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Check cache first const cached = await this.cache.get(key); if (cached !== null) { return cached; // Cache hit } // 2. Cache miss: Load from database const data = await this.database.get(key); if (data !== null) { // 3. Populate cache for future reads await this.cache.set(key, data, this.ttl); } return data; } // Write-Around Write Pattern (Database Only) async set(key: string, value: T): Promise { // Write directly to database await this.database.set(key, value); // Optionally invalidate stale cache entry await this.cache.delete(key); // Note: NOT populating cache // If this data is needed, get() will load it } // Bulk write example: Write-around shines here async bulkInsert(records: Record[]): Promise { // All writes go to database await this.database.bulkInsert(records); // Cache is NOT polluted with bulk data // Only frequently-read records will enter cache naturally } }65:T5b5,interface LatencyMetrics { writeLatency: number; readHitLatency: number; readMissLatency: number; cacheHitRate: number; } class InstrumentedWriteAroundCache { private metrics: LatencyMetrics = { writeLatency: 0, readHitLatency: 0, readMissLatency: 0, cacheHitRate: 0, }; async write(key: string, value: T): Promise { const start = performance.now(); await this.database.write(key, value); await this.cache.delete(key); // Invalidation this.metrics.writeLatency = this.ema( this.metrics.writeLatency, performance.now() - start ); } async read(key: string): Promise { const start = performance.now(); const cached = await this.cache.get(key); if (cached !== null) { this.metrics.readHitLatency = this.ema( this.metrics.readHitLatency, performance.now() - start ); this.recordHit(); return cached; } // Cache miss path const data = await this.database.read(key); if (data !== null) { await this.cache.set(key, data); } this.metrics.readMissLatency = this.ema( this.metrics.readMissLatency, performance.now() - start ); this.recordMiss(); return data; } // Exponential moving average for smooth metrics private ema(current: number, sample: number, alpha = 0.1): number { return alpha * sample + (1 - alpha) * current; } }66:T5e3,// Staleness window depends on implementation variant // Pure Write-Around: Staleness = TTL - elapsed time function pureWriteAroundStaleness(ttl: number, timeSinceWrite: number): number { // Maximum staleness equals remaining TTL return Math.max(0, ttl - timeSinceWrite); // Example: TTL=1hour, write happened 10min ago // Staleness window = 50 minutes (worst case) } // Write-Around with Invalidation: Staleness = 0 function invalidationStaleness(): number { // Invalidation removes stale entry // Next read fetches from database return 0; // No staleness after invalidation } // Practical staleness scenarios const scenarios = { // User updates profile, reads immediately profileUpdate: { pattern: "write-then-read", pureWriteAround: "May see old profile for up to TTL", withInvalidation: "Cache miss, fresh data guaranteed", recommendation: "Use invalidation for user-facing updates", }, // Analytics event written, never read by same user analyticsWrite: { pattern: "write-only", pureWriteAround: "No staleness concern (no read)", withInvalidation: "Unnecessary overhead", recommendation: "Pure write-around, skip invalidation", }, // Inventory update, read by many other users inventoryUpdate: { pattern: "write-then-read-by-others", pureWriteAround: "Other users may see stale inventory", withInvalidation: "All users see fresh data on next read", recommendation: "Use invalidation, consider short TTL", }, };43:["$","$L62",null,{"owner":"anonymous","state":{"mutations":[],"queries":[{"dehydratedAt":1792304214763,"state":{"data":{"json":{"id":"seed_4373a84707450a8d4d8f1bc1d397b2f9","slug":"write-around-caching","category":"System Design (HLD)","topic":"Caching Strategies","title":"Write-Around Caching","description":"Master write-around caching strategy—understand how writes bypass the cache to go directly to the database while reads populate the cache on-demand. Learn the mechanics, trade-offs, read miss behavior, and optimal use cases for this cache-aside pattern variant that excels in write-heavy workloads.","difficulty":"INTERMEDIATE","estimatedMinutes":55,"pages":[{"id":"seed_b048d60997a0c309f2f1a623dc51fe30","title":"How Write-Around Caching Works","order":0,"content":[{"id":"opening-hook","title":"The Cache-Bypass Philosophy","content":[{"data":{"content":"In the landscape of caching strategies, **write-around caching** represents a deliberate philosophical choice: **not all data deserves immediate cache residency**. Unlike write-through and write-back strategies that ensure written data lives in the cache, write-around takes a fundamentally different approach—it **bypasses the cache entirely for write operations**, sending data directly to the persistent storage layer.