Session consistency is a vital element when designing and implementing native applications with Microsoft Azure Cosmos DB. By utilizing session tokens, you can retain the session consistency, thereby enhancing the performance, data integrity, and user experience of your application.
Session Consistency
Unlike the customary consistency levels (Strong, Bounded staleness, Session, Consistent prefix, and Eventual) that Cosmos DB provides, session consistency is scoped to a client session, which allows all reads and writes within that session to have consistency.
Session Tokens
Session tokens capture the collection’s entire state, which means they carry a snapshot of the collection’s current state. They serve as a measure that guarantees consistency in reading operations within a single session, regardless of the number of concurrent writes to a data item.
Each client-issued read or write operation on a specific session carries a session token. Subsequent reads in that same session consume this token, making sure that they are reading the right version of the data.
Implementing Session Consistency Using Session Tokens
To implement session consistency, use session tokens for Cosmos DB operations as follows:
- Create a session token to initiate the session. This is automatically done once you start a new session.
- Every time a write operation occurs within the session, Cosmos DB updates the session token, which captures the state of the data change.
- Include the session token with each read operation to ensure the session’s consistency.
Code Example
Here is a simple example of how to use session tokens in Azure Cosmos DB.
const { CosmosClient } = require("@azure/cosmos");
const client = new CosmosClient(process.env.connectionString);
const container = client.database("myDatabaseName").container("myContainerName");
let itemToRead = "item1";
let mySession;
// Creating a new item, the Cosmos DB will generate a new session token
const { resource: createdItem, headers } = await container.items.create(
{ id: itemToRead, sessionExample: "MyExample" }
);
mySession = headers["session-token"];
// Reading the item with the session token
const { resource: readItem } = await container.item(itemToRead).read({ sessionToken: mySession });
In this example, we first create an item in the Cosmos DB and retrieve the session token from the headers of the response. We then use this session token to read the item, ensuring session consistency.
Factors to Consider
While using session tokens, few factors need to be considered:
- Always keep track of the session token after each operation in the session. Each successive operation needs to contain the latest session-token to guarantee session consistency.
- Monitor the session tokens for each Cosmos DB operation to troubleshoot or debug any consistency-related issues.
- As session tokens carry the full state, they can grow large in scenarios of high write operations.
Designing and implementing session consistency using session tokens offer a great way to maintain consistency within a single client session. Therefore, it is an essential concept to grasp when preparing for the DP-420: Designing and Implementing Native Applications Using Microsoft Azure Cosmos DB exam.
Practice Test
True or False: Session consistency in Azure Cosmos DB guarantees monotonic reads, monotonic writes, and read-your-own-write consistency settings.
- True
- False
Answer: True
Explanation: Session consistency ensures linearizability by providing monotonic reads, monotonic writes, and read-your-own-writes guarantees within a single logical session.
True or False: Session tokens in Azure Cosmos DB are managed automatically by default.
- True
- False
Answer: True
Explanation: By default, session tokens are managed automatically by the system making it transparent to the user.
In Azure Cosmos DB, how is session consistency achieved?
- By maintaining read and write sessions for each client
- By maintaining a session token for each client
- By maintaining read-only sessions for each client
- By maintaining global sessions for all clients
Answer: By maintaining a session token for each client
Explanation: Session consistency in Azure Cosmos DB is maintained by keeping a session token for each client. These tokens enable clients to read their own writes.
True or False: In Azure Cosmos DB, session consistency is bounded staleness consistency.
- True
- False
Answer: False
Explanation: Session consistency is not the same as Bounded staleness consistency. Session consistency ensures reads and writes are consistent within a single session while bounded staleness allows lag in updates for some replicas.
True or False: Session tokens are valid across all Azure Cosmos containers in a database.
- True
- False
Answer: False
Explanation: Session tokens are scoped to a specific partition and they are not valid across all Cosmos containers.
What is the primary purpose of session tokens in Azure Cosmos DB?
- To limit the number of simultaneous sessions
- To identify a client’s read and write operations
- To encrypt client data
- To provide metadata about the session
Answer: To identify a client’s read and write operations
Explanation: The primary purpose of session tokens is to identify a client’s read and write operations, thereby ensuring session consistency in Azure Cosmos DB.
True or False: It is recommended to manually handle session tokens in Azure Cosmos DB.
- True
- False
Answer: False
Explanation: By default, session tokens are automatically managed by Azure Cosmos DB. Manual handling of session tokens is not recommended as it may lead to incorrect results or exceptions.
Azure Cosmos DB’s session consistency model is ideal for scenarios where ________ is important.
