Elasticache
Chapter 24: Amazon ElastiCache - In-Memory Caching
Section titled “Chapter 24: Amazon ElastiCache - In-Memory Caching”Managed Redis and Memcached
Section titled “Managed Redis and Memcached”24.1 Overview
Section titled “24.1 Overview”Amazon ElastiCache is a fully managed in-memory data store service, compatible with Redis and Memcached.
ElastiCache Overview+------------------------------------------------------------------+| || +------------------------+ || | ElastiCache | || +------------------------+ || | || +---------------------+---------------------+ || | | | || v v v || +----------+ +----------+ +----------+ || | Redis | | Memcached| | Managed | || | | | | | Service | || | - Multi-AZ| | - Simple | | | || | - Cluster| | - Scale | | - Setup | || | - Replication| | out | | - Patch | || | - Persistence| | - Cache | | - Monitor| || +----------+ +----------+ +----------+ || || Redis: Advanced features, persistence, replication || Memcached: Simple caching, multi-threaded || |+------------------------------------------------------------------+24.2 Redis vs Memcached
Section titled “24.2 Redis vs Memcached” Redis vs Memcached Comparison+------------------------------------------------------------------+| || Feature | Redis | Memcached || -----------------|--------------------|------------------------|| Data Structures | Strings, Lists, | Simple key-value || | | Sets, Hashes, etc. | || -----------------|--------------------|------------------------|| Persistence | Yes (AOF, RDB) | No || -----------------|--------------------|------------------------|| Replication | Yes (Primary/Replica)| No || -----------------|--------------------|------------------------|| Multi-AZ | Yes | No || -----------------|--------------------|------------------------|| Clustering | Yes (sharding) | Yes (distributed) || -----------------|--------------------|------------------------|| Transactions | Yes | No || -----------------|--------------------|------------------------|| Pub/Sub | Yes | No || -----------------|--------------------|------------------------|| Sorting | Yes | No || -----------------|--------------------|------------------------|| Threading | Single-threaded | Multi-threaded || -----------------|--------------------|------------------------|| Use Case | Complex data, | Simple caching, || | | leaderboards, | session store || | | session store | || |+------------------------------------------------------------------+24.3 ElastiCache Redis Architecture
Section titled “24.3 ElastiCache Redis Architecture”Cluster Mode Disabled
Section titled “Cluster Mode Disabled” Redis Cluster Mode Disabled+------------------------------------------------------------------+| || Single Node || +----------------------------------------------------------+ || | | || | +------------------+ | || | | Primary Node | | || | | (Read/Write) | | || | +------------------+ | || | | || +----------------------------------------------------------+ || || Primary-Replica (No Cluster) || +----------------------------------------------------------+ || | | || | AZ A AZ B | || | +------------------+ +------------------+ | || | | Primary Node | | Replica Node | | || | | (Read/Write) | | (Read Only) | | || | +------------------+ +------------------+ | || | | ^ | || | +----Replication-------+ | || | | || | Features: | || | - Single shard | || | - Up to 5 replicas | || | - Automatic failover | || | - Data size: Up to node capacity | || | | || +----------------------------------------------------------+ || |+------------------------------------------------------------------+Cluster Mode Enabled
Section titled “Cluster Mode Enabled” Redis Cluster Mode Enabled+------------------------------------------------------------------+| || Sharded Cluster || +----------------------------------------------------------+ || | | || | Shard 1 Shard 2 Shard 3 | || | +----------------+ +----------------+ +--------+ | || | | Primary Replica| | Primary Replica| | Primary| | || | | Node 1 Node 1 | | Node 2 Node 2 | | Node 3 | | || | | (AZ-a) (AZ-b) | | (AZ-a) (AZ-b) | | (AZ-a) | | || | +----------------+ +----------------+ +--------+ | || | | | | | || | v v v | || | Slot 0-5460 Slot 5461-10922 Slot 10923-16383| | | || | Features: | || | - Up to 500 shards | || | - Data partitioned across shards | || | - 16384 hash slots | || | - Automatic failover per shard | || | - Scale out by adding shards | || | | || +----------------------------------------------------------+ || |+------------------------------------------------------------------+24.4 ElastiCache Memcached Architecture
