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Design - Fault-Tolerance

Overview

Describe how your system handles and recovers from failures to maintain service availability.

Data Replication

Replication Model

Strategy: [Master-Slave / Multi-Master / Peer-to-Peer]

Topology:

graph TB
    subgraph DC1["Datacenter 1"]
        M["Primary\n(Master)"]
        R1["Read Replica 1"]
    end

    subgraph DC2["Datacenter 2"]
        R2["Read Replica 2"]
        R3["Read Replica 3"]
    end

    M -->|Continuous Replication| R1
    M -->|Continuous Replication| R2
    M -->|Continuous Replication| R3

    style M fill:#ff9999
    style R1 fill:#99ccff
    style R2 fill:#99ccff
    style R3 fill:#99ccff

Replication Details

  • What Data is Replicated: [All data / Critical data only]
  • Number of Replicas: [2, 3, 5+]
  • Replication Lag: [Milliseconds / Seconds / Eventually consistent]
  • Synchronous vs Asynchronous: [Which approach?]

Consistency Between Replicas

  • Update Propagation: [How are updates sent to replicas?]
  • Conflict Detection: [How are conflicts detected?]
  • Conflict Resolution: [CRDT / Vector Clocks / Last-Write-Wins / Application-specific]

Heartbeat and Monitoring

Heartbeat Mechanism

Purpose: Detect when components fail

Implementation: - Frequency: [Every X seconds] - Timeout: [Consider dead after Y seconds] - Protocol: [HTTP, TCP, UDP, custom]

Example:

sequenceDiagram
    participant Monitor
    participant Service

    loop Every 5 seconds
        Monitor->>+Service: PING
        Service-->>-Monitor: PONG
        Note over Monitor: Service is healthy
    end

    Monitor->>+Service: PING
    Note over Service: Service is down
    Note over Monitor: Timeout! Mark as unhealthy

Components Involved

Component Monitors Heartbeat Interval Failure Threshold
[Service 1] [Monitors] [X seconds] [Y failures]
[Service 2] [Monitors] [X seconds] [Y failures]

Monitoring and Alerting

  • Monitoring Tool: [Prometheus/ELK/Datadog/Custom]
  • Alert Conditions: [CPU > 80%, Latency > 1s, etc.]
  • Alert Channels: [Email, Slack, PagerDuty, etc.]

Timeout and Retry Mechanisms

Retry Strategy

When to Retry: [Transient failures like network timeouts]

Retry Logic:

for attempt in range(1, MAX_ATTEMPTS + 1):
    try:
        result = call_remote_service()
        return result
    except TransientException:
        if attempt < MAX_ATTEMPTS:
            wait_time = exponential_backoff(attempt)
            time.sleep(wait_time)
        else:
            raise

Backoff Strategy: - Type: [Linear / Exponential / Random Exponential] - Initial Delay: [X milliseconds] - Max Delay: [Y milliseconds] - Multiplier: [2.0 for exponential]

Timeout Configuration

Operation Timeout Rationale
[Operation 1] [X seconds] [Why this timeout?]
[Operation 2] [Y seconds] [Why this timeout?]

Example:

sequenceDiagram
    participant Client
    participant Server

    Client->>+Server: Request (Timeout: 5s)
    Note over Server: Processing...
    Note over Server: Delay > 5s
    Client-->>Client: Timeout!
    Note over Client: Retry with backoff
    Server-->>-Client: Response (arrives late)

Error Handling and Recovery

Error Classification

Transient Errors (Recoverable): - Network timeouts - Temporary unavailability - Rate limiting - Action: Retry with backoff

Permanent Errors (Non-recoverable): - Authentication failures - Data validation errors - Resource not found - Action: Log and propagate to user

Cascading Failures: - One component failure triggers others - Action: Circuit breaker pattern

Circuit Breaker Pattern

Purpose: Prevent cascading failures

stateDiagram-v2
    [*] --> Closed
    Closed --> Open: Failure threshold exceeded
    Open --> Half_Open: Timeout elapsed
    Half_Open --> Closed: Test succeeds
    Half_Open --> Open: Test fails

States: - Closed: Normal operation, requests pass through - Open: Failures detected, requests fail fast - Half-Open: Testing if service recovered

Configuration: - Failure Threshold: [X consecutive failures] - Reset Timeout: [Y seconds before trying again] - Success Threshold: [Z successful requests in half-open to close]

Graceful Degradation

Degraded Mode Operation

Scenario: [Critical component fails]

Response: 1. [Detect failure] 2. [Switch to degraded mode] 3. [Reduce functionality] 4. [Notify users] 5. [Maintain core functionality]

Example:

graph TB
    Normal["Normal Operation\n(All features)"]
    Degraded["Degraded Mode\n(Core features only)"]
    Failed["Failed\n(Service unavailable)"]

    Normal -->|Component Failure| Degraded
    Degraded -->|Recovery| Normal
    Degraded -->|Cascading Failures| Failed

Fallback Strategies

Component Primary Strategy Fallback Strategy Fallback 2
[Component] [Primary] [Fallback 1] [Fallback 2]

Failover and Recovery

Failover Process

Detection Time: [How long to detect failure?]

Failover Time: [How long to complete failover?]

Data Loss: [Acceptable data loss during failover?]

Procedure:

sequenceDiagram
    participant Monitor
    participant Primary
    participant Secondary
    participant DNS

    Monitor->>Primary: Healthcheck
    Primary-->>Monitor: [No response]
    Monitor->>Monitor: Failure detected
    Monitor->>Secondary: Promote to primary
    Secondary->>DNS: Update records
    DNS->>DNS: Propagate changes
    Note over Secondary: Now receiving traffic

Automatic Recovery

  • Recovery Time Objective (RTO): [X minutes]
  • Recovery Point Objective (RPO): [Lose last Y minutes of data]
  • Automatic Restart: [Enabled/Disabled]
  • Restart Delay: [If exponential backoff, what's the policy?]

Backup and Restore

Backup Strategy

Frequency: [Daily / Hourly / Continuous]

Type: [Full / Incremental / Differential]

Location: [On-site / Off-site / Multiple locations]

Retention: [How long are backups kept?]

Example Backup Schedule:

00:00 - Full backup
06:00 - Incremental backup
12:00 - Incremental backup
18:00 - Incremental backup

Restore Procedure

  1. [Step 1: Assess data loss]
  2. [Step 2: Prepare recovery environment]
  3. [Step 3: Restore from backup]
  4. [Step 4: Verify data integrity]
  5. [Step 5: Resume normal operations]

Estimated Recovery Time: [X hours]

Disaster Recovery Plan

Disaster Scenarios

Scenario 1: [Data corruption] - Impact: [What fails?] - Detection: [How to detect?] - Recovery: [Steps to recover] - RTO/RPO: [X hours / Y hours]

Scenario 2: [Datacenter outage] - Impact: [Service completely down] - Detection: [All health checks fail] - Recovery: [Failover to secondary datacenter] - RTO/RPO: [Minutes / Seconds of data]

Scenario 3: [Cascading failures] - Impact: [Multiple components fail] - Detection: [Circuit breakers open] - Recovery: [Rolling restart, health checks] - RTO/RPO: [Variable]

Disaster Recovery Testing

  • Test Frequency: [Monthly / Quarterly / Annually]
  • Test Type: [Full / Partial / Simulation]
  • Test Results: [Document findings and improvements]