Introducing TransactionalBatch in the .NET SDK

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Matias

Recently we talked about the new Bulk support introduced in the .NET SQL SDK, but along with it, another great feature was also released: support for TransactionalBatch.

Wait, Bulk? Batch? What’s the difference?

While Bulk describes scenarios that require a high degree of throughput to process a high volume of point operations, these operations can succeed or fail independently.

TransactionalBatch describes a group of point operations that need to either succeed or fail. If all operations, in the order that are described in the TransactionalBatch, succeed, the transaction is committed. If any operation fails, the entire transaction is rolled back.

So, what is a transaction for Cosmos DB?

A transaction in a typical database can be defined as a sequence of operations performed as a single logical unit of work. Each transaction provides ACID (Atomicity, Consistency, Isolation, Durability) property guarantees.

  • Atomicity guarantees that all the operations done inside a transaction are treated as a single unit, and either all of them are committed or none of them are.
  • Consistency makes sure that the data is always in a valid state across transactions.
  • Isolation guarantees that no two transactions interfere with each other – many commercial systems provide multiple isolation levels that can be used based on the application needs.
  • Durability ensures that any change that is committed in a database will always be present.

Azure Cosmos DB supports full ACID compliant transactions with snapshot isolation for operations within the same logical partition key.

Got it. How do I create a TransactionalBatch now?

Creating a TransactionalBatch is a very descriptive operation, you basically start from a Container instance and call CreateTransactionalBatch:

And when you are ready, just call ExecuteAsync:

When the response comes back, you need to examine if it’s a success or not, and extract the results:

In case of a failure, the failing operation will have the StatusCode of its corresponding error, while all the other operations will have a 424 StatusCode (Failed Dependency). So, it’s quite easy to identify the cause of the transaction failure.

How is that TransactionalBatch executed?

When ExecuteAsync is called, all operations in the TransactionalBatch are grouped, serialized into a single payload, and sent as a single request to the Azure Cosmos DB service.

The service receives the request and executes all operations within a transactional scope, and returns a response using the same serialization protocol. This response is either a success, or a failure, and contains all the individual operation responses internally.

The SDK exposes the response for you to verify the result and, optionally, extract each of the internal operation results. Quite simple.

Why would I use TransactionalBatch?

It is known that Azure Cosmos DB supports Stored Procedures, which also provide transactional scope on its operations, but why would you use TransactionalBatch vs Stored Procedures?

  • Pick your own language – TransactionalBatch is supported on the SDK and language you work with already, while Stored Procedures need to be written in Javascript.
  • Code versioning – Versioning application code and onboarding it on your CI/CD pipeline is much more natural than orchestrating the update of a Stored Procedure and making sure the rollover happens at the right time. It also makes rolling back changes much easier.
  • Performance – We have seen reduction in latency for equivalent operations of up to 30% when comparing them with a Stored Procedure execution.
  • Content serialization – Each operation within a TransactionalBatch can leverage custom serialization options for its payload.

Are there any known limits?

Currently, there are three known limits:

  • As per the Azure Cosmos DB request size limit, the size of the TransactionalBatch payload cannot exceed 2MB, and the maximum execution time is 5 seconds.
  • There is a current limit of 100 operations per TransactionalBatch to make sure the performance is as expected and within SLAs.

Next steps

If you want to try out TransactionalBatch, you can follow our sample. Please share any feedback on our official Github repository.

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Matias Quaranta

Software Engineer, Azure Cosmos DB

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