Merge pull request #35 from kennethnym/feat/feed-engine-cache

feat: add caching to FeedEngine

packages/aris-core/src/feed-engine.test.ts

@@ -638,4 +638,290 @@ describe("FeedEngine", () => {
 			)
 		})
 	})
+
+	describe("lastFeed", () => {
+		test("returns null before any refresh", () => {
+			const engine = new FeedEngine()
+
+			expect(engine.lastFeed()).toBeNull()
+		})
+
+		test("returns cached result after refresh", async () => {
+			const location = createLocationSource()
+			location.simulateUpdate({ lat: 51.5, lng: -0.1 })
+
+			const weather = createWeatherSource()
+			const engine = new FeedEngine().register(location).register(weather)
+
+			const refreshResult = await engine.refresh()
+
+			const cached = engine.lastFeed()
+			expect(cached).not.toBeNull()
+			expect(cached!.items).toEqual(refreshResult.items)
+			expect(cached!.context).toEqual(refreshResult.context)
+		})
+
+		test("returns null after TTL expires", async () => {
+			const engine = new FeedEngine({ cacheTtlMs: 50 })
+			const location = createLocationSource()
+			location.simulateUpdate({ lat: 51.5, lng: -0.1 })
+
+			engine.register(location)
+			await engine.refresh()
+
+			expect(engine.lastFeed()).not.toBeNull()
+
+			await new Promise((resolve) => setTimeout(resolve, 60))
+
+			expect(engine.lastFeed()).toBeNull()
+		})
+
+		test("defaults to 5 minute TTL", async () => {
+			const engine = new FeedEngine()
+			const location = createLocationSource()
+			location.simulateUpdate({ lat: 51.5, lng: -0.1 })
+
+			engine.register(location)
+			await engine.refresh()
+
+			// Should still be cached immediately
+			expect(engine.lastFeed()).not.toBeNull()
+		})
+
+		test("refresh always fetches from sources", async () => {
+			let fetchCount = 0
+			const source: FeedSource = {
+				id: "counter",
+				...noActions,
+				async fetchContext() {
+					fetchCount++
+					return null
+				},
+			}
+
+			const engine = new FeedEngine().register(source)
+
+			await engine.refresh()
+			await engine.refresh()
+			await engine.refresh()
+
+			expect(fetchCount).toBe(3)
+		})
+
+		test("reactive context update refreshes cache", async () => {
+			const location = createLocationSource()
+			const weather = createWeatherSource()
+
+			const engine = new FeedEngine({ cacheTtlMs: 5000 }).register(location).register(weather)
+
+			engine.start()
+
+			// Simulate location update which triggers reactive refresh
+			location.simulateUpdate({ lat: 51.5, lng: -0.1 })
+
+			// Wait for async reactive refresh to complete
+			await new Promise((resolve) => setTimeout(resolve, 50))
+
+			const cached = engine.lastFeed()
+			expect(cached).not.toBeNull()
+			expect(cached!.items.length).toBeGreaterThan(0)
+
+			engine.stop()
+		})
+
+		test("reactive item update refreshes cache", async () => {
+			let itemUpdateCallback: (() => void) | null = null
+
+			const source: FeedSource = {
+				id: "reactive-items",
+				...noActions,
+				async fetchContext() {
+					return null
+				},
+				async fetchItems() {
+					return [
+						{
+							id: "item-1",
+							type: "test",
+							priority: 0.5,
+							timestamp: new Date(),
+							data: {},
+						},
+					]
+				},
+				onItemsUpdate(callback) {
+					itemUpdateCallback = callback
+					return () => {
+						itemUpdateCallback = null
+					}
+				},
+			}
+
+			const engine = new FeedEngine().register(source)
+			engine.start()
+
+			// Trigger item update
+			itemUpdateCallback!()
+
+			// Wait for async refresh
+			await new Promise((resolve) => setTimeout(resolve, 50))
+
+			const cached = engine.lastFeed()
+			expect(cached).not.toBeNull()
+			expect(cached!.items).toHaveLength(1)
+
+			engine.stop()
+		})
+
+		test("TTL resets after reactive update", async () => {
+			const location = createLocationSource()
+			const weather = createWeatherSource()
+
+			const engine = new FeedEngine({ cacheTtlMs: 100 }).register(location).register(weather)
+
+			engine.start()
+
+			// Initial reactive update
