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Case Study 2: A Pipelined HTTP Service
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n this case study, I ll describe a component that supports pipelined HTTP operations. The purpose of this study is to show how you can use an event-based approach to implement high-performance, multithreaded components that have no user interface. Before looking at the details, a short digression on HTTP is in order. What is HTTP If you haven t heard of it, you ve either just come out of a 15-year coma, or you ve been in solitary confinement for an extended period of time. In both cases, this chapter and the rest of the book are probably over your head. Getting back to the question, the short answer is this: HTTP is the main application-level protocol in current use on the Internet. An overwhelming proportion of Internet traffic is due to HTTP. Tim Berners-Lee submitted RFC 19451 to the Internet Engineering Task Force (IETF) in 1996. The RFC defined HTTP 1.0, a simple text-based protocol for client-server interactions. Version 1.0 was derived from version 0.9, which had been in use since 1990. HTTP is an application-level protocol built on top of TCP/IP; it supports a stateless client-server interaction model. In the standard HTTP interaction model, clients initiate interactions with servers. Servers are always passive and are designed to respond to client requests. An HTTP server has a listener service that waits for incoming requests. When a client needs to obtain information from a server, the client must establish a socket connection with the server. Once a connection is made, the client sends a request to the server and gets the information back as a response, assuming no errors occurred. Most HTTP traffic is related to HTML documents, and most HTML-related client applications are Web browsers. With versions of HTTP prior to 1.1, a client could make only a single request after connecting to the host. Upon returning the response, the host closed the connection. Opening and closing a host connection with each request was a simple technique, but one that didn t work well with a complex HTML document. Why Because when an HTTP 1.0-compliant Web browser was told to retrieve an HTML document, it first downloaded the HTML text of the document, then it scanned the HTML code and compiled a list of all the images embedded in the document. For each image, the browser established a new socket connection to the server hosting the image, downloaded the image, and then closed the connection. The connect-disconnect overhead quickly became a bottleneck for network traffic, because the establishment of a TCP socket connection requires a three-way handshake, which is fairly expensive in terms of networking usage. It is also fairly slow, taking more than 200 ms in normal Internet traffic conditions. When the images embedded in an HTML document reside on the same server as the document, it is wasteful to close and reopen connections for each image to fetch.
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1. Tim Berners-Lee, Roy Fielding, and Henrik Frystyk Nielsen, Hypertext Transfer Protocol -- HTTP/1.0 (technical report, www.w3.org/Protocols/rfc1945/rfc1945, 1996).
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CHAPTER 12 CASE STUDY 2: A PIPELINED HTTP SERVICE
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To reduce Internet congestion due to the HTTP reconnection overhead, HTTP was revised to version 1.1. The new version, which is the standard today, supports persistent connections. When a client establishes a connection, it has the option of requesting the server to keep the connection open after a response is returned. If a client needs to download additional resources from the same server, it can do so using the same connection, greatly reducing the total time required to display a complete HTML page containing embedded images or other resources. HTTP 1.1 also introduced an additional feature to improve performance: pipelined requests, which become useful when a client knows in advance that it will need to send multiple requests to the server. Without using pipelined requests, the client would have to send a request, wait for the response, send the next request, wait for the response, and so on. The host would receive one request at a time and return one response at a time. Everything happens in strict sequential order. If the connection is persistent, a client that needs to send multiple requests can use the connection more efficiently by sending all the requests in a row, one after the other, without waiting for any responses. The server might then process the requests in parallel, using separate threads, and greatly reduce the overall response time. The problem with HTTP 1.1 pipelined requests is that the protocol specifies that responses must be returned in the same order as the requests received. Using this approach is not as efficient as it could be. Ideally, a client should be able to submit a group of requests and have the server process them in parallel. The server should return responses as soon as they are available, without having to worry about their order in relationship to the requests. But how would the client know how to match up requests with responses HTTP doesn t define a standard way to do this, but there is a simple solution. HTTP uses header fields to convey information about requests and responses. The HTTP specification defines several standard header fields, but also allows you to define nonstandard header fields. HTTP proxy servers are required to forward all headers in a request and response even the nonstandard ones ensuring that they will travel correctly through the WWW infrastructure from client to server and back. To solve the request-response matching problem, all you need to do is add a custom header field that contains a sequence number. Both requests and responses use this header. Each time the client submits a new request, the sequence number is incremented, so all requests have a different sequence number. When the server sends a response back to the client, the response contains the sequence number of its request. On the client side, matching responses to requests is a simple matter of matching sequence numbers. Any pipelining implementation, regardless of whether it uses sequence numbers or not, must be multithreaded, because requests must wait for their respective responses, and those responses can return in arbitrary order. The threading details aren t too complicated, especially using an event-based design that uses coordinators to handle the threading.
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