Azure Functions - Durable Functions

What are Durable Functions?

Durable Functions is an extension of Azure Functions and Azure Logic Apps that lets you write stateful functions and stateful orchestrations in a serverless compute environment. It allows you to define and execute workflows that span across multiple function executions, maintaining their state between invocations.

This is achieved through a powerful programming model that leverages code-first orchestration. You write your orchestration logic as regular code (C#, F#, JavaScript, Python, PowerShell) and the Durable Functions extension handles the complexities of state management, checkpointing, and retries.

Key Concepts

Orchestrator Functions: Define the workflow logic, calling other functions (activities, other orchestrators, entities) and managing control flow.
Activity Functions: Perform individual units of work. These are the building blocks of your orchestration.
Entity Functions: Used for managing stateful entities. They are ideal for scenarios like counters, timers, or game state.
Client Functions: Used to start, query, and terminate orchestrations.

Why Use Durable Functions?

Simplified State Management

Automatically manages state, checkpointing progress and resuming from failures without manual intervention.

Complex Workflows

Easily build intricate workflows like human interaction, dynamic branching, fan-out/fan-in patterns, and long-running processes.

Reliability and Resiliency

Built-in error handling, retry policies, and fault tolerance ensure your workflows complete reliably.

Cost-Effective Serverless

Leverage the pay-as-you-go model of Azure Functions for efficient execution of your stateful logic.

Developer Productivity

Write orchestrations in your preferred language, using familiar programming constructs.

Common Use Cases

Human Interaction: Long-running processes that require human approval or input.
Fan-out/Fan-in: Performing multiple tasks in parallel and aggregating results.
Monitors: Periodically checking for conditions and taking action.
Event-driven Orchestrations: Responding to events from various sources and coordinating actions.
Order Processing: Managing complex order fulfillment pipelines.
Data Pipelines: Orchestrating data ingestion, transformation, and loading.

Getting Started

To get started with Durable Functions, you'll need an Azure subscription and the Azure Functions tooling installed.

Example Orchestrator (Conceptual C#)

This is a simplified example showing how an orchestrator might call activity functions.


// This is a conceptual example, not runnable code.
// Requires Durable Functions SDK for C#.

using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.DurableTask;
using System.Threading.Tasks;

public static class SampleOrchestration
{
    [FunctionName("SampleOrchestration")]
    public static async Task RunOrchestrator(
        [OrchestrationTrigger] IDurableOrchestrationContext context)
    {
        // Input can be passed to the orchestration
        string input = context.GetInput();

        // Call an activity function
        var result1 = await context.CallActivityAsync("ActivityFunction1", $"Processing {input}");

        // Make a decision based on the result
        if (result1.Contains("Success"))
        {
            // Call another activity function
            var result2 = await context.CallActivityAsync("ActivityFunction2", result1);

            // Perform a sub-orchestration
            var finalResult = await context.CallSubOrchestratorAsync("SubOrchestration", result2);

            return finalResult;
        }
        else
        {
            // Handle failure
            await context.CallActivityAsync("LogFailure", result1);
            return false;
        }
    }
}

For detailed guides, API references, and more code examples, please visit the official Azure Durable Functions Documentation.