CS 501: Software Engineering Fall 2000 Lecture 15 System Architecture II Distributed and Real Time Systems

Administration Assignment 2: Requirements Grades -- presentation, report, individual Comments at presentation Comments from teaching assistant Assignment 3: Design

Comments on Requirements Report Audience Client and design team Will be updated over time Content Level of detail -- will be used to validate the implementation Requirements, not design Precise, but not legalistic

Sequence Diagram: Notation BookBorrower libMem: LibraryMember theCopy:Copy theBook:Book borrow(theCopy) okToBorrow borrow dotted line shows object lifetime rectangle shows focus of control

Sequence Diagram: Branching BookBorrower libMem: LibraryMember theCopy:Copy theBook:Book 1:borrow(theCopy) 2:okToBorrow [ok]3:borrow 4:borrow branch [not ok]3:noborrow

Example: Distributed Database two copies of the same data

Distributed Data and Replication Distributed Data Data is held on several computer systems. A transaction may need to assemble data from several sources. Replication Several copies of the data are held in different locations. Mirror: Complete data set is replicated Cache: Dynamic set of data is replicated (e.g., most recently used) With replicated data, the biggest problem is consistency.

Example: Broadcast Search User interface server User Databases

Example: UseNet

Stateless Protocol v. Stateful Stateless protocol Example: http Open connection Send message Return reply Close connection State in http must be sent with every message (e.g., as parameter string or in a cookie)

Stateless Protocol v. Stateful Stateful (session) protocol Example: Z39.50 Open connection Begin session Interactive session End session Close connection Server remembers the results of previous transactions (e.g., authentication, partial results) until session is closed.

Firewall Public network Private network Firewall A firewall is a computer at the junction of two network segments that: Inspects every packet that attempts to cross the boundary Rejects any packet that does not satisfy certain criteria, e.g., an incoming request to open a TCP connection an unknown packet type

The Domain Name System.edu server cornell.edu server cs.cornell.edu server First attempt to resolve

Discussion of the First Attempt Problems?

The Domain Name System.edu server cornell.edu server cs.cornell.edu server Better method 3 1 almaden.ibm.com cornell.edu ece.cmu.edu ibm.com acm.org.edu 2 Local cache local DNS server

Real Time System A real time system is a software system whose correct functioning depends upon the results produced and the time at which they are produced. A soft real time system is degraded if the results are not produced within required time constraints A hard real time system fails if the results are not produced within required time constraints

Example: Web Server http message daemon spawned processes TCP port 80 The daemon listens at port 80. When a message arrives it: spawns a processes to handle the message returns to listening at port 80

Embedded Systems Software and hardware are combined to provide an integrated unit, usually dedicated to a specific task: Digital telephone Automobile engine control GPS Scientific instruments The software may be embedded in the device in a manner that can not be altered after manufacture.

Example: Autonomous Land Vehicle Sensors GPS Sonar Laser Signal processing ModelControl signals Steer Throttle Controls

Other Applications Response critical Network router Telephone switch Seat bag controller Shared systems Multi-user data processing Time sharing

Techniques Special purpose hardware Multi-threading and multi-tasking Parallel processing => digital signal processing Interrupts => levels and priorities

Multi-Threading Several similar threads operating concurrently: Re-entrant code -- separation of pure code from data for each thread Testing -- single thread and multi thread May be real time (e.g., telephone switch) or non- time critical

Real Time Executive Schedules and dispatches tasks in a real time system Real time clock Interrupt handler Scheduler Resource manager Dispatcher Must be extremely reliable

Timing Timing mechanisms Synchronous (clocked) -- periodic stimuli Asynchronous -- wait for next signal Example: Communications protocols may be synchronous or asynchronous

Hardware v. Software Design of embedded systems requires close understanding of hardware characteristics Special purpose hardware requires special tools and expertise. Some functions may be implemented in either hardware of software (e.g., floating point unit) Design requires separation of functions Distinction between hardware and software may be blurred.

Example: Dartmouth Time Shared System Communications processor Central processor Central processor Central processor I/O Mulitplexor master processor

Software Considerations Resource considerations may dictate software design and implementation: Low level language (e.g., C) where programmer has close link to machine Inter-process communication may be too slow (e.g., C fork). May implement special buffering, etc., to control timings

Example: CD Controller Input block Output block Circular buffer

Continuous Operation Many systems must operate continuously Software update while operating Hardware monitoring and repair Alternative power supplies, networks, etc. Remote operation These functions must be designed into the fundamental architecture.

Routers and Other Network Computing Interoperation with third party devices Support for several versions of protocols Restart after total failure Defensive programming -- must survive => erroneous or malicious messages => extreme loads Time outs, dropped packets, etc. Evolution of network systems