3.4 Resilience & recovery
📘CompTIA Security+ (SY0-701)
1. What is High Availability (HA)?
High Availability (HA) means keeping systems, applications, and data accessible and running even when part of the system fails.
The goal is to minimize downtime and ensure continuous service to users.
In a secure and reliable IT environment, HA is achieved by adding redundancy — meaning extra systems or components are available to take over if something goes wrong.
CompTIA Security+ expects you to understand how HA supports business continuity and how load balancing and clustering help achieve it.
2. Key Concepts of High Availability
| Concept | Description |
|---|---|
| Redundancy | Having backup components (servers, links, power, etc.) so if one fails, another takes over. |
| Failover | The automatic switching to a backup component when the main one fails. |
| Uptime | The percentage of time a system is operational (for example, 99.999% uptime is “five nines” reliability). |
| Scalability | The ability to add more systems or resources to handle increased load without downtime. |
3. Load Balancing
Definition
Load balancing is the process of distributing network traffic or workloads across multiple servers to improve performance and ensure availability.
It is mainly used in web servers, databases, and application servers to handle high volumes of requests efficiently.
Purpose
- Prevent any single server from becoming overloaded.
- Increase the performance and reliability of services.
- Provide automatic failover — if one server goes down, traffic is redirected to others.
How Load Balancing Works
A load balancer (hardware or software device) sits between clients and servers. It receives all incoming requests and decides which server should handle each one based on specific algorithms.
Common Load Balancing Methods
| Method | Description |
|---|---|
| Round Robin | Requests are distributed evenly across all servers in rotation. |
| Least Connections | New traffic is sent to the server with the fewest active connections. |
| IP Hash / Source Hash | Requests are distributed based on the client’s IP address, ensuring consistent connection to the same server. |
| Weighted Load Balancing | Some servers receive more requests based on their capacity (e.g., faster servers get more traffic). |
Load Balancing Benefits
- Improves performance by spreading load.
- Provides fault tolerance — if one server fails, others continue to serve users.
- Enables maintenance without downtime — individual servers can be updated while the service stays available.
- Supports scaling as more servers can be added to handle growth.
Load Balancing in IT Environments
- Web hosting services distribute website traffic among multiple web servers.
- Cloud environments (like AWS, Azure) automatically balance workloads among virtual machines.
- Database servers use load balancing to handle high query requests.
4. Clustering
Definition
Clustering means connecting two or more servers (nodes) to work together as a single system.
The main goal is high availability and redundancy — if one node fails, another takes over immediately with little or no disruption.
Types of Clustering
| Type | Description |
|---|---|
| Active-Active Cluster | All nodes are active and share the workload. If one fails, others continue handling requests. This improves performance and availability. |
| Active-Passive Cluster | One node is active while the other is on standby. If the active node fails, the standby node takes over (failover). |
Clustering Components
- Heartbeat: A monitoring mechanism that checks if nodes are active and healthy.
- Shared Storage: A common storage area used by all cluster nodes to keep data synchronized.
- Failover Mechanism: Automatically switches workloads from a failed node to a standby node.
Clustering Benefits
- Ensures continuous availability of critical services (for example, databases, file servers, virtual machines).
- Provides automatic recovery after a node failure.
- Simplifies maintenance, since updates can be applied to one node at a time.
Clustering in IT Environments
- Database servers (e.g., Microsoft SQL Server clusters) use clustering to ensure data is always available.
- Virtualization systems (e.g., VMware HA clusters) automatically restart virtual machines on another host if one host fails.
- File servers in enterprises use clustering to prevent data unavailability during maintenance or hardware issues.
5. Load Balancing vs Clustering – Key Differences
| Feature | Load Balancing | Clustering |
|---|---|---|
| Purpose | Distribute workload among multiple servers to improve performance and prevent overload. | Provide redundancy and failover to ensure service availability. |
| Focus | Performance and efficiency. | Reliability and uptime. |
| System Relationship | Servers work independently but share the workload. | Servers work together as one logical system. |
| Failure Handling | Redirects traffic to another healthy server. | Transfers operations automatically to another node. |
| Common Usage | Web servers, application servers. | Database servers, virtualization hosts, storage systems. |
6. Security Considerations for HA Systems
High availability solutions must also maintain security, not just uptime.
| Security Control | Description |
|---|---|
| Authentication & Encryption | Communication between load balancers, clusters, and clients should use secure protocols (e.g., HTTPS, TLS). |
| Patch Management | Apply patches carefully — ideally, one node at a time to maintain uptime. |
| Access Control | Limit administrative access to load balancers and cluster management interfaces. |
| Monitoring & Logging | Continuously monitor the health and performance of nodes to detect attacks or misconfigurations. |
7. Summary for Exam
| Key Term | Summary |
|---|---|
| High Availability (HA) | Ensures systems stay operational with minimal downtime. |
| Load Balancing | Distributes traffic evenly across servers for performance and fault tolerance. |
| Clustering | Connects multiple servers to act as one unit for redundancy and failover. |
| Active-Active Cluster | All nodes handle traffic simultaneously. |
| Active-Passive Cluster | One node works while another waits as a backup. |
| Failover | Automatic switch to a backup system when a failure occurs. |
Exam Tip
For Security+:
- Remember that load balancing = distributing workload
- Clustering = ensuring redundancy/failover
- Both are used for High Availability (HA) but serve slightly different purposes.
- You might see them mentioned in business continuity or disaster recovery contexts.