+			location.simulateUpdate({ lat: 51.5, lng: -0.1 })
+			await new Promise((resolve) => setTimeout(resolve, 50))
+
+			expect(engine.lastFeed()).not.toBeNull()
+
+			// Wait 70ms (total 120ms from first update, past original TTL)
+			// but trigger another update at 50ms to reset TTL
+			location.simulateUpdate({ lat: 52.0, lng: -0.2 })
+			await new Promise((resolve) => setTimeout(resolve, 50))
+
+			// Should still be cached because TTL was reset by second update
+			expect(engine.lastFeed()).not.toBeNull()
+
+			engine.stop()
+		})
+
+		test("cacheTtlMs is configurable", async () => {
+			const engine = new FeedEngine({ cacheTtlMs: 30 })
+			const location = createLocationSource()
+			location.simulateUpdate({ lat: 51.5, lng: -0.1 })
+
+			engine.register(location)
+			await engine.refresh()
+
+			expect(engine.lastFeed()).not.toBeNull()
+
+			await new Promise((resolve) => setTimeout(resolve, 40))
+
+			expect(engine.lastFeed()).toBeNull()
+		})
+
+		test("auto-refreshes on TTL interval after start", async () => {
+			let fetchCount = 0
+			const source: FeedSource = {
+				id: "counter",
+				...noActions,
+				async fetchContext() {
+					fetchCount++
+					return null
+				},
+				async fetchItems() {
+					return [
+						{
+							id: `item-${fetchCount}`,
+							type: "test",
+							priority: 0.5,
+							timestamp: new Date(),
+							data: {},
+						},
+					]
+				},
+			}
+
+			const engine = new FeedEngine({ cacheTtlMs: 50 }).register(source)
+			engine.start()
+
+			// Wait for two TTL intervals to elapse
+			await new Promise((resolve) => setTimeout(resolve, 120))
+
+			// Should have auto-refreshed at least twice
+			expect(fetchCount).toBeGreaterThanOrEqual(2)
+			expect(engine.lastFeed()).not.toBeNull()
+
+			engine.stop()
+		})
+
+		test("stop cancels periodic refresh", async () => {
+			let fetchCount = 0
+			const source: FeedSource = {
+				id: "counter",
+				...noActions,
+				async fetchContext() {
+					fetchCount++
+					return null
+				},
+			}
+
+			const engine = new FeedEngine({ cacheTtlMs: 50 }).register(source)
+			engine.start()
+			engine.stop()
+
+			const countAfterStop = fetchCount
+
+			// Wait past TTL
+			await new Promise((resolve) => setTimeout(resolve, 80))
+
+			// No additional fetches after stop
+			expect(fetchCount).toBe(countAfterStop)
+		})
+
+		test("reactive update resets periodic refresh timer", async () => {
+			let fetchCount = 0
+			const location = createLocationSource()
+			const countingWeather: FeedSource<WeatherFeedItem> = {
+				id: "weather",
+				dependencies: ["location"],
+				...noActions,
+				async fetchContext(ctx) {
+					fetchCount++
+					const loc = contextValue(ctx, LocationKey)
+					if (!loc) return null
+					return { [WeatherKey]: { temperature: 20, condition: "sunny" } }
+				},
+				async fetchItems(ctx) {
+					const weather = contextValue(ctx, WeatherKey)
+					if (!weather) return []
+					return [
+						{
+							id: `weather-${Date.now()}`,
+							type: "weather",
+							priority: 0.5,
+							timestamp: new Date(),
+							data: { temperature: weather.temperature, condition: weather.condition },
+						},
+					]
+				},
+			}
+
+			const engine = new FeedEngine({ cacheTtlMs: 100 })
+				.register(location)
+				.register(countingWeather)
+
+			engine.start()
+
+			// At 40ms, push a reactive update — this resets the timer
+			await new Promise((resolve) => setTimeout(resolve, 40))
+			const countBeforeUpdate = fetchCount
+			location.simulateUpdate({ lat: 51.5, lng: -0.1 })
+			await new Promise((resolve) => setTimeout(resolve, 20))
+
+			// Reactive update triggered a fetch
+			expect(fetchCount).toBeGreaterThan(countBeforeUpdate)
+			const countAfterUpdate = fetchCount
+
+			// At 100ms from start (60ms after reactive update), the original
+			// timer would have fired, but it was reset. No extra fetch yet.
+			await new Promise((resolve) => setTimeout(resolve, 40))
+			expect(fetchCount).toBe(countAfterUpdate)
+
+			engine.stop()
+		})
+	})
 })

