Cyberstudy
PDF p.197 In progress

Clustering

Open PDF at p.197 18 flashcards

Summary

PDF p.197

Clustering involves multiple redundant processing nodes that share data and accept connections, providing redundancy and high availability. If one node fails, connections can failover to a working node, making the cluster appear as a single server to clients.

In plain words

Supplementary — not from your PDF

A cluster is a group of redundant nodes that share data and appear to clients as one server, often through a shared virtual IP address. Active/passive keeps a spare node idle, so performance holds during failover but capacity is wasted. Active/active uses all nodes, but performance drops when one fails. N+1 shares one spare among several active nodes; N+M shares several spares. Application clustering shares session state, so users can move between nodes.

Detailed explanation

PDF p.197
  • Clustering vs. Load Balancing
    • Load Balancing: Distributes traffic between independent processing nodes, typically managing web traffic.
    • Clustering: Provides redundancy and high availability for systems like databases and file servers by sharing data among nodes.
  • Virtual IP
    • Definition: A shared or floating address used by multiple load balancer appliances.
    • Configuration: Instances have a private connection with "real" IP addresses and run a redundancy protocol (e.g., CARP).
    • Failover Mechanism: Heartbeat mechanism allows failover to a passive node if the active one fails.
  • Active/Passive (A/P) and Active/Active (A/A) Clustering
    • Active/Passive Clustering
      • Definition: One node is active, the other is passive.
      • Advantage: Performance is not affected during failover.
      • Disadvantage: Higher hardware and operating system costs due to unused capacity.
    • Active/Active Clustering
      • Definition: Both nodes process connections concurrently.
      • Advantage: Maximum capacity utilization.
      • Disadvantage: Performance degradation during failover.
  • N+1 and N+M Configurations
    • N+1 Configuration
      • Definition: A single passive node shared among multiple active nodes.
      • Example: Five active nodes with one passive node.
      • Benefit: Reduces the number of passive nodes needed.
    • N+M Configuration
      • Definition: Multiple passive nodes shared among multiple active nodes.
      • Example: Ten active nodes with two or three passive nodes.
      • Benefit: Balance between redundancy and cost-efficiency.
  • Application Clustering
    • Purpose: Provision fault-tolerant application services.
    • Session State Data: Allows servers to communicate session information.
    • Example: User logs in on one instance, next session can start on another instance with access to login information.

Important terms

taken from the text above
Load Balancing
Distributes traffic between independent processing nodes, typically managing web traffic.
Clustering
Provides redundancy and high availability for systems like databases and file servers by sharing data among nodes.
Virtual IP
A shared or floating address used by multiple load balancer appliances.
Failover Mechanism
Heartbeat mechanism allows failover to a passive node if the active one fails.
Active/Passive Clustering
One node is active, the other is passive.
Active/Active Clustering
Both nodes process connections concurrently.
N+1 Configuration
A single passive node shared among multiple active nodes.
N+M Configuration
Multiple passive nodes shared among multiple active nodes.
Session State Data
Allows servers to communicate session information.
A/P Active/Passive A/A Active/Active

Examples & real-world scenarios

Supplementary — not from your PDF
  • Two database servers in an active/passive cluster.
  • A pair of load balancers sharing a virtual IP with a heartbeat.
  • Five active web nodes and one spare (N+1).

Scenario

An active/active pair runs at 80% load each. When one fails, the survivor must carry 160%, so users see slowdowns. Active/active clusters must be sized for failover.

Common mistakes

Supplementary — not from your PDF
  • Mixing up clustering (shared data, redundancy) and load balancing (independent nodes, traffic distribution).
  • Forgetting the performance hit in active/active failover.

Practical skills

Supplementary — not from your PDF
  • Compare A/P, A/A, N+1 and N+M for cost and resilience.

What I should remember

Key Points PDF p.197
  • Clustering vs. Load Balancing
    • Load Balancing: Manages web traffic.
    • Clustering: Redundancy for databases, file servers.
  • Virtual IP
    • Shared Address: Used by multiple appliances.
    • Redundancy Protocol: CARP, heartbeat mechanism.
  • Active/Passive and Active/Active Clustering
    • Active/Passive: One active, one passive.
    • Active/Active: Both nodes active.
  • N+1 and N+M Configurations
    • N+1: Single passive node for multiple active nodes.
    • N+M: Multiple passive nodes for multiple active nodes.
  • Application Clustering
    • Fault-Tolerant Services: Communicate session information.
    • Example: Session continuity across instances.