Projector Examples

Read-side projectors from the poker domain. All code is from the actual examples/ directory.

Projectors subscribe to events and produce read-optimized views: text logs, database tables, search indexes, or external API calls.

---

Output Projector

The Output Projector subscribes to events from multiple domains (player, table, hand) and writes formatted game logs to a file. This demonstrates a multi-domain projector.

examples/cpp/prj-output/src/main.cpp

Projectors are classes with @projects / [Projects] decorators on event-handling methods — the same OO pattern as aggregates and sagas. All six languages use this.

---

Multi-Domain Subscription

Projectors can subscribe to events from multiple domains. The Output projector subscribes to player, table, and hand domains:

flowchart LR
    P[player domain] --> O[Output Projector]
    T[table domain] --> O
    H[hand domain] --> O
    O --> F[hand_log.txt]

Each language’s ProjectorHandler/ProjectorBase accepts multiple domain names:

Python

handler = ProjectorHandler("output", "player", "table", "hand")

---

Projector Principles

1. Read-only — Projectors never modify domain state, only create read views
2. Idempotent — Same events always produce same projections
3. Catchup safe — Can replay full event history to rebuild state
4. Domain aware — Subscribe to specific domains, filter unwanted events
5. Stateful OK — Can maintain local state (caches, maps) for rendering

---

Running Projectors

With ProjectorHandler (All Languages)

# Python
cd examples/python && python -m prj-output.main

# Go
cd examples/go && go run ./prj-output

# Rust
cd examples/rust && cargo run --bin prj-output

# Java
cd examples/java && ./gradlew prj-output:run

# C#
cd examples/csharp && dotnet run --project Prj/Output

# C++
cd examples/cpp && ./build/prj-output

---

Next Steps

• Aggregates — Command handling examples
• Sagas — Cross-domain coordination