This document summarizes a presentation about stateful patterns in Azure Functions using Durable Functions. The presentation introduces Durable Functions as a way to add state management to Azure Functions. It discusses common stateful patterns like function chaining, fan-in/fan-out, and human interaction and how Durable Functions addresses issues with implementing these patterns with regular stateless functions through orchestrations, activities, and entities. The presentation concludes by emphasizing how Durable Functions solves concurrency issues but may not always be the right choice depending on requirements around latency.

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Speaker Slide:
Massimo Bonanni
Azure Technical Trainer @ Microsoft
I spend my time to help customers to empower their Azure skills to achieve more and leverage the
power of Azure in their solutions.
I’m also a technical speaker both for local and international events and a user-group guy.
I founded Aa couple of communities in Italy and collaborated with most of the Italian communities.
Finally, I is also passionate about biking, reading, and dogs!!

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Stateful patterns in Azure Functions
Massimo Bonanni
Azure Technical Trainer @ Microsoft

4. Be aware of others
Be friendly and
patient
Be welcoming and
respectful
Be open to all
questions and
viewpoints
Be understanding of
differences
Be kind and
considerate to others

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What are Azure Functions?
Author functions in
C#, F#, Node.JS, Java,
and more
Code
Events
React to timers, HTTP, or
events from your favorite
Azure services, with more
on the way
Outputs
Send results to an
ever-growing
collection of services

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Functions must be stateless
Functions cannot call other functions
Functions should do only one thing
Principles and best practices…

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Workflow manages state
Workflow is interactions between
components
Workflows must do more than one thing
… and workflows!?!?!

8. The magic is
Durable Functions!!

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Azure Functions
Extension
Based on Azure
Functions
Adds new Triggers and
Bindings
Manages state,
checkpoints, and
restarts
Durable Task
Framework
Long running persistent
workflows in C#
Used within various
teams at Microsoft to
reliably orchestrate long
running operations
Languages
C#
JavaScript
F#
Python
Powershell
What are Durable Functions?

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Durable Function components
Client
Is the triggered
functions that will
create new instances
of an orchestration.
It is the entry point for
creating an instance of
a durable orchestration
Orchestrator
Is the heart of a
durable function.
Orchestrator functions
describe the way and
order actions are
executed.
Activity
Is the basic unit of
work in a durable
orchestration.
An activity function
must be triggered by
an activity trigger.

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Function chaining
Relations between functions and queues aren’t clearly identifying
Queues are an implementation detail
Operation context management is difficult
Error handling is difficult

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Function chaining in Durable Functions
Orchestrator Function
Activity Functions

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Function chaining in Durable Functions

14. The magic is
Event Sourcing!!

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1. var x = await context.CallActivityAsync<int>("F1", null);
2. var y = await context.CallActivityAsync<int>("F2", x);
3. return await context.CallActivityAsync<int>("F3", y);
Orchestrator Function
Client
Orchestrator
Activity
F1 => return 42;
F2 => return value + 1;
F3 => return value + 2;
Event History
Orchestrator Started
Task Scheduled, F1
Task Completed, F1 => 42
Task Scheduled, F2
Task Completed, F2 => 43
Task Scheduled, F3
Task Completed, F3 => 45
Orchestrator Completed => 45
Storage managed by
Durable Task Framework

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Orchestrator MUST be deterministic
Never write logic that depends on random
numbers, current date/time, delay, etc.
Never do I/O in the orchestrator function
Never start custom thread in the orchestrator
function
Do not write infinite loops

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FanIn-FanOut
FanIn is simple, but FanOut is more complicated
The platform must track progress of all work
All the same issues of Function Chain

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FanIn-FanOut in Durable Functions

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FanIn-FanOut in Durable Functions

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Human interaction
Handling race conditions between timeouts and
approval
Need mechanism for implementing and cancelling
timeout events
Same issues as the other pattern

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Human Interaction in Durable Functions

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Human Interaction in Durable Functions

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Aggregator
Storing the state
Correlation of event for a particular state
Syncronization of access to the state

25. The magic is
Durable Entities!!

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Actor model

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Entity Functions define
operations for reading and
updating small piece of
state
Entity Functions are
functions with special
trigger
Entity Functions are
accessed using:
• Entity Name
• Entity key
Entity Functions expose
operations that can be accessed
using:
• Entity Key
• Operation Name
• Operation Input
• Scheduled time
Durable Entities aka Entity Functions

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Accessing the Entities
Two-way (round-trip) communication.
You send an operation message to the entity, and then
wait for the response message before you continue.
Calling
(round-
trip)
One-way (fire and forget) communication.
You send an operation message but don't wait for a
response.
Signaling
(fire-and-
forget)
Two-way communication.
You can retrieve the state of an entity
State
Orchestrator
Orchestrator
Client
Entity
Client

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Anatomy of an Entity
Properties (state)
Operations
Entry Function

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Designed for reliability, not for latency
Workflow by code
Durable Entities are similar to Virtual Actor
Solve the concurrency issues, but is it the
right choice?
Takeaways

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Thanks for your
attention!!!!!
Massimo Bonanni
Azure Technical Trainer
massimo.bonanni@microsoft.com
@massimobonanni
Connect with me on LinkedIn
linkedin.com/in/massimobonanni/
http://bit.ly/MasteringServerless

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References
Photo by Jaredd Craig on Unsplash
https://docs.microsoft.com/en-US/azure/azure-functions/
https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-
overview?tabs=csharp
https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-dotnet-entities
https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-
entities?tabs=csharp
https://docs.microsoft.com/en-us/azure/azure-functions/durable/quickstart-python-vscode
https://github.com/Azure/durabletask
https://github.com/massimobonanni/StatefulPatternFunctions