Distributed Computing Systems CSCI 6900/4900

Review Distributed system –A collection of independent computers that appears to its users as a single coherent system. Need for distributed systems –Distributed clients –Distributed data/resources –Distributed organizations –Enabling technology

Distributed System Organization A distributed system organized as middleware. Note that the middleware layer extends over multiple machines. 1.1

Design Goals Make resources available to users –Enable resource sharing in a controlled manner Transparency –Hide distributed nature Openness –Complies to standards with regard to syntax and semantics –Interoperability Security & privacy Scalability & performance

Transparency in a Distributed System TransparencyDescription Access Hide differences in data representation and how a resource is accessed LocationHide where a resource is located MigrationHide that a resource may move to another location Relocation Hide that a resource may be moved to another location while in use ReplicationHide that a resource may have multiple copies Concurrency Hide that a resource may be shared by several competitive users FailureHide the failure and recovery of a resource Persistence Hide whether a (software) resource is in memory or on disk How much transparency is needed? How much can we achieve?

Openness Security & Privacy Openness – Follows rules that describe syntax and semantics –Interface definition language –Interoperability, portability –Mechanism-policy separation Security –Access control –Data integrity –Data/service availability –Copyright protection

Scalability ConceptExample Centralized servicesA single server for all users Centralized dataA single on-line telephone book Centralized algorithmsDoing routing based on complete information Scalability with respect to size Scalability with respect to geographical location Scalability with respect to management Scalability Problems

Scalability Techniques Hiding communication latencies –Asynchronous communication –Reduce amount of data transmitted Distribution –Spreading work across system Caching and replication –Make copies of data and services Balance load –Avoid hot spots

Scaling Techniques 1.4 The difference between letting: a)a server or b)a client check forms as they are being filled

Scaling Techniques (2) 1.5 An example of dividing the DNS name space into zones.

Hardware Classification Based on address space –Multiprocessor –Multicomputer Based on communication infrastructure –Bus –Switched Based on uniformity –Homogenous –Heterogeneous

Hardware Concepts 1.6 Different basic organizations and memories in distributed computer systems

Multiprocessors (1) 1.7 Key Property: All CPUs have direct access to shared memory Coherent memory: Reads and writes are sequential A bus-based multiprocessor. Scalability Ensuring coherency

Multiprocessors (2) Crossbar switch Omega switching network

Homogeneous Multicomputer Systems Hypercube Grid Also known as System Area Networks (SANs) Bus-based (Shared multi-access N/W such as Ethernet) Switch-based (Massively Parallel Processors, Cluster of Workstations)

Heterogeneous Multicomputer Systems Most widely used Individual computers can vary widely Some node might be multiprocessor machines or homogenous multicomputer machines Hierarchical systems (multicomputer systems on top of existing multicomputer systems). Nodes lack global view Cannot assume identical or even similar performance Providing transparency, performance & scalability are key challenges.

Software Concepts Functionalities –Resource managers –Hiding intricacies and heterogeneity Two kinds of operating systems –Tightly coupled (distributed operating system) –Loosely coupled (network operating system) Middleware –Providing transparency for loosely coupled systems