Command-Query Responsibility Segregation (CQRS) in Rust with Actix

Adam Szpilewicz

4 min read

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May 18, 2025

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“One model to read, one model to write — CQRS makes this explicit. Rust makes it safe.”

🧭 What Is CQRS?

CQRS (Command Query Responsibility Segregation) is an architectural pattern that separates read operations (queries) from write operations (commands). In Rust, where strong typing and ownership are first-class, this pattern maps cleanly to the language’s strengths.

Use cases include:

• Auditable systems (event sourcing, logging)
• Complex domain logic for writes vs performance-optimized reads
• Decoupling bounded contexts (e.g., microservices)

Let’s implement a simple user registration system using Actix-Web, showcasing how CQRS can be cleanly modeled in idiomatic Rust.

🧱 System Overview

We’ll build:

• Command Handler: Creates users.
• Query Handler: Fetches users.
• State Storage: In-memory DB (mocked with Mutex<HashMap>).

🧩 Dependencies

# Cargo.toml
[dependencies]
actix-web = "4"
serde = { version = "1", features = ["derive"] }
serde_json = "1"
tokio = { version = "1"…