Merge branch 'feature/ext-cache/core-and-caffeine' into feature/extensions/validation-scheduler-cache

# Conflicts: # AGENTS.md # flash-extensions/pom.xml # pom.xml
2026-09-09 12:37:52 +00:00
parent dbf057f493 24bb10175d
commit 9de2800a83
17 changed files with 753 additions and 1 deletions
@@ -0,0 +1,85 @@
+# flash-ext-cache-caffeine
+
+In-process caching backed by [Caffeine](https://github.com/ben-manes/caffeine). Implements
+[`flash-ext-cache-core`](../../flash-ext-cache-core/docs/README.md).
+
+## Dependency
+
+```xml
+<dependency>
+    <groupId>dev.relism</groupId>
+    <artifactId>flash-ext-cache-caffeine</artifactId>
+    <version>${flash.version}</version>
+</dependency>
+```
+
+## Quick start
+
+```java
+FlashApp.create(8080)
+    .install(new CaffeineCacheExtension())
+    .scan("dev.example.api");
+```
+
+```java
+@GET("/api/users/{id}")
+public final class GetUser extends RequestHandler {
+
+    private Cache<String, User> users;
+    private UserRepository repo;
+
+    @Override protected void onInit() {
+        repo  = require(UserRepository.class);
+        users = require(CacheManager.class).build("users", spec -> spec
+                .maxSize(10_000)
+                .ttl(Duration.ofMinutes(10)));
+    }
+
+    @Override public Object handle(Request req, Response res) {
+        return users.get(req.param("id"), repo::findById);
+    }
+}
+```
+
+The extension takes no configuration. Each cache declares its own size and TTL where it is built.
+
+## Why Caffeine and not a `LinkedHashMap`
+
+An LRU on top of `LinkedHashMap` is about sixty lines, and for a cache that is genuinely
+low-traffic it is the right answer — `ConcurrentHashMap::computeIfAbsent` is one line and has no
+hit rate to get wrong.
+
+This module exists for the case where that stops being true. Caffeine's W-TinyLFU admission,
+striped frequency counters and amortised eviction are not a weekend's work to reproduce, and the
+failure mode of getting them wrong is a cache that is *slower* than no cache — lock contention on
+every lookup, or an eviction policy that throws away exactly the entries you were about to want.
+
+## Lifecycle
+
+Caches are released through `FlashContext.onClose`, so `app.stop()` drops every entry. That is
+invisible in production with one app per process and matters immediately under test, where many
+apps start and stop in one JVM.
+
+## Statistics
+
+```java
+CacheStats stats = users.stats();
+stats.hitRate();   // 0.0 until something is looked up
+```
+
+Requires `recordStats()` on the spec. Without it you get `CacheStats.DISABLED`, which is honest
+about being unmeasured rather than reporting zeroes that look like a cold cache.
+
+`manager.names()` lists every cache built so far, for an ops endpoint.
+
+## What this is not
+
+**HTTP caching.** If what you want is for the *client* to stop asking — `Cache-Control`, `ETag`,
+`304 Not Modified` — that is a middleware, not an object cache, and it saves the whole request
+rather than the lookup inside it. Reach for that first: it is cheaper, and the two solve different
+problems.
+
+**A shared cache.** Every replica has its own. Two instances will hold different values for the
+same key, and an invalidation on one does not reach the other. When that becomes a problem the
+answer is a networked backend — see the note on `flash-ext-cache-redis` — and the semantics change
+with it: a cache that can fail is no longer transparent.
@@ -0,0 +1,39 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <parent>
+        <groupId>dev.relism</groupId>
+        <artifactId>flash-extensions</artifactId>
+        <version>2.1.0-SNAPSHOT</version>
+    </parent>
+
+    <artifactId>flash-ext-cache-caffeine</artifactId>
+
+    <dependencies>
+        <dependency>
+            <groupId>dev.relism</groupId>
+            <artifactId>flash-ext-cache-core</artifactId>
+        </dependency>
+        <!--
+          Caffeine rather than a hand-rolled LRU: W-TinyLFU admission, striped counters and
+          amortised eviction are not a weekend's work to get right, and getting them wrong is a
+          cache that is slower than no cache.
