We started to see an increase in WebSocket traffic recently without a substantial increase in HTTP traffic. Here is what we figured out:
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Googlebot has more than doubled the crawl of javascript compared to the last week of April.
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Googlebot now executes Javascript in real-time. Previously, the majority of our JS crawl was through sockjs endpoints, which resulted in 404s (this was due to Googlebot crawling JS but executing them later—sometimes up to a week delay—and the sockjs endpoints no longer exist). Now, all sockjs traffic results in 200s. Blocking sockjs resulted in search console-related features of Google failing (we assume that these bots use a Googlebot that executes javascript in real-time).
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Google updated its documentation related to Javascript. Dynamic rendering (e.g. using Prerender.io) is no longer recommended. What is recommended: server-side rendering, static rendering, hydration.
Google Search Makes Minor Updates To JavaScript Documentation & Drops Dynamic Rendering Workaround
Dynamic Rendering as a workaround | Google Search Central | Documentation | Google for Developers