Cache eviction policies — LRU, LFU, and TTL explained

What Eviction Is

When a cache is full and a new item needs to be stored, the cache must evict something. The eviction policy determines what gets removed.

LRU — Least Recently Used

Evicts the item that hasn't been accessed for the longest time. Best for workloads with temporal locality — recently accessed items are likely to be accessed again soon.

# LRU in Python using OrderedDict
from collections import OrderedDict

class LRUCache:
    def __init__(self, capacity):
        self.cache = OrderedDict()
        self.capacity = capacity

    def get(self, key):
        if key in self.cache:
            self.cache.move_to_end(key)
            return self.cache[key]
        return -1

    def put(self, key, value):
        if key in self.cache:
            self.cache.move_to_end(key)
        self.cache[key] = value
        if len(self.cache) > self.capacity:
            self.cache.popitem(last=False)

LFU — Least Frequently Used

Evicts the item accessed fewest times total. Better for workloads with stable hot items (popular products, frequent queries). But new items are vulnerable — they start at frequency 1 and may be evicted before becoming popular.

TTL — Time To Live

Items expire after a fixed time regardless of access. Simple, predictable. Use for data with known staleness tolerance: weather data (TTL 10 min), session tokens (TTL 24h), product catalog (TTL 1h).