\n\nThis might initially seem counterintuitive. Isn't the whole point of caching to keep frequently accessed data in fast storage? The insight behind write-around is that **writes don't predict reads**. Just because data was written doesn't mean it will be read soon—or ever. Write-around recognizes this reality and optimizes accordingly."},"type":"text"},{"data":{"icon":"Target","title":"What You Will Learn","content":"By the end of this page, you will understand the complete mechanics of write-around caching—how data flows through the system, why the cache is bypassed during writes, how reads trigger cache population, and the architectural patterns that emerge from this strategy. You'll develop deep intuition for when write-around is the optimal choice.","variant":"info"},"type":"callout"}]},{"id":"fundamental-mechanics","title":"Fundamental Mechanics of Write-Around","content":[{"data":{"content":"Write-around caching implements a simple but powerful principle: **writes go to the database; reads populate the cache**. This creates an asymmetric data flow where the cache serves purely as a read accelerator rather than a write buffer.\n\n**The Core Data Flow:**\n\nUnderstanding write-around requires visualizing two distinct paths—the write path and the read path—that operate independently of each other."},"type":"text"},{"data":{"left":[{"data":{"type":"ordered","items":["**Application issues write request** — Create, update, or delete operation initiated","**Request goes directly to database** — Cache is completely bypassed, no cache interaction","**Database persists the data** — Storage layer handles durability","**Confirmation returned to application** — Write is complete","**Cache remains unchanged** — Existing cache entries untouched (potential staleness)"],"title":{"icon":"ArrowDown","text":"Write Path (Cache Bypass)","type":"icon","color":"primary","variant":"outline-circle"},"variant":"accent"},"type":"list"}],"right":[{"data":{"type":"ordered","items":["**Application issues read request** — Data retrieval operation initiated","**Cache checked first** — Look for existing cached entry (cache hit/miss)","**On cache miss → Database query** — Fetch data from persistent storage","**Data loaded into cache** — Populate cache with retrieved data","**Return data to application** — Subsequent reads served from cache"],"title":{"icon":"ArrowUp","text":"Read Path (Cache Population)","type":"icon","color":"secondary","variant":"outline-circle"},"variant":"accent"},"type":"list"}]},"type":"two-column"},{"data":{"content":"**The Key Insight:**\n\nThe write path and read path are **completely decoupled**. A write operation has no effect on the cache—it doesn't add data, doesn't update existing entries, and doesn't invalidate stale data. Only read operations interact with the cache, and only when data isn't already present (cache miss) does the cache get populated.\n\nThis decoupling is both the strength and the weakness of write-around caching. It prevents cache pollution from transient writes but creates the potential for stale reads immediately after writes."},"type":"text"},{"data":{"code":"$63","title":"Write-Around Caching Implementation","language":"typescript","highlightedLines":[7,8,9,22,23,24,25]},"type":"code"},{"data":{"icon":"AlertCircle","title":"The Staleness Window","content":"Notice that after a write, the cache may contain stale data. If key 'user:123' is in the cache with value A, and we write value B using write-around, the cache still contains A. Any reads before the cache entry expires will return stale data. This is the fundamental trade-off of write-around—you must decide if this staleness window is acceptable for your use case.","variant":"warning"},"type":"callout"}],"titleType":"number","titleNumber":1},{"id":"architectural-data-flow","title":"Architectural Data Flow Patterns","content":[{"data":{"content":"Write-around caching creates distinct architectural patterns based on how the system handles the interplay between written data, cached data, and read requests. Understanding these patterns is essential for designing robust systems.\n\n**Pattern 1: Pure Write-Around (No Invalidation)**\n\nIn its purest form, write-around never touches the cache during writes. This creates maximum write performance but relies entirely on TTL (Time-To-Live) for eventual consistency."},"type":"text"},{"data":{"code":"// Pure Write-Around: Maximum write performance\nasync function pureWriteAround(key: string, data: Data): Promise {\n // Single database write - cache completely ignored\n await database.write(key, data);\n \n // Staleness resolved by:\n // 1. TTL expiration on cache entries (eventually consistent)\n // 2. Natural read patterns bringing fresh data\n // 3. Background refresh processes (if implemented)\n}\n\n// Implications:\n// - Writes: O(1) database operation only\n// - Reads after write: May return stale data until TTL expires\n// - Best for: Write-heavy workloads with tolerance for eventual consistency","title":"Pure Write-Around Pattern","language":"typescript"},"type":"code"},{"data":{"content":"**Pattern 2: Write-Around with Invalidation**\n\nA more conservative approach adds cache invalidation to the write path. While still bypassing cache population, it removes stale entries to ensure subsequent reads fetch fresh data from the database."