1 Principles and characteristics of distributed systems and environments
2 Definition of a distributed system Distributed system is a collection of independent computers that appears to its users as a single coherent system. Distributed computing is decentralised and parallel computing, using two or more computers communicating over a network to accomplish a common objective or task. The types of hardware, programming languages, operating systems and other resources may vary drastically. Leslie Lamport: A distributed system is one I which the failure of a computer you didn t even know existed can render your computer unusable.
3 Definition cont. 2 aspects Deals with hardware: machines are autonomous Deals with software: the users think they are dealing with a single system Important characteristics Communication is hidden from users Applications can interact with uniform and consistent way Relatively easy to expand or scale (DS will normally be continuously available) A distributed system should be functionally equivalent to the systems of which it is composed.
4 Distributed system as Middleware service If the system as a whole looks and acts like a classical single-processor timesharing system, it qualifies as a distributed system.
5 Goals A DS should easily connect users to resources; it should hide the fact that resources are distributed across a network; should be open and scalable. 4 goals: Connecting users and resources Transparency Openness Scalability
7 Transparency Transparent ~ present itself to users and applications as if it were only a single computer system Different forms of transparency in a distributed system ANSA Reference Manual  and ISO Reference Model For Open Distributed Processing (RM-ODP) [1992/7]
8 Openness System that offers services according to standard rules that describe the syntax and semantics Generally specified through interfaces, often described in Interface Definition Language (IDL) Interoperability characterizes two implementations of system can co-exists and work together Portability characterizes to what extent an application developed for a distributed system A can be executed, without modification, on distributed system B that implements the same interface as A Small replaceable and adaptable parts - provide definitions of not only the highest-level interfaces, those used by applications, but also definitions for interfaces to internal parts
9 Scalability Measure at least 3 different dimensions Size easily add more users and resources to system Geographically scalable system lie far apart Administratively scalable independent administrative organizations Scalability in one or more dimensions often means loss of performance should not be significant.
10 Scalability cont. Example of scalability limitations Performance Congestion of communication links Only decentralized algorithms should be used (no machine has complete information; decisions based on local information; failure of one machine does not ruin the algorithm; no implicit assumption of global clock)
11 Scaling techniques 3 techniques for scaling Hiding communication latencies - Asynchronous communication Distribution - domains Replication caching -> consistency problems!
12 Why Distributed Systems? Geographically distributed environment Speed up Parallel vs. Distributed Systems (synchronous SIMD or MIMD; asynchronous process) Resource sharing HW and SW (databases) Fault tolerance Detection, Recover Logical distribution of functional capabilities: multiple processes, interprocess communication, disjoint address space, collective goal
13 Issues in Distributed Systems Knowledge of process Identity, Identity of it s immediate neighbors, communication channel to neighbors Network topology Completely or sparsely connected, message routing, channel uni/bi-directional Degree of synchronization Clock drift, propagation delay Failures Type of failure and duration of failure Scalabilty Time and space complexity (O(log N) ~ excellent, O(N) ~ pure)
14 Common Subproblems Leader election Mutual exclusion Time synchronization Global state Replica management
15 RM-ODP Reference Model Open Distributed Processing ITU-T X.9xx standards, X.901 Overview Viewpoints (and their languages) Enterprise Information Computational Engineering Technology
16 Hardware concepts DS consist of multiple CPU several different ways of HW organization Multiprocessors have shared memory Multicomputers without shared memory
17 Hardware concepts cont. Architecture of interconnection network Bus Switched Distributed computer system (significant for multicomputers) Homogenous single interconnection network Heterogeneous different networks (LAN connected through FDDI, ATM)
18 Multiprocessors Coherent Overloaded bus > cache memory between CPU and bus (hit rate) Bus suitable until 256 CPUs
19 Multiprocessors cont. Crossbar switch crosspoint switch n CPUs and n memories = n 2 crosspoint switches Omega network 2x2 switches, each two inputs and two outputs Fast switching, not cheap NUMA (NonUniform Memory Access) Fast access to local memory, slow to other s Better average access times than omega networks
21 Homogenous Multicomputer Systems Relative easy acording multiprocessors CPU-to-CPU and CPU-to-memory communication System Area Networks (SANs) High performance interconnection network Bus-based Fast Ethernet nodes Switch-based messages routed Mesh, hypercube Vary widely from Massively Parallel Processors (MPPs) to Clusters of Workstations (COWs)
22 Heterogenous Multicomputer Systems Most distributed systems build on it Due to scale, inherent heterogeneity, and most of all, lack of global system view, sophisticated software is needed to build apps for heterogeneous multicomputers. Provide transparency for apps. Example DAS I-way project
23 Software concepts Determines what a DS actually looks like Resource manager for underlying hardware Operating systems for distributed computers Tightly-coupled distributed operating system (multiprocessors, homogenous multicomputer systems) Loosely-coupled network operating system (heterogeneous multicomputer systems) Middleware enhancements to services of network operating systems distribution transparency
