Drupal 8’s Dynamic Page Cache

Drupal 8 now has a Dynamic Page Cache. The Page Cache module only works for anonymous users, the Dynamic Page Cache module takes that a step further: it works for any user.

Since April 8, Drupal 8 had its Page Cache enabled by default. Exactly 5 months later, on September 8, the Dynamic Page Cache¹ module was added to Drupal 8, and also enabled by default.

What?

The Page Cache module caches fully rendered HTML responses — it assumes only one variant of each response exists, which is only true for anonymous users². The innovation in 8 on top of 7’s Page Cache is the addition of cache tags, which allow one to use the Page Cache but still have instantaneous updates: no more stale content.

The Dynamic Page Cache module caches mostly rendered HTML responses — because it does not assume only a single variant exists: thanks to cache contexts, it knows the different variants that exist of each part of the page, and thus also of the final (fully assembled) page. During the rendering process, auto-placeholdering ensures that the parts of the page that are too dynamic to cache³ or are simply uncacheable are replaced with placeholders. These placeholders are only rendered at the very last moment. The Dynamic Page Cache module caches the page just before those placeholders are replaced.

So, conceptually speaking:

This table also illustrates quite clearly the strengths and weaknesses of both: Page Cache is faster because it contains the final response, unlike Dynamic Page Cache which only contains a partial response, but thanks to that response not yet being personalized it can be used across users.

In other words: Dynamic Page Cache saves us less work, but in doing so, allows us to apply it in many more cases.

Benchmark

Drupal 8’s Page Cache can respond in constant time. So, for anonymous users, Drupal 8 already responds in constant (amortized) time.

With Dynamic Page Cache, Drupal 8 is now able to respond in constant amortized time + the time it takes to render the placeholders. Which means that most parts of most pages can be cached in the constant part (and it doesn’t matter how much content is in there!). The parts of the page that end up as placeholders are the ones you want to optimize, and if possible, avoid. (Also see BigPipe further down.)

On my machine (ab -c1 -n 1000, PHP 5.5.11, Intel Core i7 2.8 GHz laptop, warm caches, 15 nodes with one comment each, 10 of which are listed on the frontpage):

• Without Dynamic Page Cache
  • Front page: 61.3 ms/request (16 requests/s)
  • node/1: 92.3 ms/request (10.8 requests/s)
• With Dynamic Page Cache
  • Front page: 38.3 ms/request (26 requests/s)
  • node/1: 73.8 ms/request (13.5 requests/s)

Analysis

The front page only contains a single placeholder (for messages). This makes it is a strong indicator of the lowest response times to expect: roughly 38 ms on my machine.

• ~38 ms goes to bootstrapping, routing, route access checking, and of course retrieving the cached response from the Dynamic Page Cache.⁴
• Compare that with the node/1 case: 35.5 more milliseconds are spent replacing placeholders there. Which means that we do some very expensive rendering there… and that is the comment form.⁵

As you can see: Dynamic Page Cache actually becomes a helpful assistant while developing: it surfaces the impossible-to-cache-and-expensive-to-render parts of the page!

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