},"type":"text"},{"data":{"code":"// Write-Around with Invalidation: Balance of performance and consistency\nasync function writeAroundWithInvalidation(key: string, data: Data): Promise {\n // Step 1: Write to database\n await database.write(key, data);\n \n // Step 2: Invalidate cache entry (but don't populate)\n await cache.delete(key); // Fast O(1) operation\n \n // Next read will be a cache miss → fetch from DB → populate cache\n}\n\n// Implications:\n// - Writes: O(1) database + O(1) cache delete\n// - Reads after write: Cache miss guaranteed, fresh data loaded\n// - Slightly slower writes, but immediate consistency on next read","title":"Write-Around with Invalidation","language":"typescript","highlightedLines":[5,6]},"type":"code"},{"data":{"content":"**Pattern 3: Write-Around with Background Refresh**\n\nAdvanced implementations combine write-around with background processes that proactively refresh stale cache entries based on database change events or scheduled refreshes."},"type":"text"},{"data":{"code":"// Write-Around with Background Refresh\nclass WriteAroundWithRefresh {\n private changeLog: ChangeEvent[] = [];\n \n async write(key: string, data: Data): Promise {\n await database.write(key, data);\n \n // Emit change event for background processing\n this.changeLog.push({ \n key, \n timestamp: Date.now(), \n action: 'write' \n });\n }\n \n // Background worker refreshes hot cache entries\n async backgroundRefreshWorker(): Promise {\n while (true) {\n const changes = await this.consumeChanges();\n \n for (const change of changes) {\n const isHotKey = await this.isFrequentlyAccessed(change.key);\n \n if (isHotKey) {\n // Proactively refresh cache for hot keys\n const freshData = await database.read(change.key);\n await cache.set(change.key, freshData);\n }\n // Cold keys can wait for natural read-through\n }\n \n await sleep(REFRESH_INTERVAL);\n }\n }\n}","title":"Write-Around with Background Refresh","language":"typescript"},"type":"code"},{"data":{"rows":[["Pure Write-Around","Lowest","Eventually consistent (TTL-based)","Minimal","Logging, analytics, metrics"],["With Invalidation","Low-Medium","Consistent on next read","Low","User profiles, configurations"],["With Background Refresh","Lowest","Near real-time for hot data","High","Mixed workloads, critical hot data"]],"caption":"Write-Around Pattern Comparison","headers":["Pattern","Write Latency","Read Consistency","Complexity","Best Use Case"]},"type":"table"}],"titleType":"number","titleNumber":2},{"id":"cache-aside-relationship","title":"Write-Around and Cache-Aside: A Unified View","content":[{"data":{"content":"Write-around caching is often implemented as the **write policy** within a broader **cache-aside** (also called lazy-loading) architecture. Understanding this relationship clarifies how write-around fits into the larger caching ecosystem.\n\n**Cache-Aside Architecture:**\n\nIn cache-aside, the application is responsible for managing the cache—the cache doesn't automatically handle reads or writes. The application must:\n1. Check the cache before reading from the database\n2. Populate the cache after a cache miss\n3. Decide what to do with the cache during writes\n\nWrite-around provides the answer to #3: **do nothing** (or invalidate)."},"type":"text"},{"data":{"items":["**Write-Through** — Write to cache AND database synchronously. Cache always has fresh data but writes are slower.","**Write-Back (Write-Behind)** — Write to cache first, async write to database. Fastest writes but risk of data loss.","**Write-Around** — Write to database only, cache bypassed. Prevents cache pollution from transient writes."],"title":{"icon":"GitBranch","text":"Cache-Aside Write Policies","type":"icon","color":"primary","variant":"outline-circle"},"variant":"accent"},"type":"list"},{"data":{"content":"**Why Write-Around is Natural for Cache-Aside:**\n\nCache-aside already assumes the application manages cache population lazily—on read misses. Write-around extends this philosophy to writes: if we don't know that data will be read, why eagerly cache it? Let the read path handle cache population when (and if) the data is actually needed.\n\nThis creates a **demand-driven cache** where cache contents precisely reflect actual read patterns rather than write patterns. The cache becomes a true LRU (Least Recently Used) read cache."