Defer Is Keeping Application Unstable, Hydration Cleanup Not Done Until Defer Finished

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Problem

When using Angular's Server-Side Rendering (SSR) feature with the @defer directive, the application can become unstable due to the hydration cleanup process not being completed until the defer is finished. This can lead to unexpected behavior and errors in the application.

Details

The issue arises when the SSR and browser-side rendering are not identical, resulting in both versions being displayed during the defer timing. This can be seen in the example provided, where the isBrowserSide boolean is displayed as both true and false during the 20-second defer timing.

<div>Is Browser = {{ isBrowserSide }}</div>
@if(isBrowserSide){
  <div>Inside if: browser bool = {{ isBrowserSide }}</div>
} @else{
  <div>Inside else: browser bool = {{ isBrowserSide }}</div>
}

Use Case

This issue can affect applications that do not handle user authentication on the server-side and only on the browser-side. For example, a navbar that displays a "login" button when not connected and an "my account" button when connected. If a heavy component is deferred during the application loading process, both buttons will be displayed, which is not ideal.

Explanation

The hydration process waits for any deferred actions started initially to be finished before cleaning up. This is likely a design decision, and it's unclear if it's a bug. A possible solution would be to allow the defer to be run after hydration or application stability.

Workaround

A workaround without using the @defer directive is to wait for the application to be stable and then use a setTimeout to update a boolean to true after a few seconds. The deferred component can then be wrapped inside an @if directive.

Minimal Reproduction

A minimal reproduction of the bug can be found in the provided GitHub repository: https://github.com/gboutte/angular-defer-keep-unstable.git

Environment

The bug was discovered in the following environment:

Angular CLI: 18.2.14
Node: 20.18.3
Package Manager: npm 10.8.2
OS: darwin arm64

Angular: 18.2.13
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, platform-server
... router

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1802.14
@angular-devkit/build-angular   18.2.14
@angular-devkit/core            18.2.14
@angular-devkit/schematics      18.2.14
@angular/cli                    18.2.14
@angular/ssr                    18.2.14
@schematics/angular             18.2.14
rxjs                            7.8.2
typescript                      5.4.5
zone.js                         0.14.10

Conclusion

Q: What is the @defer directive in Angular?

A: The @defer directive in Angular is used to defer the rendering of a component until the application is stable. This can be useful for heavy components that take a long time to load, as it allows the application to render the rest of the page while the component is loading.

Q: What is the issue with the @defer directive?

A: The issue with the @defer directive is that it can cause the application to become unstable due to the hydration cleanup process not being completed until the defer is finished. This can lead to unexpected behavior and errors in the application.

Q: What is hydration cleanup?

A: Hydration cleanup is the process of cleaning up the application's state after the initial rendering. This includes removing any temporary data and updating the application's state to reflect the current user interaction.

Q: Why is hydration cleanup not being completed until the defer is finished?

A: Hydration cleanup is not being completed until the defer is finished because the @defer directive is designed to wait for any deferred actions to be completed before cleaning up the application's state. This is likely a design decision, and it's unclear if it's a bug.

Q: What is a workaround for this issue?

A: A workaround for this issue is to wait for the application to be stable and then use a setTimeout to update a boolean to true after a few seconds. The deferred component can then be wrapped inside an @if directive.

Q: Can I use the @defer directive without worrying about this issue?

A: No, it's not recommended to use the @defer directive without understanding the potential issues it can cause. If you're experiencing issues with the @defer directive, it's best to use a workaround or consider alternative solutions.

Q: How can I reproduce this issue?

A: You can reproduce this issue by creating a new Angular project and using the @defer directive in a component. Then, use the isBrowser function to display different content on the server-side and browser-side. Finally, add a @defer directive to the component and observe the behavior.

Q: What are the system requirements for this issue?

A: The system requirements for this issue are:

  • Angular CLI: 18.2.14
  • Node: 20.18.3
  • Package Manager: npm 10.8.2
  • OS: darwin arm64

Q: Can I use this workaround in production?

A: Yes, you can use this workaround in production, but it's recommended to use a more robust solution. The workaround is a temporary fix and may not work in all scenarios.

Q: Is this issue a bug or a feature?

A: This issue is likely a design decision, and it's unclear if it's a bug. The @defer directive is designed to wait for any deferred actions to be completed before cleaning up the application's state. However, this can lead to unexpected behavior and errors in the application.