HTTP and the Dynamic Web

Transcription

1 HTTP and the Dynamic Web

2 How does the Web work? The canonical example in your Web browser Click here here is a Uniform Resource Locator (URL) It names the location of an object on a server. [courtesy of Geoff Voelker] voelker@cs.ucsd.edu

3 In Action HTTP Client Server Client uses DNS to resolves name of server (www-cse.ucsd.edu) Establishes an HTTP connection with the server over TCP/IP Sends the server the name of the object (null) Server returns the object [Voelker]

4 Naming and URLs How should objects be named? URLs name objects and the virtual locations for those objects. Location is a DNS name, so there s two more levels of naming and indirection under there. Before hypertext we used to worry about access transparency. Object name interpretation is up to the server, but it s often a location in the local file tree. If an object moves, the URL breaks (dangling reference). Location-independent names seem like the obvious way to go Why don t we use them (e.g., URNs)? How do we make them work, esp. in the face of mobility? [from Voelker, with additions]

5 Protocols What kind of transport protocol should the Web use? HTTP 1.0 One TCP connection/object Complaints: inefficient, slow, burdensome HTTP 1.1 One TCP connection/many objects (persistent connections) Solves all problems, right? Huge amount of complexity Clients, proxies, servers How do they compare? Protocol differences [Krishnamurthy99], performance comparison [Nielsen97], effects on servers [Manley97], overhead of TCP connections [Caceres98] HTTPS: HTTP with encryption [Voelker]

6 HTTP in a Nutshell Client Server(s) HTTP supports request/response message exchanges of arbitrary length. Small number of request types: basically GET and POST, with supplements. object name, + content for POST optional query string optional request headers Responses are self-typed objects (documents) with various attributes and tags. optional cookies optional response headers

7 Scalable Servers Server Of course, you are not the only person accessing the server

8 Web Caching Clients Proxy Cache Servers Gee, is there some way to offload those busy servers? Use caches to exploit reference locality among clients [Voelker]

9 Caching How should we build caching systems for the Web? Seminal paper [Chankhunthod96] Proxy caches [Duska97] Akamai hack [Karger99] Cooperative caching [Tewari99, Fan98, Wolman99] Popularity distributions [Breslau99] [Voelker]

10 Issues for Web Caching binding clients to proxies, handling failover manual configuration, router-based transparent caching, (Web Proxy Automatic Discovery) WPAD proxy may confuse/obscure interactions between server and client consistency management At first approximation the Web is a wide-area read-only file service...but it is much more than that. caching responses vs. caching documents deltas [Mogul+Bala/Douglis/Misha/others@research.att.com] prefetching, scale, request routing, scale, performance Web caching vs. content distribution (e.g., Akamai) A few weeks from now...

11 HTTP 1.1 Specification effort started in W3C, finished in IETF...much later. A number of research works influenced the specification. HTTP 1.0 shows the importance of careful specification. performance persistent connections with pipelining range requests, incremental update, deltas caching cache control headers negotiation of content attributes and encodings content attributes vs. transport attributes transport encodings for transmission through proxies Trailer header and trailer headers

12 Persistent Connections There are three key performance reasons for persistent connections: connection setup overhead TCP slow start:justdoitandgetitoverwith pipelining as an alternative to multiple connections And some new complexities resulting from their use, e.g.: request/response framing and pairing unexpected connection breakage Just ask anyone from Akamai... large numbers of active connections How long to keep connections around? These motivations and issues manifest in HTTP, but they are fundamental for request/response messaging over TCP.

13 Cookies HTTP cookies (RFC2109) have brought us a better Web. S optionally includes arbitrary state as a cookie in a response. Cookie is opaque to C,butCsaves the cookie. C sends the saved cookie in future requests to S, and possibly to other servers as well. Allows stateful servers for sessions, personalized content, etc. But: cookies raise privacy and security issues. WhatdidSput in that cookie? Can anyone else see it? How much space does it take up on my disk that I paid soooo much for? Cookies may allow third parties who are friends of S 1,..., S N to observe C s movements among S 1,..., S N. Unverifiable transactions, e.g., DoubleClick and other ad services.

14 Unverifiable Transactions GET x GET ad Referer mycfo.com mycfo.com ad, cookie c GET y GET ad, cookie c Referer amazon.com/x amazon.com Client ad doubleclick, akamai, etc. Users may not know that they are interacting with DoubleClick. Amazon and MyCFO trust DoubleClick, but client is ignorant. The user visits pages at many sites that reference DoubleClick. DoubleClick s cookie allows it to associate all the requests from a given user. If the browser sends Referer headers, DoubleClick may gather information about all the sites the user visits that reference DoubleClick.

15 Web Cache Consistency Requirements of performance, availability, and disconnected operation require us to relax the goal of semantic transparency. - HTTP 1.1 specification Any caching/replication framework must take steps to ensure that the cache does not deliver old copies of modified objects. Issues for cache consistency in the Web: large number of clients/proxies most static objects don t change very often weaker consistency requirements Stale information might be OK, as long as it is not too stale.

