Challenges of Distributed Systems

• Heterogenity

• Openness

• Security

• Scalability

• Failure Handling

• Concurrency

• Transparency

Heterogenity

Distributed Systems may use completely different hardware and software. These listed above are examples of differring hardware/software:

• Networks

• Computer Hardware

• Operating Systems

• Programming Languages

• Implementation by different developers

  These problems have some solutions such as:

• Using standard protocols

• Using agreed upon message formats and data types

• Adhering to an API

• Middleware

• Portable code (code that is not tighly coupled to specific hardware)

Middleware

Middleware is a software layer between the distributed application and the operating system that:

• provides a programming abstraction and

• masks the heterogenity of underlying platform

Middleware Models

• Distributed File Systems

• Remote Procedure Call (RPC)

• Remote Method Invocation (RMI)

• Distributed Documents

• Distributed Databases

Openness

Openness refers to the ability to extend the system in different ways by adding hardware or software resources. Following are some approaches to address openness:

• Publishing key interfaces

• Allowing a uniform communication mechanism to communicate over the published interfaces.

• Ensuring all implementations adhere to the published standards.

Security

There are three aspects of security:

1. Confidentiality

2. Integrity

3. Availability

Security Mechanisms:

1. Encryption

2. Authentication

3. Authorization

Scalability

A system should be able to handle growth in users.

Scalability Challenges

• Cost of physical resources

• Controlling performance loss

• Resources should not run out

• Avoiding performance bottlenecks

Failure Handling

• Detecting : some types of failures can be detected and some are hard to be certain about.

• Masking : some failures that have been detected can be hidden or made less severe;

• Tolerating : it is sometimes impractical to try and handle every failure that occurs

• Recovery : some failures can recover, for example a rollback mechanism

• Redundancy : some failures can be made to tolerate failure using redundant components (eg multiple services that provide the same service(failover))

Concurrency

• Multiple clients can access the same resource at the same time, in some cases for updates.

• One approach to handling concrrency is making access sequential - slows down the system.

• Semaphores supported by the operating system is a well accepted mechanism to handle concurrency.

Transparency

Hiding certain aspects from the user and application programmer

• Access

• Location

• Concurrency

• Replication

• Failure

• Mobility

• Performance

• Scaling