Improve post processor response cache #1325

hellofromtonya · 2018-08-07T22:53:02Z

Randomly generated content can occur on a web page. When that happens, the response will differ for each request causing cache misses for the plugin's AMP response cache. As @westonruter notes in Issue #1239:

the object cache will get polluted with values that never get used.

This PR detects continual cache misses by caching the page's URL and then comparing the number of cached URLs to a threshold. When that threshold is exceeded, it disables the response cache and then notifies the admin.

TODO

Detect continual cache misses
Disable caching
Notify
Provide a UI to reset the option's value and re-enable response caching.

Closes #1239.

@todo

We are capturing each response's URL and storing it into the cache. @todo turn off response caching when the number of caches for the response's URL exceeds the cache miss threshold.

@todo

When the number of cached URLs for the given response exceeds the cache miss threshold, disable the response caching. It also disables continuing to cache the response's URLs. @todo add getter for response cache to test the response.

Added the caching keys as public properties. Why? 1. Eliminates passing it into the caching closure. 2. Exposes both of them for testing. Then we use these keys to valid the caching conditions before and after we hit the threshold.

hellofromtonya · 2018-08-07T22:57:52Z

@westonruter Commits through to 1d1f399 are a prototype per our discussions here. It's implementing:

Cache the URLs.
Then determine if we should turn off the response caching.
If caching is enabled, then inside of the caching closure, we'll cache the new URL.

Take a look and see what you think. BTW Notices are not included yet.

westonruter · 2018-08-08T02:07:43Z

includes/class-amp-theme-support.php

+		self::$post_processor_cache_key = null;
+		if ( true === $args['enable_response_caching'] ) {
+			self::$post_processor_cache_key = md5( $current_url );
+			$caches_for_url                 = wp_cache_get( self::$post_processor_cache_key, self::POST_PROCESSOR_CACHE_EFFECTIVENESS );


Sone of the subsequent checks could perhaps be simplified by checking if this is an array here, and if not, just set it to be an empty array.

westonruter · 2018-08-08T02:08:54Z

includes/class-amp-theme-support.php

+	 *
+	 * @var string
+	 */
+	public static $response_cache_key;


Is this necessary? Aren't references limited to a single method? Since this is a static class it would be better of we limited the state that is persisted in it's class variables IMO.

@westonruter I added this public property for the following purposes:

to give us the ability to deeply test by knowing what the cache key is and then checking for it in the cache.

to simplify access to the logic within the caching closure, i.e. meaning that we don't have to pass it via use.

It serves no other external purpose as part of an API.

Let's think about it.

From a testing point of view, we can infer that we disabled the cache by the state of the post-processor cache. While that's not a direct test on the response cache, both are within the closure. Therefore, it's a minimal risk to test by inference.

Hmm, I like that approach better actually. We get the benefit only exposing what is needed externally while avoided the extra work of resetting persistent state.

I'll make that change.

westonruter · 2018-08-08T02:28:02Z

includes/class-amp-theme-support.php

+				if ( empty( $caches_for_url ) ) {
+					$caches_for_url = array( AMP_Theme_Support::$response_cache_key );
+				} else {
+					$caches_for_url[] = AMP_Theme_Support::$response_cache_key;


Per above, this could be the sole line instead of the if/else.

…tic property.

…ic static property.

When there are no URLs cached, we get `false` back. To simplify the code, if we do not get an array back, the variable is initialized to an empty array.

westonruter · 2018-08-09T20:48:36Z

For the notice, are you thinking there would be a new key added to the amp-options option which would flag whether or not the response cache is enabled?

hellofromtonya · 2018-08-09T21:18:42Z

For the notice, are you thinking there would be a new key added to the amp-options option which would flag whether or not the response cache is enabled?

Well that's a good question @westonruter. I think to find the solution, we need to explore what we want to report back to the developer/admin.

We are disabling the cache on a page-by-page basis. Do we want to give a list of URLs (i.e. that have cache disabled)? Or do we want a general notice? Or both?

It's a more broad discussion for the DX/UX.

westonruter · 2018-08-09T21:35:50Z

@hellofromtonya humm, how about showing the notice on the general settings page, similar to how we are showing a notice for not having the object cache active.

I didn't think about letting the cache be disabled on a page-by-page basis. I realize that is how it is implemented now. I think that once we get to the point where the uncached count is greater than CACHE_MISS_THRESHOLD we can just set an option to stop the caching. In the future we could go to a page-by-page basis, but I think that could be overkill for now.

westonruter · 2018-08-09T21:43:47Z

Also, currently the $post_processor_cache_key is specific to the current URL, so there would be no way to query for all URLs that are surpassing the CACHE_MISS_THRESHOLD, since the object cache cannot be queried. Maybe a compromise is that we could store the URL that causes the CACHE_MISS_THRESHOLD to be surpassed, and then use that URL in the notice.