+        -->
+        <dependency>
+            <groupId>com.github.ben-manes.caffeine</groupId>
+            <artifactId>caffeine</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>dev.relism</groupId>
+            <artifactId>flash-testing</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>
@@ -0,0 +1,41 @@
+package dev.relism.flash.ext.cache.caffeine;
+
+import dev.relism.flash.ext.cache.Cache;
+import dev.relism.flash.ext.cache.CacheStats;
+
+import java.util.function.Function;
+
+/**
+ * {@link Cache} over a Caffeine cache. A thin adapter by design: every method delegates directly,
+ * adding no wrapper object, no copy and no synchronisation of its own.
+ */
+final class CaffeineCache<K, V> implements Cache<K, V> {
+
+    private final com.github.benmanes.caffeine.cache.Cache<K, V> delegate;
+    private final boolean statsRecorded;
+
+    CaffeineCache(com.github.benmanes.caffeine.cache.Cache<K, V> delegate, boolean statsRecorded) {
+        this.delegate = delegate;
+        this.statsRecorded = statsRecorded;
+    }
+
+    @Override
+    public V get(K key, Function<? super K, ? extends V> loader) {
+        // Caffeine's own get(key, mappingFunction) already guarantees the loader runs once per key
+        // across concurrent callers; wrapping it in anything of ours would only add a race.
+        return delegate.get(key, loader);
+    }
+
+    @Override public V getIfPresent(K key)      { return delegate.getIfPresent(key); }
+    @Override public void put(K key, V value)   { delegate.put(key, value); }
+    @Override public void invalidate(K key)     { delegate.invalidate(key); }
+    @Override public void invalidateAll()       { delegate.invalidateAll(); }
+    @Override public long estimatedSize()       { return delegate.estimatedSize(); }
+
+    @Override
+    public CacheStats stats() {
+        if (!statsRecorded) return CacheStats.DISABLED;
+        com.github.benmanes.caffeine.cache.stats.CacheStats snapshot = delegate.stats();
+        return new CacheStats(snapshot.hitCount(), snapshot.missCount(), snapshot.evictionCount());
+    }
+}
@@ -0,0 +1,33 @@
+package dev.relism.flash.ext.cache.caffeine;
+
+import dev.relism.flash.ext.cache.CacheManager;
+import dev.relism.flash.extension.FlashContext;
+import dev.relism.flash.extension.FlashExtension;
+import dev.relism.flash.extension.FlashRegistrar;
+
+/**
+ * Installs an in-process {@link CacheManager} backed by Caffeine.
+ *
+ * <pre>{@code
+ * FlashApp.create(8080)
+ *     .install(new CaffeineCacheExtension())
+ *     .scan("dev.example.api");
+ * }</pre>
+ *
+ * <p>No configuration. Each cache declares its own size and TTL where it is built, because those
+ * are properties of what is being cached, not of the process caching it.
+ *
+ * <p>Caches are dropped through {@link FlashContext#onClose}, so a stopped app does not keep its
+ * values alive — which matters when many apps start and stop in one JVM, as they do under test.
+ */
+public final class CaffeineCacheExtension implements FlashExtension {
+
+    @Override
+    public void configure(FlashRegistrar<?> app, FlashContext ctx) {
+        ctx.supply(CacheManager.class, services -> {
+            CaffeineCacheManager manager = new CaffeineCacheManager();
+            services.onClose(manager::clear);
+            return manager;
+        });
+    }
+}
@@ -0,0 +1,66 @@
+package dev.relism.flash.ext.cache.caffeine;
+
+import com.github.benmanes.caffeine.cache.Caffeine;
+import dev.relism.flash.ext.cache.Cache;
+import dev.relism.flash.ext.cache.CacheManager;
+import dev.relism.flash.ext.cache.CacheSpec;
+
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+
+/** In-process {@link CacheManager} backed by Caffeine. */
+final class CaffeineCacheManager implements CacheManager {
+
+    private final Map<String, Entry> caches = new ConcurrentHashMap<>();
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public <K, V> Cache<K, V> build(String name, java.util.function.Consumer<CacheSpec> configure) {
+        CacheSpec spec = CacheSpec.of();
+        configure.accept(spec);
+
+        Entry entry = caches.computeIfAbsent(name, key -> new Entry(describe(spec), create(spec)));
+        // Two handlers sharing a cache is the point; two handlers disagreeing about its size or
+        // TTL is a bug that would otherwise resolve to whichever one ran first.
