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Working with Data in the Surveys Application : Saving Survey Response Data

6/20/2011 5:33:43 PM
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When a user completes a survey, the application must save the user’s answers to the survey questions to storage where the survey creator can access and analyze the results.

1. Goals and Requirements

The format that application uses to save the summary response data must enable the Surveys application to meet the following three requirements:

  • The owner of the survey must be able to browse the results.

  • The application must be able to calculate summary statistics from the answers.

  • The owner of the survey must be able to export the answers in a format that enables detailed analysis of the results.

Tailspin expects to see a very large number of users completing surveys; therefore, the process that initially saves the data should be as efficient as possible. The application can handle any processing of the data after it has been saved by using an asynchronous worker process.

Transaction costs will be significant because calculating summary statistical data and exporting survey results will require the application to read survey responses from storage.

The focus here is on the way the Surveys application stores the survey answers. Whatever type of storage the Surveys application uses, it must be able to support the three requirements listed earlier. Storage costs are also a significant factor in the choice of storage type because survey answers account for the majority of the application’s storage requirements; both in terms of space used and by the number of storage transactions.

2. The Solution

To meet the requirements, the developers at Tailspin analyzed two possible storage solutions: a delayed write pattern using queues and table storage, and a solution that saves directly to BLOB storage. In both cases, the application first saves the survey responses to storage, and then it uses an asynchronous task in a worker role to calculate and save the summary statistics.


Note:

The Surveys application saves each survey response as a BLOB.


2.1. Solution 1: The Delayed Write Pattern

Figure 2 shows the delayed write pattern that the Surveys application could use to save the results of a filled out survey to Windows Azure table storage.

Figure 2. Delayed write pattern for saving survey responses in the Surveys application


In this scenario, a user browses to a survey, fills it out, and then submits his or her answers back to the Surveys website. The Surveys website puts the survey answers into a message on a queue and returns a “Thank you” message to the user as quickly as possible, minimizing the value of Tp in Figure 5-2. A task in a worker role is then responsible for reading the survey answers from the queue and saving them to table storage. This operation must be idempotent, to avoid any possibility of double counting and skewing the results.


Note:

You could use separate worker roles, one to calculate and save the summary statistics, and one to save the survey results to table storage if you need to scale the application.


Surveys is a “geo-aware” application. For example, the Surveys website and queue could be hosted in a data center in the U.S., and the worker role and table storage could be hosted in a data center in Europe.

There is an 8-kilobyte (KB) maximum size for a message on a Windows Azure queue, so this approach works only if the size of each survey response is less than that maximum. Figure 3 shows how you could modify this solution to handle survey results that are greater than 8 KB in size.

Figure 3. Handling survey results greater than 8 KB in size


Figure 3 includes an optimization, whereby the application places messages that are smaller than 8 KB directly onto a queue, as in the previous example. For messages that are larger than 8 KB in size, the application saves them to Windows Azure BLOB storage and places a message on the “Big Surveys” queue to notify the worker role. The worker role now contains two tasks: Task 1 retrieves and processes small surveys from the “Small Surveys” queue; Task 2 polls the “Big Surveys” queue for notifications of large surveys that it retrieves and processes from BLOB storage.

2.2. Solution 2: Writing Directly to BLOB Storage

As you saw in the previous section, the delayed write pattern becomes more complex if the size of a survey answer can be greater than 8 KB. In this case, it is necessary to save the response as a BLOB and notify the worker role of the new response data by using a message on a queue. The developers at Tailspin also analyzed a simpler approach to saving and processing query responses using only BLOB storage. Figure 4 illustrates this alternative approach.

When you calculate the size of messages, you must consider the effect of any encoding, such as Base64, that you use to encode the data before you place it in a message.

Figure 4. Saving survey responses directly to BLOB storage


As you can see from the sequence diagram in Figure 4, the first stages of the saving survey response process are the same as for the delayed write pattern. In this approach, there is no queue and no table storage, and the application stores the survey results directly in BLOB storage. The worker role now generates the summary statistical data directly from the survey responses in BLOB storage.

Figure 5 illustrates a variation on this scenario where the subscriber has chosen to host a survey in a different data center from his or her account.

Figure 5. Saving the responses from a survey hosted in a different data center


In this scenario, there is an additional worker role. This worker role is responsible for moving the survey response data from the data center where the subscriber chose to host the survey to the data center hosting the subscriber’s account. This way, the application transfers the survey data between data centers only once, instead of every time the application needs to read it; this minimizes the costs associated with this scenario.

2.3. Comparing the Solutions

The second solution is much simpler than the first. However, you also need to check whether keeping the survey responses in BLOBs instead of tables adds complexity to any of the processes that use the survey results. In the Surveys application, using BLOBs does not add significantly to the complexity of generating summary statistics, enabling the survey owner to browse the responses, or exporting the data to SQL Azure.

The application reads survey response data when it calculates the statistics, when a user browses through the responses, and when it exports the data to SQL Azure.