Something else: if there are logged-in users and the REST API is being used on the frontend, it is likely that the API nonce is going to be output. This nonce will be unique for each user. This being the case, it is very likely that the CACHE_MISS_THRESHOLD will be very quickly passed, since each user landing on the homepage would then cause the increment to go up. Maybe this is expected and is proper behavior, but it's also something to be aware of as it could make the cache very prone to being turned off.

westonruter · 2018-08-09T21:47:18Z

Maybe such caching should just be disabled entirely if the user is logged in.

Also, I just realized something else, when we do:

$caches_for_url[] = $response_cache_key;

We could easily cause for the homepage, for example, to trigger the cache to get disabled once you've published 3 posts, each causing the homepage to change. So what I think we're missing here is a timestamp for when the cache MISS happened. And then what we'd want to do is only consider such entries that have happened within a recent time period (e.g. 10 minutes) for whether the CACHE_MISS_THRESHOLD has been surpassed.

hellofromtonya · 2018-08-09T21:55:32Z

Also, currently the $post_processor_cache_key is specific to the current URL, so there would be no way to query for all URLs that are surpassing the CACHE_MISS_THRESHOLD, since the object cache cannot be queried.

Correct.

Maybe a compromise is that we could store the URL that causes the CACHE_MISS_THRESHOLD to be surpassed, and then use that URL in the notice.

I agree. That's the next step as we think about how to notify.

We could easily cause for the homepage, for example, to trigger the cache to get disabled once you've published 3 posts, each causing the homepage to change. So what I think we're missing here is a timestamp for when the cache MISS happened. And then what we'd want to do is only consider such entries that have happened within a recent time period (e.g. 10 minutes) for whether the CACHE_MISS_THRESHOLD has been surpassed.

How about instead if we set the cache expiration to say 10 minutes?

westonruter · 2018-08-09T21:58:58Z

How about instead if we set the cache expiration to say 10 minutes?

Yeah, that sounds good.

hellofromtonya · 2018-08-09T22:20:32Z

I didn't think about letting the cache be disabled on a page-by-page basis. I realize that is how it is implemented now. I think that once we get to the point where the uncached count is greater than CACHE_MISS_THRESHOLD we can just set an option to stop the caching. In the future we could go to a page-by-page basis, but I think that could be overkill for now.

Maybe a compromise is that we could store the URL that causes the CACHE_MISS_THRESHOLD to be surpassed, and then use that URL in the notice.

@westonruter okay so instead of page-by-page, once we hit the threshold, we:

turn off the cache for all pages/URLs
store that URL in an option
display a notice with a link to that URL

That's a simple solution. And as you said, page-by-page is likely overkill right now.

Instead of storing cache miss URLs page-by-page, this commit stores it on occurrence, using a key and group constant.

1. Refactored into a separate method as there's a lot going on. 2. Check if cache is turned off. 3. If no, check if we've exceeded the threshold. 4. If yes, store the URL.

westonruter · 2018-08-10T17:30:17Z

includes/options/class-amp-options-manager.php

+
+		printf(
+			'<div class="notice notice-warning"><p>%s <a href="%s">%s</a></p></div>',
+			esc_html__( 'Response caching was disabled due to exceeding the cache miss threshold.', 'amp' ),


We'll probably need to figure out a way to explain what this means to non-developers. Maybe there should be a link to the Wiki with more information about what causes this?

Also, should the notice be dismissible?

Should there be a checkbox to re-enable? If so, then the notice could be displayed inline with the checkbox.

We'll probably need to figure out a way to explain what this means to non-developers. Maybe there should be a link to the Wiki with more information about what causes this?

Agreed. We need more work on the messaging. I've noted it as a @todo.

should the notice be dismissible?

I think so. I think it's okay that the developer or admin dismisses this warning.

Thinking through it, I'd like to use this top-level warning as an alert: "Hey, we automatically turned off caching. Go to the AMP > General page for more information." Use it to alert them that we did something automatically and it needs their attention.

Should there be a checkbox to re-enable? If so, then the notice could be displayed inline with the checkbox.

Agreed. This is the next step, i.e. adding a re-enable checkbox that appears when caching is disabled.

