Differences between in-process and isolated worker process .NET Azure Functions

Functions supports two process models for .NET class library functions:

Execution mode Description
In-process Your function code runs in the same process as the Functions host process. Supports only Long Term Support (LTS) versions of .NET. To learn more, see Develop .NET class library functions.
Isolated worker process Your function code runs in a separate .NET worker process. Use with supported versions of .NET and .NET Framework. To learn more, see Develop .NET isolated worker process functions.

This article describes the current state of the functional and behavioral differences between the two models.

Execution mode comparison table

Use the following table to compare feature and functional differences between the two models:

Feature/behavior In-process3 Isolated worker process
Supported .NET versions Long Term Support (LTS) versions All supported versions + .NET Framework
Core packages Microsoft.NET.Sdk.Functions Microsoft.Azure.Functions.Worker
Binding extension packages Microsoft.Azure.WebJobs.Extensions.* Microsoft.Azure.Functions.Worker.Extensions.*
Durable Functions Supported Supported (public preview)
Model types exposed by bindings Simple types
JSON serializable types
Service SDK types such as BlobClient
IAsyncCollector (for output bindings)
Simple types
JSON serializable types
HTTP trigger model types HttpRequest / ObjectResult HttpRequestData / HttpResponseData
Output binding interaction Return values (single output only)
out parameters
Return values (expanded model with single or multiple outputs)
Imperative bindings1 Supported Not supported
Dependency injection Supported Supported
Middleware Not supported Supported
Logging ILogger passed to the function
ILogger<T> via dependency injection
ILogger/ILogger<T> obtained from FunctionContext or via dependency injection
Application Insights dependencies Supported Supported (public preview)
Cancellation tokens Supported Supported
Cold start times2 (Baseline) Additionally includes process launch
ReadyToRun Supported Supported

1 When you need to interact with a service using parameters determined at runtime, using the corresponding service SDKs directly is recommended over using imperative bindings. The SDKs are less verbose, cover more scenarios, and have advantages for error handling and debugging purposes. This recommendation applies to both models.

2 Cold start times may be additionally impacted on Windows when using some preview versions of .NET due to just-in-time loading of preview frameworks. This applies to both the in-process and out-of-process models but may be noticeable when comparing across different versions. This delay for preview versions isn't present on Linux plans.

3 C# Script functions also run in-process and use the same libraries as in-process class library functions. For more information, see the Azure Functions C# script (.csx) developer reference.

Next steps

To learn more, see: