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The Cache module provides a high-performance, effect-based caching solution with automatic time-to-live (TTL) management, capacity limits, and customizable lookup functions.

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Overview

A Cache<Key, A, E, R> is a mutable key-value store that:
  • Automatically populates missing entries using a lookup function
  • Enforces capacity limits with automatic eviction
  • Supports time-to-live (TTL) for cache entries
  • Integrates seamlessly with Effect for safe, composable caching
  • Handles concurrent access without race conditions

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Creating Caches

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Basic Cache

import { Cache, Effect } from "effect"

const program = Effect.gen(function*() {
  const cache = yield* Cache.make<string, number>({
    capacity: 100,
    lookup: (key: string) => Effect.succeed(key.length)
  })
  
  const length1 = yield* Cache.get(cache, "hello")
  console.log(length1) // 5
  
  const length2 = yield* Cache.get(cache, "hello") // cached
  console.log(length2) // 5
})

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Cache with TTL

import { Cache, Effect } from "effect"

const userCache = Effect.gen(function*() {
  const cache = yield* Cache.make<number, User, Error>({
    capacity: 1000,
    lookup: (userId: number) => fetchUserFromDatabase(userId),
    timeToLive: "5 minutes" // Entries expire after 5 minutes
  })
  
  return cache
})

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Cache with Error Handling

import { Cache, Effect } from "effect"

const program = Effect.gen(function*() {
  const cache = yield* Cache.make<string, number, string>({
    capacity: 10,
    lookup: (key: string) =>
      key === "error"
        ? Effect.fail("Lookup failed")
        : Effect.succeed(key.length)
  })
  
  // Successful lookup
  const success = yield* Cache.get(cache, "test")
  
  // Failed lookup - error is cached temporarily
  const failure = yield* Effect.exit(Cache.get(cache, "error"))
})

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Advanced Cache Creation

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Dynamic TTL with makeWith

import { Cache, Effect, Exit } from "effect"

const program = Effect.gen(function*() {
  const cache = yield* Cache.makeWith<string, number, string>({
    capacity: 100,
    lookup: (key) =>
      key === "fail"
        ? Effect.fail("error")
        : Effect.succeed(key.length),
    timeToLive: (exit, key) => {
      // Different TTL for successes vs failures
      if (Exit.isFailure(exit)) {
        return "1 minute" // Short TTL for errors
      }
      // Different TTL based on key
      return key.startsWith("temp") ? "5 minutes" : "1 hour"
    }
  })
  
  return cache
})

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TTL Based on Value

import { Cache, Effect, Exit } from "effect"

const userCache = Effect.gen(function*() {
  const cache = yield* Cache.makeWith<
    number,
    { id: number; active: boolean },
    never
  >({
    capacity: 1000,
    lookup: (id) => Effect.succeed({ id, active: id % 2 === 0 }),
    timeToLive: (exit) => {
      if (Exit.isSuccess(exit)) {
        const user = exit.value
        // Active users cached longer
        return user.active ? "1 hour" : "5 minutes"
      }
      return "30 seconds"
    }
  })
  
  return cache
})

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Using Caches

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Get Values

import { Cache, Effect } from "effect"

const program = Effect.gen(function*() {
  const cache = yield* Cache.make<string, number>({
    capacity: 100,
    lookup: (key) => Effect.succeed(key.length)
  })
  
  // Get a value (populates if missing)
  const value = yield* Cache.get(cache, "hello")
  console.log(value) // 5
})

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Invalidate Entries

import { Cache, Effect } from "effect"

const program = Effect.gen(function*() {
  const cache = yield* Cache.make<string, number>({
    capacity: 100,
    lookup: (key) => Effect.succeed(Date.now())
  })
  
  const time1 = yield* Cache.get(cache, "timestamp")
  
  // Invalidate specific key
  yield* Cache.invalidate(cache, "timestamp")
  
  const time2 = yield* Cache.get(cache, "timestamp")
  // time2 will be different (cache was invalidated)
})

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Refresh Values

import { Cache, Effect } from "effect"

const program = Effect.gen(function*() {
  const cache = yield* Cache.make<string, number>({
    capacity: 100,
    lookup: (key) => Effect.succeed(Date.now())
  })
  
