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Your mission is to build a component, which I ll call HttpService, that accepts requests from local client applications and forwards them to remote HTTP servers. The traffic between HttpService and the remote servers must consist of HTTP 1.1 requests and responses, using persistent connections. When a client sends multiple requests concurrently to the same remote server, HttpService must pipeline the requests and use a custom Sequence-Number HTTP header to match responses with requests. HttpService must support concurrent requests from multiple clients, as shown in Figure 12-1.
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CHAPTER 12 CASE STUDY 2: A PIPELINED HTTP SERVICE
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Pipelined Requests and Responses Application HttpService HTTP Server
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Figure 12-1. HttpService supports multiple clients, each sending pipelined requests. Each client must be able to send requests to any arbitrary remote server. Those sent concurrently to the same server must be pipelined. The custom Sequence-Number HTTP header field must carry a value that is different for each request. A request using this header must look something like Listing 12-1. Listing 12-1. A Simple HTTP Request Using the Sequence-Number Header POST /MyPath/MyResource HTTP/1.1 Host: 192.168.0.2:8020 Sequence-Number: 1 Connection: Keep-Alive Content-Length: 9 MyMessage The HTTP request must use the verb POST, followed by a resource path. The code in this case study will ignore the path, but typically you would use the path to indicate what type of request to execute. The last word of the path might be a command code. Per the HTTP spec, all header lines are terminated with a carriage-return/line-feed (CR/LF) combination. The HTTP header ends with a blank line, terminated again with a CR/LF pair, so the header is actually terminated by two CR/LF pairs. The rest of the HTTP request is the message body. The HTTP Content-Length header refers only to the length (in bytes) of the message body. The server returns a response with the same sequence number as the request. Listing 12-2 shows a simple response. Listing 12-2. A Simple HTTP Response Containing the Sequence-Number Header HTTP/1.1 200 OK Sequence-Number: 1 Content-Length: 10 MyResponse
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CHAPTER 12 CASE STUDY 2: A PIPELINED HTTP SERVICE
HttpService must expose a single method called Send that forwards a request to the designated remote server and blocks until a response is returned. A Send operation must time out if a response isn t returned within 20 seconds. If a time-out doesn t occur, the Send method must return the response. A multithreaded client must be able to call Send concurrently from different threads, and HttpService must be able to support multiple clients at the same time.
System Design
The requirements are simple enough to allow you to package all the functionality of HttpService in a single component. When a client sends a request to a remote server, you need to create a connection to the server, send the request, and wait for the response. When the response comes back, you keep the connection open in case further requests need to go to the same server. To manage the connections left open, keep them in a pool. If you don t use a connection for a certain amount of time, close it and remove it from the pool. You can design HttpService using three basic blocks, as shown in Figure 12-2.
HttpService
Send to Socket Send Send
Communication Management
Receive From Socket
Coordinator Request Handler
GetConnection
ConnectionPool Management
Figure 12-2. The block diagram of HttpService CoordinatorRequestHandler is a singleton object that receives incoming requests and passes them to the communication management block. This block manages all the details of transmitting/receiving data to/from the remote server. Most of the functionality of the system is inside the Communication Management block, designed as a Coordinator team. The team consists of a top-level coordinator with six workers. The coordinator handles the interactions between the workers and is coupled to most of them, as shown in Figure 12-3.
CHAPTER 12 CASE STUDY 2: A PIPELINED HTTP SERVICE
CoordinatorConnection +Send()
BlockedRequests
Receiver
Connection
HttpRequest
HttpResponse
HttpResponseHeader
Figure 12-3. The coupling diagram of the Coordinator team that represents the Communication Management block All the workers except HttpResponse are completely decoupled from each other and from the rest of the system. On the other hand, the Coordinator is coupled to five of the six workers. While it is perfectly possible to design the system so that the Coordinator is decoupled from the workers, I chose to leave the Coordinator coupled to avoid distracting you with separate builders and binders, which would have been necessary. In this case study, the Coordinator builds and binds the workers directly to avoid introducing unnecessary complexity. The Coordinator exposes a single public method called Send. When CoordinatorRequestHandler calls this method, the Coordinator uses the workers to carry out the task. Noteworthy is the fact that HttpService supports concurrent operations, but none of the workers contains any threading logic. The workers are completely unaware of which thread they run in. The Coordinator handles all threading details, allowing the workers to focus only on specific tasks, without regard to the timing of those tasks in relationship to the rest of the system. When the Coordinator team establishes a connection to a remote server, the connection is kept open and saved in a pool. Connections remain in the pool unless they re idle for a certain amount of time. The task of managing the pool and monitoring for idle connections is assigned to a coordinator called CoordinatorConnectionPool that has no workers. The Coordinator uses an internal Hashtable to hold the collection of active connections. Now you re in a position to see the entire signal diagram of the system, shown in Figure 12-4.
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