Durable functions offer an interesting programming model for building workflows. Whether you need to sometimes split and do multiple things or wait for user input, a lot of things are possible. They do present some challenges as well, and the limitations of orchestrator functions can make working with Durable seem very complicated. In this talk we will go through the basics of Durable Functions along with strategies for deploying and monitoring them. A sample application will be presented where users can send documents for electronic signature. A Durable Functions workflow will power the signing process.

1. Building a document e-
signing workflow with
Durable Functions
Joonas Westlin

2. Speaker Intro
• Joonas Westlin
• Developer / Architect @ Zure
• Azure MVP
• Global #9 on Stack Overflow for Azure
answers
@JoonasWestlin
joonasw.net

3. Contents
• An e-signing workflow
• Workflows? Orchestrations? Activities?
• What is Durable Functions and what can you use it for?
• Demo: Building the signing workflow
• Useful things to know
• Conclusions and Q&A
• Links

4. An e-signing workflow
User uploads
document
Send email to
signers
Wait for signer
response
Create signed
document
Send
completion
email
Users download
signed
document

5. Workflows? Orchestrations? Activities?
• A workflow is an orchestrated sequence of activities that describes a process
• An activity is a step in a workflow
• Orchestration refers to automatic management of workflows
• In this context at least

6. What is Durable Functions?
• Framework built on Azure Functions and the Durable Task framework
• Build durable serverless workflows in C# / JavaScript
• State persisted in Table Storage -> Durable
• Storage Queue messages used under the hood to communicate
• Three types of functions: Orchestrator, Activity, and Entity
• Orchestrators can be started by any other Azure Function

7. Use cases for
Durable Functions

8. Function chaining

9. Fan out – fan in

10. External events and human interaction
Approved
Rejected
Wait for
approval

11. Monitoring
Poll status
Repeat

12. Why orchestrators and activities?
• Orchestrator functions:
• Define steps of the workflow
• State is persisted in history table
• Code is replayed as the workflow progresses
• Require code to be deterministic
• Can start sub-orchestrators and activities
• Activity functions:
• Steps in the workflow
• Can do I/O
• Can be non-deterministic

13. Code replay
• When an orchestrator hits an await/yield for a sub-orchestrator or activity:
• Message is posted to a queue
• The orchestrator is suspended
• Once result is available, a queue message wakes up the orchestrator
• The code is executed again; this time it will continue past the await/yield call
• History table tells it that the activity was completed
• Can check if we are currently replaying from the context object

14. Sample orchestrator
Execution history
Execution started
Orchestrator started
Activity scheduled
Orchestrator completed
Activity completed
Orchestrator started
Activity scheduled
Orchestrator completed
Activity completed
Orchestrator started
Activity scheduled
Orchestrator completed
Activity completed
Orchestrator started
Orchestrator completed
Execution completed

15. Determinism
• Required in orchestrator function code due to replay
• With the same inputs, the same code path is taken and the same output is produced
• DateTime.UtcNow is non-deterministic, because it gives a different result
depending on time
• Other examples of things you cannot do/use:
• Randomness
• Guid.NewGuid()
• I/O, database queries
• CPU-intensive computation, multi-threading, thread blocking
• Orchestration context object has deterministic versions of some of these
• HTTP API calls are also possible, but flexibility is limited

16. Sample activity
• Activities are where
you will do:
• Database queries
• API calls with
maximum flexibility
• Other non-
deterministic things
• Their results are
persisted in the
history table

17. Workflow that we will build
Waitfor
signaturesub-
orchestrator
Mainorchestrator
Mark workflow
started in database
Send emails to
signers
Set instance id to
database
Wait for signing
event
Signer
responded on
time?
Yes
Set signing data in
database
Did allsigners
sign?
No
No
Send completion
email
Start
Yes
Create signed
document
Mark workflow
completed in
database
End

18. Demo: Building the signing
workflow

19. Useful things to know

20. Error handling
• Use try-catch around activity / sub-orchestrator calls to catch errors thrown
in them
• Compensate or re-throw
• Use retries: CallActivityWithRetryAsync / CallSubOrchestratorWithRetryAsync
• Retry with optional back-off coefficient

21. Monitoring Durable Functions
• Application Insights works, but end-to-
end tracing does not work correctly
• Use context.CreateReplaySafeLogger()
to prevent duplicate log messages

22. Use the .NET analyzers
• Microsoft.Azure.WebJobs.Extensions.DurableTask.Analyzers
• Gives you warnings if you:
• Break determinism in orchestrators
• Use the wrong parameter type for activities or sub-orchestrators
• And much more

23. Handling code complexity
• Divide and conquer
• Split complex orchestrator functions into sub-orchestrators
• Can also split into methods; Use [Deterministic] on them
• A long activity function might be doing too much work, split it

24. Updating Durable Functions
• When deploying new versions, you must think about existing orchestrations
• Having the orchestrator code change would be a violation of determinism
• Versioning strategies:
• Let the old orchestrations fail by changing the function name
• Create the new version side-by-side as a new function and delete the old one later
• Deploy new function app
• https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-
versioning

25. Summary and takeaways
• Build workflows in code with Durable Functions
• Implement patterns like function chaining, fan-out / fan-in, and human
interaction
• Use orchestrator functions to build the overall workflow and call activities
• Orchestrator code must be deterministic
• Use activity functions to do database queries, send emails, heavy
computation etc.
• Monitoring is currently not the best but is improving
• Be careful when updating orchestrators

26. Q&A

27. Links
• https://westl.in/globalazure2020
• https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-
functions-overview
• Concepts section is awesome
• https://markheath.net/category/durable%20functions
• https://dontcodetired.com/blog/?tag=durable+functions
• https://github.com/juunas11/Joonasw.ElectronicSigningDemo