COSC Software Engineering. Lecture 23: Multilingual and Component Programming

Transcription

1 COSC Software Engineering Lecture 23: Multilingual and Component Programming

2 Isolation Scale Change Errors These are core issues in Software Engineering Isolation is our main way to deal with them Components with narrow interfaces are one way to get isolation.

3 Component-Based Software Building programs out of pieces Possibly concurrent Possibly distributed Possible multiple languages Independent development Isolation Messages, not procedures Copying, not sharing

4 Web browser example HTML rendering PDF rendering Audio playing Video playing Database holding session data Plugins Do not all have to be in one process Do not all have to come from one supplier Can be augmented at run time Do not have to be in the same language

5 Old UNIX example #!/bin/sh awk F [^[:alpha:]]+ \ {for (i=1;i<=nf;i++)if($i!= )print tolower($i)} $@ \ sort u \ comm /usr/dict/words Uses generalised filter component, sorting component, and sorted set union/difference/intersection component. A spelling checker in 3 lines.

6 In One Address Space Pointer errors in one component can corrupt others Very hard to kill and restart faulty component Hard to upgrade components High level languages (Javascript, ML, Haskell, Prolog, Smalltalk, Erlang) may have run time systems that do not get on with each other (who is in charge of garbage collection?) Low level languages may have incompatible data representations (row major vs column major for arrays, padding for records, alignment of doubles) When it all works, it can be fast

7 Poplog, JVM, and.net Poplog, 1980s, Sussex University. Pop-11, ML, Prolog, and Common Lisp, any mix, in one address space; also Fortran, Pascal, C via foreign function interface. Low level code can corrupt anything. Compile high level code to common VM, then back end takes that to native code. Java, 1990s, Sun. Java compiled to JVM to native code. Ada, AWK, Basic, COBOL, Lisp, Forth, Go, Lua, Pascal, Prolog, Python, REXX, Ruby, Scheme, TCL, Mercury, Smalltalk + languages designed for/after JVM..NET, 2000s, Microsoft. C#, F#, Eiffel#,

8 Issues Use a safe VM so bad pointers and arrays overflows cannot happen Low level languages cannot be used unrestrictedly Common data representation for all languages Leads to big performance hit for Lisp, Smalltalk, Prolog which like to treat small numbers specially Even AWK is worse in JVM than interpreted in C Write once run everywhere means can t use special features like SIMD or GPGPU (directly) Leaks, stack overflows, failure to free resources in one component still harm others Programs tend to look like C# in drag BUT cheap communication of rich structures

9 In multiple address spaces How do they communicate? Shared memory (man shmget System V; man shm_open POSIX) is fast Limits you to a single computer Complex management (distributed GC) All shared-data concurrency bugs still apply Components need a common representation for common data Nearly as tightly bound as linked together Kill+restart easier Dynamic attachment easier

10 Messages Asynchronous message: send message and do not wait for reply Synchronous message: send message and wait for reply; rather like a procedure call RPC remote procedure call tries to make message passing look like procedure calls. Client issues call, handled by rpcgen-generated stub, sends message, caught by rpcgen-generated stub, calls server code. Response goes back the other way. Messages aren t procedure calls; RPC can only hide so much.

11 How do you decode messages? The components can agree on a common ad hoc binary format. The components can use a language-specific serialisation format (like Java or PowerShell) The components can use types declared in an Interface Definition Language as in Internet RPC, Corba, Web IDL, ODMG IDL. Languageindependent as long as it s C++/Java. The components can use a self-describing format like XML Or JSON; language-independent as long as it s Javascript(-ish)

12 The document object model W3C specifications at Procedural interface to an object model for XML documents (web pages, basically) Four versions so far (2,4,8 years apart) Based on old Netscape browser: implementation in Java but used by Javascript Uses Web IDL (107 page specification) to specify interfaces Implementation may be single address space or multiple address space

13 A DOM interface fragment interface EventTarget { void addeventlistener(domstring type, EventListener? callback, optional boolean capture = false); void removeeventlistener(domstring type, EventListener? callback, optional boolean capture = false); boolean dispatchevent(event event); }; callback interface EventListener { void handleevent(event event); };

14 We never get it right first time THE ROAD TO WISDOM Well, it s plain and simple to express. Err and err and err again, but less and less and less. Piet Hein.

15 Protocols need versioning Networked File System; version 1, Sun, in house NFS v2, Internet RFC 1094, March Only allowed 2GiB files, ran over UDP NFS v3, RFC 1813, June bit sizes and offsets, asynchronous writes, more info in replies, ran over UDP or TCP NFS v4, RFC 3010, December 2000, faster, stronger security, stateful revised in RFC 3530 NFS 4.1, RFC 5661, January 2010, scalable parallel access Communicating parties must agree on a version

16 When to use components? Components are, or can be, separate. What needs to be tightly integrated? What doesn t? Components are, or can be, replaceable. What might need to be replaced? What mustn t be? Components can be located at run time. What has to be present all the time? What can be loaded only when needed? Components can be on different machines. What can be distributed securely? What can be moved to its data for performance? What has high data rates so can t be moved?