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chromium / chromium / src / 5db31b51f232cfd6b7b99a8919e739c7c45d0f55 / . / content / browser / preloading / prerender
tree: 9cc46154af761784d0c1be8163fcc5be4b8eeec1 [path history] [tgz]
  1. COMMON_METADATA
  2. devtools_prerender_attempt.cc
  3. devtools_prerender_attempt.h
  4. DIR_METADATA
  5. OWNERS
  6. prerender_attributes.cc
  7. prerender_attributes.h
  8. prerender_browsertest.cc
  9. prerender_commit_deferring_condition.cc
  10. prerender_commit_deferring_condition.h
  11. prerender_final_status.cc
  12. prerender_final_status.h
  13. prerender_handle_impl.cc
  14. prerender_handle_impl.h
  15. prerender_host.cc
  16. prerender_host.h
  17. prerender_host_registry.cc
  18. prerender_host_registry.h
  19. prerender_host_registry_unittest.cc
  20. prerender_host_unittest.cc
  21. prerender_metrics.cc
  22. prerender_metrics.h
  23. prerender_metrics_unittest.cc
  24. prerender_navigation_throttle.cc
  25. prerender_navigation_throttle.h
  26. prerender_navigation_utils.cc
  27. prerender_navigation_utils.h
  28. prerender_new_tab_handle.cc
  29. prerender_new_tab_handle.h
  30. prerender_subframe_navigation_throttle.cc
  31. prerender_subframe_navigation_throttle.h
  32. prerender_trigger_type_impl.cc
  33. prerender_trigger_type_impl.h
  34. README.md
content/browser/preloading/prerender/README.md

This directory contains the Prerender2 implementation (https://crbug.com/1126305).

If you're interested in relevant code changes, join the prerendering-reviews@chromium.org group.

Summary

Prerendering is “pre”-rendering, it's about pre-loading and rendering a page before the user actually navigates to it. The main goal of prerendering is to make the next page navigation faster, or ideally nearly instant.

The Prerender2 is the new implementation of prerendering.

Terminology

  • Trigger: “Trigger” is an entry point to start prerendering. Currently, <script type="speculationrules"> is the only trigger.
  • Activate: The Prerender2 runs navigation code twice: navigation for prerendering a page, and navigation for displaying the prerendered page. “Activate” indicates the latter navigation.
  • Legacy Prerender: “Legacy Prerender” is the previous implementation of prerendering that does not use NoStatePrefetch. This is already deprecated (https://crbug.com/755921).
  • NoStatePrefetch: An internal mechanism for speculative prefetching of pages and their subresources that are on a critical path of page loading without executing any JavaScript or creating a complex state of the web platform (https://www.chromestatus.com/feature/5928321099497472). This mechanism is not purely “no state” because the HTTP cache allows to create cookies and other state related to validating cache entries. The current prerendering uses this mechanism, that is, it does not actually render pages, while the Prerender2 renders pages.
  • Activation-gated APIs: Web platform APIs that are dependent on user activation. Prerendered pages never have user activation, so the activation-gated APIs automatically fail or no-op in the prerendered pages. The known activation-gated APIs are listed here.

UKM Source Ids

As previously mentioned, there are two navigations involved in Prerender2: the prerendering navigation and the activation navigation. As a result, there are two UKM Source IDs that are currently used for a prerendered page; one derived from each navigation's ID.

The SourceId for the prerendering navigation is currently returned by RenderFrameHost::GetPageUkmSourceId(), and by NavigationHandle::GetNextPageUkmSourceId() when called on the NavigationHandle for the prerendering navigation. This source ID is not associated with a URL by SourceUrlRecorder (the data collection policy disallows recording it before prerender activation), so any metrics recorded using it will not be associated with a URL.

The SourceId for the activation navigation is associated with a URL by SourceUrlRecorder. The value is currently returned by NavigationHandle::GetNextPageUkmSourceId() when called on the NavigationHandle for the activation navigation, and by PageLoadMetricsObserverDelegate::GetPageUkmSourceId (and is used by all PLMOs that record UKMs for prerendered pages).

There are 2 patterns recording metrics for prerendering pages used currently in content/:

  1. Use the source ID of the most recently navigated primary page (obtained from NavigationHandle::GetNextPageUkmSourceId(), which uses the activation navigation ID for prerender activations). This UKM source ID is always associated with a URL. This approach is currently used by Precog (the UKM infrastructure in content/browser/preloading) to record the Preloading_Prediction and Preloading_Attempt UKMs.

  2. Use the triggering page‘s UKM source ID to record metrics for a prerendering page. PrerenderHost uses this approach currently to record the PrerenderPageLoad UKM, which it reports even if the prerendered page isn’t activated. Precog also uses this approach to record the Preloading_Attempt_PreviousPrimaryPage UKM. They both currently use RenderFrameHost::GetPageUkmSourceId to retrieve the source ID, which will be associated with a URL in most cases, except when the triggering page itself was prerendered.

Another possible approach is to collect/remember metrics while prerendering and only record them using the activation navigation's source ID after the prerender is activated (a similar pattern is used by PageLoadMetricsObservers outside content/). In the future, we may want to support recording UKMs after activation with both source IDs. This will require registering the prerender navigation source ID with a URL after activation.

References

The date is the publication date, not the last updated date.

  • Prerender2 (Oct, 2020): Introduces how Prerender2 works and more detailed designs such as Mojo Capability Control.