Section titled “24.4 ElastiCache Memcached Architecture” Memcached Architecture+------------------------------------------------------------------+| || Memcached Cluster || +----------------------------------------------------------+ || | | || | Application | || | +------------------+ | || | | | | || | +------------------+ | || | | | || | v | || | Configuration Endpoint | || | +------------------+ | || | | mycache.cfg.cache| | || | | .amazonaws.com | | || | +------------------+ | || | | | || | +----+----+ | || | | | | | || | v v v | || | +----++----++----+ | || | |Node||Node||Node| | || | | 1 || 2 || 3 | | || | +----++----++----+ | || | | || | Features: | || | - No replication | || | - Each node independent | || | - Client-side sharding | || | - Auto-discovery via config endpoint | || | | || +----------------------------------------------------------+ || |+------------------------------------------------------------------+24.5 Caching Patterns
Section titled “24.5 Caching Patterns”Lazy Loading (Cache-Aside)
Section titled “Lazy Loading (Cache-Aside)” Lazy Loading Pattern+------------------------------------------------------------------+| || Read Flow: || +----------------------------------------------------------+ || | | || | Application | || | | | || | v | || | +----------+ | || | | Check | | || | | Cache | | || | +----------+ | || | | | || | +----+----+ | || | | | | || | v v | || | HIT MISS | || | | | | || | v v | || | Return +----------+ | || | Data | Query | | || | | Database| | || | +----------+ | || | | | || | v | || | +----------+ | || | | Write to | | || | | Cache | | || | +----------+ | || | | | || | v | || | Return Data | || | | || +----------------------------------------------------------+ || || Pros: Only requested data cached || Cons: Cache miss penalty, stale data possible || |+------------------------------------------------------------------+Write-Through
Section titled “Write-Through” Write-Through Pattern+------------------------------------------------------------------+| || Write Flow: || +----------------------------------------------------------+ || | | || | Application | || | | | || | v | || | +----------+ | || | | Write to | | || | | Cache | | || | +----------+ | || | | | || | v | || | +----------+ | || | | Write to | | || | | Database | | || | +----------+ | || | | | || | v | || | Return Success | || | | || +----------------------------------------------------------+ || || Pros: Cache always fresh || Cons: Write latency, wasted cache for unread data || |+------------------------------------------------------------------+Write-Behind (Write-Back)
Section titled “Write-Behind (Write-Back)” Write-Behind Pattern+------------------------------------------------------------------+| || Write Flow: || +----------------------------------------------------------+ || | | || | Application | || | | | || | v | || | +----------+ | || | | Write to | | || | | Cache | | || | +----------+ | || | | | || | v | || | Return Success (immediate) | || | | | || | v (async) | || | +----------+ | || | | Write to | | || | | Database | | || | | (async) | | || | +----------+ | || | | || +----------------------------------------------------------+ || || Pros: Fast writes, reduced database load || Cons: Data loss risk, complexity || |+------------------------------------------------------------------+24.6 Redis Features
Section titled “24.6 Redis Features”Data Structures
Section titled “Data Structures” Redis Data Structures+------------------------------------------------------------------+| || Strings || +----------------------------------------------------------+ || | SET key value | || | GET key | || | INCR counter | || | Use: Caching, counters, session data | || +----------------------------------------------------------+ || || Hashes || +----------------------------------------------------------+ || | HSET user:1 name "John" email "john@ex.com" | || | HGET user:1 name | || | HGETALL user:1 | || | Use: User profiles, product info | || +----------------------------------------------------------+ || || Lists || +----------------------------------------------------------+ || | LPUSH queue task1 | || | RPOP queue | || | LRANGE queue 0 -1 | || | Use: Message queues, activity feeds | || +----------------------------------------------------------+ || || Sets || +----------------------------------------------------------+ || | SADD tags "redis" "database" | || | SMEMBERS tags | || | SINTER set1 set2 | || | Use: Tags, unique items, social graphs | || +----------------------------------------------------------+ || || Sorted Sets || +----------------------------------------------------------+ || | ZADD leaderboard 100 "player1" | || | ZRANGE leaderboard 0 -1 WITHSCORES | || | ZREVRANK leaderboard "player1" | || | Use: Leaderboards, rankings, rate limiting | || +----------------------------------------------------------+ || |+------------------------------------------------------------------+Persistence Options
