Caching: ElastiCache, CloudFront and DAX (DynamoDB Accelerator)#

The fastest query is the one you never make. Caching questions on the exam come down to: what layer (CDN, application, database) and which engine (Redis vs Memcached vs DAX).

Cache placement - pick the layer#

ElastiCache: Redis vs Memcached#

Exam shortcut: “failover / durability / data structures / sorted leaderboard” → Redis. “Pure cache, simplest and fastest horizontal scaling” → Memcached.

Cache strategies (know the trade-offs)#

• Lazy loading (cache-aside): cache only what’s requested; cheap on cache size, but first request after expiry = cache miss → DB hit
• Write-through: write to cache and DB together; consistent, but writes are slower and everything gets cached
• TTL (time to live) management: expired = stale data gone; “the product name is wrong after update” → missing invalidation / TTL question
• Session state in Redis = the standard “sticky sessions are bad, where do sessions go” answer

DynamoDB DAX#

• Write-through cache for DynamoDB, API-compatible (change the endpoint, done)
• Microsecond reads for eventually-consistent hot data; survives node failures (multi-AZ cluster)
• Not for writes-heavy or full table scans - it’s a read cache (see dynamodb-deep-dive)

CloudFront caching knobs#

• TTL per behavior (origin sets Cache-Control: max-age; you can override)
• Invalidation to purge objects early (see cloudfront-invalidation)
• Signed URLs / cookies for private content (presigned-urls)
• Cuts data transfer costs: edge serves repeats, origin serves misses

flowchart LR
    user[User] --> cf[CloudFront edge cache]
    cf -->|miss| elb[ALB]
    elb --> app[App]
    app -->|miss| redis[ElastiCache Redis]
    app -->|miss| rds[(RDS)]

Not: “kullaniciya hizli goster” katmani CloudFront, “veritabanina gitmesin” katmani ElastiCache, “DynamoDB sorgusu mikrosaniyeye insin” = DAX. Ucunu ayni mimaride birlikte gormek normal.

Exam check#

Q1: A product catalog API runs the same SQL queries thousands of times per minute against RDS (Relational Database Service). What gives the biggest performance gain?

• A) Switch RDS to Multi-AZ
• B) Add ElastiCache in front of the database
• C) Add read replicas
• D) Enable RDS Performance Insights

Q2: Session data must survive the failure of a node and support sorted-set leaderboards. Which engine?

• A) Memcached
• B) Redis
• C) DynamoDB
• D) S3 (Simple Storage Service)

Ozet#

• CloudFront = istemciye en yakin statik katman (maliyet de dusurur)
• ElastiCache = uygulama onu cache, DB yukunu alir
• Redis = replikasyon, kalici, veri yapilari, failover; Memcached = saf basit cache
• Lazy loading = sadece istenen cache’lenir, ilk istek miss
• Write-through = tutarli ama yavas yazma
• DAX = DynamoDB icin write-through cache, mikrosaniye okuma
• TTL / cache invalidation = bayat veri sorularinin cevabi