One of the most intriguing characteristics it offers is the multi-model API where it provides capabilities to interact with data by utilizing different APIs like SQL, MongoDB, Cassandra, Graph, and Table API.
One thing that’s often overlooked, but of crucial importance, while working with Cosmos DB is consistency levels. Cosmos DB provides five consistency models – Strong, Bounded staleness, Session, Consistent prefix, and Eventual. By default, consistency is set at the account level during the creation of the Cosmos DB account. However, one can override this default consistency by using Query Request Options.
Understanding the default consistency and the process of overriding it using query request options is crucial for candidates appearing for the DP-420 Designing and Implementing Native Applications Using Microsoft Azure Cosmos DB exam.
Default Consistency in Azure Cosmos DB
The consistency model in Azure Cosmos DB is a significant departure from the consistency models in traditional databases. Here is a brief overview:
- Strong: Guarantees linearizability and is the strongest consistency model.
- Bounded staleness: Guarantees a specified lag between reads and writes.
- Session: Guarantees monotonic reads, monotonic writes, and read-your-own-writes.
- Consistent prefix: Guarantees that reads never see out-of-order writes.
- Eventual: No ordering guarantees for reads relative to writes.
Each of these consistency models offers a trade-off between consistency, availability, latency, and throughput.
Overriding Default Consistency Using Query Request Options
Azure Cosmos DB provides an option to override the default consistency level using Query Request Options at runtime. This is particularly useful when you want to achieve a different consistency level for a single operation without changing the default account-level consistency setting.
An example of how to set the consistency level using .NET SDK could look like the following:
// Create a new CosmosClient with strong consistency
CosmosClient cosmosClient = new CosmosClient(
“
new CosmosClientOptions()
{
ConsistencyLevel = ConsistencyLevel.Strong,
});
// Query with default strong consistency set at the client level
FeedIterator
“SELECT * FROM c”,
requestOptions: new QueryRequestOptions());
// Override with eventuality consistency
FeedIterator
“SELECT * FROM c”,
requestOptions: new QueryRequestOptions()
{
ConsistencyLevel = ConsistencyLevel.Eventual,
});
In the above code, we have created a `CosmosClient` with Strong consistency. When running a query, by default, it will use Strong consistency considering the client-level setting (`iteratorStrong`). However, while creating `iteratorEventual`, we pass a `QueryRequestOptions` object where we set the `ConsistencyLevel` to `Eventual`. This query will then run with Eventual consistency, overriding the client-level Strong consistency.
It’s worth mentioning that consistency defined by `QueryRequestOptions` is not permanent and does not change the default consistency level for the Cosmos DB account. It only applies to the query request where it’s defined.
Conclusion
Understanding consistency in Azure Cosmos DB and being able to override the default consistency levels is a crucial aspect of the DP-420 exam. It’s crucial for building and designing efficient Azure native applications that make the best use of Azure Cosmos DB’s multi-model APIs and globally distributed capacity.
Practice Test
True or False: Azure Cosmos DB follows the default consistency level that you set at the account level.
- True
- False
Answer: True
Explanation: Azure Cosmos DB has a default consistency level which is defined during the database creation at the account level.
True or False: The default consistency level in Azure Cosmos DB cannot be overridden.
- True
- False
Answer: False
Explanation: The default consistency level in Azure Cosmos DB can be overridden by using the x-ms-consistency-level header.
What is the default consistency level in Azure Cosmos DB?
- a) Eventual
- b) Session
- c) Bounded staleness
- d) Strong
Answer: b) Session
Explanation: Session consistency is the default level for all Azure Cosmos DB accounts.
Where can the default consistency level in Azure Cosmos DB be overridden?
- a) Application queries
- b) At the account level
- c) Both A and B
- d) None of the above
Answer: a) Application queries
Explanation: You can override the default consistency level on a per request basis for read and query operations only.
Which HTTP header is used to override the default consistency level in Azure Cosmos DB for a specific request?
- a) x-ms-consistency-level
- b) x-db-consistency-level
- c) x-azure-consistency-level
- d) x-cosmos-consistency-level
Answer: a) x-ms-consistency-level
Explanation: The x-ms-consistency-level header in the HTTP request can be used to override the default consistency level for a specific request.
True or False: Consistency level is not a critical factor in deciding the application performance, availability, and durability of your data.
- True
- False
Answer: False
Explanation: Consistency level is an important factor in deciding the application performance, availability, and durability of data in Azure Cosmos DB.
True or False: It is possible to use multi-region writes with stricter consistency levels.
- True
- False
Answer: True
Explanation: Azure Cosmos DB offers multi-region writes, and these can still be combined with stronger consistency models.
Does eventual consistency in Azure Cosmos DB provide the lowest latency?