},"type":"text"},{"data":{"code":"$64","title":"Cache-Aside with Write-Around Implementation","language":"typescript"},"type":"code"},{"data":{"icon":"Lightbulb","title":"The Lazy Philosophy","content":"Write-around embraces 'lazy' as a virtue. Why do work that might never be needed? By deferring cache population to actual read demand, the system automatically optimizes cache contents for real access patterns. This is analogous to lazy evaluation in functional programming—compute only what's necessary.","variant":"tip"},"type":"callout"}],"titleType":"number","titleNumber":3},{"id":"timing-latency","title":"Timing and Latency Characteristics","content":[{"data":{"content":"Understanding the timing profile of write-around caching is crucial for capacity planning and setting performance expectations. Let's analyze the latency characteristics for different operations."},"type":"text"},{"data":{"rows":[["Write (pure)","None","5-20ms","Network → Database write → Response"],["Write (with invalidation)","Delete only","5-25ms","Network → Database write + Cache delete → Response"],["Read (cache hit)","Read only","1-5ms","Network → Cache read → Response"],["Read (cache miss)","Read + Write","10-30ms","Network → Cache read → DB read → Cache write → Response"],["First read after write","Miss + Populate","10-30ms","Always a cache miss (guaranteed fresh data)"]],"caption":"Write-Around Latency Profile","headers":["Operation","Cache Involvement","Expected Latency","Components"]},"type":"table"},{"data":{"content":"**The First-Read Penalty:**\n\nA defining characteristic of write-around is the **first-read penalty** after writes. When data is written, the cache either doesn't have it (pure write-around) or has been invalidated (write-around with invalidation). Either way, the first read operation pays the full cost of a cache miss:\n\n```\nWrite → [Database stores data]\nRead → [Cache miss] → [Database read] → [Cache population] → [Return data]\n```\n\nThis is the price paid for write-path simplicity. Whether this trade-off is acceptable depends on the read-after-write latency requirements of your application."},"type":"text"},{"data":{"code":"$65","title":"Measuring Write-Around Latency","language":"typescript"},"type":"code"},{"data":{"icon":"Info","title":"Amortized Performance","content":"While individual operations may vary, the amortized performance of write-around depends heavily on your read/write ratio. In read-heavy workloads (90%+ reads), the cache hit rate dominates and overall latency is excellent. In write-heavy workloads, the cache provides less benefit but also causes less overhead. This balance is why write-around often outperforms alternatives in aggregate metrics.","variant":"note"},"type":"callout"}],"titleType":"number","titleNumber":4},{"id":"consistency-model","title":"Consistency Model Analysis","content":[{"data":{"content":"Write-around caching implements an **eventually consistent** model with configurable staleness windows. Understanding the consistency guarantees—and limitations—is essential for choosing this strategy appropriately."},"type":"text"},{"data":{"items":["**Write Durability** — Writes go directly to the database, ensuring data is persisted before acknowledgment. No data loss risk from cache failures.","**Eventual Consistency** — Reads eventually reflect all writes, bounded by TTL or explicit invalidation timing.","**Read-Your-Writes (Conditional)** — Only guaranteed if invalidation is used. Pure write-around may return stale data to the same session.","**Monotonic Reads (Not Guaranteed)** — A reader may see newer data, then older data if the cache serves a stale entry.","**Causal Consistency (Not Guaranteed)** — No ordering guarantees between writes and reads from different sessions."],"title":{"icon":"Shield","text":"Consistency Guarantees","type":"icon","color":"primary","variant":"outline-circle"},"variant":"glass"},"type":"list"},{"data":{"content":"**Staleness Window Analysis:**\n\nThe staleness window is the period during which cache reads may return outdated data after a write. Understanding this window is critical for determining if write-around is appropriate."},"type":"text"},{"data":{"code":"$66","title":"Staleness Window Calculation","language":"typescript"},"type":"code"},{"data":{"rows":[["User session data","Zero (security)","Write-Through or Invalidation","Short TTL + invalidation"],["Product catalog","Minutes acceptable","Pure Write-Around","5-15 minute TTL"],["Inventory counts","Seconds","Invalidation","30 second TTL + invalidation"],["Analytics/logs","Hours/Never read","Pure Write-Around","Long TTL or no caching"],["Social feed","Seconds to minutes","Invalidation + Background","1-5 minute TTL"],["Configuration","Zero","Write-Through","Long TTL + version-based invalidation"]],"caption":"Consistency Requirements by Use Case","headers":["Use Case","Staleness Tolerance","Recommended Variant","TTL Strategy"]},"type":"table"},{"data":{"icon":"AlertCircle","title":"The Stale Read Trap","content":"Be careful with pure write-around for user-facing updates. If a user changes their email address and immediately sees their old email displayed (from cache), they'll assume the update failed. This UX degradation may outweigh the performance benefits. For user-initiated updates with immediate visibility requirements, prefer write-around with invalidation.","variant":"warning"},"type":"callout"}],"titleType":"number","titleNumber":5},{"id":"strategy-comparison","title":"Comparison with Other Caching Strategies","content":[{"data":{"content":"To truly understand write-around, we must compare it with its siblings: write-through and write-back caching. Each strategy makes different trade-offs along the dimensions of latency, consistency, and failure resilience."