The DP-420 exam, “Designing and Implementing Native Applications Using Microsoft Azure Cosmos DB,” covers various topics, one of which is implementing a query operation using a continuation token. This functionality plays an essential role in dealing with large data sets and provides a way to handle paginating through the results in Azure Cosmos DB.
Understanding Continuation Tokens
Before we delve into the practical implementation, let’s understand what a continuation token is. It’s a mechanism that allows queries to resume where they left off. This is particularly useful when dealing with a large data set that can’t be retrieved all at once due to client-side limitations or maintaining efficient memory usage.
For example, imagine querying a database that contains millions of documents. Retrieving them all at once might not be feasible due to memory constraints or the amount of data that your application can handle, leading to an overload and poor application performance. This is where the continuation token becomes a handy tool, enabling you to retrieve smaller, manageable subsets of data.
Implementing a Query Operation Using a Continuation Token
Implementing a query operation using a continuation token involves creating a query, executing the query, and using the continuation token to paginate the results from Azure Cosmos DB.
Here’s how you can implement this in C#:
First, create an instance of the `FeedOptions` class and set the `MaxItemCount` property, which determines the maximum number of items to return in a single call:
csharp
FeedOptions queryOptions = new FeedOptions { MaxItemCount = -1 };
Next, you initiate the query process, as shown below:
csharp
FeedResponse
documentsLink, queryOptions)
.AsDocumentQuery();
In the code below, the `while` loop enables you to keep retrieving the next data subset till you exhaust the data:
csharp
while (queryResultSet.HasMoreResults)
{
foreach (Document doc in await queryResultSet.ExecuteNextAsync())
{
Console.WriteLine(doc);
}
}
The `ExecuteNextAsync` method retrieves the next set of results. This method also returns a `FeedResponse` object that has a `ResponseContinuation` property. This property holds the continuation token that you can use to resume where you left off during the next query execution.
csharp
string continuation = queryResultSet.ResponseContinuation;
The next time you want to query, you resume from the continuation token:
csharp
queryOptions.RequestContinuation = continuation;
It’s important to store the continuation tokens securely if there is a need to halt and resume a running query. This functionality is integral to cloud-native app design where being able to handle large datasets efficiently and effectively is key.
Conclusion
In summary, a continuation token is a robust tool for working with large data sets in Azure Cosmos DB. It allows applications to handle data pagination and resource throttling, making it a valuable knowledge point for the DP-420 exam. By understanding the above examples and how to apply continuation tokens, you can take a step closer to becoming proficient in designing and implementing native applications using Microsoft Azure Cosmos DB. The utilization and handling of continuation tokens are key competencies expected of a successful DP-420 candidate, mastered through practice and application.
Practice Test
True or False: The continuation token in Azure Cosmos DB is used to manage paging in query results.
- True
- False
Answer: True
Explanation: The continuation token is a feature in Azure Cosmos DB that allows developers to manage paging through query results, allowing for an efficient and smooth browsing experience.
True or False: The continuation token in Azure Cosmos DB is stateless.
- True
- False
Answer: False
Explanation: The continuation token maintains the state of the query operation, and is used to resume the query from where it was last left.
What primary purpose does a continuation token serve in Azure Cosmos DB?
- a. Record insertion
- b. Query execution
- c. Execution of stored procedures
- d. Pagination of query results
Answer: d. Pagination of query results
Explanation: The continuation token helps in the pagination of query results. It is used to manage the state between successive query segments.
Which of the following are true about a continuation token in Azure Cosmos DB?
- a. It is used for executing stored procedures
- b. It aids in managing pagination of query results
- c. It’s returned in the HTTP response headers
- d. It doesn’t support partially served queries
Answer: b. It aids in managing pagination of query results, c. It’s returned in the HTTP response headers
Explanation: The continuation token is used to paginate query results and is returned in the HTTP response headers to be used in the next query fetch.
True or False: A continuation token in Azure Cosmos DB is returned in the HTTP response body.
- True
- False
Answer: False
Explanation: A continuation token is returned in the HTTP response header, not the body.
True or False: A continuation token is no longer valid once the collection state changes.
