Azure Cosmos DB is a globally-distributed database service designed to enable you to elastically and independently scale throughput and storage across any number of geographical regions. As a developer, you have to manage conflicts in the data in a way that is consistent with the application’s business rules.
Azure Cosmos DB has built-in support for database conflicts, which can occur due to concurrent updates to the same record in different regions. By default, Cosmos DB uses the “Last Writer Wins” (LWW) policy, which resolves conflicts based on a system-defined timestamp value.
However, there may be cases where the built-in LWW policy doesn’t suit your application needs, and in such cases, you might want to implement a custom conflict resolution policy instead.
Implementing Custom Conflict Resolution
To customize conflict resolution, you will need to define a stored procedure that the system will automatically invoke whenever a conflict is detected.
Below is an example of a stored procedure for a custom conflict resolution policy. It resolves the conflict by merging the conflicting documents’ properties. When the same property has different values in the conflicting documents, the property in the source document overwrites the target document.
function mergeProcedure(targetDocBody, sourceDocBody, conflictId) {
for (var attrname in sourceDocBody) {
targetDocBody[attrname] = sourceDocBody[attrname];
}
var context = getContext();
var collection = context.getCollection();
var collectionLink = collection.getSelfLink();
// Overwrite the target document with the merged document
collection.replaceDocument(targetDocBody._self,
targetDocBody,
function (err, doc, options) {
if (err) throw err;
collection.deleteConflict(conflictId,
{},
function (err, doc, options) {
if (err) throw err;
else context.getResponse().setBody('Conflict resolved');
});
});
}
In this example, the function `mergeProcedure` takes three arguments: `targetDocBody`, `sourceDocBody`, and `conflictId`. It then replaces the target document with the merged document and deletes the detected conflict.
Remember, the custom resolution strategy may vary based on your application’s need. It’s important to consider the requirements of your application before deciding on a conflict resolution strategy.
Conclusion
Conflict resolution in Azure Cosmos DB is an essential matter when designing and implementing native applications using Microsoft Azure Cosmos DB, particularly in the multi-region scenarios where conflicts are likely to occur more frequently. The built-in LWW policy may not always be sufficient, and implementing a custom conflict resolution policy could provide a more effective solution. Be mindful of your application’s requirements and choose a strategy that fits best.
It’s worth mentioning that no matter what policy you choose, it always involves a trade-off. The LWW policy has the advantage of being simple and efficient, but it may overlook some updates. On the other hand, a custom policy could provide more control but may introduce additional complexity into your database system. It is always important to balance between your application’s needs and the complexity you’re willing to introduce to manage conflicts.
Practice Test
True/False: Azure Cosmos DB supports both automatic and custom conflict resolution.
- True
- False
Answer: True
Explanation: Azure Cosmos DB provides built-in support for conflict resolution which is automatic, and it also allows developers to implement a custom conflict resolution policy.
In a custom conflict resolution policy for Azure Cosmos DB, which of the following can be done?
- a. Changing the winning item
- b. Ignoring the conflict
- c. Selecting the losing item
- d. None of the above
Answer: a. Changing the winning item, b. Ignoring the conflict
Explanation: Developers can change the winning item or decide to ignore a conflict during a custom conflict resolution policy implementation. Selection of the losing item is not an option.
Multiple Select: Which procedures are recommended when implementing a custom conflict resolution policy?
- a. Define the conflict resolution policy
- b. Choose replication mode
- c. Set the ConflictResolutionMode property as Custom
- d. Implement onChanged and getResolutionProcedure functions
Answer: a. Define the conflict resolution policy, c. Set the ConflictResolutionMode property as Custom, d. Implement onChanged and getResolutionProcedure functions
Explanation: Setting the ConflictResolutionMode property as Custom, defining the conflict resolution policy, and implementing relevant functions are essential steps in the process of creating a custom conflict resolution policy.
Single Select: In Azure Cosmos DB, the Last Writer Wins (LWW) policy resolves conflicts by…
- a. Choosing the item with the highest property value
- b. Choosing the item that was last updated
- c. Randomly selecting an item
- d. Applying a user-defined logic
Answer: b. Choosing the item that was last updated
Explanation: The Last Writer Wins (LWW) policy in Azure Cosmos DB resolves conflicts based on which item was last updated.
True/False: The default conflict resolution policy in Azure Cosmos DB is Custom.
- True
- False
Answer: False
Explanation: The default conflict resolution policy in Azure Cosmos DB is Last Writer Wins.
Single Select: What is the role of getResolutionProcedure function in custom conflict resolution policy in Azure Cosmos DB?
