---
title: "Fault tolerance and types of faults"
subject: "Distributed System"
chapter: 9
key: "semester_7/distributed_system:fault-types"
source: https://www.bctengineer.com/q/semester_7/distributed_system/fault-types
site: BCT Engineering
---

# Fault tolerance and types of faults

Distributed System · Chapter 9: Fault Tolerance

- Asked in: 2076 Asoj
- Video: [Types of faults in distributed systems](https://www.youtube.com/watch?v=LIQWk_-aydc) — Simplified Computer Science Concepts-Prof. Rutuja
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/distributed_system/fault-types
- Whole chapter: https://www.bctengineer.com/md/semester_7/distributed_system/chapter-9

## Question

What is fault tolerance? Explain different type of faults that may occur in a distributed system.

## Short answer

Fault tolerance is the ability of a system to continue correct operation despite faults. Faults in distributed systems include crash (server halts), omission (lost messages), timing (response outside deadline), response (incorrect value), and arbitrary/Byzantine (malicious or arbitrary behavior).

## Answer

#### Fault tolerance
**Fault tolerance** is the property of a system to keep working correctly in the presence of faults, so that a fault does not become a failure of the whole service. It is achieved by redundancy and recovery.

#### Types of faults
A **fault** is the defect or cause (a broken component, a software bug, a bad link) that may lead to an **error**, an incorrect internal state. An error that reaches the service interface and makes the system deviate from its specification is a **failure**. By duration, faults are **transient** (occur once and vanish), **intermittent** (appear and disappear repeatedly) or **permanent** (persist until the component is repaired).

By the way a server fails, the failures are classified as:

| Type | Behaviour |
|---|---|
| **Crash failure** | The server halts, but worked correctly until it halted |
| **Omission failure** | Fails to respond to requests (receive omission) or to send replies (send omission) |
| **Timing failure** | Response comes outside the specified time interval |
| **Response failure** | Wrong value (value failure) or wrong state transition |
| **Arbitrary (Byzantine) failure** | Produces arbitrary responses at arbitrary times; may be malicious |

By source, faults can also be **hardware, software, network or operator** faults. In a distributed system, network faults such as lost messages, link failures and partitions are especially important.

Example: a server that stops responding has a crash failure, and a link that drops 5% of packets has an omission failure.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. An error that reaches the service interface and violates the specification is called a:
   A. Fault
   B. Failure
   C. Checkpoint
   D. Omission
   Answer: B. Fault causes error, which may become failure. (Key point: Fault, error, failure)

2. A fault that appears and disappears repeatedly is:
   A. Transient
   B. Intermittent
   C. Permanent
   D. Byzantine
   Answer: B. Transient occurs once; intermittent recurs; permanent persists until repair. (Key point: Transient vs intermittent)

3. A crash failure is when:
   A. A server returns wrong values
   B. The server halts but worked correctly until it halted
   C. The reply is late
   D. It acts maliciously
   Answer: B. Crash means fail-stop after correct behaviour. (Key point: Crash failure)

4. A link that drops 5% of packets is an example of:
   A. Crash failure
   B. Timing failure
   C. Omission failure
   D. Response failure
   Answer: C. Failing to deliver messages is an omission failure. (Key point: Omission example)

## Related in Distributed System

- [Replication, fault tolerance, scalability](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Areplication-scalability-ft) — Distributed System, chapter 7
- [Fault tolerance techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Afault-tolerance-techniques) — Distributed System, chapter 9
- [Fault tolerance; Byzantine failure](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Abyzantine-failure-agreement) — Distributed System, chapter 9
- [K-fault tolerant system; recovery](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Ak-fault-recovery) — Distributed System, chapter 9
- [Autonomous vs distributed system](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aautonomous-vs-distributed) — Distributed System, chapter 1
- [Models of distributed system](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Amodels-of-ds) — Distributed System, chapter 1

## Related in other subjects

- [Availability and fault tolerance in GFS](https://www.bctengineer.com/question.md?key=semester_8%2Fbig_data_technologies%3Agfs-availability-fault-tolerance) — Big Data Technologies, chapter 2
- [Fault tolerance and redundancy for reliability](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Afault-tolerance-schemes) — Instrumentation II, chapter 7, asked 4×
- [Software fault tolerance and recovery](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Asw-fault-tolerance-rollback) — Instrumentation II, chapter 9, asked 2×
- [Distributed system in big data example](https://www.bctengineer.com/question.md?key=semester_8%2Fbig_data_technologies%3Adistributed-system-example) — Big Data Technologies, chapter 1