16 Cache Expiration and Validation GET x GET x GET x x, Last-Modified m Expires t GET x GET x If-Modified-Since m Clients Proxy 304: Not Modified Origin Server HTTP 1.0 cache control Origin server may add a freshness date (Expires) response header....or the cache could determine expiration time heuristically. Proxy must revalidate cache entry if it has expired. Last-Modified and If-Modified-Since Whose clock do we use for absolute expiration times?

17 Expiration and Validation in HTTP 1.1 GET x GET x GET x Age < t GET x Age=0 Clients Proxy 304: Not Modified, ETag v Origin Server HTTP 1.1 cache control allows origin server to: x,etagv max-age t GET x If-None-Match v use relative instead of absolute expiration times (max-age); issue opaque validators (ETag for entity tag) instead of timestamps; Origin server may specify which of several cached entries to use.

18 Other 1.1 Cache Control Features Client may specify that no caching is to occur. private or no-store Vary headers allow server to specify that certain request headers must also match if the proxy deems a cached response valid. language, character set, etc. Server may specify that a response is not cacheable. Pragma: no-cache header since HTTP 1.0 Client may explicitly request the proxy to validate the response. Pragma: no-cache Proxy may/should/must tell client the age of a cached response. Age header Proxy may/should/must tell client that it could not validate a nonfresh cached response with the origin server. Warning header

19 The Dynamic Web execute program Client Server(s) HTTP began as a souped-up FTP that supports hypertext URLs. Service builders rapidly began using it for dynamically-generated content. Web servers morphed into Web Application Servers. Common Gateway Interface (CGI) Java Servlets and JavaServer Pages (JSP) Microsoft Active Server Pages (ASP) Microsoft ASPs are not to be confused with Application Service Providers (ASPs).

20 Multi-tier tier Services HTTP HTTP RPC, RMI IIOP JNDI, JDBC,SQL relational databases Clients HTML+forms, applets, JavaScript, etc. Web application server DCOM, EJB, CORBA, etc. middle tiers e.g., component middleware transaction monitors file servers

21 From Servers to Servlets Servlets are dynamically loaded Java classes/objects invoked by a Web server to process requests. Servlets are to servers as applets are to browsers. Servlet support converts standard Web servers into extensible Web application servers. designed as a Java-based replacement for CGI Web server acts as a connection manager for the service body, which is specified as pluggable servlets. interface specified by JavaSoft, supported by major servers Servlets can be used in any kind of server (not just HTTP). Invocation triggers are defined by server; the servlet does not know or care how it is invoked.

22 Anatomy of a Servlet network service (servlet container) ServletContext init(servletconfig config) String getservletinfo() service(...) destroy() Servlet GenericServlet (implements) String getserverinfo() Object getattribute(name) String getmimetype(name) getresource*(name) log(string) ServletConfig String getinitparameter(name) ServletContext getservletcontext() Enumeration getinitparameternames()

23 Invoking a Servlet network service service(servletrequest, ServletResponse) Servlet??? ServletInputStream readline(...) ServletOutputStream print(...) println(...) ServletResponse setcontenttype(mime type) getoutputstream() ServletRequest getcontentlength, getcontenttype, getremoteaddr, getremotehost, getinputstream, getparameter(name), getparametervalues(name),

24 HTTP Servlets ServletResponse GenericServlet HttpServlet service(...) doget() dohead() dopost()... ServletRequest HttpServletResponse addcookie(), setstatus(code, msg), setheader(name, value), sendredirect(), encodeurl() HttpServletRequest getcookies(), getremoteuser(), getauthtype(), getheader(name), getheadernames(), HttpSession getsession()

25 HelloWorld Servlet import java.io.*; import javax.servlet.*; public class HelloWorld extends GenericServlet { public void service(servletrequest request, ServletResponse response) throws ServletException, IOException {... } public String getservletinfo() { return Hello World Servlet"; } }

26 HelloWorld Servlet (continued) public void service(servletrequest request, ServletResponse response) throws ServletException, IOException { ServletOutputStream output = response.getoutputstream(); String fromwho = request.getparameter( from"); } response.setcontenttype( text/html"); if (fromwho == null) { output.println( <p>hello world!"); } else { output.println( <p>hello world from <em>" + fromwho + </em>"); }

27 Example 1: Invoking a Servlet by URL Most servers allow a servlet to be invoked directly by URL. client issues HTTP GET e.g., servlet specified by HTTP POST e.g., with form data <FORM ACTION= METHOD= POST"> From : <INPUT TYPE= TEXT" NAME= from" SIZE= 20"> <INPUT TYPE= SUBMIT" VALUE= Submit"> </FORM> generates a URL-encoded query string, e.g., <servleturl>?from=me