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Building a Scalable, Multi-Tenant Application for Windows Azure : Scaling the Surveys Application

6/18/2011 11:55:54 AM
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This section describes how Tailspin designed one functional area of the Surveys application for scalability. Tailspin anticipates that some surveys may have thousands, or even hundreds of thousands of respondents, and Tailspin wants to make sure that the public website remains responsive for all users at all times. At the same time, survey owners want to be able to view summary statistics calculated from the survey responses to date.

1. Goals and Requirements

The Surveys application currently supports three question types: free text, numeric range (values from one to five), and multiple choice. Survey owners must be able to view some basic summary statistics that the application calculates for each survey, such as the total number of responses received, histograms of the multiple-choice results, and aggregations such as averages of the range results. The Surveys application provides a pre-determined set of summary statistics that cannot be customized by subscribers. Subscribers who want to perform a more sophisticated analysis of their survey responses can export the survey data to a SQL Azure instance.

Calculating summary statistics is an expensive operation if there are a large number of responses to process.

Because of the expected volume of survey response data, Tailspin anticipates that generating the summary statistics will be an expensive operation because of the large number of storage transactions that must occur when the application reads the survey responses. However, Tailspin does not require the summary statistics to be always up to date and based on all of the available survey responses. Tailspin is willing to accept a delay while the application calculates the summary data if this reduces the cost of generating them.

The public site where respondents fill out surveys must always have fast response times when users save their responses, and it must record the responses accurately so that there is no risk of any errors in the data when a subscriber comes to analyze the results.

There are also integration tests that verify the end-to-end behavior of the application using Windows Azure storage.

The developers at Tailspin also want to be able to run comprehensive unit tests on the components that calculate the summary statistics without any dependencies on Windows Azure storage.

2. The Solution

To meet the requirements, the developers at Tailspin decided to use a worker role to handle the task of generating the summary statistics from the survey results. Using a worker role enables the application to perform this resource-intensive process as a background task, ensuring that the web role responsible for collecting survey answers is not blocked while the application calculates the summary statistics.

Based on the framework for worker roles that the previous section outlined, this asynchronous task is one that will by triggered on a schedule, and it must be run as a single instance process because it updates a single set of results.

The application can use additional tasks in the same worker role to perform any additional processing on the response data; for example, it can generate a list of ordered answers to enable paging through the response data.

To calculate the survey statistics, Tailspin considered two basic approaches. The first approach is for the task in the worker role to retrieve all the survey responses to date at a fixed time interval, recalculate the summary statistics, and then save the summary data over the top of the existing summary data. The second approach is for the task in the worker role to retrieve all the survey response data that the application has saved since the last time the task ran, and use this data to adjust the summary statistics to reflect the new survey results.

The first approach is the simplest to implement, because the second approach requires a mechanism for tracking which survey results are new. The second approach also depends on it being possible to calculate the new summary data from the old summary data and the new survey results without re-reading all the original survey results.

You can use a queue to maintain a list of all new survey responses. This task is still triggered on a schedule that determines how often the task should look at the queue for new survey results to process.


Note:

You can recalculate all the summary data in the Surveys application using the second approach. However, suppose you want one of your pieces of summary data to be a list of the 10 most popular words used in answering a free-text question. In this case, you would always have to process all of the survey answers, unless you also maintained a separate list of all the words used and a count of how often they appeared. This adds to the complexity of the second approach.


The key difference between the two approaches is cost. The graph in Figure 1 shows the result of an analysis that compares the costs of the two approaches for three different daily volumes of survey answers. The graph shows the first approach on the upper line with the Recalculate label, and the second approach on the lower line with the Merge label.

Figure 1. Comparison of costs for alternative approaches to calculating summary statistics


The graph clearly shows how much cheaper the merge approach is than the recalculate approach after you get past a certain volume of transactions. The difference in cost is due almost entirely to the transaction costs associated with the two different approaches. Tailspin decided to implement the merge approach in the Surveys application.


Note:

The vertical cost scale on the chart is logarithmic. The analysis behind this chart makes a number of “worst-case” assumptions about the way the application processes the survey results. The chart is intended to illustrate the relative difference in cost between the two approaches; it is not intended to give “hard” figures.


It is possible to optimize the recalculate approach if you decide to sample the survey answers instead of processing every single one when you calculate the summary data. You would need to perform some detailed statistical analysis to determine what proportion of results you need to select to calculate the summary statistics within an acceptable margin of error.

