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of 30What is High Availability and why is it important?
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High Availability (HA) refers to systems designed to remain operational and accessible for extended periods, typically measured as uptime percentages (99.9%, 99.99%, etc.). HA is crucial because:
- Business Continuity: Minimizes revenue loss from downtime
- User Experience: Ensures consistent service availability
- Reputation: Prevents damage from service outages
- Compliance: Meets regulatory requirements for uptime
The goal is to eliminate single points of failure through redundancy, failover mechanisms, and robust system design. For example, 99.9% availability means approximately 8.77 hours of downtime per year, while 99.99% allows only 52.6 minutes annually.
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Explain the difference between RTO and RPO.
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RTO (Recovery Time Objective) and RPO (Recovery Point Objective) are critical disaster recovery metrics:
- RTO: Maximum acceptable time to restore service after a failure. This is about how fast you can recover.
- RPO: Maximum acceptable amount of data loss measured in time. This is about how much data you can afford to lose.
Example:
If a database fails at 2:00 PM:
- RTO of 1 hour means service must be restored by 3:00 PM
- RPO of 15 minutes means you can only lose data from 1:45 PM onward
These metrics drive backup frequency, infrastructure investment, and recovery strategy decisions.
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What are the different levels of availability (9s) and their practical implications?
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Availability levels are expressed as "nines":
- 99% (Two 9s): 3.65 days downtime/year - Basic consumer services
- 99.9% (Three 9s): 8.77 hours downtime/year - Small business applications
- 99.99% (Four 9s): 52.6 minutes downtime/year - Enterprise applications
- 99.999% (Five 9s): 5.26 minutes downtime/year - Critical systems (telecom, financial)
- 99.9999% (Six 9s): 31.5 seconds downtime/year - Ultra-critical systems
Each additional "9" exponentially increases complexity and cost. Achieving five 9s requires redundant everything: power, network, hardware, software, and often geographically distributed infrastructure.
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What is a single point of failure and how do you eliminate it?
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A Single Point of Failure (SPOF) is any component whose failure would cause the entire system to fail. Common SPOFs include:
- Hardware: Single server, network switch, power supply
- Software: Single database instance, application server
- Network: Single internet connection, DNS server
- Human: Single administrator with exclusive knowledge
Elimination strategies:
- Redundancy: Multiple instances of critical components
- Load Balancing: Distribute traffic across multiple nodes
- Clustering: Group servers to act as one logical unit
- Geographic Distribution: Multiple data centers
- Documentation: Ensure knowledge is shared among team members
Example: Instead of one web server, deploy three servers behind a load balancer with health checks.
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What is session persistence (sticky sessions) and when would you use it?
How do health checks work in load balancers?
What are the considerations for power redundancy in HA systems?
What are the key metrics to monitor in an HA environment?
What is clustering and what are the main types?
Explain the concept of quorum in clustering.
What is split-brain and how do you prevent it?
Describe heartbeat mechanisms in clustering.
Explain different load balancing algorithms.
What is the difference between Layer 4 and Layer 7 load balancing?
Explain database replication types: synchronous vs asynchronous.
What is database clustering and how does it differ from replication?
What is a database proxy and how does it help with HA?
What is network bonding/teaming and how does it provide HA?
Explain VRRP (Virtual Router Redundancy Protocol).
How do you implement effective alerting for HA systems?
Describe blue-green deployment strategy and its role in achieving high availability.
How do you achieve high availability in cloud environments (AWS/Azure/GCP)?
What is auto-scaling and how does it contribute to high availability?
Explain containerization's impact on high availability strategies.
What is fencing in clustering and why is it necessary?
Explain the concept of database sharding.
Explain the CAP theorem and its implications for distributed systems.
What are consensus algorithms and why are they important in distributed systems?
What is circuit breaker pattern and how does it improve system resilience?
What are the key considerations for disaster recovery in HA systems?
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High Availability & Clustering cheatsheet
High Availability (HA) System Administration Cheat Sheet
- 📌 Core Concepts01
- 🏗️ HA Architecture Patterns02
- ⚖️ Load Balancing Strategies03
- 🔄 Clustering Technologies04
- 💾 Data Replication Strategies05
- 🔍 Monitoring & Alerting06
- 🛠️ HA Tools & Technologies07
- 📋 Best Practices08
- 🎯 Common Interview Questions09
- 💡 Quick Tips for Interviews10
- 🔗 Key Technologies to Know11
- 📝 Final Interview Prep Checklist12
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