REPLAY Command Overview
Purpose
The REPLAY command enables clients to request historical events from a specific point in time or sequence. This capability is essential for data recovery, reprocessing, and ensuring consistency across distributed systems after downtime or failures.
Key Capabilities
Historical Event Access
The REPLAY command provides access to previously published events stored in the system's durable storage, allowing clients to retrieve and process historical data.
Position-Based Replay
Clients can specify an exact position (timestamp, sequence number, or event ID) from which to begin replaying events, ensuring precise control over the replay process.
Gap Detection and Recovery
When a client detects gaps in its event stream due to disconnections or processing failures, it can use REPLAY to retrieve the missing events and maintain data consistency.
System Reconstruction
New services or reinstalled instances can use REPLAY to rebuild their state from historical events, implementing event sourcing patterns for robust state management.
Technical Implementation
The REPLAY command works by:
Authenticating the client using their client ID
Verifying that the client has permission to access the requested event types
Retrieving events from durable storage based on the specified position parameters
Delivering the events to the client in the original publication order
Optionally updating the client's position tracking after successful replay
This implementation ensures that clients can reliably access historical events while maintaining proper access controls and event ordering.
Integration Points
The REPLAY command integrates with:
Authentication System: Verifies client identity and permissions
Event Storage: Retrieves historical events based on position parameters
Position Tracking System: Optionally updates client position after replay
Rate Limiting System: Controls replay speed to prevent system overload
Use Cases
Recovering from service outages or network partitions
Rebuilding service state after deployment or failure
Implementing event sourcing patterns for state management
Analyzing historical data for debugging or auditing
Synchronizing new service instances with existing system state
Reprocessing events after fixing bugs or adding new features