In the Surveys application, it would also be possible to generate the summary statistics by using an approach based on MapReduce. The advantage of this approach is that it is possible to use multiple task instances to calculate the summary statistics. However, Tailspin is willing to accept a delay in calculating the summary statistics, so performance is not critical for this task.

3. Inside the Implementation

Now is a good time to walk through the code that implements the asynchronous task that calculates the summary statistics in more detail. As you go through this section, you may want to download the Visual Studio solution for the Tailspin Surveys application from http://wag.codeplex.com/.

3.1. Using a Worker Role to Calculate the Summary Statistics

The team at Tailspin decided to implement the asynchronous background task that calculates the summary statistics from the survey results by using a merge approach. Each time the task runs, it processes the survey responses that the application has received since the last time the task ran; it calculates the new summary statistics by merging the new results with the old statistics.

The worker role in the TailSpin.Workers.Surveys project periodically scans the queue for pending survey answers to process.

The following code example from the UpdatingSurvey ResultsSummaryCommand class shows how the worker role processes each temporary survey answer and then uses them to recalculate the summary statistics.

private readonly IDictionary<string, SurveyAnswersSummary>
surveyAnswersSummaryCache;
private readonly ISurveyAnswerStore surveyAnswerStore;
private readonly ISurveyAnswersSummaryStore
surveyAnswersSummaryStore;

public UpdatingSurveyResultsSummaryCommand(
IDictionary<string, SurveyAnswersSummary>
surveyAnswersSummaryCache,
ISurveyAnswerStore surveyAnswerStore,
ISurveyAnswersSummaryStore surveyAnswersSummaryStore)
{
this.surveyAnswersSummaryCache =
surveyAnswersSummaryCache;
this.surveyAnswerStore = surveyAnswerStore;
this.surveyAnswersSummaryStore =
surveyAnswersSummaryStore;
}
public void PreRun()
{
this.surveyAnswersSummaryCache.Clear();
}

public void Run(SurveyAnswerStoredMessage message)
{
this.surveyAnswerStore.AppendSurveyAnswerIdToAnswersList(
message.Tenant,
message.SurveySlugName,
message.SurveyAnswerBlobId);

var surveyAnswer =
this.surveyAnswerStore.GetSurveyAnswer(
message.Tenant,
message.SurveySlugName,
message.SurveyAnswerBlobId);

var keyInCache = string.Format(CultureInfo.InvariantCulture,
"{0}_{1}", message.Tenant, message.SurveySlugName);
SurveyAnswersSummary surveyAnswersSummary;

if (!this.surveyAnswersSummaryCache.ContainsKey(keyInCache))
{
surveyAnswersSummary = new
SurveyAnswersSummary(message.Tenant,
message.SurveySlugName);
this.surveyAnswersSummaryCache[keyInCache] =
surveyAnswersSummary;
}
else
{
surveyAnswersSummary =
this.surveyAnswersSummaryCache[keyInCache];
}

surveyAnswersSummary.AddNewAnswer(surveyAnswer);
}

public void PostRun()
{
foreach (var surveyAnswersSummary in
this.surveyAnswersSummaryCache.Values)
{
var surveyAnswersSummaryInStore =
this.surveyAnswersSummaryStore
.GetSurveyAnswersSummary(surveyAnswersSummary.Tenant,
surveyAnswersSummary.SlugName);
surveyAnswersSummary.MergeWith(
surveyAnswersSummaryInStore);

this.surveyAnswersSummaryStore
.SaveSurveyAnswersSummary(surveyAnswersSummary);
}
}


The Surveys application uses the Unity Application Block (Unity) to initialize an instance of the UpdatingSurveyResultsSummary Command class and the surveyAnswerStore and surveyAnswers SummaryStore variables. The surveyAnswerStore variable is an instance of the SurveyAnswerStore type that the Run method uses to read the survey responses from BLOB storage. The survey AnswersSummaryStore variable is an instance of the Survey AnswersSummary type that the PostRun method uses to write summary data to BLOB storage. The surveyAnswersSummaryCache dictionary holds a SurveyAnswersSummary object for each survey.


Note:

Unity is a lightweight, extensible dependency injection container that supports interception, constructor injection, property injection, and method call injection. You can use Unity in a variety of ways to help decouple the components of your applications, to maximize coherence in components, and to simplify design, implementation, testing, and administration of these applications.

For more information about Unity and to download the application block, see the patterns & practices Unity page on CodePlex (http://unity.codeplex.com/).


