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Network Applications: HTTP; High-Perf HTTP Server 2/2/2012
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2 Outline Admin and recap r HTTP r High performance HTTP server
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3 Recap: TCP Socket state: listening address: {*.6789, *:*} completed connection queue: sendbuf: recvbuf: state: listening address: {*.25, *:*} completed connection queue: sendbuf: recvbuf: state: established address: {128.36.232.5:6789, 198.69.10.10.1500} sendbuf: recvbuf:
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4 Recap: FTP: Client-Server with Separate Control, Data Connections r Two parallel TCP connections opened: m control: exchange commands, responses between client, server. “out of band control” m data: file data to/from server FTP client FTP server TCP control connection port 21 at server TCP data connection server:20 clientip:cport PORT clientip:cport RETR file.dat
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5 Recap: the HTTP Protocol PC running Explorer Server running Apache Web server Linux running Firefox http request http response GET /somedir/page.html HTTP/1.0 Host: www.somechool.edu Connection: closewww.somechool.edu User-agent: Mozilla/4.0 Accept: text/html, image/gif Accept-language: en (extra carriage return, line feed)
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6 Design Exercise Workflow of an HTTP server processing a GET request: GET /somedir/page.html HTTP/1.0 Host: www.somechool.edu
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Simple HTTP Server TCP socket space state: listening address: {*.6789, *.*} completed connection queue: sendbuf: recvbuf: 128.36.232.5 128.36.230.2 state: listening address: {*.25, *.*} completed connection queue: sendbuf: recvbuf: state: established address: {128.36.232.5:6789, 198.69.10.10.1500} sendbuf: recvbuf: connSocket = accept() Create ServerSocket(6789) read request from connSocket Map URL to file Read from file/ write to connSocket close connSocket
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8 Dynamic Content Pages r There are multiple approaches to make dynamic web pages: m Embedding code into pages http server includes an interpreter for the type of pages m Invoke external programs Q: how to integrate an external program’s output http://www.cs.yale.edu/index.shtml http://www.cs.yale.edu/cgi-bin/ureserve.pl http://www.google.com/search?q=Yale&sourceid=chrome
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9 Invoking External Programs r Two issues m Pass HTTP request parameters to the external program m Redirect external program output to socket
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10 Example: CGI r Configuration indicates that a mapped file is executable m Web server sets up environment variables http://httpd.apache.org/docs/2.2/env.html CGI standard: http://www.ietf.org/rfc/rfc3875 m Starts the executable as a child process m Redirects input/output of the child process to the socket
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11 Example: CGI r Example: m GET /search?q=Yale&sourceid=chrome HTTP/1.0 m mapped file search is an executable m setup environment variables, in particular $QUERY_STRING= q=Yale&sourceid=chrome m start search and redirect its input/output http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/ProcessBuilder.html http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Process.html
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12 Client Dynamic Pages r There is no need to change the server r See ajax.html for client code example
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HTTP Message Extension: POST r if an HTML page contains forms or parameter too large, they are sent using POST and encoded in message body 13
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14 HTTP Message Flow Extensions: Keeping State r Why do we need to keep state? r How does FTP keep state (e.g., current dir) and why does HTTP not use it?
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15 User-server Interaction: Cookies Goal: no explicit application level session r Server sends “cookie” to client in response msg Set-cookie: 1678453 r Client presents cookie in later requests Cookie: 1678453 r Server matches presented-cookie with server-stored info m authentication m remembering user preferences, previous choices client server usual http request msg usual http response + Set-cookie: # usual http request msg Cookie: # usual http response msg usual http request msg Cookie: # usual http response msg cookie- specific action cookie- specific action
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16 User-Server Interaction: Authentication Authentication goal: control access to server documents r stateless: client must present authorization in each request r authorization: typically name, password Authorization: header line in request m if no authorization presented, server refuses access, sends WWW-authenticate: header line in response client server usual http request msg 401: authorization req. WWW-authenticate: usual http request msg + Authorization:line usual http response msg usual http request msg + Authorization:line usual http response msg time Browser caches name & password so that user does not have to repeatedly enter it.