  // Get current value
  const time1 = yield* Cache.get(cache, "timestamp")
  
  // Refresh - runs lookup and updates cache
  yield* Cache.refresh(cache, "timestamp")
  
  // Get refreshed value
  const time2 = yield* Cache.get(cache, "timestamp")
})

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Set Values Manually

import { Cache, Effect } from "effect"

const program = Effect.gen(function*() {
  const cache = yield* Cache.make<string, number>({
    capacity: 100,
    lookup: (key) => Effect.succeed(0)
  })
  
  // Manually set a value
  yield* Cache.set(cache, "count", 42)
  
  const value = yield* Cache.get(cache, "count")
  console.log(value) // 42
})

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Capacity and Eviction

import { Cache, Effect } from "effect"

const program = Effect.gen(function*() {
  // Cache with limited capacity
  const cache = yield* Cache.make<number, string>({
    capacity: 5, // Only holds 5 items
    lookup: (n) => Effect.succeed(`value-${n}`)
  })
  
  // Fill cache beyond capacity
  for (let i = 0; i < 10; i++) {
    yield* Cache.get(cache, i)
  }
  
  // Oldest entries are evicted
  const size = yield* Cache.size(cache)
  console.log(size) // At most 5
})

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Inspecting Cache State

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Get Size

import { Cache, Effect } from "effect"

const size = Effect.gen(function*() {
  const cache = yield* Cache.make<string, number>({
    capacity: 100,
    lookup: (key) => Effect.succeed(key.length)
  })
  
  yield* Cache.get(cache, "hello")
  yield* Cache.get(cache, "world")
  
  const currentSize = yield* Cache.size(cache)
  console.log(currentSize) // 2
})

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Get Keys

import { Cache, Effect } from "effect"

const program = Effect.gen(function*() {
  const cache = yield* Cache.make<string, number>({
    capacity: 100,
    lookup: (key) => Effect.succeed(key.length)
  })
  
  yield* Cache.get(cache, "foo")
  yield* Cache.get(cache, "bar")
  
  const keys = yield* Cache.keys(cache)
  console.log(keys) // ["foo", "bar"]
})

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Advanced Patterns

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Cache with Complex Keys

import { Cache, Data, Effect } from "effect"

// Use Data.Class for structural equality
class UserId extends Data.Class<{ id: number }> {}

const program = Effect.gen(function*() {
  const userCache = yield* Cache.make<UserId, string>({
    capacity: 1000,
    lookup: (userId: UserId) =>
      Effect.succeed(`User-${userId.id}`)
  })
  
  const name = yield* Cache.get(
    userCache,
    new UserId({ id: 123 })
  )
  console.log(name) // "User-123"
})

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Cache as a Service

import { Cache, Effect, Layer, ServiceMap } from "effect"

class UserCache extends ServiceMap.Service<UserCache, {
  getUser(id: number): Effect.Effect<User, Error>
}>()("UserCache") {
  static readonly layer = Layer.effect(
    UserCache,
    Effect.gen(function*() {
      const cache = yield* Cache.make<number, User, Error>({
        capacity: 1000,
        lookup: (id) => fetchUserFromDatabase(id),
        timeToLive: "5 minutes"
      })
      
      return UserCache.of({
        getUser: (id) => Cache.get(cache, id)
      })
    })
  )
}

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Best Practices

  1. Choose appropriate capacity: Set capacity based on memory constraints
  2. Use TTL for frequently changing data: Prevent stale data with appropriate TTL
  3. Handle lookup errors: Consider error caching strategy (short TTL for errors)
  4. Monitor cache performance: Track hit/miss ratios and adjust capacity
  5. Use structural equality for complex keys: Use Data.Class or similar for complex key types
  6. Wrap in services for reusability: Expose caches through service layers

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Performance Considerations

  • Cache operations are lock-free and thread-safe
  • TTL is evaluated on access, not via background processes
  • Eviction happens synchronously when capacity is exceeded
  • Consider using separate caches for different data types to optimize capacity usage

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Next Steps

  • Learn about Effect for building effectful programs
  • Explore Layer for service composition
  • Understand Schedule for cache refresh strategies

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