CS 501: Software Engineering Fall 2000

CS 501: Software EngineeringFall 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 libMem: LibraryMember theBook:Book BookBorrower theCopy:Copy dotted line shows object lifetime borrow(theCopy) okToBorrow borrow rectangle shows focus of control borrow

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

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 Databases User interface server User

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 Private network Public 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 First attempt to resolve www.cs.cornell.edu .edu server 1 cornell.edu server 2 3 cs.cornell.edu server

Discussion of the First Attempt Problems?

The Domain Name System Better method local DNS server .edu server 1 2 cornell.edu server almaden.ibm.com cornell.edu ece.cmu.edu ibm.com acm.org .edu 3 Local cache cs.cornell.edu 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 TCP port 80 spawned processes 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 GPS Steer Sonar Model Control signals Throttle Laser Controls Sensors Signal processing

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 master processor Central processor Communications processor I/O Mulitplexor Central processor Communications processor Central 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 3 4 1 2 Input block 5 Output block 6 7 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

CS 501: Software Engineering Fall 2000