The PreRun method runs before the task reads any messages from the queue and initializes a temporary cache for the new survey response data.

The Run method runs once for each new survey response. It uses the message from the queue to locate the new survey response, and then it adds the survey response to the SurveyAnswersSummary object for the appropriate survey by calling the AddNewAnswer method. The AddNewAnswer method updates the summary statistics in the surveyAnswersSummaryStore instance. The Run method also calls the AppendSurveyAnswerIdToAnswersList method to update the list of survey responses that the application uses for paging.

The PostRun method runs after the task reads all the outstanding answers in the queue. For each survey, it merges the new results with the existing summary statistics, and then it saves the new values back to BLOB storage.

The worker role uses some “plumbing” code developed by Tailspin to invoke the PreRun, Run, and PostRun methods in the Updating SurveyResultsSummaryCommand class on a schedule. The following code example shows how the Surveys application uses the “plumbing” code in the Run method in the worker role to run the three methods that comprise the job.

public override void Run()
{
var updatingSurveyResultsSummaryJob =
this.container.Resolve
<UpdatingSurveyResultsSummaryCommand>();
var surveyAnswerStoredQueue =
this.container.Resolve
<IAzureQueue<SurveyAnswerStoredMessage>>();
BatchProcessingQueueHandler
.For(surveyAnswerStoredQueue)
.Every(TimeSpan.FromSeconds(10))
.Do(updatingSurveyResultsSummaryJob);

var transferQueue = this.container
.Resolve<IAzureQueue<SurveyTransferMessage>>();
var transferCommand = this
.container.Resolve<TransferSurveysToSqlAzureCommand>();
QueueHandler
.For(transferQueue)
.Every(TimeSpan.FromSeconds(5))
.Do(transferCommand);

while (true)
{
Thread.Sleep(TimeSpan.FromSeconds(5));
}
}


This method first uses Unity to instantiate the UpdatingSurvey ResultsSummaryCommand object that defines the job and the AzureQueue object that holds notifications of new survey responses.

The method then passes these objects as parameters to the For and Do “plumbing” methods. The Every “plumbing” method specifies how frequently the job should run. These methods cause the plumbing code to invoke the PreRun, Run, and PostRun method in the UpdatingSurveyResultsSummaryCommand class, passing a message from the queue to the Run method.

The preceding code example also shows how the worker role initializes the task defined in the TransferSurveysToSqlAzure Command class that dumps survey data to SQL Azure. This task is slightly simpler and only has a Run method.

You should tune the frequency at which these tasks run based on your expected workloads by changing the value passed to the Every method.

Finally, the method uses a while loop to keep the worker role instance alive.


Note:

The For, Every, and Do methods implement a fluent API for instantiating tasks in the worker role. Fluent APIs help to make the code more legible.


3.2. The Worker Role “Plumbing” Code

The “plumbing” code in the worker role enables you to invoke commands of type IBatchCommand or ICommand by using the Do method, on a Windows Azure queue of type IAzureQueue by using the For method, at a specified interval. Figure 2 shows the key types that make up the “plumbing” code.

Figure 2. Key “plumbing” types


Figure 2 shows both a BatchProcessingQueueHandler class and a QueueHandler class. The QueueHandler class runs tasks that implement the simpler ICommand interface instead of the IBatch Command interface. The discussion that follows focuses on the Batch ProcessingQueueHandlerTask that the application uses to create the summary statistics.

The worker role first invokes the For method in the static Batch ProcessingQueueHandler class, which invokes the For method in the BatchProcessingQueueHandler<T> class to return a Batch ProcessingQueueHandler<T> instance that contains a reference to the IAzureQueue<T> instance to monitor. The “plumbing” code identifies the queue based on a queue message type that derives from the AzureQueueMessage type. The following code example shows how the For method in the BatchProcessingQueueHandler<T> class instantiates a BatchProcessingQueueHandler<T> instance.

private readonly IAzureQueue<T> queue;
private TimeSpan interval;

protected BatchProcessingQueueHandler(IAzureQueue<T> queue)
{
this.queue = queue;
this.interval = TimeSpan.FromMilliseconds(200);
}

public static BatchProcessingQueueHandler<T> For(
IAzureQueue<T> queue)
{
if (queue == null)
{
throw new ArgumentNullException("queue");
}

return new BatchProcessingQueueHandler<T>(queue);
}

Next, the worker role invokes the Every method of the BatchProcessingQueueHandler<T> object to specify how frequently the task should be run.