Section titled “Persistence Options” Redis Persistence+------------------------------------------------------------------+| || RDB (Redis Database) || +----------------------------------------------------------+ || | | || | Features: | || | - Point-in-time snapshots | || | - Compact file | || | - Faster recovery | || | | || | Configuration: | || | save 900 1 # Save after 900s if >= 1 changes | || | save 300 10 # Save after 300s if >= 10 changes | || | save 60 10000 # Save after 60s if >= 10000 changes | || | | || +----------------------------------------------------------+ || || AOF (Append Only File) || +----------------------------------------------------------+ || | | || | Features: | || | - Logs every write operation | || | - Higher durability | || | - Larger file size | || | | || | Configuration: | || | appendonly yes | || | appendfsync everysec # Sync every second | || | appendfsync always # Sync every write (slowest) | || | | || +----------------------------------------------------------+ || || Recommended: Enable both RDB and AOF || |+------------------------------------------------------------------+24.7 Practical Configuration
Section titled “24.7 Practical Configuration”ElastiCache with Terraform
Section titled “ElastiCache with Terraform”# ============================================================# ElastiCache Subnet Group# ============================================================
resource "aws_elasticache_subnet_group" "main" { name = "main-cache-subnet" subnet_ids = var.private_subnet_ids
tags = { Name = "main-cache-subnet-group" }}
# ============================================================# ElastiCache Parameter Group# ============================================================
resource "aws_elasticache_parameter_group" "redis" { name = "redis-params" family = "redis7"
parameter { name = "maxmemory-policy" value = "allkeys-lru" }
parameter { name = "timeout" value = "300" }
tags = { Name = "redis-parameter-group" }}
# ============================================================# Redis Cluster Mode Disabled# ============================================================
resource "aws_elasticache_replication_group" "redis" { replication_group_id = "main-redis" replication_group_description = "Main Redis cluster"
# Engine engine = "redis" engine_version = "7.0" parameter_group_name = aws_elasticache_parameter_group.redis.name
# Node type node_type = "cache.r6g.large"
# Cluster mode disabled num_cache_clusters = 2 # Primary + 1 replica
# Network subnet_group_name = aws_elasticache_subnet_group.main.name security_group_ids = [aws_security_group.redis.id]
# Availability multi_az_enabled = true automatic_failover_enabled = true
# Encryption at_rest_encryption_enabled = true transit_encryption_enabled = true auth_token = var.redis_password
# Snapshot snapshot_retention_limit = 7 snapshot_window = "03:00-05:00"
# Maintenance maintenance_window = "Mon:05:00-Mon:07:00"
tags = { Name = "main-redis" }}
# ============================================================# Redis Cluster Mode Enabled# ============================================================
resource "aws_elasticache_replication_group" "cluster" { replication_group_id = "cluster-redis" replication_group_description = "Redis cluster mode enabled"
engine = "redis" engine_version = "7.0" parameter_group_name = aws_elasticache_parameter_group.redis.name
node_type = "cache.r6g.large"
# Cluster mode enabled cluster_mode { replicas_per_node_group = 1 num_node_groups = 3 # 3 shards }
# Network subnet_group_name = aws_elasticache_subnet_group.main.name security_group_ids = [aws_security_group.redis.id]
# Availability automatic_failover_enabled = true multi_az_enabled = true
# Encryption at_rest_encryption_enabled = true transit_encryption_enabled = true auth_token = var.redis_password
tags = { Name = "cluster-redis" }}
# ============================================================# Memcached Cluster# ============================================================
resource "aws_elasticache_cluster" "memcached" { cluster_id = "main-memcached" engine = "memcached" engine_version = "1.6.22"
node_type = "cache.r6g.large" num_cache_nodes = 3
# Network subnet_group_name = aws_elasticache_subnet_group.main.name security_group_ids = [aws_security_group.memcached.id]
# Parameter group parameter_group_name = "default.memcached1.6"
# Maintenance maintenance_window = "Mon:05:00-Mon:07:00"
tags = { Name = "main-memcached" }}
