Distributed vs Decentralized Systems in Computer Networks

Questions and Answers

What is an expansive view in the context of networked computer systems?

Extending an existing networked computer system with additional computers.

What is the primary difference between a decentralized and a distributed system?

In a decentralized system, processes and resources are necessarily spread across multiple computers, whereas in a distributed system, they are sufficiently spread across multiple computers.

What is an example of a system that is logically centralized but physically distributed?

The Domain Name System (DNS).

Why is the statement 'Centralized solutions do not scale' considered a misconception?

Because some centralized solutions can be designed to scale, such as the root of the Domain Name System. Signup and view all the answers

What is a common misconception about centralized solutions?

That they always have a single point of failure. Signup and view all the answers

What is the significance of the distinction between logically centralized and physically centralized systems?

It highlights the importance of understanding the underlying architecture of a system, rather than just its surface-level characteristics. Signup and view all the answers

What is one technique used to scale applications and hide communication latencies?

Asynchronous communication with separate handlers for incoming responses. Signup and view all the answers

How can computations be scaled across multiple machines?

By partitioning data and computations across multiple machines. Signup and view all the answers

What is one way to facilitate solution scalability by moving computations to clients?

Using Java applets and scripts. Signup and view all the answers

What is the definition of trust in the context of security mechanisms?

One entity can be assured that another will perform particular actions according to a specific expectation. Signup and view all the answers

What is the primary purpose of encryption and decryption in symmetric cryptosystems?

To secure data using encryption and decryption keys. Signup and view all the answers

What is the benefit of decentralized naming services like DNS?

Scalability through partitioning of data and computations across multiple machines. Signup and view all the answers

What is the main problem with replication and caching?

Inconsistencies arise when modifying one copy of the data, making it different from the rest. Signup and view all the answers

What is the relationship between the encryption key (EK) and decryption key (DK) in symmetric cryptosystems?

EK = DK, and both keys must be kept secret. Signup and view all the answers

What is the primary difference between symmetric and asymmetric cryptosystems?

Asymmetric cryptosystems use a public key (PK) and a private (secret) key (SK), whereas symmetric cryptosystems use the same key for encryption and decryption. Signup and view all the answers

What is one technique used to make data available at different machines?

Replication and caching. Signup and view all the answers

What is the benefit of using replicated file servers and databases?

Scalability and availability of data through replication. Signup and view all the answers

What is the purpose of secure hash functions in security mechanisms?

To return a fixed-length string that cannot be reversed or tampered with. Signup and view all the answers

What is the purpose of web caches in browsers and proxies?

To reduce latency and improve performance by storing frequently accessed data locally. Signup and view all the answers

What happens when data is modified and passed through a secure hash function?

The resulting hash value will be completely different from the original hash value. Signup and view all the answers

What is the primary goal of secure hashing in security mechanisms?

To ensure data integrity and prevent tampering. Signup and view all the answers

What is the computational complexity of finding the original data from a given hash value?

It is computationally impossible to find the original data from a given hash value. Signup and view all the answers

What is the primary difference between reliability and availability in the context of dependability?

Reliability refers to the continuity of service delivery, while availability refers to the readiness for usage. Signup and view all the answers

What is the mean time to repair (MTTR) in the context of dependability metrics?

The average time needed to repair a component. Signup and view all the answers

What is the relationship between fault, error, and failure in the context of dependability?

A fault causes an error, which in turn causes a failure. Signup and view all the answers

What is the primary focus of the safety requirement in the context of dependability?

The very low probability of catastrophes. Signup and view all the answers

What is the definition of reliability R(t) of a component C?

The conditional probability that C has been functioning correctly during [0,t) given C was functioning correctly at the time T = 0. Signup and view all the answers

What is the mean time between failures (MTBF) in the context of dependability metrics?

MTTF + MTTR. Signup and view all the answers

What is the characteristic that defines distributed pervasive systems?

Small, mobile, and often embedded in a larger system, blending into the user's environment. Signup and view all the answers

What is the primary difference between ubiquitous computing systems and mobile computing systems?

The emphasis on continuous interaction between system and user in ubiquitous computing systems, versus the emphasis on the mobility of devices in mobile computing systems. Signup and view all the answers

What is the main characteristic of sensor (and actuator) networks?

Collaborative sensing and actuation of the environment. Signup and view all the answers

How do distributed pervasive systems typically interact with users?

There is a continuous interaction between the system and user. Signup and view all the answers

What is the significance of the size of nodes in distributed pervasive systems?

The nodes are small, which allows them to be embedded in a larger system. Signup and view all the answers

What is the primary feature of mobile computing systems?

Devices are inherently mobile. Signup and view all the answers

How do distributed pervasive systems integrate with their environment?

They naturally blend into the user's environment. Signup and view all the answers

What is the relationship between the three subtypes of distributed pervasive systems?

They are overlapping. Signup and view all the answers

Study Notes

Distributed Systems Overview

Expansive view: a networked computer system is extended with additional computers
Decentralized system: processes and resources are necessarily spread across multiple computers
Distributed system: processes and resources are sufficiently spread across multiple computers

Common Misconceptions

Centralized solutions do not scale
Make distinction between logically and physically centralized systems
The root of the Domain Name System: logically centralized, physically distributed, and decentralized across several organizations
Centralized solutions have a single point of failure; generally not true (e.g., the root of DNS)

Dependability

Requirements related to dependability:
- Availability: readiness for usage
- Reliability: continuity of service delivery
- Safety: very low probability of catastrophes
- Maintainability: how easy can a failed system be repaired
Reliability versus availability
Traditional metrics:
- Mean Time To Failure (MTTF): average time until a component fails
- Mean Time To Repair (MTTR): average time needed to repair a component
- Mean Time Between Failures (MTBF): MTTF + MTTR

Security

Trust: one entity can be assured that another will perform particular actions according to a specific expectation
Security mechanisms:
- Symmetric cryptosystem: encryption and decryption keys are the same and should be kept secret
- Asymmetric cryptosystem: public key and private (secret) key
- Secure hashing: any change from data to data* will lead to a completely different string H(data*)

Scalability

Techniques for scaling:
- Hide communication latencies: asynchronous communication and separate handlers for incoming responses
- Facilitate solution by moving computations to clients
- Partition data and computations across multiple machines
- Replication and caching: make copies of data available at different machines
The problem with replication: modifying one copy makes that copy different from the rest, leading to inconsistencies

Pervasive Systems

Definitions:
- Ubiquitous computing systems: pervasive and continuously present, with continuous interaction between system and user
- Mobile computing systems: pervasive, but emphasis is on the fact that devices are inherently mobile
- Sensor (and actuator) networks: pervasive, with emphasis on actual (collaborative) sensing and actuation of the environment

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