+        String requested = describe(spec);
+        if (!entry.signature.equals(requested))
+            throw new IllegalStateException("Cache '" + name + "' already exists as " + entry.signature
+                    + " but was requested as " + requested);
+        return (Cache<K, V>) entry.cache;
+    }
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public <K, V> Cache<K, V> cache(String name) {
+        Entry entry = caches.get(name);
+        return entry == null ? null : (Cache<K, V>) entry.cache;
+    }
+
+    @Override
+    public Set<String> names() {
+        return Set.copyOf(caches.keySet());
+    }
+
+    /** Releases every entry so a stopped app does not keep its values alive. */
+    void clear() {
+        caches.values().forEach(entry -> entry.cache.invalidateAll());
+        caches.clear();
+    }
+
+    private static CaffeineCache<Object, Object> create(CacheSpec spec) {
+        Caffeine<Object, Object> builder = Caffeine.newBuilder();
+        if (spec.bounded()) builder.maximumSize(spec.maxSize());
+        if (spec.ttl() != null) builder.expireAfterWrite(spec.ttl());
+        if (spec.ttlAfterAccess() != null) builder.expireAfterAccess(spec.ttlAfterAccess());
+        if (spec.statsRecorded()) builder.recordStats();
+        return new CaffeineCache<>(builder.build(), spec.statsRecorded());
+    }
+
+    private static String describe(CacheSpec spec) {
+        return "maxSize=" + spec.maxSize() + " ttl=" + spec.ttl()
+                + " ttlAfterAccess=" + spec.ttlAfterAccess() + " stats=" + spec.statsRecorded();
+    }
+
+    private record Entry(String signature, CaffeineCache<Object, Object> cache) {}
+}
@@ -0,0 +1,160 @@
+package dev.relism.flash.ext.cache.caffeine;
+
+import dev.relism.flash.ext.cache.Cache;
+import dev.relism.flash.ext.cache.CacheManager;
+import dev.relism.flash.ext.cache.CacheStats;
+import dev.relism.flash.extension.FlashApp;
+import dev.relism.flash.extension.FlashConfiguration;
+import dev.relism.flash.testing.FlashTest;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+import java.time.Duration;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CaffeineCacheTest {
+
+    private static final AtomicInteger loads = new AtomicInteger();
+
+    @RegisterExtension
+    static FlashTest app = FlashTest.of(configured -> {
+        configured.install(new CaffeineCacheExtension());
+        configured.ctx().onReady(() -> {
+            Cache<String, String> users = configured.ctx().require(CacheManager.class)
+                    .build("users", spec -> spec.maxSize(100).ttl(Duration.ofMinutes(5)).recordStats());
+            configured.get("/users/{id}", (req, res) ->
+                    users.get(req.param("id"), id -> "loaded:" + id + ":" + loads.incrementAndGet()));
+        });
+    });
+
+    private static CacheManager manager() {
+        return app.app().ctx().require(CacheManager.class);
+    }
+
+    @Test
+    void aRepeatedRequestIsServedFromCache() {
+        String first = app.get("/users/alice").expectStatus(200).body();
+        String second = app.get("/users/alice").expectStatus(200).body();
+
+        assertEquals(first, second);
+        assertTrue(first.startsWith("loaded:alice:"));
+    }
+
+    @Test
+    void distinctKeysLoadSeparately() {
+        assertNotEquals(app.get("/users/bob").body(), app.get("/users/carol").body());
+    }
+
+    @Test
+    void statsCountHitsAndMisses() {
+        Cache<String, String> cache = manager().build("stats-probe", spec -> spec.maxSize(10).recordStats());
+        cache.get("k", key -> "v");
+        cache.get("k", key -> "v");
+
+        CacheStats stats = cache.stats();
+        assertEquals(1, stats.misses());
+        assertEquals(1, stats.hits());
+        assertEquals(0.5, stats.hitRate());
+    }
+
+    @Test
+    void statsAreDisabledUnlessAskedFor() {
+        Cache<String, String> cache = manager().build("no-stats", spec -> spec.maxSize(10));
