Definition of a Distributed System

• A distributed system is:

A collection of independent computers that appears to its users as a single coherent system.

• User Programmer User Application Program

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Definition of a Distributed System

A distributed system organized as middleware. The middleware layer extends over multiple machines, and offers each application the same interface.

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2.2 Examples of distributed systems

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GOALS

• Making Resources accessible• Distribution Transparency• Openness• Scalability• Fault Tolerance• Concurrency• Security

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Making Resources Available• Positivity:

Allow user to access remote resources in controlled and efficient way.- Save high cost- Connecting users and resources whether existing or virtually- Easy availability of products.

• Negativity: IntrusionSecurity Encrypted Passwords

Tracking communications

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Distributed Transparency

2. Degree of Transparency

1. Type of Transparency

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Openness

- Easy to add new components or replace existing ones- Portability of applications - Component should provide optimal policy for a user

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Scalability

• Adapted to change in components• Two types - Vertical- Horizontal• Measured in 3 ways- w.r.t. size- Geographically- Administrative

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Scalibilty Problems• W.r.t. size: - services are centralized• Geographically:- Communication in LAN, WAN• Administrative: - Conflict in policies- DS has to protect against malicious attacks from

new domain- New domain has to protect itself against DS.

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Scaling Techniques• Hiding Communication latencies

• Distribution: split the components• Replication: helps to balance the loadIsha Padhy, Department of CSE

Fault Tolerance

• 3 kinds of problems (fault, errors, failures)• Types of faults(Transient, Intermittent,

permanent)• Fault Tolerance• Steps of fault tolerance- Fault detection- Fault diagnosis- Assessment- Recovery- Redundancy

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Security

• 3 components- Confidentiality: keeping secret.- Integrity: protection against alteration.- Availability: protection against interference.• Identification of the user• Security challenges- Denial of service attack- Security of mobile code

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Trends in DS

• Distributed computing systems: used for high performance computing task.

- Cluster computing: Underlying components are simple and similar.

- Cloud computing: Relies on sharing of resources

- Grid computing: Run an application on different machines.

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• Distributed Information systems(EPI): - Transaction Processing system: captures, enters,

stores, retrieves the relevant details of business events and generates information required.

- Enterprise application integration: moderning and co-ordinating applications.

• Distributed pervasive systems: Embedding microprocessors in everyday objects.

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