@todo

@todo Change the messaging/description to be more user-friendly and helpful.

hellofromtonya · 2018-08-13T20:29:30Z

includes/options/class-amp-options-manager.php

+		printf(
+			'<div class="notice notice-warning is-dismissible"><p>%s <a href="%s">%s</a></p></div>',
+			esc_html__( "The AMP plugin's response cache disabled due to detecting randomly generated content.", 'amp' ),
+			esc_url( 'https://github.com/Automattic/amp-wp/wiki/Response-cache#automatically-disabling-of-the-response-cache' ),


@westonruter I opened a new Wiki page and started the process of explaining the cache. Once we get the UI done, then screenshots can be added to the page to provide users/developers more clarity.

hellofromtonya · 2018-08-13T20:38:05Z

includes/options/class-amp-options-manager.php

@@ -48,6 +49,7 @@ public static function register_settings() {

 		add_action( 'update_option_' . self::OPTION_NAME, array( __CLASS__, 'maybe_flush_rewrite_rules' ), 10, 2 );
 		add_action( 'admin_notices', array( __CLASS__, 'persistent_object_caching_notice' ) );
+		add_action( 'admin_notices', array( __CLASS__, 'render_cache_miss_notice' ) );


@westonruter As we are automatically turning off the response cache, I'm thinking we may want to notify not only on this settings page, but on other pages too. Why? We can't be certain the admin/developer will come to this page. At the very least, I'm thinking this notice should be on all AMP settings' pages. What do you think?

What if we limit it to the general settings page but add an indicator to the admin menu item so that the user knows there has something changed on this page?

@westonruter I like that approach in general. I think it spans more than just this PR and edge case. Thinking about, it seems we could have a general indicator to alert of something that needs attention. That indicator could link to the plugin's settings page where more information lives.

If we go for that approach, I think that's a separate feature, issue, and PR.

westonruter · 2018-08-15T05:16:47Z

includes/options/class-amp-options-manager.php

+		// Handle the caching option.
+		$options['enable_response_caching'] = ! empty( $new_options['enable_response_caching'] );
+		if ( $options['enable_response_caching'] ) {
+			do_action( 'amp_reenable_response_cache' );


Wouldn't it not be better to call the method here rather than indirectly trigger the action that the method is is connected to? Is there any utility for the amp_reenable_response_cache action generally?

Upon review, you're right. There is not need for a separation action here. Switching to call direct.

westonruter · 2018-08-15T05:28:53Z

includes/options/class-amp-options-menu.php

@@ -109,6 +109,17 @@ public function add_menu_items() {
 			)
 		);

+		add_settings_field(


This setting should only be added if wp_using_ext_object_cache() returns true, as we should only do response caching if we know that the cached value will persist across requests.

True. Good point.

westonruter · 2018-08-15T05:30:05Z

includes/options/class-amp-options-manager.php

@@ -31,6 +31,7 @@ class AMP_Options_Manager {
 		'disable_admin_bar'       => false,
 		'all_templates_supported' => true,
 		'supported_templates'     => array( 'is_singular' ),
+		'enable_response_caching' => true,


The default value here can be the return value of wp_using_ext_object_cache() (which wouldn't work as-is right here in this static context)

One way to accomplish this is to set the default state within get_options():

self::$defaults['enable_response_caching'] = wp_using_ext_object_cache(); return array_merge( self::$defaults, $options );

This change is implemented in commit 752de78.

westonruter · 2018-08-15T05:31:26Z

includes/options/class-amp-options-manager.php

@@ -198,6 +200,12 @@ public static function validate_options( $new_options ) {
 		// Store the current version with the options so we know the format.
 		$options['version'] = AMP__VERSION;

+		// Handle the caching option.
+		$options['enable_response_caching'] = ! empty( $new_options['enable_response_caching'] );


The condition here should probably be wp_using_ext_object_cache() && ! empty( $new_options['enable_response_caching'] )

…nse_caching'. The $defaults array is a static property. Therefore, we set the default value within the `get_options()` method just before we merge defaults and the stored options.

…reshold.

hellofromtonya · 2018-08-23T02:16:17Z

@westonruter When we changed the conditions by using wp_using_ext_object_cache(), the notices were displaying incorrectly compared to the state. In the commit e575cde, I fixed it so that the notice appears when:

wp_using_ext_object_cache() is enabled.
The enable_response_caching option is disabled.
The cache miss threshold was exceeded.

hellofromtonya · 2018-08-23T22:59:34Z

includes/options/class-amp-options-manager.php

@@ -355,8 +355,8 @@ public static function render_cache_miss_notice() {

 		printf(
 			'<div class="notice notice-warning is-dismissible"><p>%s <a href="%s">%s</a></p></div>',
-			esc_html__( "The AMP plugin's response cache disabled due to detecting randomly generated content.", 'amp' ),
-			esc_url( 'https://github.com/Automattic/amp-wp/wiki/Response-cache#automatically-disabling-of-the-response-cache' ),
+			esc_html__( "The AMP plugin's post-processor cache disabled due to the detection of highly-variable content.", 'amp' ),


Much better @westonruter. Thank you 👍

westonruter · 2018-08-23T23:05:33Z

@hellofromtonya Great work. This was tough!

hellofromtonya · 2018-08-23T23:07:27Z

Collaborative effort between both of us. Thanks for your help and for first identifying this as an issue.

