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Beginner Topic 7 of 51 Distributed Systems & Cloud

Availability and Reliability

Fraction of time working and consistency of correct operation.

How it works

Availability is the fraction of time a system is operational and able to serve requests, often expressed in nines: 99.9 percent allows about 8.8 hours of downtime per year, 99.99 percent about 53 minutes. Reliability is the probability that it performs correctly over a period, measured by mean time between failures, while recovery time and mean time to repair determine how quickly service returns. Redundancy, failover, health checks, graceful degradation and avoiding single points of failure improve both. Service level objectives set targets.

Where it is used

Cloud services publish SLAs, and architects compute availability of composed systems: serial dependencies multiply availabilities, while redundant components improve them.

Common pitfall

Each added nine costs much more. Dependencies lower overall availability, so a service depending on five 99.9 percent services cannot exceed about 99.5 percent. Untested failover often fails when needed. Correlated failures defeat redundancy. Define SLOs from user impact, practice failure drills and monitor error rates.

Where it fits in the course

Availability and Reliability is topic 7 of 51 in Distributed Systems & Cloud, rated Beginner. It follows Latency vs Throughput. The next topic is Remote Procedure Calls.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Distributed Systems & Cloud quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Availability and Reliability?
Fraction of time working and consistency of correct operation. It belongs to Distributed Systems & Cloud and is a beginner-level topic.
Where do people use Availability and Reliability?
Cloud services publish SLAs, and architects compute availability of composed systems: serial dependencies multiply availabilities, while redundant components improve them.
What is the most common mistake with Availability and Reliability?
Each added nine costs much more.
What should I learn before this topic?
Start with Latency vs Throughput, the topic just before it in Distributed Systems & Cloud.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Distributed Systems & Cloud quiz.
Is Availability and Reliability hard to learn?
It is a Beginner topic, so it works as an entry point. If the terms are new, read the explanation twice and try the visualizer before the quiz.
How long does it take to learn Availability and Reliability?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Availability and Reliability?
Latency vs Throughput, Remote Procedure Calls, Vertical vs Horizontal Scaling are closely connected. The full list is in the related topics section of this page.
What should I study after Availability and Reliability?
Continue with Remote Procedure Calls, the next topic in Distributed Systems & Cloud.

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