ExtractorChrome: reduce request duplication between browser and frontier #416
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By intercepting the browser's request and fulfilling it using the response previously recorded by FetchHTTP we avoid sending duplicate requests for the CrawlURI to the web server. A size limit (maxReplayLength) is applied as a safety measure since the browser's Fetch.fulfillRequest API requires us to load the entire response body into memory. Note: This only applies to the main CrawlURI. The browser can still make duplicate requests when loading sub-resources. Solving this for sub-resources will require implementing the ability to read back previously written WARC records.
This ensures we discover links in subresources even if the browser doesn't happen to load them. For example a CSS file might link to images that the browser won't load as they're gated by media queries.
Since we now run extractors on subresources there's no reason to schedule and refetch them again. Note that duplicate fetches can still occur if the URI was already scheduled or if the browser itself refetches the resource.
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By intercepting the browser's request and fulfilling it using the response previously recorded by FetchHTTP we avoid the browser sending a duplicate request for the main CrawlURI to the web server.
By running the link extractors on subresources captured by the browser we can now also tell the frontier not to bother scheduling them for classic fetching.
Note: The browser will still make duplicate requests for previously downloaded subresources. Solving this will require implementing resource caching or some way to read back previously written WARC records.