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sponse time of the operationThis main factor is the size of the response, and it is important in terms of both the raw number of bytes returned and the number of attributes requested For each call, a base level of overhead determines minimum response timeThis minimum response time is the time it takes for small requests to be satisfiedTypically, it is between 20 and 50 milliseconds for the vast majority of small requestsThis base level is on the order of double-digit milliseconds, as a ballpark figure In the world of sequential calls, waiting 50 milliseconds for a response is more than twice as bad as waiting only 20 milliseconds However, when you are able to make the calls in parallel at a rate of more than 1,000 per second, the incremental cost of making additional requests averages out to be very small Figure 9-1 depicts the difference between sequential and concurrent callsThe upper portion represents the time taken by six calls that each return very quickly, but each does not begin until the prior call is completeThe bottom portion shows six calls that have more variability in response time and are all generally slower than the top depiction However, the fact that the requests are all in flight at the same time makes up for both the variability and the speed of the requests
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Figure 9-1
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The time taken by sequential and concurrent calls
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This approach scales differently than the sequential callsThe time taken by any individual call is much more important when there is no concurrency since the overall time taken is directly related to every individual call In contrast, the response time of most of the concurrent calls does not change the total time taken since the total time is usually dominated by the longest requestAs the number of calls increases for a given process, the total time taken increases much more slowly for concurrent calls compared to sequential
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calls Figure 9-2 shows what happens when you double the number of calls shown in Figure 9-1
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Figure 9-2
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Scaling the number of calls sequentially and concurrently
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Leveraging Concurrency In order to take advantage of the concurrency that SimpleDB is optimized for, you have to have a client that supports itThree of the common approaches taken by SimpleDB clients in exposing the concurrency are with futures, collections, and threadsAt a basic level, a SimpleDB client can be implemented in a thread-safe wayThis permits you to invoke operations on the client from different threads without breaking itThis can be awkward because it requires the application to handle all the threading issues properly A more friendly way for a SimpleDB client to expose the concurrency is with some form of API that allows requests to be submitted in collections In the case of GetAttributes, the client may provide a get method that allows an arbitrary number of item names to be passed and returns all the results together as collection of results In this scenario, all the concurrency is handled by the client behind the scenes, and the applica-
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tion does not have the need to implement or even be aware of the concurrencyThe API method is called with a collection of item names, the caller is blocked until all the results have come back, and then those results are returned all togetherThe calling code is not required to know the details about the implementation used by the API to make the calls The approach of using futures as a placeholder for the results of a pending operation is becoming more commonWhen this is the case, the API methods will return immediately after the request has been queued, rather than waiting for the response to come back Instead of returning the actual values, a wrapper object is returnedThis wrapper object is initially empty, but behind the scenes, it will be updated with the response data once it has been receivedThe benefit of this pattern is that the application does not need to wait around in a blocked state for the operation to come back; it can continue with other work that must be done and retrieve the response data laterThis permits the application to not only execute SimpleDB requests in parallel, but also execute application code in parallel without any explicit need to manage the threading or synchronization at the application level In a simplistic example, an application may need to fetch a dozen SimpleDB items to populate a web page that has been requested Using a SimpleDB client with futures, the dozen requests can be made, and immediately a dozen empty futures are returnedThe application can then proceed to the next stage of assembling the final web page, which may include tasks that do not depend on the responses from SimpleDBThis processing could consist of selecting an HTML template to use, building some HTML based on application-wide settings, and pulling in HTML page fragments from a cache Once the application reaches a point where the data responses must be used, calls to get that data from the futures may already be populated If any are not, they will block until the response comes back Having a client that supports asynchronous SimpleDB operations gets you part of the way there, but you must also have an application design that supports itAs a practical matter, there are usually some dependencies between data itemsThese dependencies prevent you from fully exploiting the available concurrencyWhen the data in one item determines what other items you need to get, it acts as a bottleneck, preventing concurrency until that first response comes backThis is something to keep in mind:The way data flows through your application will have an impact on the level of concurrency you can achieveThe degree to which performance might suffer from sequential access will depend on how much data you need to collect for a single process If you only need to get a few items, it is unlikely to matter much Limiting the Use of Consistent Get The ability to set the consistency flag on a GetAttributes request is an important featureThere are many situations where this is exactly what is neededAnother great aspect of consistent get is that frequently it is just as fast as the default eventually consistent get However, be cautious about setting the consistent flag for all calls to GetAttributes, or even a majority of callsThe performance characteristics are less predictable; sometimes it
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