packages/aris-core/src/feed-engine.ts

@@ -16,6 +16,14 @@ export interface FeedResult<TItem extends FeedItem = FeedItem> {
 
 export type FeedSubscriber<TItem extends FeedItem = FeedItem> = (result: FeedResult<TItem>) => void
 
+const DEFAULT_CACHE_TTL_MS = 300_000 // 5 minutes
+const MIN_CACHE_TTL_MS = 10 // prevent spin from zero/negative values
+
+export interface FeedEngineConfig {
+	/** Cache TTL in milliseconds. Default: 300_000 (5 minutes). Minimum: 10. */
+	cacheTtlMs?: number
+}
+
 interface SourceGraph {
 	sources: Map<string, FeedSource>
 	sorted: FeedSource[]
@@ -59,6 +67,29 @@ export class FeedEngine<TItems extends FeedItem = FeedItem> {
 	private cleanups: Array<() => void> = []
 	private started = false
 
+	private readonly cacheTtlMs: number
+	private cachedResult: FeedResult<TItems> | null = null
+	private cachedAt: number | null = null
+	private refreshTimer: ReturnType<typeof setTimeout> | null = null
+
+	constructor(config?: FeedEngineConfig) {
+		this.cacheTtlMs = Math.max(config?.cacheTtlMs ?? DEFAULT_CACHE_TTL_MS, MIN_CACHE_TTL_MS)
+	}
+
+	/**
+	 * Returns the cached FeedResult if available and not expired.
+	 * Returns null if no refresh has completed or the cache TTL has elapsed.
+	 */
+	lastFeed(): FeedResult<TItems> | null {
+		if (this.cachedResult === null || this.cachedAt === null) {
+			return null
+		}
+		if (Date.now() - this.cachedAt > this.cacheTtlMs) {
+			return null
+		}
+		return this.cachedResult
+	}
+
 	/**
 	 * Registers a FeedSource. Invalidates the cached graph.
 	 */
@@ -124,7 +155,10 @@ export class FeedEngine<TItems extends FeedItem = FeedItem> {
 
 		this.context = context
 
-		return { context, items: items as TItems[], errors }
+		const result: FeedResult<TItems> = { context, items: items as TItems[], errors }
+		this.updateCache(result)
+
+		return result
 	}
 
 	/**
@@ -138,7 +172,7 @@ export class FeedEngine<TItems extends FeedItem = FeedItem> {
 	}
 
 	/**
-	 * Starts reactive subscriptions on all sources.
+	 * Starts reactive subscriptions on all sources and begins periodic refresh.
 	 * Sources with onContextUpdate will trigger re-computation of dependents.
 	 */
 	start(): void {
@@ -168,13 +202,16 @@ export class FeedEngine<TItems extends FeedItem = FeedItem> {
 				this.cleanups.push(cleanup)
 			}
 		}
+
+		this.scheduleNextRefresh()
 	}
 
 	/**
-	 * Stops all reactive subscriptions.
+	 * Stops all reactive subscriptions and the periodic refresh timer.
 	 */
 	stop(): void {
 		this.started = false
+		this.cancelScheduledRefresh()
 		for (const cleanup of this.cleanups) {
 			cleanup()
 		}
@@ -279,11 +316,14 @@ export class FeedEngine<TItems extends FeedItem = FeedItem> {
 
 		items.sort((a, b) => b.priority - a.priority)
 
-		this.notifySubscribers({
+		const result: FeedResult<TItems> = {
 			context: this.context,
 			items: items as TItems[],
 			errors,
-		})
+		}
+		this.updateCache(result)
+
+		this.notifySubscribers(result)
 	}
 
 	private collectDependents(sourceId: string, graph: SourceGraph): string[] {
@@ -307,11 +347,46 @@ export class FeedEngine<TItems extends FeedItem = FeedItem> {
 		return graph.sorted.filter((s) => result.includes(s.id)).map((s) => s.id)
 	}
 
+	private updateCache(result: FeedResult<TItems>): void {
+		this.cachedResult = result
+		this.cachedAt = Date.now()
+		if (this.started) {
+			this.scheduleNextRefresh()
+		}
+	}
+
+	private scheduleNextRefresh(): void {
+		this.cancelScheduledRefresh()
+		this.refreshTimer = setTimeout(() => {
+			this.refresh()
+				.then((result) => {
+					this.notifySubscribers(result)
+				})
+				.catch(() => {
+					// Periodic refresh errors are non-fatal; schedule next attempt
+					if (this.started) {
+						this.scheduleNextRefresh()
+					}
+				})
+		}, this.cacheTtlMs)
+	}
+
+	private cancelScheduledRefresh(): void {
+		if (this.refreshTimer !== null) {
+			clearTimeout(this.refreshTimer)
+			this.refreshTimer = null
+		}
+	}
+
 	private scheduleRefresh(): void {
 		// Simple immediate refresh for now - could add debouncing later
-		this.refresh().then((result) => {
+		this.refresh()
-			this.notifySubscribers(result)
+			.then((result) => {
-		})
+				this.notifySubscribers(result)
+			})
+			.catch(() => {
+				// Reactive refresh errors are non-fatal
+			})
 	}
 
 	private notifySubscribers(result: FeedResult<TItems>): void {

packages/aris-core/src/index.ts

@@ -13,7 +13,7 @@ export type { FeedItem } from "./feed"
 export type { FeedSource } from "./feed-source"
 
 // Feed Engine
-export type { FeedResult, FeedSubscriber, SourceError } from "./feed-engine"
+export type { FeedEngineConfig, FeedResult, FeedSubscriber, SourceError } from "./feed-engine"
 export { FeedEngine } from "./feed-engine"
 
 // =============================================================================