1. Distributed Systems and Concurrency: Distributed Systems
Majeed Kassis

2. What is a distributed system?
A distributed system is a collection of independent computers that appear to the users of the system as a single system.
They communicate by using a network by passing messages.
Successful distributed system requires:
Networking
Processors
Memory and Storage
Communication Protocols

3. Key Characteristics of Distributed Systems
Resource Sharing
Openness
Concurrency
Scalability
Fault Tolerance
Transparency

4. Resource Sharing Resource Sharing Freedom Controlled Access
Ability to use any hardware, software or data anywhere in the system.
Controlled Access
Resource manager controls concurrent access to resources
Resource Sharing Protocol
How resources are provided - provider
How to acquire resources - user
How user provider interaction is defined
The systems needs a well defined protocol for resource sharing

5. Openness Changes Visibility Interface Visibility Integration
System changes and improvements must be visible to users!
Interface Visibility
Detailed interfaces of components need to be published.
Integration
New components have to be integrated with existing components.
Backward Computability
Differences in data representation of interface types on different processors (of different vendors) have to be resolved.

6. Concurrency Concurrent Execution Concurrent Access
Components in distributed systems are executed in concurrent processes.
Concurrent Access
Components access and update shared resources concurrently
Concurrent Integrity
Concurrent access to shared resources must be coordinated
To avoid data loss
To avoid inconsistent state
Inconsistent state of the system :
-> partial database modification of a transaction without rollback
Inconsistent state of memory:
-> constructor that crashes
-> a list of set() that fails in the middle

7. Scalability Adaption of distributed systems to accommodate: Done by:
more users
respond faster
Done by:
Upgrading hardware
Upgrading software
Upgrading protocols (networking protocols, storage protocols)
Components should not need to be changed when scale of a system increases.
Design components to be scalable!
Upgrading hardware: more computers, more ram, more disk space
Upgrading software: improved OS, file systems, storage systems, etc
Upgrading protocols: newer connectivity protocols (Ethernet), better storage protocols

8. Fault Tolerance Failures in distributed systems are not uncommon
Software failure – program crashes
Hardware failure – HDDs die, routers fail, RAM gets corrupt, etc
Distributed systems achieve fault tolerance by:
Recovery
Recovery of failed process
Redundancy
Data is copied to multiple places
Solves data loss in case of hardware failure

9. Transparency
Distributed systems should be perceived by users and application programmers as a whole rather than as a collection of cooperating components.
The users need not know that the system is distributed.
The users interaction with a distributed system is exactly with that of a centralized system.

10. Types of Transparency Type Explanation Access Transparency
Accessing local and remote data is done using the same commands
Location Transparency
Accessing local or remote data should not be affected by its location
Concurrency Transparency
Executing several processes operating on shared data should not cause conflicts
Replication Transparency
Duplication of data should not require the users or applications for special care
Failure Transparency
Failure of process or hardware should not hinder the users from completing their tasks
Migration Transparency
Migrating data should not affect the regular operation of the system
Performance Transparency
Allows the system to be reconfigured to improve performance (load balancing)
Scaling Transparency
Expanding the system should not require system structure/implementation change