.

Typical use cases

LRU cache is useful for reducing the network interactions while working with data that does not fit into service memory and rarely changes. Examples:

• caching user ids by their emails
• caching localized strings by their ids

Expirable LRU cache is useful for reducing the network interactions while working with data does not fit into service memory and changes in predetermined intervals. Examples:

• caching tokens that expire in an hour
• caching road average traffic situation for an area for a few minutes
• caching cab rating for a median travel time

Usage

Using cache::LruCacheComponent is quite straightforward. Write a component that derives from it:

#include <userver/cache/lru_cache_component_base.hpp>
#include <userver/components/component_list.hpp>
 
using Key = std::string;
using Value = std::size_t;
 
class ExampleCacheComponent final : public cache::LruCacheComponent<Key, Value> {
public:
    static constexpr std::string_view kName = "example-cache";
 
    ExampleCacheComponent(const components::ComponentConfig& config, const components::ComponentContext& context)
        : ::cache::LruCacheComponent<Key, Value>(config, context) {}
 
private:
    Value DoGetByKey(const Key& key) override { return GetValueForExpiredKeyFromRemote(key); }
 
    Value GetValueForExpiredKeyFromRemote(const Key& key);
};

After that, get the component using the components::ComponentContext::FindComponent() and call cache::LruCacheComponent::GetCache(). Use the returned cache::LruCacheWrapper.

Low level primitives

cache::LruCacheComponent should be your choice by default for implementing all the LRU caches. However, in some cases there is a need to avoid using the component system, or to avoid synchronization, or to control the expiration logic more precisely.

Here's a brief introduction into LRU types:

• Concurrency-safe expirable component cache::LruCacheComponent. Call cache::LruCacheComponent::GetCache() to get a cache::LruCacheWrapper that provides actual interface to work with the cache.
• Concurrency-safe expirable cache::LruCacheWrapper is a smart pointer that provides simplified interface to the cache::ExpirableLruCache.
• Concurrency-safe expirable container cache::ExpirableLruCache with precise control over the expiration logic.
• Concurrency-safe non-expirable container cache::NWayLRU.
• Non-expirable container cache::LruMap that provides the same concurrency guarantees as the standard library containers.
• Non-expirable cache::LruSet that provides the same concurrency guarantees as the standard library containers.

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