- a) Yes
- b) No
Answer: a) Yes
Explanation: Eventual consistency offers the lowest latency because it does not need to wait for acknowledgment from other regions before a write is confirmed.
True or False: If you want no lag time or latency delay whatsoever, you should choose “strong” as your default consistency level in Azure Cosmos DB.
- True
- False
Answer: False
Explanation: Despite strong consistency offering the most linearizability, it can also result in higher latency since it requires replication of the data across all regions before the write operation gets confirmed.
Can you set different default consistency levels for different databases in the same Azure Cosmos DB account?
- a) Yes
- b) No
Answer: b) No
Explanation: The default consistency level is set for the entire Azure Cosmos DB account and not at individual database level.
True or False: You can change the default consistency level in Azure Cosmos DB anytime after the database has been created.
- True
- False
Answer: True
Explanation: The default consistency level in Azure Cosmos DB can be changed after database creation, offering flexibility based on your needs.
True or False: The bounded staleness remains indefinite until overwritten in Azure Cosmos DB.
- True
- False
Answer: True
Explanation: The bounded staleness remains until the specified number of lag operations or time period is satisfied, hence it is not indefinite.
Can the Eventual consistency level guarantee read-your-own-write (RYOW) consistency?
- a) Yes
- b) No
Answer: b) No
Explanation: Only session, bounded staleness, and strong consistency levels guarantee read-your-own-write (RYOW) consistency.
Is it possible to modify the default consistency level using the Azure portal?
- a) Yes
- b) No
Answer: a) Yes
Explanation: The default consistency level can be modified using either Azure portal or Azure Cosmos DB .NET SDK.
True or False: All consistency levels are available in multi-region Azure Cosmos DB account.
- True
- False
Answer: True
Explanation: All five consistency levels are available in the multi-region replication setup for an Azure Cosmos DB account.
Interview Questions
How can one override the default consistency level in Azure Cosmos DB when sending a query request?
One can override the default consistency level by using the RequestOptions object and setting its ConsistencyLevel property to the desired level when sending the query request.
What are some of the consistency levels available in Azure Cosmos DB?
Consistency levels available in Azure Cosmos DB include Strong, Bounded staleness, Session, Consistent prefix, and Eventual.
Is it possible to override the default consistency level for a single request in Azure Cosmos DB?
Yes, it is possible to override the default consistency level for a single request in Azure Cosmos DB by using query request options.
What is the significance of using RequestOptions in Azure Cosmos DB?
RequestOptions in Azure Cosmos DB are used to allow customization of operations like Read, Delete, Replace, Upsert, and ExecuteStoredProcedure. This includes setting the consistency level for a specific request.
In which scenarios might you want to override the default consistency level in Azure Cosmos DB?
For read-heavy and latency-sensitive applications, you might want to override the default strong consistency level for better performance. Similarly, for write-heavy applications, you might want to set a lower consistency level to allow higher throughput.
How does the Session consistency level in Azure Cosmos DB work?
Session consistency level in Azure Cosmos DB provides a guarantee that reads and writes are always consistent within a single session.
Does setting RequestOptions on a query request in Azure Cosmos DB affect other concurrent queries or operations?
No. Setting RequestOptions on a query affects only that particular query and does not affect other concurrent operations or queries.
What command in .NET SDK would you use to set a desired consistency level for a specific read operation?
In .NET SDK, we would use the SetConsistencyLevel method on the RequestOptions object to set a desired consistency level for a specific read operation.
What is the default consistency level in Azure Cosmos DB?
The default consistency level in Azure Cosmos DB is Session.
Can the consistency level be overridden for stored procedure execution in Azure Cosmos DB?
Yes, the consistency level can be overridden for a stored procedure execution in Azure Cosmos DB using request options.
Can the override of the default consistency level at request level in Azure Cosmos DB give better performance?
Yes, overriding the default consistency level at request level in Azure Cosmos DB can provide better performance depending on the requirement of the specific application and operations.
Which consistency level provides the highest availability in Azure Cosmos DB?
The Eventual consistency level provides the highest availability in Azure Cosmos DB.
How do you override the default consistency level using Azure Cosmos DB’s Python SDK?
In Python SDK, one can use the ‘request_options’ parameter and set its ‘session_token’ or ‘consistency_level’ properties to override the default consistency level.
Can the default consistency level be changed after an Azure Cosmos DB account is created?
Yes, the default consistency level can be changed after an Azure Cosmos DB account is created, either through Azure portal, or programmatically through SDKs.
Which Azure Cosmos DB operations support consistency level override?
Consistency level override is supported in read, delete, replace, upsert, and executeStoredProcedure operations in Azure Cosmos DB.
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