Redis Isn't PostgreSQL: Building a Hybrid Change Data Capture Runtime in Ruby

I Built Commercial Redis CDC Source Drivers for Ruby — Here's What I Learned For the past couple of years I've been building a Change Data Capture (CDC) ecosystem for Ruby. Like many CDC projects, it started with PostgreSQL. PostgreSQL's Write-Ahead Log (WAL) is an excellent source of truth: durable, ordered, replayable, and well understood. It provides exactly the properties you want when you're building reliable event pipelines. But the deeper I went into distributed systems, the more I realized something important. Many systems don't observe change from PostgreSQL first. They observe it from Redis. Redis often sits at the front of modern architectures: Redis Streams carry application events. Pub/Sub distributes transient state changes. Keyspace notifications react to cache invalidation and key expiry. Redis Cluster routes events across multiple primaries. In many systems, Redis sees a change before PostgreSQL ever commits it. That raised an interesting question: Can Redis become a first-class Change Data Capture source? The obvious answer is "yes." The interesting answer is "yes—but not in the same way PostgreSQL does." That distinction eventually became cdc-redis-pro , a commercial Redis source driver for the Ruby CDC ecosystem. This article isn't a product announcement. It's an engineering write-up about the architectural decisions behind the project, the tradeoffs Redis forces you to make, and the execution model that ultimately emerged. Redis Doesn't Have One CDC Interface One misconception I frequently encounter is the assumption that Redis has an equivalent of PostgreSQL's WAL. It doesn't. Instead, Redis exposes several completely different mechanisms for observing change. Source Delivery Replay Streams At-least-once Yes Pub/Sub At-most-once No Sharded Pub/Sub At-most-once No Keyspace Notifications At-most-once No At first glance they all look like "events." Operationally they're completely different systems. Streams are durable. Pub/Sub isn't. Keyspace not