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Working with Data in the Surveys Application : Displaying Data (part 1) - Paging through Survey Results

6/29/2011 6:07:27 PM
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This section describes several interesting scenarios in the Surveys application where the application displays data to users and how the underlying data model supports this functionality.

1. Paging through Survey Results

The owner of a survey must be able to browse through the survey results, displaying a single survey response at a time. This feature is in addition to being able to view summary statistical data, or being able to analyze the results using SQL Azure. The Surveys application contains a Browse Responses page for this function.

1.1. Goals and Requirements

The design of this feature of the application must address two specific requirements. The first requirement is that the application must display the survey responses in the order that they were originally submitted. The second requirement is to ensure that this feature does not adversely affect the performance of the web role.

1.2. The Solution

The developers at Tailspin considered two solutions, each based on a different storage model. The first option assumed that the application stored the survey response data in table storage. The second option, which was the one chosen, assumed that the application stored the survey response data in BLOB storage.

1.2.1. Paging with Table Storage

The developers at Tailspin looked at two features of the Windows Azure table storage API to help them design this solution. The first feature is the continuation token that you can request from a query that enables you to execute a subsequent query that starts where the previous query finished. You can use a stack data structure to maintain a list of continuation tokens that you can use to go forward one page or back one page through the survey responses. You must then keep this stack of continuation tokens in the user’s session state to enable navigation for the user.

The second useful API feature is the ability to run asynchronous queries against Windows Azure table storage. This can help avoid thread starvation in the web server’s thread pool in the web role by offloading time-consuming tasks to a background thread.

1.2.2. Paging with Blob Storage

The assumption behind this solution is that each survey answer is stored in a separate BLOB. To access the BLOBs in a predefined order, you must maintain a list of all the BLOBs. You can then use this list to determine the identity of the previous and next BLOBs in the sequence and enable the user to navigate backward and forward through the survey responses.

To support alternative orderings of the data, you must maintain additional lists.

1.2.3. Comparing the Solutions

The previous section, which discusses alternative approaches to saving survey response data, identified lower transaction costs as the key advantage of saving directly to BLOB storage instead of using a delayed write pattern to save to table storage. Paging with table storage is complex because you must manage the continuation stack in the user’s session state.

The obvious solution (in this case to use table storage) is not always the best solution.

However, you must consider the costs and complexity associated with maintaining the ordered list of BLOBs in the second of the two alternative solutions. This incurs two additional storage transactions for every new survey; one as the list it retrieved from BLOB storage, and one as it is saved back to BLOB storage. This is still fewer transactions per survey response than the table-based solution. Furthermore, it’s possible to avoid using any session state by embedding the links to the next and previous BLOBs directly in the web page.

1.3. Inside the Implementation

Now is a good time to walk through the data paging functionality that Tailspin implemented 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/.

This walkthrough is divided into two sections. The first section describes how the application maintains an ordered list of BLOBs. The second section describes how the application uses this list to page through the responses.

1.3.1. Maintaining the Ordered List of Survey Responses

The surveys application already uses an asynchronous task in a worker role to calculate the summary statistical data for each survey. This task periodically process new survey answers from a queue, and as it processes each answer, it updates the ordered list of BLOBs containing survey results. The application assigns each BLOB an ID that is based on the tick count when it is saved, and the application adds a message to a queue to track new survey responses.


Note:

Surveys uses an asynchronous task in a worker role to maintain the ordered list of BLOBs.


The following code example from the SurveyAnswerStore class shows how the application creates a BLOB ID, saves the BLOB to the correct BLOB container for the survey, and adds a message to the queue that tracks new survey responses.

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
});
}

The Run method in the UpdatingSurveyResultsSummaryCommand class in the worker role calls the AppendSurveyAnswerIdToAnswerList method for each survey response in the queue of new survey responses.

The following code example shows how the AppendSurveyAnswerIdToAnswerList method in the SurveyAnswerStore class.

public void AppendSurveyAnswerIdToAnswersList(string tenant,
string slugName, string surveyAnswerId)
{
string id = string.Format(CultureInfo.InvariantCulture,
"{0}-{1}", tenant, slugName);
var answerIdList = this.surveyAnswerIdsListContainer.Get(id)
?? new List<string>(1);
answerIdList.Add(surveyAnswerId);
this.surveyAnswerIdsListContainer.Save(id, answerIdList);
}

The application stores list of survey responses in a List object, which it serializes in the JSON format and stores in a BLOB. There is one BLOB for every survey.

The application adds new responses to the queue in the order that they are received. When it retrieves messages from the queue and adds the BLOB IDs to the list, it preserves the original ordering.

1.3.2. Implementing the Paging

When the Surveys application displays a survey response, it finds the BLOB that contains the survey response by using a BLOB ID. It can use the ordered list of BLOB IDs to create navigation links to the next and previous survey responses.

The following code example shows the BrowseResponses action method in the SurveysController class in the TailSpin.Web project.

public ActionResult BrowseResponses(string tenant,
string surveySlug, string answerId)
{
SurveyAnswer surveyAnswer = null;
if (string.IsNullOrEmpty(answerId))
{
answerId = this.surveyAnswerStore
.GetFirstSurveyAnswerId(tenant, surveySlug);
}

if (!string.IsNullOrEmpty(answerId))
{
surveyAnswer = this.surveyAnswerStore
.GetSurveyAnswer(tenant, surveySlug, answerId);
}

var surveyAnswerBrowsingContext = this.surveyAnswerStore
.GetSurveyAnswerBrowsingContext(tenant,
surveySlug, answerId);
var browseResponsesModel = new BrowseResponseModel
{
SurveyAnswer = surveyAnswer,
PreviousAnswerId =
surveyAnswerBrowsingContext.PreviousId,
NextAnswerId = surveyAnswerBrowsingContext.NextId
};

var model = new TenantPageViewData<BrowseResponseModel>
(browseResponsesModel);
model.Title = surveySlug;
return this.View(model);
}


This action method uses the GetSurveyAnswer method in SurveyAnswerStore class to retrieve the survey response from BLOB storage and the GetSurveyAnswerBrowsingContext method to retrieve a SurveyBrowsingContext object that contains the BLOB IDs of the next and previous BLOBs in the sequence. It then populates a model object with this data to forward on to the view.

Other -----------------
- Microsoft Azure: Enterprise Application Development - Queue Storage
- Microsoft : Azure Enterprise Application Development : Web Role
- Microsoft Azure: Enterprise Application Development - Worker Roles
- Working with Data in the Surveys Application : Saving Survey Response Data
- 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
 
 
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