+        cache.get("k", key -> "v");
+
+        assertEquals(CacheStats.DISABLED, cache.stats());
+    }
+
+    @Test
+    void theLoaderRunsOncePerKeyUnderConcurrency() throws Exception {
+        Cache<String, String> cache = manager().build("single-flight", spec -> spec.maxSize(10));
+        AtomicInteger invocations = new AtomicInteger();
+        int threads = 16;
+        CountDownLatch start = new CountDownLatch(1);
+        CountDownLatch done = new CountDownLatch(threads);
+
+        for (int i = 0; i < threads; i++) {
+            Thread.ofVirtual().start(() -> {
+                try {
+                    start.await();
+                    cache.get("hot", key -> {
+                        invocations.incrementAndGet();
+                        return "value";
+                    });
+                } catch (InterruptedException e) {
+                    Thread.currentThread().interrupt();
+                } finally {
+                    done.countDown();
+                }
+            });
+        }
+        start.countDown();
+        assertTrue(done.await(5, java.util.concurrent.TimeUnit.SECONDS));
+
+        assertEquals(1, invocations.get(), "concurrent callers must share one load, not race");
+    }
+
+    @Test
+    void aNullLoaderResultStoresNothing() {
+        Cache<String, String> cache = manager().build("nulls", spec -> spec.maxSize(10));
+
+        assertNull(cache.get("missing", key -> null));
+        assertNull(cache.getIfPresent("missing"));
+    }
+
+    @Test
+    void invalidateDropsOneKeyAndInvalidateAllDropsEverything() {
+        Cache<String, String> cache = manager().build("invalidation", spec -> spec.maxSize(10));
+        cache.put("a", "1");
+        cache.put("b", "2");
+
+        cache.invalidate("a");
+        assertNull(cache.getIfPresent("a"));
+        assertEquals("2", cache.getIfPresent("b"));
+
+        cache.invalidateAll();
+        assertNull(cache.getIfPresent("b"));
+    }
+
+    @Test
+    void buildIsIdempotentPerName() {
+        Cache<String, String> first = manager().build("shared", spec -> spec.maxSize(10));
+        Cache<String, String> second = manager().build("shared", spec -> spec.maxSize(10));
+
+        assertSame(first, second, "two handlers asking for one cache must get one cache");
+        assertSame(first, manager().cache("shared"));
+    }
+
+    @Test
+    void disagreeingOnASharedCacheIsARejectedMistakeNotASilentWinner() {
+        manager().build("contested", spec -> spec.maxSize(10));
+
+        IllegalStateException conflict = assertThrows(IllegalStateException.class,
+                () -> manager().build("contested", spec -> spec.maxSize(999)));
+        assertTrue(conflict.getMessage().contains("contested"), conflict.getMessage());
+    }
+
+    @Test
+    void unknownNameReturnsNullRatherThanBuildingOne() {
+        assertNull(manager().cache("never-built"));
+    }
+
+    /** Why CaffeineCacheExtension registers onClose: values must not outlive the app holding them. */
+    @Test
+    void stoppingTheAppReleasesEveryCache() {
+        FlashApp standalone = FlashApp.create(FlashConfiguration.builder()
+                .port(0).host("127.0.0.1").shutdownDrainTimeoutMs(250).build())
+                .install(new CaffeineCacheExtension());
+        standalone.start();
+        CacheManager manager = standalone.ctx().require(CacheManager.class);
+        manager.build("scoped", spec -> spec.maxSize(10)).put("k", "v");
+        assertEquals(1, manager.names().size());
+
+        standalone.stop().join();
+
+        assertTrue(manager.names().isEmpty(), "caches must be released when the app stops");
+    }
+}
@@ -0,0 +1,64 @@
+# flash-ext-cache-core
+
+The caching contract, shared across backends. Like `flash-ext-data-core`, this module talks to
+nothing: it defines the abstractions and a backend implements them.
