To Mediate is the Message (a brief history of indirection) Glen Daniels Independent Technologist

Transcription

1 To Mediate is the Message (a brief history of indirection) Glen Daniels Independent Technologist

2 Your Presenter

3 What You re Going To Learn Today A Brief History of Indirection and Abstraction Programming patterns and SOA patterns How to use mediation to do SOA right Things to watch out for

4 An Aphorism Any problem in computer science can be solved with another layer of indirection. David Wheeler

5 An Aphorism (the unexpurgated version) Any problem in computer science can be solved with another layer of indirection. But that usually will create another problem. David Wheeler

6 Where it all began 0 4 mov (0), ex ;ex = 4 mov (ex), ax ;ax = memory

7 Call by Value vs. Call by Reference 5 24 y x (to y) (to x) 4 dist stack frame stack frame float distfromorigin(int x, int y) void moveleft(int dist, int *x, int *y)

8 Pointers to Functions typedef float (*DiscountFunc)(float); int fivedollarsoff(float price) { return (price 5.0); } int tenpercentoff(float price) { return (price * 0.9); } DiscountFunc mydiscount = &tenpercentoff; Now it gets interesting.

9 Abstraction: Programming to an Interface Device Data read() write(data) SecureDevice Data read() write(data) setacl(acl) ACL getacl() NetworkDevice Data read() write(data) connect(address) disconnect() DiskDevice Data read() write(data)

10 Proxy Objects interface Device { Data read() throws IOException; void write(data data) throws IOException; } class SecureDevice implements Device { Device delegate; public SecureDevice(Device delegate) { this.delegate = delegate; } public Data read() { checkpermission(read); return delegate.read(); } } public void write(data data) { checkpermission(write); delegate.write(data); } Device mydevice = new SecureDevice(myRawDevice);

11 Dynamic Proxies class LoggingHandler implements InvocationHandler { final Object actual; public LoggingHandler(Object actual) { this.actual = actual; } } public Object invoke(object proxy, Method method, Object[] args) throws Throwable { print( Calling + method.getname() + ( ); for (int i=0; args!= null && i < args.length; i++) { if (i!= 0) print(", "); print(args[i]); } print(")"); Object ret = method.invoke(actual, args); print(" -> + ret); return ret; } LoggingHandler proxyhandler = new LoggingHandler(realDevice); Device device = (Device)Proxy.newProxyInstance(classLoader, Device.class.getInterfaces(), proxyhandler);

12 So What Does Indirection Buy Us? Separation of concerns Points of control / enforcement / logging Extensibility Loose coupling But there are costs

13 Abstracting Indirection

14 Abstracting Indirection

15 Interactions in a Nutshell To talk to something, you need to: Obtain a reference to it Know it s interface This is as true for Java as it is for Web Services

16 Another View of Interaction Registry Publish Find Provider Bind Consumer

17 Obtaining References Create it yourself Look it up in a directory/registry Allow someone to hand it to you Dependency Injection

18 Dependency Injection and OSGi DI frameworks bind concrete implementations to injection points which expect interfaces OSGi offers a service registry, versioning, etc.

19 Who controls the indirection? Client chooses to use a mediator Client would normally go direct, knows this endpoint Client Service Proxy

20 Who controls the indirection? Provider decides to expose a mediator In other words influences the discovery mechanism Client Proxy Service Client thinks this is the real endpoint domain of control

21 Who controls the indirection? A third party domain of control Client Proxy Service Client thinks this is the real endpoint domain of control

22 Example : NAT Remember the 80s? We re running out of addresses!!! NAT to the rescue

23 Pattern : Caching Squid, ESB, etc. The Intarwebs

24 Example : Nexus % mvn clean install cache Maven Central Repository Nexus settings.xml points here

25 Pattern : Load Balancing Distribute requests across a group of providers Sometimes affinity is important (i.e. sessions)

26 ESBs An ESB is essentially a mediation infrastructure Client Client Client ESB DB Service

27 ESBs In a Nutshell Adapters for various transports / infrastructures Flexible, composable mediation components Compositions can be light or heavyweight Security, reliability, etc. Transformations Routing Policy Enforcement Point Some can run in-process or in v. lightweight form

28 ESB Example ESB checks incoming msgs for client ID client ESB service client ESB ESB client ESB service ESBs decorate msgs with client ID backend is insulated from security checks

29 ESB Example client ESB service client ESB ESB client ESB service rogue client

30 Pattern : Versioning Client ESB Service V1 ESB exposes V1 interface

31 Pattern : Versioning Old Client Transformer XSLT Rules New Client ESB ESB exposes both interfaces

32 Pattern : Transformation / Adapter Relevant for data, transport, policies Client Transformer XSLT Service <patient> <name> John Q. Public </name> </patient> <client> <name> <first>john</first> <middle>q</middle> <last>public</last> </client>

33 Pattern : Pre-emptive Mediation client ESB Service If I directly expose endpoints, I also expose myself to pain when things need to change The effort of designing mediation in early tends to pay for itself later do-nothing at first Modern ESBs are fast Hardware solutions can be even faster for some uses

34 Some Anti-Patterns

35 AntiPattern #1 : The Big Slowdown

36 Performance Tips Get a fast mediator Watch out for too many layers Consider in-proc solutions (Interceptors/Handlers) Decentralize use clusters/networks of mediators Utilize the caching pattern where possible Test, test, test

37 AntiPattern #2 : Debugging Nightmare

38 Debugging Tips Log everything you can Don t forget the correlation ID! Tools can help SOA Management systems, syslog, home-grown stuff You can only debug what you can see Debugging at the ESB often gets you most of the messages Need correlation at the endpoints if you want to log there too

39 AntiPattern #3 : Reneging on a Contract Any time another actor comes into play, you have a risk of breaking SLAs or implicit agreements. Performance fits in here too, but also Privacy Retention policies Security Cost

40 Policy Enforcement Tips KNOW what the contracts are Both a technical and a business issue Use metadata generously The more metrics you have, the better How do you know when you ve broken things? Once again testing does wonders Aim as close to the real thing as you can get

41 Summing Up History shows that judiciously applied indirection beats tightly-coupled / static design any day If you re doing SOA, mediation is your friend Separate concerns, policies, caching, versioning/evolution Get your feet wet - try the many quality free/opensource mediators Watch out for common pitfalls - and test, test, test

42 Some FOSS Packages to Try Apache Synapse ( and WSO2 ESB ( JBoss ESB ( Apache ServiceMix ( and Progress/IONA FUSE ( Glassfish ESB ( It s quite easy to get up and running with modern ESBs

43 Questions Go forth and mediate!