dsfx/docs/requirements/fr-5-replication-and-redundancy-management.md

# FR-5: Replication and Redundancy Management

## 1. Overview

This document specifies the requirements for ensuring high availability and fault tolerance through replication and redundancy of file chunks across multiple nodes. The aim is to guarantee that file data remains accessible even when individual nodes encounter failures or network disruptions. By automatically replicating data and maintaining redundancy, the system can support continuous file access and preserve data integrity under both expected and adverse conditions.

## 2. Objectives & Scope

### 2.1 Objectives

- **High Data Availability:** Ensure that file chunks are stored redundantly across multiple nodes so that failures of individual nodes do not result in loss of data.
- **Fault Tolerance:** Enable automatic recovery from node, network, or disk failures through robust replication mechanisms.
- **Seamless Replication:** Perform replication without user intervention, ensuring that data redundancy remains up-to-date.
- **Efficient Resource Usage:** Optimize replication to reduce unnecessary data transfer and storage overhead, using techniques such as deduplication and reference counting.
- **Integration with Deterministic Simulation:** Validate replication and redundancy mechanisms under simulated conditions such as network latency and disk failures.

### 2.2 Scope

- Applies to all file chunks managed by the system.
- Covers the automatic replication process that spreads file chunks to multiple nodes.
- Includes verification of replicated data using integrity checks (e.g., SHA-256 hashes).
- Integrates with the system’s deterministic simulation framework for testing under controlled adverse conditions.

## 3. Detailed Requirements

### 3.1 Automatic Replication Mechanism

- **Replication Trigger:**

  - The system shall automatically monitor the availability and integrity of file chunks.
  - On detecting a node failure or data inconsistency, the system must trigger a replication process to copy affected chunks to other healthy nodes.

- **Predefined Replication Factor:**
  - Configuration must allow specifying a minimum replication factor (e.g., a chunk should exist on at least three different nodes) for fault tolerance.
  - In cases where a chunk is stored on fewer nodes than specified, a replication process is initiated to restore the desired redundancy level.

### 3.2 Data Integrity and Verification

- **Integrity Checks:**

  - Each file chunk must be associated with a cryptographic hash (e.g., SHA-256) that is used to verify the chunk’s integrity before and after replication.
  - During the replication process, a verification step must confirm that the replicated data matches the expected hash.

- **Auditability:**
  - All replication operations, including successful replications and error events, must be recorded in an immutable, tamper-evident log.
  - Logs should include details such as timestamps, node identifiers, replication actions, and outcomes to support audit and troubleshooting activities.

### 3.3 Transparent Redundancy Management

- **Automatic Monitoring:**

  - A built-in monitoring service (or daemon) should continuously assess the health and availability of file chunks across nodes.
  - The monitoring service should also check for outdated replicas and trigger rebalancing if necessary.

- **User Transparency:**
  - The replication mechanism must operate in the background, without requiring manual configuration or intervention by the user during normal operations.
  - Users should, however, be provided with a summary view of replication status and data redundancy levels via the user interface.

### 3.4 Handling Node Failures and Network Disruptions

- **Resilience under Failure:**

  - In the event of a node or disk failure, the system must automatically redistribute file chunks based on the pre-configured replication policy.
  - The replication strategy must include fallback mechanisms that ensure replication continues despite intermittent network connectivity or node unavailability.

- **Retry and Recovery:**
  - Replication operations should include retry logic and exponential backoff strategies in case of transient errors.
  - After restoration of connectivity or node recovery, the system must reassess and adjust the replication factor if necessary.

### 3.5 Integration with Deterministic Simulation Testing

- **Simulated Failure Conditions:**

  - The replication process must be fully testable within a deterministic simulation framework that emulates network latency, packet loss, and disk failures.
  - Testing scenarios should include verifying that replication operations complete successfully despite simulated adverse conditions, and that logs accurately reflect the replication events.

- **Performance Benchmarks:**
  - The system should meet performance targets under simulated failure conditions, ensuring that replication delays do not significantly impact overall system responsiveness (see NFR-2 for performance benchmarks).

## 4. Measurable Criteria & Test Cases

### 4.1 Automated End-to-End Replication Tests

- **Replication Factor Test:**

  - Simulate a scenario where one or more nodes become unavailable. Verify that file chunks are replicated up to the pre-defined replication factor.
  - Confirm that the system automatically detects insufficient replication and initiates additional replication cycles.

- **Data Integrity Test:**

  - For each replicated chunk, compare the cryptographic hash of the replica against the original value.
  - Ensure that any mismatch triggers an error recovery process and logs the event for audit purposes.

- **Recovery and Retry Test:**
  - Simulate transient network outages and node failures. Verify that the replication process successfully recovers once the system conditions stabilize and that all affected chunks reach the required replication level.

### 4.2 Performance and Stress Testing

- **Replication Throughput Test:**

  - Measure the time taken to replicate a set of file chunks under normal conditions versus under simulated network latency/disruptions.
  - Ensure that performance remains within acceptable bounds as outlined in the NFR-2 specifications.

- **Logging and Audit Test:**
  - Validate that all replication actions are recorded in the system log with accurate timestamps, node IDs, and operation details.
  - Confirm that auditor reviews can successfully trace and verify all replication events.

## 5. Dependencies & Integration Points

- **Storage Backend:**

  - Reliance on the underlying storage management system (see ADR-0012) which manages local file systems and content-addressable storage.

- **Manifest and Audit Log:**

  - Integration with the immutable manifest (ADR-0006) ensures that replication events are incorporated into the overall audit trail.

- **Deterministic Simulation Framework:**

  - Testing and validation of the replication mechanisms under simulated adverse conditions are critical, as defined by the deterministic simulation framework requirements.

- **Monitoring and Diagnostic Tools:**
  - The replication system should integrate with internal monitoring tools to continuously track node health, replication status, and resource utilization.

## 6. Security Considerations

- **Data Confidentiality:**

  - Replicated data must remain encrypted at all times, with no decryption occurring during normal replication operations.

- **Tamper-Evident Logging:**

  - All replication-related events must be logged securely, ensuring that logs cannot be altered or tampered with to hide replication failures or anomalies.

- **Access Controls:**
  - Only authorized system components and administrators should be able to trigger or modify replication operations. Proper authentication measures must be enforced.

## 7. Conclusion

FR-5 establishes a robust replication and redundancy management mechanism that is essential for ensuring high data availability and fault tolerance in the system. By automatically replicating file chunks across multiple nodes while maintaining strict integrity checks and tamper-evident logging, the system efficiently safeguards user data against node, network, or disk failures. Integration with deterministic simulation further ensures that these mechanisms are reliable under both normal and adverse operating conditions.