The Azure Cosmos DB Change Feed acts as a persistent transaction log that records document insertions and updates in sorted order per logical partition. It provides an efficient mechanism for triggering asynchronous microservices and maintaining search indexes.

Change Feed Processor vs. Azure Functions Trigger

Developers usually choose between writing custom C# services with the Change Feed Processor library or using the managed Azure Functions Cosmos DB trigger. Azure Functions handles checkpointing, scaling, and partition management automatically behind the scenes.

Implementing an Azure Function Trigger

Below is a production-ready C# Azure Function consuming document updates directly from Cosmos DB:

public static class DocumentProcessorFunction
{
    [FunctionName("ProcessContainerChanges")]
    public static async Task Run(
        [CosmosDBTrigger(
            databaseName: "InventoryDb",
            containerName: "Products",
            Connection = "CosmosDBConnectionString",
            LeaseContainerName = "leases",
            CreateLeaseContainerIfNotExists = true)] IReadOnlyList<MyDocument> documents,
        ILogger log)
    {
        // Check if any documents were updated or inserted in this batch
        if (documents != null && documents.Count > 0)
        {
            log.LogInformation($"Processing batch of {documents.Count} modified documents.");
            
            foreach (var doc in documents)
            {
                // Process individual item change, e.g., publish to Event Grid
                await ProcessItemChangeAsync(doc);
            }
        }
    }
}

A dedicated lease container tracks progress across partition leases, enabling multiple function instances to scale horizontally without duplicate processing.