How Google improved ads performance with stale-while-revalidate
Find out how loading third-party resources faster can increase revenue.
This case study highlights how improving the performance of third-party resources can boost business metrics. While a previous study measured the cost of added ads latency, this study demonstrates the value of a real-world performance improvement:
0.5%
Revenue lift for publishers
2%
Increase in early ad script loads
Source: Google Internal Data, June to July 2019.
Background #
The Google Publisher Tag (GPT) is the ad tagging script for Google Ad Manager that requests and renders display ads on the web. By implementing a simple stale-while-revalidate HTTP header for GPT, the GPT team was able to improve the speed and performance of Google display ads for its publisher partners. This same technique can be applied to any other scenario where loading scripts as quickly as possible is more important than loading the freshest code.
The problem #
GPT is deployed as a bootstrapping script, gpt.js, which is given a short time to live (TTL) of 15 minutes. This short TTL allows the script to be updated or rolled back quickly. Once loaded, gpt.js requests and loads additional implementation scripts, which have a longer TTL.
Once the 15 minute TTL expires, the version of gpt.js in cache goes stale and needs to be revalidated. Previously, this revalidation process involved making a synchronous network request to fetch a fresh copy of the script, adding latency to the first ad request.
The solution #
The stale-while-revalidate attribute is used by the Cache-Control header and defines an extra window of time during which a cache can use a stale asset while the asset is revalidated asynchronously. This helps developers balance between immediacy—loading cached content right away—and
freshness—ensuring updates to the cached content are used in the future.
Google display ads case study #
The GPT team added this Cache-Control header in the gpt.js HTTP response in 2016, in anticipation of browsers implementing stale-while-revalidate:
cache-control: private, max-age=900, stale-while-revalidate=3600This setting means that if gpt.js is requested between 15 and 60 minutes after the previous cached value, then the cached value will be used to fulfill the request even though it's stale. At the same time, a revalidation request will be made in the background to populate the cache with a fresh value for future use.
Chrome rolled out stale-while-revalidate in version 75 to 99% of all traffic, leaving 1% of traffic with the feature disabled temporarily to measure its impact. The GPT team logged metrics from this 1% (the experimental group) as well as a 1% sample of traffic with the feature enabled (the control group), to test the effectiveness of stale-while-revalidate for ad scripts. Over the course of 2 weeks of metrics logged from a sample size of 5.2 billion Google display ad impressions, the control group observed:
- 0.3% increase in ad impressions.
- 0.5% increase in revenue.
- 2% increase in early ad script loads (<500ms from the start of page load).
- 1.1% increase in successful ad script loads overall.
As shown in the chart above, the results of this experiment can be attributed to an increase in successful ad script loads, with a majority occurring early in the page load process.
Implementing stale-while-revalidate on your site #
The GPT team has seen that making a relatively simple change to HTTP headers with stale-while-revalidate can improve speed and boost business metrics. Check out the Keeping things fresh with stale-while-revalidate post for more on implementing stale-while-revalidate on your own site.