Scalable IO in Java -Doug Lea.pdf

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u d e . o g e w s o . s c . e e g / / : p t t h Scalable IO in Java Doug Lea State University of New York at Oswego dl@cs.oswego.edu http://gee.cs.oswego.edu

Outline " Scalable network services " Event-driven processing " Reactor pattern Basic version Multithreaded versions Other variants u d e . o g e w s o . s c . e e g / / : p t t h " Walkthrough of java.nio nonblocking IO APIs

Network Services " Web services, Distributed Objects, etc " Most have same basic structure: Read request Decode request Process service Encode reply Send reply " But differ in nature and cost of each step XML parsing, File transfer, Web page generation, computational services, ... u d e . o g e w s o . s c . e e g / / : p t t h

Classic Service Designs u d e . o g e w s o . s c . e e g / / : p t t h client client Server client read read read decode compute encode handler decode compute encode handler decode compute encode handler send send send Each handler may be started in its own thread

Classic ServerSocket Loop class Server implements Runnable { public void run() { try { ServerSocket ss = new ServerSocket(PORT); while (!Thread.interrupted()) new Thread(new Handler(ss.accept())).start(); // or, single-threaded, or a thread pool } catch (IOException ex) { /* ... */ } } static class Handler implements Runnable { final Socket socket; Handler(Socket s) { socket = s; } public void run() { try { byte[] input = new byte[MAX_INPUT]; socket.getInputStream().read(input); byte[] output = process(input); socket.getOutputStream().write(output); } catch (IOException ex) { /* ... */ } } private byte[] process(byte[] cmd) { /* ... */ } } } Note: most exception handling elided from code examples u d e . o g e w s o . s c . e e g / / : p t t h

Scalability Goals " Graceful degradation under increasing load (more clients) " Continuous improvement with increasing resources (CPU, memory, disk, bandwidth) " Also meet availability and performance goals Short latencies Meeting peak demand Tunable quality of service " Divide-and-conquer is usually the best approach for achieving any scalability goal u d e . o g e w s o . s c . e e g / / : p t t h

Divide and Conquer " Divide processing into small tasks Each task performs an action without blocking " Execute each task when it is enabled Here, an IO event usually serves as trigger decode compute encode send read handler " Basic mechanisms supported in java.nio Non-blocking reads and writes Dispatch tasks associated with sensed IO events " Endless variation possible A family of event-driven designs u d e . o g e w s o . s c . e e g / / : p t t h

Event-driven Designs " Usually more efficient than alternatives Fewer resources " Don't usually need a thread per client Less overhead " Less context switching, often less locking But dispatching can be slower " Must manually bind actions to events " Usually harder to program Must break up into simple non-blocking actions " Similar to GUI event-driven actions " Cannot eliminate all blocking: GC, page faults, etc Must keep track of logical state of service u d e . o g e w s o . s c . e e g / / : p t t h

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