- a. To define the custom logic for conflict resolution
- b. To update the conflict resolution mode
- c. To get the current state of conflict resolution mode
- d. To set the property for the Last Writer Wins policy
Answer : a. To define the custom logic for conflict resolution
Explanation: The getResolutionProcedure function is used to define the custom logic that will be applied when conflicts occur.
Single Select: In which scenarios may a conflict occur in Azure Cosmos DB?
- a. When data is read from the DB
- b. When data is written to two or more regions at the same time
- c. When data is deleted from the DB
- d. When data is replicated across regions
Answer: b. When data is written to two or more regions at the same time
Explanation: Conflicts in Azure Cosmos DB occur in the scenario of multi-master replication, when the same item is updated in more than one region concurrently.
Multiple Select: Implementing a custom conflict resolution policy in Azure Cosmos DB may be beneficial in:
- a. Optimizing performance
- b. Handling dynamically changing data
- c. Managing conflicts in a distributed system
- d. Ensuring data consistency
Answer: b. Handling dynamically changing data, c. Managing conflicts in a distributed system, d. Ensuring data consistency
Explanation: A custom conflict resolution policy allows developers to define specific rules for handling changing data, managing conflicts, and ensuring data consistency in a distributed system with multi-region writes.
Single Select: Azure Cosmos DB allows using a custom Stored Procedure to resolve conflicts.
- a. True
- b. False
Answer: a. True
Explanation: A Stored Procedure can be used as a custom resolution function in Azure Cosmos DB to handle conflicts.
True/False: Conflict resolution in Azure Cosmos DB is independent of the multi-region distribution and replication.
- True
- False
Answer: False
Explanation: The method of conflict resolution becomes particularly important in Azure Cosmos DB when using multi-region distribution and replication as it leads to situations where conflicts can occur.
Interview Questions
What is the primary approach for conflict resolution in Azure Cosmos DB?
The primary conflict resolution approach in Azure Cosmos DB is a concept called “automatic resolution,” in which the most recent write operation wins.
What are the conflict resolution policies supported by Azure Cosmos DB?
Azure Cosmos DB supports two types of conflict resolution policies: the Last Writer Wins (LWW) and custom conflict resolution.
How does the Last Write Wins (LWW) resolution policy work in Azure Cosmos DB?
In the Last Write Wins (LWW) policy, a system field _ts (a timestamp) is used for conflict detection. The write with the latest timestamp wins in the case of a conflict.
How is a custom conflict resolution policy implemented in Azure Cosmos DB?
A custom conflict resolution policy is implemented by defining a stored procedure to handle conflicts in a manner that suits the specific application’s requirements.
What information is provided by the conflict feed in Azure Cosmos DB?
The conflict feed in Azure Cosmos DB provides information about the conflicting operations, including the type of operation, the conflicting resources, and the source region of the conflict.
What are the prerequisites for enabling a custom conflict resolution policy in Azure Cosmos DB?
You need to disable the multi-region writes on your Azure Cosmos account before you can enable and implement a custom conflict resolution policy.
What happens if the stored procedure for the custom conflict resolution policy fails or throws an error?
If the stored procedure for the custom conflict resolution policy fails or throws an error, the conflict remains in the conflicts feed to be resolved later manually.
What is the maximum lifespan of a conflict in the conflict feed in Azure Cosmos DB?
The maximum lifespan of a conflict in the conflict feed is 24 hours. If it is not resolved within this time, it will be removed automatically.
When does Azure Cosmos DB trigger the stored procedure for a custom conflict resolution policy?
Azure Cosmos DB triggers the stored procedure for a custom conflict resolution policy whenever it detects a write conflict.
What operation type does the operationType property in the conflict object represent?
The operationType property in the conflict object represents the type of operation that resulted in the conflict – Create, Replace, Upsert, or Delete.
How can you access and process the conflict feed in Azure Cosmos DB?
You can access and process the conflict feed in Azure Cosmos DB using the Azure SDKs or REST APIs.
What is the recommended way to ensure a smooth roll-out of a custom conflict resolution policy?
Testing the custom conflict resolution policy thoroughly in a pre-production or staging environment is recommended before rolling it out in the production environment.
How does Azure Cosmos DB support multi-region writes?
Azure Cosmos DB supports multi-region writes by replicating data across multiple regions and maintaining multiple writable replicas.
Which consistency models does Azure Cosmos DB’s conflict resolution policies work with?
Azure Cosmos DB’s conflict resolution policies work together with its five pre-defined consistency models: Strong, Bounded staleness, Session, Consistent prefix, and Eventual.
What is the default conflict resolution policy in Azure Cosmos DB?
The default conflict resolution policy in Azure Cosmos DB is the Last Writer Wins (LWW) policy.
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