Saga Pattern
- thierry sinassamy
Problem
Data consistency is a major issue in distributed transaction scenarios.
Cross-service data consistency requires a cross-service transaction management strategy : transactions must be atomic, consistent, isolated and durable (ACID).
If a transaction (step) fails, the pattern will execute compensating transactions that counteract the preceding transactions.
ย
Solution
Orchestration is a way to coordinate sagas where a centralized controller tells the saga participants waht local transactions to execute.
Orchestrator executes saga requests, stores and interprets the states of each task, and handles failure recovery with compensating transactions.
Saga In Practice
Example of orchestration with Azure Function Apps and 2 different triggers : Call API from Web App (client application) or a change in a blob Storage.
Example of Triggering with Azure Blob Storage
public static class BlobTriggerStart
{
[FunctionName("BlobTriggerStart")]
public static async Task HttpBlobStart(
[BlobTrigger("photoscontainer/{name}", Connection = "StorageConnectionString")] CloudBlockBlob myCloudBlob, string name, ILogger log,
[DurableClient] IDurableOrchestrationClient starter)
{
try
{
log.LogInformation($"Started orchestration trigged by BLOB trigger. A blob item with name = '{name}'");
log.LogInformation($"BLOB Name {myCloudBlob.Name}");
// Function input comes from the request content.
if (myCloudBlob != null)
{
var newUploadedBlobItem = new CloudBlobItem
{
Name = myCloudBlob.Name,
BlobUrl = myCloudBlob.Uri.AbsoluteUri.ToString(),
Metadata = (Dictionary<string, string>)myCloudBlob.Metadata,
FileType = myCloudBlob.BlobType.ToString(),
Size = myCloudBlob.Name.Length.ToString(),
ETag = myCloudBlob.Properties.ETag.ToString()
};
var instanceId = await starter.StartNewAsync("AzureStorageOrchestrator", newUploadedBlobItem);
log.LogInformation($"Started orchestration with ID = '{instanceId}'.");
}
else
{
log.LogError($"The blob was trigged but myCloudBlob was empty");
}
}
catch (Exception ex)
{
//Errorhandling
log.LogError("Something went wrong. Error : " + ex.InnerException);
throw;
}
}
}