Redis

Redis provides lowest latency for high-throughput scenarios where some durability trade-offs are acceptable.

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Why Redis

Strength	Benefit
Sub-millisecond latency	Fastest event writes
Horizontal scaling	Redis Cluster for partitioning
Memory-first	No disk I/O on hot path
Pub/Sub native	Can double as event bus

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Trade-offs

Concern	Consideration
Durability	Configurable (AOF, RDB, none)
Memory cost	Events live in RAM until evicted
Complexity	Cluster mode adds operational overhead

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Configuration

[storage]
backend = "redis"

[storage.redis]
url = "redis://localhost:6379"
# For cluster mode
# urls = ["redis://node1:6379", "redis://node2:6379"]

Environment Variables

export REDIS_URL="redis://localhost:6379"
export STORAGE_BACKEND="redis"

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Data Model

Redis stores events in sorted sets keyed by aggregate:

events:{domain}:{edition}:{root}
  └── ZADD with sequence as score
      └── Member: serialized event

positions:{handler}:{domain}:{edition}:{root}
  └── String: sequence number

snapshots:{domain}:{edition}:{root}
  └── Hash: { sequence, state }

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Durability Options

AOF (Append-Only File)

redis.conf

appendonly yes
appendfsync everysec  # fsync every second (default)
# appendfsync always  # fsync every write (slower, safest)

RDB Snapshots

redis.conf

save 900 1    # Save if 1 key changed in 900 seconds
save 300 10   # Save if 10 keys changed in 300 seconds
save 60 10000 # Save if 10000 keys changed in 60 seconds

Production Recommendation

Use both AOF and RDB for durability with fast recovery:

appendonly yes
appendfsync everysec
save 900 1

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Helm Deployment

values.yaml

storage:
  backend: redis

redis:
  enabled: true
  host: redis.cache.svc.cluster.local
  port: 6379
  credentials:
    secretName: redis-credentials
    passwordKey: password

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Testing

# Run Redis tests (requires testcontainers)
cargo test --test storage_redis --features redis

# Requires podman socket
systemctl --user start podman.socket

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When to Use Redis

• Gaming/real-time — Latency-critical applications
• Ephemeral data — Acceptable to lose on crash
• Caching layer — In front of durable storage
• Development — Fast iteration without disk I/O

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Next Steps

• PostgreSQL — Durable alternative
• Testcontainers — Container-based testing