The current implementation uses a single queue, but you could modify the BatchProcessingQueue-Handler to read from multiple queues instead of only one. According to the benchmarks published at http://azurescope.cloupapp.net, the maximum write throughput for a queue is between 500 and 700 items per second. If the Surveys application needs to handle more than approximately 2 million survey responses per hour, the application will hit the threshold for writing to a single queue. You could change the application to use multiple queues, perhaps with different queues for each subscriber.

Next, the worker role invokes the Do method of the BatchProcessingQueueHandler<T> object, passing an IBatch Command object that identifies the command that the “plumbing” code should execute on each message in the queue. The following code example shows how the Do method uses the Task.Factory.StartNew method from the Task Parallel Library (TPL) to run the PreRun, ProcessMessages, and PostRun methods on the queue at the requested interval.


Note:

Use Task.Factory.StarNew in preference to ThreadPool.Queue UserWorkItem.


public virtual void Do(IBatchCommand<T> batchCommand)
{
Task.Factory.StartNew(() =>
{
while (true)
{
this.Cycle(batchCommand);
}
}, TaskCreationOptions.LongRunning);
}

protected void Cycle(IBatchCommand<T> batchCommand)
{
try
{
batchCommand.PreRun();

bool continueProcessing;
do
{
var messages = this.queue.GetMessages(32);
ProcessMessages(this.queue, messages,
batchCommand.Run);

continueProcessing = messages.Count() > 0;
}
while (continueProcessing);

batchCommand.PostRun();

this.Sleep(this.interval);
}
catch (TimeoutException)
{
}
}

The Cycle method repeatedly pulls up to 32 messages from the queue in a single transaction for processing until there are no more messages left.

The following code example shows the ProcessMessages method in the GenericQueueHandler class.

protected static void ProcessMessages(IAzureQueue<T> queue,
IEnumerable<T> messages, Action<T> action)
{
...

foreach (var message in messages)
{
var success = false;

try
{
action(message);
success = true;
}
catch (Exception)
{
success = false;
}
finally
{
if (success || message.DequeueCount > 5)
{
queue.DeleteMessage(message);
}
}
}
}


This method uses the action parameter to invoke the custom command on each message in the queue. Finally, the method checks for poison messages by looking at the DequeueCount property of the message; if the application has tried more than five times to process the message, the method deletes the message.


Note:

Instead of deleting poison messages, you should send them to a dead message queue for analysis and troubleshooting.


3.3. Testing the Worker Role

The implementation of the “plumbing” code in the worker role, and the use of Unity, makes it possible to run unit tests on the worker role components using mock objects instead of Windows Azure queues and BLOBs. The following code from the BatchProcessingQueue HandlerFixture class shows two example unit tests.

[TestMethod]
public void ForCreatesHandlerForGivenQueue()
{
var mockQueue = new Mock<IAzureQueue<StubMessage>>();

var queueHandler = BatchProcessingQueueHandler
.For(mockQueue.Object);

Assert.IsInstanceOfType(queueHandler,
typeof(BatchProcessingQueueHandler<StubMessage>));
}

[TestMethod]
public void DoRunsGivenCommandForEachMessage()
{
var message1 = new StubMessage();
var message2 = new StubMessage();
var mockQueue = new Mock<IAzureQueue<StubMessage>>();
mockQueue.Setup(q =>
q.GetMessages(32)).Returns(
() => new[] { message1, message2 });
var command = new Mock<IBatchCommand<StubMessage>>();
var queueHandler =
new BatchProcessingQueueHandlerStub(mockQueue.Object);

queueHandler.Do(command.Object);

command.Verify(c => c.Run(It.IsAny<StubMessage>()),
Times.Exactly(2));
command.Verify(c => c.Run(message1));
command.Verify(c => c.Run(message2));
}
public class StubMessage : AzureQueueMessage
{
}

private class BatchProcessingQueueHandlerStub :
BatchProcessingQueueHandler<StubMessage>
{
public BatchProcessingQueueHandlerStub(
IAzureQueue<StubMessage> queue) : base(queue)
{
}

public override void Do(
IBatchCommand<StubMessage> batchCommand)
{
this.Cycle(batchCommand);
}
}


The ForCreateHandlerForGivenQueue unit test verifies that the static For method instantiates a BatchProcessingQueueHandler correctly by using a mock queue. The DoRunsGivenCommand ForEachMessage unit test verifies that the Do method causes the command to be executed against every message in the queue by using mock queue and command objects.

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