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17 HTTP/1.0 Delay r For each object: m TCP handshake --- 1 RTT m client request and server responds --- at least 1 RTT (if object can be contained in one packet) r Discussion: how to reduce delay? TCP SYN TCP/ACK; HTTP GET TCP ACK base page TCP SYN TCP/ACK; HTTP GET TCP ACK image 1
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18 HTTP Message Flow: Persistent HTTP r Default for HTTP/1.1 r On same TCP connection: server parses request, responds, parses new request, … r Client sends requests for all referenced objects as soon as it receives base HTML r Fewer RTTs
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19 Browser Cache and Conditional GET r Goal: don’t send object if client has up-to-date stored (cached) version r client: specify date of cached copy in http request If-modified-since: r server: response contains no object if cached copy up- to-date: HTTP/1.0 304 Not Modified client server http request msg If-modified-since: http response HTTP/1.0 304 Not Modified object not modified http request msg If-modified-since: http response HTTP/1.1 200 OK … object modified
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20 Summary: HTTP r HTTP message format m ASCII (human-readable format) requests, header lines, entity body, and responses line r HTTP message flow m stateless server each request is self-contained; thus cookie and authentication, are needed in each message m reducing latency persistent HTTP –the problem is introduced by layering ! conditional GET reduces server/network workload and latency cache and proxy reduce traffic and latency - Is the application extensible, scalable, robust, secure?
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WebServer Implementation TCP socket space state: listening address: {*.6789, *.*} completed connection queue: sendbuf: recvbuf: 128.36.232.5 128.36.230.2 state: listening address: {*.25, *.*} completed connection queue: sendbuf: recvbuf: state: established address: {128.36.232.5:6789, 198.69.10.10.1500} sendbuf: recvbuf: connSocket = accept() Create ServerSocket(6789) read request from connSocket read local file write file to connSocket close connSocket Discussion: what does each step do and how long does it take?
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Test r telnet locahost 6789 r Start a browser to connect to the server
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Writing High Performance Servers: Major Issues r Many socket/IO operations can cause a process to block, e.g., accept : waiting for new connection; read a socket waiting for data or close; write a socket waiting for buffer space; I/O read/write for disk to finish r Thus a crucial perspective of network server design is the concurrency design (non-blocking) m for high performance m to avoid denial of service r Concurrency is also important for clients!
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Using Multi-Threads for Servers r A thread is a sequence of instructions which may execute in parallel with other threads r A multi-thread server is a concurrent program as it has multiple threads that are active at the same time, e.g., m we can have one thread for each client connection m thus, only the flow (thread) processing a particular request is blocked
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Multi-Threaded Server r A multithreaded server might run on one CPU m The CPU alternates between running different threads m The scheduler takes care of the details m Switching between threads might happen at any time r Might run in parallel on a multiprocessor machine 25
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Java Thread Model r Every Java application has at least one thread m The “main” thread, started by the JVM to run the application’s main() method m Most JVM’s use POSIX threads to implement Java threads r main() can create other threads m Explicitly, using the Thread class m Implicitly, by calling libraries that create threads as a consequence (RMI, AWT/Swing, Applets, etc.) 26
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Thread vs Process 27
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Java Thread Class Concurrency is introduced through objects of the class Thread m Provides a ‘handle’ to an underlying thread of control r Threads are organized into thread group A thread group represents a set of threads activeGroupCount (); m A thread group can also include other thread groups to form a tree m Why thread group? 28 http://java.sun.com/javase/6/docs/api/java/lang/ThreadGroup.html
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Some Main Java Thread Methods Thread(Runnable target) Allocates a new Thread object. ThreadRunnable Thread(String name) Allocates a new Thread object. ThreadString Thread(ThreadGroup group, Runnable target) Allocates a new Thread object. ThreadThreadGroup Runnable r start() Start the processing of a thread; JVM calls the run method 29
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Creating Java Thread r Two ways to implement Java thread Extend the Thread class Overwrite the run() method of the Thread class Create a class C implementing the Runnable interface, and create an object of type C, then use a Thread object to wrap up C A thread starts execution after its start() method is called, which will start executing the thread’s (or the Runnable object’s) run() method A thread terminates when the run() method returns 30 http://java.sun.com/javase/6/docs/api/java/lang/Thread.html