Tonya Mork added 3 commits August 7, 2018 17:11

Add post-processor cache effectiveness.

0a5e3f2

We are capturing each response's URL and storing it into the cache. @todo turn off response caching when the number of caches for the response's URL exceeds the cache miss threshold.

hellofromtonya requested a review from westonruter August 7, 2018 22:53

westonruter reviewed Aug 8, 2018

View reviewed changes

Tonya Mork added 3 commits August 9, 2018 13:15

Convert response cache key to internal variable instead of public sta…

7e6f874

…tic property.

Convert post-processor cache key to internal variable instead of publ…

eab2d95

…ic static property.

Init $caches_for_url to an empty array.

d7afb0d

When there are no URLs cached, we get `false` back. To simplify the code, if we do not get an array back, the variable is initialized to an empty array.

westonruter changed the title ~~[WIP] Improve post processor response cache~~ Improve post processor response cache Aug 9, 2018

westonruter changed the title ~~Improve post processor response cache~~ [WIP] Improve post processor response cache Aug 9, 2018

Set post-processor cache time to 10 minutes.

73e72b9

Tonya Mork added 3 commits August 9, 2018 17:40

Convert post-processor cache to occurrence instead of page-by-page.

7316ee0

Instead of storing cache miss URLs page-by-page, this commit stores it on occurrence, using a key and group constant.

Store the cache miss URL in an option. Then disable.

773bfee

1. Refactored into a separate method as there's a lot going on. 2. Check if cache is turned off. 3. If no, check if we've exceeded the threshold. 4. If yes, store the URL.

Add notice to AMP general screen.

75611f5

westonruter reviewed Aug 10, 2018

View reviewed changes

Tonya Mork added 6 commits August 13, 2018 12:48

Add caching section to the AMP General sub page.

bb39b89

@todo Change the messaging/description to be more user-friendly and helpful.

Merge branch 'develop' into improve/post-processor-response-cache

2c760bd

Automatically disable response caching when threshold is exceeded.

efd9d93

Handle re-enabling the response cache.

f37e7ff

Top level notice is dismissible.

b0dc43d

Improve top level notice to be user friendly and link to Wiki.

8e553e9

Improve field description and inline warning.

1e6a8af

hellofromtonya commented Aug 13, 2018

View reviewed changes

westonruter reviewed Aug 15, 2018

View reviewed changes

Tonya Mork added 4 commits August 22, 2018 15:29

Merge branch 'develop' into improve/post-processor-response-cache

b05a38f

Call AMP_Theme_Support::reset_cache_miss_url_option() directly.

0274715

Check wp_using_ext_object_cache() as part of setting and enable.

ecb1587

Use wp_using_ext_object_cache() as the default value of 'enable_respo…

752de78

…nse_caching'. The $defaults array is a static property. Therefore, we set the default value within the `get_options()` method just before we merge defaults and the stored options.

hellofromtonya changed the title ~~[WIP] Improve post processor response cache~~ Improve post processor response cache Aug 23, 2018

hellofromtonya added this to the v1.0 milestone Aug 23, 2018

hellofromtonya added the Bug Something isn't working label Aug 23, 2018

Tonya Mork added 3 commits August 22, 2018 21:00

Show notice only when the cache has been disabled due to exceeding th…

e575cde

…reshold.

Add description for the enable checkbox.

f6716ec

Fix self reference for local constant.

bf3dd20

westonruter added 2 commits August 23, 2018 15:50

Rename response cache to post-processor cache

cdd0506

Bump CACHE_MISS_THRESHOLD from 3 to 20

65ad9fa

hellofromtonya commented Aug 23, 2018

View reviewed changes

westonruter approved these changes Aug 23, 2018

View reviewed changes

westonruter merged commit 234b722 into develop Aug 23, 2018

westonruter deleted the improve/post-processor-response-cache branch August 23, 2018 23:03

westonruter mentioned this pull request Apr 8, 2019

Detect high cache miss rate for obtaining parsed CSS from variable stylesheets #2092

Closed

westonruter mentioned this pull request Jan 25, 2020

Eliminate post-processor cache #4178

Closed

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Improve post processor response cache #1325

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Improve post processor response cache #1325

Improve post processor response cache #1325

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