+
+## Components
+
+- `Cache<K, V>` — a named cache. `get(key, loader)` is the method that matters.
+- `CacheManager` — creates and hands back named caches.
+- `CacheSpec` — size and expiry for one cache.
+- `CacheStats` — hit/miss/eviction counters.
+
+Install a backend, not this module: [`flash-ext-cache-caffeine`](../../flash-ext-cache-caffeine/docs/README.md)
+for in-process caching.
+
+## The one shape that matters
+
+```java
+User user = users.get(id, repo::findById);
+```
+
+Compute-if-absent is the only cache operation most code needs, and the only one that is hard to
+get right — the loader runs **once per key** across concurrent callers, and the rest wait rather
+than each computing their own. `getIfPresent`, `put`, `invalidate` and `invalidateAll` exist for
+what it cannot express.
+
+A loader returning `null` stores nothing and returns `null`. Caching absence is a decision, not a
+default; wrap it in an `Optional` or a sentinel if you want it.
+
+## Naming and sharing
+
+`CacheManager.build(name, spec)` is idempotent per name: two handlers asking for `"users"` get one
+cache, not two, so nobody has to coordinate who creates it first.
+
+If they disagree about the spec, that throws. The alternative is a cache whose size depends on
+which handler happened to initialise first, which is the kind of bug that only shows up under
+load.
+
+## Specs
+
+```java
+CacheSpec.of()
+    .maxSize(10_000)
+    .ttl(Duration.ofMinutes(10))
+    .recordStats();
+```
+
+Every field is optional, but a spec that sets neither `maxSize` nor `ttl` is an unbounded cache
+that never expires — a memory leak wearing a hat. Set at least one.
+
+`recordStats()` is off by default: counting costs a pair of atomic increments on every lookup, and
+a cache nobody is measuring should not pay for numbers nobody reads. Without it, `stats()` returns
+`CacheStats.DISABLED` rather than silently zero.
+
+## Where the spec lives
+
+On the cache, at the point it is built — not in application config. Size and TTL are properties of
+*what is being cached*, not of the process doing the caching, and a TTL in a config file is a TTL
+nobody can relate back to the data it governs.
+
+## Writing a backend
+
+Implement `CacheManager` and `Cache`, provide the manager from a `FlashExtension`, and register
+cleanup with `FlashContext.onClose` so a stopped app does not keep its values alive.
@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <parent>
+        <groupId>dev.relism</groupId>
+        <artifactId>flash-extensions</artifactId>
+        <version>2.1.0-SNAPSHOT</version>
+    </parent>
+
+    <artifactId>flash-ext-cache-core</artifactId>
+
+    <dependencies>
+        <dependency>
+            <groupId>dev.relism</groupId>
+            <artifactId>flash</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+        </dependency>
+    </dependencies>
+</project>
@@ -0,0 +1,50 @@
+package dev.relism.flash.ext.cache;
+
+import java.util.function.Function;
+
+/**
+ * A named cache. Obtained from a {@link CacheManager}, safe to hold in a handler field and share
+ * across threads.
+ *
+ * <pre>{@code
+ * User user = users.get(id, repo::findById);
+ * }</pre>
+ *
+ * <p>{@link #get} is the only method most code needs. The rest exist for the cases it cannot
+ * express: reading without populating, writing a value computed elsewhere, and invalidating.
+ *
+ * @param <K> key type — must have a stable {@code hashCode}/{@code equals}
+ * @param <V> value type
+ */
+public interface Cache<K, V> {
+
+    /**
+     * Returns the cached value, computing and storing it with {@code loader} if absent.
+     *
+     * <p>The loader runs at most once per key across concurrent callers; the others wait for it
+     * rather than each computing their own. A loader returning {@code null} stores nothing and
+     * {@code null} is returned.