# ============================================================# Security Groups# ============================================================
resource "aws_security_group" "redis" { name = "redis-sg" description = "Security group for Redis" vpc_id = var.vpc_id
ingress { description = "Redis from application" from_port = 6379 to_port = 6379 protocol = "tcp" security_groups = [aws_security_group.app.id] }
egress { from_port = 0 to_port = 0 protocol = "-1" cidr_blocks = ["0.0.0.0/0"] }
tags = { Name = "redis-sg" }}
resource "aws_security_group" "memcached" { name = "memcached-sg" description = "Security group for Memcached" vpc_id = var.vpc_id
ingress { description = "Memcached from application" from_port = 11211 to_port = 11211 protocol = "tcp" security_groups = [aws_security_group.app.id] }
egress { from_port = 0 to_port = 0 protocol = "-1" cidr_blocks = ["0.0.0.0/0"] }
tags = { Name = "memcached-sg" }}
# ============================================================# Global Datastore (Redis)# ============================================================
# Primary regionresource "aws_elasticache_replication_group" "global_primary" { provider = aws.primary
replication_group_id = "global-redis" replication_group_description = "Global Redis"
engine = "redis" engine_version = "7.0" node_type = "cache.r6g.large" num_cache_clusters = 2
subnet_group_name = aws_elasticache_subnet_group.main.name security_group_ids = [aws_security_group.redis.id]
automatic_failover_enabled = true
# Global datastore global_replication_group_id = aws_elasticache_global_replication_group.main.id}
# Secondary regionresource "aws_elasticache_replication_group" "global_secondary" { provider = aws.secondary
replication_group_id = "global-redis-secondary" replication_group_description = "Global Redis Secondary"
# Reference global replication group global_replication_group_id = aws_elasticache_global_replication_group.main.global_replication_group_id primary_cluster_id = aws_elasticache_replication_group.global_primary.primary_cluster_id
subnet_group_name = aws_elasticache_subnet_group.secondary.name security_group_ids = [aws_security_group.redis_secondary.id]}
resource "aws_elasticache_global_replication_group" "main" { global_replication_group_id_suffix = "global" primary_replication_group_id = aws_elasticache_replication_group.global_primary.id}24.8 Best Practices
Section titled “24.8 Best Practices” ElastiCache Best Practices+------------------------------------------------------------------+| || 1. Instance Selection || +----------------------------------------------------------+ || | - Use cache-optimized instances (r6g family) | || | - Consider network bandwidth | || | - Right-size based on data volume | || +----------------------------------------------------------+ || || 2. Eviction Policy || +----------------------------------------------------------+ || | - allkeys-lru: Evict least recently used | || | - volatile-lru: Evict LRU among keys with TTL | || | - allkeys-random: Evict random keys | || | - noeviction: Return error when memory full | || +----------------------------------------------------------+ || || 3. Connection Management || +----------------------------------------------------------+ || | - Use connection pooling | || | - Set appropriate timeouts | || | - Handle connection failures gracefully | || +----------------------------------------------------------+ || || 4. Security || +----------------------------------------------------------+ || | - Enable encryption in-transit and at-rest | || | - Use AUTH password | || | - Deploy in private subnets | || | - Use security groups | || +----------------------------------------------------------+ || || 5. Monitoring || +----------------------------------------------------------+ || | - Monitor CPU, memory, connections | || | - Set up CloudWatch alarms | || | - Monitor cache hit ratio | || +----------------------------------------------------------+ || |+------------------------------------------------------------------+24.9 Exam Tips
Section titled “24.9 Exam Tips”
- Redis vs Memcached: Redis has persistence, replication, data structures
- Cluster Mode Disabled: Single shard, up to 5 replicas
- Cluster Mode Enabled: Up to 500 shards, data partitioned
- Lazy Loading: Cache on read, stale data possible
- Write-Through: Write to cache and DB, always fresh
- Eviction Policies: LRU, volatile-LRU, noeviction
- Persistence: RDB (snapshots), AOF (append-only)
- Multi-AZ: Automatic failover for Redis
- Encryption: At-rest and in-transit for Redis
- Global Datastore: Cross-region replication for Redis
Next Chapter
Section titled “Next Chapter”Chapter 25: Other AWS Database Services
Last Updated: February 2026