Compare Scenarios

Migration Step 1

Both scenarios define a migration step at this stage. Read-Heavy SaaS API: triggered by 'Connection pool exhaustion or p99 read latency > 200ms under'. API Gateway Platform: triggered by 'PostgreSQL hot path query p99 > 2ms under sustained request '.

Migration Step 2

Both scenarios define a migration step at this stage. Read-Heavy SaaS API: triggered by 'Primary CPU > 70% during peak read hours'. API Gateway Platform: triggered by 'Rate limit enforcement allowing requests above the configure'.

Migration Step 3

API Gateway Platform has a defined migration; Read-Heavy SaaS API does not at this stage.

p99 database latency rising; connection wait queue growing; requests timing out with "too many connections" or pool queue full errors

Tier 1: Connection Exhaustion: Database connection pool saturated or max_connections exceeded. Recommended evolution: Add PgBouncer connection pooler in transaction mode.

Database CPU > 80% sustained; read query p99 rising; cache miss rate stable but overall latency increasing

Tier 2: Read Throughput Ceiling: Single PostgreSQL primary saturated with read traffic. Recommended evolution: Add one or more streaming read replicas; implement lag-aware replica routing.

Redis command latency p99 > 0.5ms; gateway hot path p99 exceeding 2ms with Redis as the bottleneck (not upstream service); Redis CPU > 60% sustained; Lua script execution visible in SLOWLOG at > 0.1ms frequency

Tier 1: Redis Rate Limit Throughput: Single Redis instance processing all rate limit Lua scripts serially for all tenants across all gateway replicas. Recommended evolution: Shard rate limit counters across Redis Cluster nodes by hashing tenant_id to a cluster slot; this distributes Lua script execution across nodes proportional to tenant count; ensure tenant_id-keyed counters use hash tags ({tenant_id}) so all keys for a tenant land on the same slot and Lua scripts can operate on them atomically; do not use Redis Cluster without testing Lua script compatibility against your cluster topology first .

Tenant reports that rate limit increase takes > 30 seconds to take effect across all gateway replicas; configuration change audit log shows primary PostgreSQL write completing, but gateway replicas still routing to old backend endpoints beyond the expected cache TTL window

Tier 2: Configuration Propagation Latency: Local in-process cache TTL too long, or cache invalidation signal (Redis pub/sub or Kafka) not reaching all replicas. Recommended evolution: Implement configuration change notification via Redis pub/sub: PostgreSQL configuration writes also publish a config_invalidated event to a Redis channel; each gateway replica subscribes to this channel and flushes the affected local cache key on receipt; this reduces propagation latency from TTL duration to sub-second pub/sub delivery without eliminating the local cache that protects Redis from per-request configuration lookups .