+     */
+    V get(K key, Function<? super K, ? extends V> loader);
+
+    /** The cached value, or {@code null} if absent. Never invokes a loader. */
+    V getIfPresent(K key);
+
+    /** Stores {@code value}, replacing any existing entry. */
+    void put(K key, V value);
+
+    /** Drops {@code key}. Does nothing if it was absent. */
+    void invalidate(K key);
+
+    /** Drops every entry. */
+    void invalidateAll();
+
+    /** Approximate entry count. Approximate because eviction is asynchronous in most backends. */
+    long estimatedSize();
+
+    /**
+     * Hit/miss counters since this cache was built, or {@link CacheStats#DISABLED} when the
+     * backend was not asked to record them.
+     */
+    CacheStats stats();
+}
@@ -0,0 +1,31 @@
+package dev.relism.flash.ext.cache;
+
+import java.util.function.Consumer;
+
+/**
+ * Creates and hands back named caches. Resolve it with {@code require(CacheManager.class)}; a
+ * backend extension such as {@code flash-ext-cache-caffeine} provides the implementation.
+ *
+ * <pre>{@code
+ * @Override protected void onInit() {
+ *     users = require(CacheManager.class).build("users", spec -> spec
+ *             .maxSize(10_000)
+ *             .ttl(Duration.ofMinutes(10)));
+ * }
+ * }</pre>
+ *
+ * <p>{@link #build} is idempotent per name: calling it twice returns the same cache rather than
+ * two, so several handlers can share one without coordinating who creates it. The spec of the
+ * first call wins; a later call with a different spec is a configuration mistake and throws.
+ */
+public interface CacheManager {
+
+    /** Creates the named cache, or returns the existing one. */
+    <K, V> Cache<K, V> build(String name, Consumer<CacheSpec> spec);
+
+    /** The named cache, or {@code null} if {@link #build} has not been called for it. */
+    <K, V> Cache<K, V> cache(String name);
+
+    /** Every cache name built so far, for an ops endpoint. */
+    java.util.Set<String> names();
+}
@@ -0,0 +1,64 @@
+package dev.relism.flash.ext.cache;
+
+import java.time.Duration;
+
+/**
+ * How one cache should behave. Every field is optional — a spec that sets nothing gives an
+ * unbounded cache that never expires, which is a memory leak wearing a hat, so set at least one
+ * of {@link #maxSize} or {@link #ttl}.
+ *
+ * <pre>{@code
+ * CacheSpec.of().maxSize(10_000).ttl(Duration.ofMinutes(10))
+ * }</pre>
+ *
+ * <p>Mutable builder rather than a record with {@code withX} copies: it is constructed once at
+ * boot inside a lambda and never shared.
+ */
+public final class CacheSpec {
+
+    private long maxSize = -1;
+    private Duration ttl;
+    private Duration ttlAfterAccess;
+    private boolean recordStats;
+
+    private CacheSpec() {}
+
+    public static CacheSpec of() {
+        return new CacheSpec();
+    }
+
+    /** Maximum entries before the backend starts evicting. Negative means unbounded. */
+    public CacheSpec maxSize(long maxSize) {
+        this.maxSize = maxSize;
+        return this;
+    }
+
+    /** Entries expire this long after they were written. */
+    public CacheSpec ttl(Duration ttl) {
+        this.ttl = ttl;
+        return this;
+    }
+
+    /** Entries expire this long after they were last read or written. */
+    public CacheSpec ttlAfterAccess(Duration ttlAfterAccess) {
+        this.ttlAfterAccess = ttlAfterAccess;
+        return this;
+    }
+
+    /**
+     * Records hit/miss counters for {@link Cache#stats()}.
+     *
+     * <p>Off by default: counting costs a pair of atomic increments on every lookup, and a cache
+     * nobody is measuring should not pay for numbers nobody reads.