- True
- False
Answer: True
Explanation: Once the collection state changes, for instance, when a document update or deletion happens, the continuation token is invalidated.
In what scenario would we use a continuation token in Azure Cosmos DB?
- a. When we want to fetch a specific chunk of data
- b. When we want to perform batch processing
- c. When we want to retrieve updated records only
- d. All of the above
Answer: d. All of the above
Explanation: A continuation token is mainly used to fetch a specific chunk of data, perform batch processing, and to retrieve updated records during the sequential query executions.
A continuation token is always required for query operations in Azure Cosmos DB.
- a. True
- b. False
Answer: b. False
Explanation: A continuation token is not always required. It is only used when we are dealing with large data sets that require pagination.
Which HTTP response contains the continuation token?
- a. x-ms-continuation
- b. x-ms-token-continuation
- c. x-ms-docdb-continuation
- d. x-cosmos-continuation
Answer: a. x-ms-continuation
Explanation: The HTTP response header “x-ms-continuation” contains the continuation token.
How should you use a continuation token if your query performs multiple page reads?
- a. Send it with each subsequent request
- b. Include it in the first request only
- c. Use it to skip pages that have already been read
- d. Ignore it
Answer: a. Send it with each subsequent request
Explanation: If your query performs multiple page reads, you should use the continuation token in each subsequent request to read the next page.
Interview Questions
What is the purpose of a continuation token in Azure Cosmos DB?
A continuation token in Azure Cosmos DB is used to handle paged results. It’s given back to you from the server to use in subsequent requests to continue reading from where the last operation left off. This guarantees consistent query execution over a logical partition of data.
In which scenarios is a continuation token useful?
A continuation token is particularly useful when dealing with large datasets where it’s not practical to return all results in a single request due to performance considerations. It allows to parse data in manageable chunks, improving application performance and efficiency.
How to use a continuation token in Cosmos DB queries?
The ‘.HasMoreResults’ property tells you if there’re more results to iterate through. If it returns true, you use the ‘.ExecuteNextAsync’ method which executes the next operation in the query and also returns a continuation token to use in the next query operation.
What does the Function “GetItemQueryStreamIterator” return in Cosmos DB?
The “GetItemQueryStreamIterator” function will return the iterator used to execute the query against the stream, providing results in batches and a continuation token for the next batch retrieval.
How is a continuation token stored and passed in Cosmos DB?
A continuation token is a string that is generated by Cosmos DB which you can store in your application. It’s passed in the x-ms-continuation request header for subsequent read/query requests.
What happens if a continuation token is not included in the subsequent query operation in Cosmos DB?
If a continuation token is not provided in a subsequent query operation, the Azure Cosmos DB service will start reading from the beginning of the result set.
Can you use continuation tokens for cross-partition queries in Cosmos DB?
Yes, Cosmos DB supports the use of continuation tokens for both single partition and cross-partition queries.
What happens if you present an expired continuation token in Cosmos DB?
If you present an expired continuation token in a query request, the Azure Cosmos DB service will return a 404 error.
How long is a continuation token in Cosmos DB valid for?
A continuation token is valid until the collection is explicitly deleted or until the token has not been used for 2 hours.
How do you deal with a continuation token that has expired?
When a continuation token has expired, you need to initiate a new read/query request without the continuation token. Therefore, the Azure Cosmos DB service will start reading from the beginning of the result set.
Can you manually create a continuation token in Cosmos DB?
No, the continuation token is system-generated and cannot be manually created.
When using a continuation token in Cosmos DB, what happens if the number of items in result set is less than the page size?
If the number of items in the result set is less than the page size, the response will not include a continuation token because all results have been included in the response.
Can a continuation token be reused or shared across different queries in Cosmos DB?
No. Continuation tokens are specific to a query and its current state, so they can’t be reused or shared across different queries in Cosmos DB.
Can a continuation token be used for different logical partitions in Cosmos DB?
No, a continuation token is specific to a query and its partition, it cannot be used for different logical partitions.
How does Cosmos DB handle continuation tokens for server-side filtering?
Cosmos DB performs server-side filtering and includes only the results that satisfy the filter condition in the response, and the continuation token points to the next item that satisfies the filter condition.
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