- Strong consistency
- Scalability
- Low latency
- All of the above
Answer: All of the above
Explanation: Azure Cosmos DB’s session consistency model provides a balance between strong consistency and latency, while also ensuring scalability.
What happens when a request is made with a session token older than the current session token in the Azure Cosmos DB?
- The request is always allowed
- The request is denied
- A new token is issued
- The system automatically updates the token
Answer: The request is denied
Explanation: If a request is made with a session token that is older than the current token, the request is rejected, ensuring linearizability within a session.
True or False: Session tokens are persisted in Azure Cosmos DB even if the application crashes or is restarted.
- True
- False
Answer: False
Explanation: Session tokens are not persisted beyond the lifetime of the session. If an application restarts or crashes, its session token will no longer be valid.
In Azure Cosmos DB, session tokens are passed in the _______ of the request.
- Body
- URL
- Header
- Query parameters
Answer: Header
Explanation: Session tokens are passed in the header of the request when interacting with Azure Cosmos DB.
True or False: Session tokens can be used to achieve global sessions in Azure Cosmos DB.
- True
- False
Answer: False
Explanation: Session tokens ensure consistency within a single logical session, and they cannot be used to achieve consistency across global sessions.
Is it possible to achieve session consistency across multiple devices in Azure Cosmos DB?
- Yes, by synchronizing session tokens
- Yes, by using global sessions
- No, session consistency is device-specific
- No, unless all devices share a single partition
Answer: Yes, by synchronizing session tokens
Explanation: Session consistency can be achieved across multiple devices by synchronizing session tokens among those devices. The synchronization needs to be handled manually.
True or False: Session tokens change after every read operation in Azure Cosmos DB.
- True
- False
Answer: False
Explanation: Session tokens only change after write operations. They remain the same after read operations.
What does Azure Cosmos DB provide to track a session’s changes accurately and efficiently?
- Sequence numbers
- Write timestamps
- Operation logs
- Both A and B
Answer: Sequence numbers
Explanation: Azure Cosmos DB provides sequence numbers for session tokens, which are used to track a session’s changes accurately and efficiently.
Interview Questions
What is session consistency in Microsoft Azure Cosmos DB?
Session consistency in Microsoft Azure Cosmos DB guarantees monotonic reads, monotonic writes, and read your own writes consistency in a single session.
What are session tokens in Azure Cosmos DB?
Session tokens are opaque strings that are passed along with every request to ensure consistency in the scope of a single logical session.
How do session tokens work in Azure Cosmos DB?
Session tokens in Azure Cosmos DB are associated with each write operation in the session and are returned to the client. The client can then use that token in subsequent read operations to ensure comparable session consistency.
What type of consistency does using session tokens ensure?
Using a session token ensures session consistency within a single logical session.
What happens when the Azure Cosmos DB client SDK maintains the session token?
When the Azure Cosmos DB client SDK maintains the session token, it ensures that all subsequent read and write operations in the session are sent with the latest session token.
How can you achieve session consistency during multi-region writes with Azure Cosmos DB?
Session consistency during multi-region writes is achieved by including the session token with every request. This ensures that writes in one region are reflected in reads in another region within the same session.
How can the consumption of RU/s be reduced in Azure Cosmos DB?
By using session consistency and session tokens, the consumption of RU/s can be reduced as it avoids the need to make additional network calls, thus reducing latency.
What is the primary function of session consistency?
The primary function of session consistency is to ensure read-your-own-write (RYOW) capabilities within a single, logical session of interactions.
How are session tokens generated in Azure Cosmos DB?
Session tokens in Azure Cosmos DB are automatically generated for each unique session and are returned to the client with each write operation.
What are the advantages of using session consistency in Azure Cosmos DB?
When using session consistency, there is less traffic between regions, reduced latency, reduced consumption of RU/s, and data is always consistent within the scope of the session.
What happens when a session token is lost in a client-side failover?
If a session token is lost in a client-side failover, the client may experience inconsistency with read and write operations in the session since Azure Cosmos DB may not be able to ensure session consistency.
How can you manually pass a session token in Azure Cosmos DB?
You can manually pass a session token in the RequestOptions or FeedOptions for a particular operation using the sessionToken property.
What is the difference between session consistency and eventual consistency in Azure Cosmos DB?
In session consistency, read operations reflect the results of write operations within the same session, whereas in eventual consistency, reads may not immediately reflect the latest write operations.
How does the Azure Cosmos DB client handle session tokens?
The Azure Cosmos DB client automatically handles sending session tokens when making requests, but developers can manually pass a session token if needed.
What is the consequence of not using session tokens with session consistency in Azure Cosmos DB?
Not using session tokens with session consistency might lead to data inconsistency within the same logical session in Azure Cosmos DB.
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