24 Software concepts cont.
25 Uniprocessor Operating Systems Implement a virtual machine Apps protected each other OS should be in full control of how hw resources are used and shared Kernel mode User mode Architecture Monolithic kernel Microkernel
26 Multiprocessor Operating Systems Support for multiple CPUs having access to shared memory IPC done through memory protect against simultaneous access Synchronization primitives Semaphore atomic operations down and up Monitor private variable and public procedures (operations)
27 Multicomputer Operating Systems Different structure and complexity than multiprocessor operating systems Communication done by message passing Software implementation of shared memory not always Message passing primitives may vary widely in different OS
28 Multicomputer Operating Systems cont.
29 Multicomputer Operating Systems cont. Reliability of message delivery?
30 Distributed Shared Memory Systems Page-based distributed shared memory Address space divided into pages (4kB or 8kB) When CPU references address not present locally, trap occurs, OS fetches the page Essential normal paging
31 Distributed Shared Memory Systems cont. (a) Pages in DSM (b) CPU 1 references page 10 (c) Situation if page 10 is read only and replication used
32 Network Operation Systems Do not assume that underlying hardware is homogeneous and that it should be managed as if it were a single system Facilities to allow users to make use of the services available on specific machine Service accessed commonly using commands rlogin machine rcp machine1:file1 machine2:file2
33 Network Operation Systems cont. Network operating systems are clearly more primitive than distributed operating systems. The main distinction between the two types of operating systems is that distributed operating systems make serious attempt to realize full transparency, that is, provide a single-system view.
34 Middleware DOS and NOS don t qualify definition of DS Modern distributed systems are constructed by means of an additional layer called middleware. Additional layer of software between applications and the network operating system
35 Middleware cont. Goal is to hide heterogeneity of the underlying platforms from applications
36 Middleware cont. Middleware models (paradigm) Everything as file (simple) Plan 9, UNIX Distributed File Systems reasonable scalable which contributes popularity Remote Procedure Calls unaware of network communication Distributed Objects unaware of network communication Distributed documents - WWW
37 Middleware cont. Middleware services high-level Communication facilities - Access transparency Naming allows entities to be shared and accessed Persistence through distributed file system or database Distributed transactions atomic multiple read and write operations; write operation succeeds or fails Security can t rely on the underlying local operating system, must be implemented in middleware, security has turned out to be one of the hardest services to implement
38 Middleware and Openness Modern distributed systems as middleware Applications are independent of operating system Independence replaced by strong dependency of specific middleware Need that middleware protocols and interfaces are the same in different implementations
39 Comparison between Systems
40 Client Server Model Server process implementing a specific service Client process that requests service from server by sending request and waiting for response
41 Client Server Model cont. Application layering Distinction of 3 levels The user-interface level The processing level The data level
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