Distributed Control Systems At SSRL
Distributed Control Systems at SSRLConstraints for Software Development StrategiesTimothy M. McPhillipsStanford Synchrotron Radiation Laboratory
OverviewComputing Environment at our Beam Linesl Need for cross-platform development.l Need for distributed architecture.Beam Line Softwarel Need for collaborative software.l Need for high performance software.Software Development Strategyl Constraints on our strategy.l Our current strategy.
User Environment at Beam Line
Network Architecture of a Beam Line
Problem 1: Multiple Hardware Hosts
Problem 2: Multiple, Simultaneous User Interfaces
Solution: Distributed Control System (DCS)
DCS Server (DCSS)
Beam Line Universal Integrated Configuration Environment (BLU-ICE)
BLU-ICE Motor Scan Windows
BLU-ICE Motor Configuration Windows
Advantages of Writing BLU-ICE in Tcl/TkCommand Prompt with ScriptinglTcl was originally designed to be an embedded scripting language, so it is easy to give theuser a command prompt and a full featured programming language for scripting.lUser can script any operation in BLU-ICE using control structures, variables, procedures,and even classes.Platform Independent GUIlTcl/Tk runs on any Unix, VMS, Mac, and 32-bit Windows computer.lScripts can be distributed without compilation and run on any computer Tcl/Tk has beeninstalled on.lScripts can also be bundled with Tcl/Tk binaries and distributed as a single executablefile. In this case, Tcl/Tk does not have to be installed on the target machine.Rapid DevelopmentlTcl/Tk GUIs can be written with only a fraction of the code necessary in typical systemprogramming languages such as C, C , or Java.lGUIs can be quickly written and are easy to maintain in Tcl.lThis characteristic is critical in the rapidly changing environments of our beam lines.Object OrientationlThe [Incr Tcl] extension to Tcl provides object-oriented features such as classes.lThe [Incr Widgets] extension provides an object oriented framework for building complexwidgets from built-in Tcl widgets.Extensible in C/C lTcl was designed to be extended readily in C. Extensions can be loaded dynamically.lHigh performance code, multiple threads and so on are best implemented in extensions.
Data Collection with the New BLU-ICE
DCSS Performance Now Critical
Cross-Operating System Library (XOS)FeatureslSupports portable, multithreaded, distributed programs Network communication using a much simplified socket object. Thread creation and synchronization with mutexes and semaphores. Memory mapped files and hash tables. Interthread communication using message queues and Win32-style messages.lCompile-time approach Header file xos.h loads appropriate, system-dependent include files. Objects hide architectural differences.AdvantageslPortability Compile code on Digital Unix, IRIX, OpenVMS, Windows NT/95. Easy to port to new platforms similar to any of the above.lReliability Simpler APIs leads to more reliable code. Less need to study different platforms.: Performance Native system calls on each platform for maximum performance No runtime overhead for platform independence.
Constraints for Software Development StrategiesCross-PlatformlMultiple operating systems needed at beam lines; future needs unknown.lRemote users of the collaboratory may have many different operating systems.lOther synchrotron labs and even users’ home labs may use our software.lMust support VMS because other SSRL beam lines use it nearly exclusively.è Use XOS (Cross-Operating System) Library for low-level software and Tcl/Tk forGUI components when feasible.DistributedlApplications must integrate services provided by different computing platforms.lUser interfaces must be kept separate from other components.è Use TCP/IP socket interfaces between all application components.High-Performance Server ProcesseslServer software must be extremely fast and take advantage of multiple processors.è Write multithreaded C programs with XOS for portability.Open SourcelMust be able to distribute all software freely without licensing issues.lInstallation distributions, source code and documentation should be nicely packaged.lDocument well enough that other groups can use and extend our solutions on their own.Low Maintenance OverheadlWrite packages in layers that mix and match.lWrap packages in clean APIs that do not require knowledge of underlying code.lAvoid requiring complex infrastructures. Make it easy for novice programmers.
l Tcl/Tk runs on any Unix, VMS, Mac, and 32-bit Windows computer. l Scripts can be distributed without compilation and run on any co mputer Tcl/Tk has been installed on. l Scripts can also be bundled with Tcl/Tk binaries and distributed as a single executable file. In this case, Tcl
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