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Option 1: Extending Java Thread 31 class PrimeThread extends Thread { long minPrime; PrimeThread(long minPrime) { this.minPrime = minPrime; } public void run() { // compute primes larger than minPrime... } } PrimeThread p = new PrimeThread(143); p.start();
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Option 1: Extending Java Thread 32 class RequestHandler extends Thread { RequestHandler(Socket connSocket) { // … } public void run() { // process request } … } Thread t = new RequestHandler(connSocket); t.start();
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Option 2: Implement the Runnable Interface 33 class PrimeRun implements Runnable { long minPrime; PrimeRun(long minPrime) { this.minPrime = minPrime; } public void run() { // compute primes larger than minPrime... } } PrimeRun p = new PrimeRun(143); new Thread(p).start();
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Option 2: Implement the Runnable Interface 34 class RequestHandler implements Runnable { RequestHandler(Socket connSocket) { … } public void run() { // } … } RequestHandler rh = new RequestHandler(connSocket); Thread t = new Thread(rh); t.start();
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Example: a Multi-threaded TCPServer 35 r The program creates a thread for each request
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Per-Request Thread Server 36 main() { ServerSocket s = new ServerSocket(port); while (true) { Socket conSocket = s.accept(); Thread t = new RequestHandler(conSocket); t.start(); } Try the per-request-thread TCP server: TCPServerMT.java main thread thread starts thread ends thread ends
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Summary: Scaling Servers r Many server socket/IO operations can cause a process to block, e.g., accept : waiting for new connection; read a socket waiting for data or close; write a socket waiting for buffer space; I/O read/write for disk to finish r Thus a crucial perspective of large- scale server design is the concurrency design (non-blocking) r To support concurrency m Multiplexing: A single server program at a given port can handle multiple connections simultaneously m Thread or other non-blocking execution techniques connSocket = accept() Create ServerSocket(6789) read request from connSocket Processing request close connSocket
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Modeling Per-Request Thread Server 38 01kN p0p0 p1p1 pkpk k+1 p k+1 pNpN Welcome Socket Queue (k+1)
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Problem of Per-Request Thread Server r High thread creation/deletion overhead r Too many threads resource overuse throughput meltdown response time explosion
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Backup Slides
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41 Web Caches (Proxy) r User sets browser: Web accesses via web cache r Client sends all http requests to web cache m if object at web cache, web cache immediately returns object in http response m else requests object from origin server, then returns http response to client Goal: satisfy client request without involving origin server client Proxy server client http request http response http request http response http request http response origin server origin server
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42 Benefits of Web Caching Assume: cache is “close” to client (e.g., in same network) r smaller response time: cache “closer” to client r decrease traffic to distant servers m link out of institutional/local ISP network often bottleneck origin servers public Internet institutional network 10 Mbps LAN 1.5 Mbps access link institutional cache
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43 Proxy Caches Users Regional Network Rest of Internet Bottleneck... Cache Sharing: Internet Cache Protocol (ICP)
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44 Cache Sharing via ICP When one proxy has a cache miss, send queries to all siblings (and parents): “do you have the URL?” Whoever responds first with “Yes”, send a request to fetch the file If no “Yes” response within certain time limit, send request to Web server Parent Cache (optional) Discussion: where is the performance bottleneck of ICP?
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45 Summary Cache r Basic idea: m let each proxy keep a directory of what URLs are cached in every other proxy, and use the directory as a filter to reduce number of queries r Problem: storage requirement m solution: compress the directory => imprecise, but inclusive directory
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46 The Problem abc.com/index.html xyz.edu/................ Proxy A Proxy B Compact Representation ?
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47 Bloom Filters r Support membership test for a set of keys r To check if URL x is at B, compute H 1 (x), H 2 (x), H 3 (x), H 4 (x), and check V B URL u Bit Vector V B k hash functions H (u) = P 11 22 33 44 1 1 1 1 m bits
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48 No Free Lunch: Problems of Web Caching r The major issue of web caching is how to maintain consistency r Two ways m pull Web caches periodically pull the web server to see if a document is modified m push whenever a server gives a copy of a web page to a web cache, they sign a lease with an expiration time; if the web page is modified before the lease, the server notifies the cache
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