Configuration for read-your-writes consistency guarantees in eventual consistency systems.

Used in query operations to wait for projections/read models to catch up to a specific version after a write operation. This enables clients to immediately read their own writes despite projection lag in CQRS/Event Sourcing architectures.

Consistency Modes

MinVersion (Recommended - Read-Your-Writes)

Waits for projection to reach AT LEAST the specified version. Will use newer data if available. Use this after mutations to ensure you can immediately read what you just wrote.

ExactVersion (Strict - Reproducible Snapshots)

Waits for projection to reach EXACTLY the specified version. Rejects if version is newer. Use this only when you need reproducible queries at a specific point-in-time (audits, reports).

Usage Pattern

1. Client performs mutation (e.g., CreateOrder, UpdateInventory)
2. Server returns stream_version (e.g., version 5)
3. Client immediately queries with Consistency:
   • min_version { version: 5 } (recommended - read-your-writes)
   • exact_version { version: 5 } (strict - reproducible snapshot)
4. Server retries query until projection reaches version or timeout

Example

// Mutation response
message CreateOrderResponse {
  string order_id = 1;
  uint64 stream_version = 2;  // Returns: 5
}

// Read-your-writes (recommended)
message GetOrderRequest {
  string order_id = 1;
  trogon.consistency.v1alpha1.Consistency consistency = 2;
}

GetOrderRequest {
  order_id: "order-123",
  consistency: {
    min_version: { version: 5 },
    timeout_duration: "1s",
    delay_duration: "100ms"
  }
}

Server Implementation Guidelines

Servers should:

• Enforce reasonable timeout limits (e.g., default 1s, max 5s)
• Enforce reasonable delay limits (e.g., default 100ms, max 500ms)
• Return unavailable status on timeout with appropriate error metadata
• Return failed precondition status on snapshot expired (ExactVersion only)
• Log when clamping client-provided values

Reference

Inspired by SpiceDB's Consistency patterns (at_least_as_fresh and at_exact_snapshot modes). See: https://authzed.com/docs/spicedb/concepts/consistency