+     */
+    public CacheSpec recordStats() {
+        this.recordStats = true;
+        return this;
+    }
+
+    public long maxSize()            { return maxSize; }
+    public Duration ttl()            { return ttl; }
+    public Duration ttlAfterAccess() { return ttlAfterAccess; }
+    public boolean statsRecorded()   { return recordStats; }
+    public boolean bounded()         { return maxSize >= 0; }
+}
@@ -0,0 +1,20 @@
+package dev.relism.flash.ext.cache;
+
+/**
+ * Hit/miss counters for one cache.
+ *
+ * @param hits      lookups that found a value
+ * @param misses    lookups that had to load
+ * @param evictions entries dropped to respect {@link CacheSpec#maxSize()}
+ */
+public record CacheStats(long hits, long misses, long evictions) {
+
+    /** Returned when {@link CacheSpec#recordStats()} was not set — all zero, and says so. */
+    public static final CacheStats DISABLED = new CacheStats(0, 0, 0);
+
+    /** Hits divided by lookups, or 0 when nothing has been looked up yet. */
+    public double hitRate() {
+        long total = hits + misses;
+        return total == 0 ? 0 : (double) hits / total;
+    }
+}
@@ -0,0 +1,39 @@
+# flash-ext-cache-redis — planned
+
+Not implemented. This directory holds the design so the decision is written down rather than
+rediscovered; there is deliberately **no module, no pom and no source**, because an empty module
+that builds an empty jar is dead weight in the reactor and in everyone's dependency tree.
+
+Add it when there is a second replica that actually needs shared state.
+
+## What it would implement
+
+`CacheManager` and `Cache` from [`flash-ext-cache-core`](../../flash-ext-cache-core/docs/README.md),
+so switching backend is an install-line change:
+
+```java
+.install(new RedisCacheExtension(RedisConfig.of("redis://localhost:6379")))
+```
+
+## The part that is not a drop-in
+
+`flash-ext-cache-caffeine` cannot fail. A networked cache can, and that changes the contract in
+ways an adapter cannot hide:
+
+- **`get(key, loader)` can fail before reaching the loader.** The honest default is to fall
+  through to the loader and serve the value uncached, so Redis being down degrades throughput
+  rather than taking the application with it. That has to be a decision, not an accident.
+- **Values must be serialized.** Caffeine stores references. A `byte[]` codec belongs in the spec,
+  and the natural default is whatever `flash-ext-jackson` is already configured with.
+- **`invalidateAll()` is not free.** Against a shared keyspace it is either a scan or a key
+  prefix per cache name. The prefix is the right answer, and it means cache names become part of
+  the wire contract.
+- **Stats are per-client, not per-cache.** Hit rate stays meaningful; eviction count does not,
+  because Redis evicts on its own policy.
+
+## Why it is not built yet
+
+Nothing in the codebase has two replicas sharing cache state. Building it now would mean choosing
+a client library, a serialization format and a failure policy with no real usage to check them
+against — and the failure policy in particular is the kind of decision that is wrong until a
+production incident tells you otherwise.
@@ -29,6 +29,8 @@
        <module>flash-ext-data-core</module>
        <module>flash-ext-data-jdbc</module>
        <module>flash-ext-data-hibernate</module>
+        <module>flash-ext-cache-core</module>
+        <module>flash-ext-cache-caffeine</module>
    </modules>

    <dependencyManagement>
@@ -43,6 +45,21 @@
                <artifactId>flash-ext-scheduler</artifactId>
                <version>${project.version}</version>
            </dependency>
+            <dependency>
+                <groupId>dev.relism</groupId>
+                <artifactId>flash-ext-cache-core</artifactId>
+                <version>${project.version}</version>
+            </dependency>
+            <dependency>
+                <groupId>dev.relism</groupId>
+                <artifactId>flash-ext-cache-caffeine</artifactId>
+                <version>${project.version}</version>
+            </dependency>
+            <dependency>
+                <groupId>com.github.ben-manes.caffeine</groupId>
+                <artifactId>caffeine</artifactId>
+                <version>${caffeine.version}</version>
+            </dependency>
            <dependency>
                <groupId>dev.relism</groupId>
                <artifactId>flash-testing</artifactId>