},"type":"text"},{"data":{"rows":[["Write latency","High (cache + DB sync)","Low (cache only)","Medium (DB only)"],["Cache pollution","High (all writes cached)","High (all writes cached)","Low (reads only)"],["Data loss risk","None (DB confirmed)","High (cache loss = data loss)","None (DB confirmed)"],["Read-after-write consistency","Guaranteed","Guaranteed","Eventual (or invalidate)"],["Cache hit rate (write-heavy)","High (but wasted)","High (but wasted)","Low (but efficient)"],["Implementation complexity","Low","High (async + recovery)","Low"]],"caption":"Caching Strategy Comparison Matrix","headers":["Dimension","Write-Through","Write-Back","Write-Around"]},"type":"table"},{"data":{"content":"**When Write-Around Wins:**\n\nWrite-around excels in scenarios that penalize the other strategies:\n\n1. **High write volume with sparse reads** — If you're writing 100,000 records but only 1% will ever be read, write-through wastes cache space on 99,000 entries that will expire unused. Write-around keeps the cache lean.\n\n2. **Cost-sensitive cache infrastructure** — Cache memory is expensive. If your workload would fill the cache with write-through but only reads a fraction, write-around provides better cost efficiency.\n\n3. **Write bursts and bulk operations** — Batch imports, ETL processes, and event logging can overwhelm caches. Write-around lets these operations complete without cache interference.\n\n4. **Strict durability requirements** — Unlike write-back, write-around confirms database persistence before acknowledging writes. There's no window for data loss."},"type":"text"},{"data":{"items":["**Predictable read-after-write patterns** — If data is always read immediately after writing (e.g., checkout confirmation pages), write-through eliminates the read miss penalty.","**Real-time requirements** — Systems requiring immediate consistency cannot tolerate any staleness window.","**Cache-as-primary-storage** — Some architectures use cache as the write target with async DB sync. Write-around doesn't fit this model.","**Session state management** — User sessions require read-your-writes guarantees that pure write-around cannot provide."],"title":{"icon":"XCircle","text":"When Write-Around is Wrong","type":"icon","color":"destructive","variant":"outline-circle"},"variant":"accent"},"type":"list"},{"data":{"icon":"Lightbulb","title":"Hybrid Approaches","content":"Many production systems use hybrid strategies—write-through for hot, frequently-read data and write-around for cold, rarely-read data. The key is understanding your access patterns and applying the appropriate strategy to each data category.","variant":"tip"},"type":"callout"}],"titleType":"number","titleNumber":6},{"id":"summary-key-mechanics","title":"Summary: Write-Around Mechanics","content":[{"data":{"content":"Write-around caching implements a simple but powerful philosophy: **let read patterns drive cache contents**. By bypassing the cache during writes, this strategy prevents cache pollution and optimizes for workloads where writes don't predict reads."},"type":"text"},{"data":{"type":"ordered","items":["**Writes bypass the cache** — Data goes directly to the database with no cache interaction (or only invalidation).","**Reads populate the cache** — Cache entries are created lazily, only when data is actually requested.","**Staleness is the trade-off** — Without invalidation, cached data may become stale after writes until TTL expires.","**Three variants exist** — Pure (no invalidation), with invalidation, and with background refresh—each with different consistency/performance profiles.","**Natural fit for cache-aside** — Write-around extends the lazy-loading philosophy to the write path.","**Prevents cache pollution** — Write-heavy workloads don't fill the cache with data that may never be read."],"title":{"icon":"CheckCircle","text":"Key Takeaways","type":"icon","color":"primary","variant":"outline-circle"},"variant":"accent"},"type":"list"},{"data":{"content":"**What's Next:**\n\nNow that you understand how write-around works at a mechanical level, the next page explores **why writes go directly to the database**—examining the architectural motivations, durability guarantees, and failure mode analysis that make database-first writes the right choice for write-around systems."},"type":"text"},{"data":{"icon":"CheckCircle","title":"Page Complete","content":"You now understand the fundamental mechanics of write-around caching—the data flow patterns, timing characteristics, consistency model, and how it compares to other strategies. 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