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With a Request instance populated with parameters and ready to sign, it is time to start opening HTTP connectionsThere are various HTTP client options available, from writing your own to using full-featured third-party library codeThe approach taken for the SimpleDB client in this chapter is to make use of the humble javanetURLConnection
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that comes standard with JavaAs a result, there are not any advanced features available, and neither do you get much fine-grained control However, you get to see how to make things work simply and without having to spend the time hunting for a third-party download and learning yet another API Like the Request class, the HTTPClient class that encapsulates all the network I/O is in the comsimpledbbookcore packageThe SimpleDB class instantiates an instance of HTTPClient in its constructor and uses it for all subsequent requests Listing 10-10 gives the full implementation of the class
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Listing 10-10 HTTPClientjava A Class to Handle All the Network I/O to SimpleDB
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package comsimpledbbookcore; import javaio*; import javanet*; public class HTTPClient { private static final String CONTENT_TYPE = "application/x-www-form-urlencoded; charset=utf-8"; public Response fetch(Request request) { requestsign(); URLConnection con = getConnection(request); writeRequest(request, con); String body = readInput(con); int code = getResponseCode(con); Response result = new Response(body, code); return result; } private URLConnection getConnection(Request request) { try { URLConnection con = requestgetURL()openConnection(); consetRequestProperty("Content-Type", CONTENT_TYPE); String contentLength = requestgetParamString()length() + ""; consetRequestProperty("Content-Length", contentLength); consetDoOutput(true); conconnect(); return con; } catch (IOException e) { throw new RuntimeException("Error opening connection", e); } } private void writeRequest(Request request, URLConnection con) { try { OutputStream outputStream = congetOutputStream();
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10 Writing a SimpleDB Client: A Language-Independent Guide
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OutputStreamWriter wr = new OutputStreamWriter(outputStream); wrwrite(requestgetParamString()); wrflush(); } catch (Exception e) { eprintStackTrace(); } } private String readInput(URLConnection con) { InputStream in = null; try { in = congetInputStream(); } catch (Exception e) { HttpURLConnection http = (HttpURLConnection) con; in = httpgetErrorStream(); } return readFully(in); } private String readFully(InputStream in) { try { BufferedReader br = new BufferedReader( new InputStreamReader(in)); StringBuilder page = new StringBuilder(); String line = null; while ((line = brreadLine()) != null) { pageappend(line); } inclose(); return pagetoString(); } catch (IOException e) { throw new RuntimeException("Error reading from stream", e); } } private int getResponseCode(URLConnection con) { try { return ((HttpURLConnection)con)getResponseCode(); } catch (IOException e) { return -1; } } }
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Instances of HTTPClient maintain no state and have a single public method: fetch() It is designed to be thread safeWith no shared state and each call to fetch() operating in isolation, multiple threads can be performing I/O concurrently The body of the fetch() method is a high-level overview of what needs to be done All the remaining methods in the class play a support roleThe first thing that happens is the request signing It can be done here because it is now clear that no more parameters are being added to the request After that, a connection is opened, the data is written out, the XML response is read back, and then a new Response object is returned, holding that XML and the HTTP response codeAll of it is straightforward, with the exception of two points that apply specifically to SimpleDB When using POST, you have to be sure to set the headers correctlyThis is done within getConnection(), and there are two headers to be set Content-Type must be set to application/x-www-form-urlencoded; charset=utf-8 and the Content-Length must be set to the length of the parameter string, which is all that is being sent The second thing to notice is the way errors are handled in readInput()An exception thrown while trying to retrieve the URLConnection input stream indicates an abnormal responseAt this point, attention is shifted from the input stream to the error stream and the code continues as if nothing happenedThis is because a proper XML response is being returned regardless of the source; it is just a matter of reading it in at this point The need to switch to reading from a different stream is an idiosyncrasy of the underlying URL connection; however, the concept applies to all languages and librariesYou must handle any errors in such a way that processing can continue and the full error response can be retrieved This type of handling can be seen in the writeRequest() method If an exception is thrown during the write, the stack trace is printed, but it is otherwise ignoredThis is not an oversight; the error message needs to be read, and that happens in the very next method call Checking the HTTP Response Code
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When everything is successful with your SimpleDB requests, the response code will always be 200 When there is a problem, a different code is returned The type of code returned can give insight into the type of problem, and it is a good practice to make these codes available to your users Codes in the 400s indicate a problem with your request that retry attempts do not fix This includes signature problems, invalid SelectExpression issues, and conditional check failure, just to name a few Alternately, response codes in the 500 s indicate an issue on the SimpleDB end that is not the result of a client error These requests can be retried later This includes the indeterminate InternalError, as well as the Service Unavailable error Although the 500 class codes indicate a retry, the specifics depend on the situation 503 Service Unavailable in response to a BatchPutAttributes hints at an overloaded service with a prescription for a dose of exponential back-off However, 503 Service Unavailable from a ConsistentRead or conditional write hints at a rare failure scenario or outage that may take time to repair
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