Although the second solution does not limit the functionality that the Surveys application requires, this design may be limiting in other applications. Using the delayed write pattern means that you can easily perform operations on the data before it’s saved to a table, so in scenarios where the raw data requires some processing to make it usable, the first solution may be more appropriate. Secondly, storing data in tables makes it much easier to access the data with dynamically constructed queries.


Note:

The delayed write pattern enables you to transform the data before saving it without affecting the performance of the web role.


The third difference between the solutions is the storage costs. The following table summarizes this difference, showing the number of storage transactions that the application must perform in order to save a single survey response.

Solution 1 The delayed write patternSolution 2 Writing directly to BLOB storage
1 save to BLOB

1 add message to queue

1 get message from queue

1 read BLOB

1 save to table
1 save to BLOB
Total 5 storage transactionsTotal 1 storage transactions

You should also verify that the second solution does not add to the number of storage transactions that your application needs to perform when it processes or uses the saved data.

3. Inside the Implementation

Now is a good time to walk through the code that saves the survey responses 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. Saving the Survey Response Data to a Temporary Blob

The following code from the SurveysController class in the TailSpin. Web.Survey.Public project shows how the application initiates saving the survey response asynchronously.

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Display(string tenant, string surveySlug,
SurveyAnswer contentModel)
{
var surveyAnswer = CallGetSurveyAndCreateSurveyAnswer(
this.surveyStore, tenant, surveySlug);

...

for (int i = 0; i < surveyAnswer.QuestionAnswers.Count; i++)
{
surveyAnswer.QuestionAnswers[i].Answer =
contentModel.QuestionAnswers[i].Answer;
}

if (!this.ModelState.IsValid)
{
var model =
new TenantPageViewData<SurveyAnswer>(surveyAnswer);
model.Title = surveyAnswer.Title;
return this.View(model);
}

this.surveyAnswerStore.SaveSurveyAnswer(surveyAnswer);

return this.RedirectToAction("ThankYou");
}


The surveyAnswerStore variable holds a reference to an instance of the SurveyAnswerStore type. The application uses Unity to initialize this instance with the correct IAzureBlob and IAzureQueue instances. The BLOB container stores the answers to the survey questions, and the queue maintains a list of new survey answers that haven’t been included in the summary statistics or the list of survey answers.

Make sure that the storage connection strings in your deployment point to storage in the deployment’s geographical location. The application should use local queues and BLOB storage to minimize latency.

The SaveSurveyAnswer method writes the survey response data to the BLOB storage and puts a message onto the queue. The action method then immediately returns a “Thank you” message.

The following code example shows the SaveSurveyAnswer method in the SurveyAnswerStore class.

public void SaveSurveyAnswer(SurveyAnswer surveyAnswer)
{
var surveyBlobContainer = this.surveyAnswerContainerFactory
.Create(surveyAnswer.Tenant, surveyAnswer.SlugName);
surveyBlobContainer.EnsureExist();
DateTime now = DateTime.UtcNow;
surveyAnswer.CreatedOn = now;
var blobId = now.GetFormatedTicks();
surveyBlobContainer.Save(blobId, surveyAnswer);
this.surveyAnswerStoredQueue.AddMessage(
new SurveyAnswerStoredMessage
{
SurveyAnswerBlobId = blobId,
Tenant = surveyAnswer.Tenant,
SurveySlugName = surveyAnswer.SlugName
});
}

This method first checks that the BLOB container exists and creates it if necessary. It then creates a unique BLOB ID by using a tick count and saves the BLOB to the survey container. Finally, it adds a message to the queue. The application uses the queue to track new survey responses that must be included in the summary statistics and list of responses for paging through answers.


Note:

It is possible, but very unlikely, that the application could try to save two BLOBs with the same ID if two users completed a survey at exactly the same time. The code should check for this possibility and, if necessary, retry the save with a new tick count value.


It’s possible that the role could fail after it adds the survey data to BLOB storage but before it adds the message to the queue. In this case, the response data would not be included in the summary statistics or the list of responses used for paging. However, the response would be included if the user exported the survey to SQL Azure.
Other -----------------
- Working with Data in the Surveys Application : Testing and Windows Azure Storage
- Working with Data in the Surveys Application : A Data Model for a Multi-Tenant Application
- Enterprise Application Development : Azure Monitoring and Diagnostics
- Enterprise Application Development : Azure Diagnostics­ under the hood & Enabling diagnostic logging
- Building a Scalable, Multi-Tenant Application for Windows Azure : Scaling the Surveys Application
- Building a Scalable, Multi-Tenant Application for Windows Azure : Scaling Applications by Using Worker Roles
- Building a Scalable, Multi-Tenant Application for Windows Azure : On-Boarding for Trials and New Customers
- Introduction to SQL Azure : Creating our database
- Introduction to SQL Azure : Migrating schema and data
- Introduction to SQL Azure
 
 
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