In order to be able to tell when a thumbnail has been prerendered and is only being pulled from swift to varnish, we need to track this extra response header. Age and Date, which are also needed, are already logged.
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- rMEXT63e47d0af961: Updated mediawiki/extensions Project: mediawiki/extensions/MultimediaViewer…
rMW0813fbddddfb: Backport Last-Modified performance logging for Media Viewer
rEMMV628dd8a87854: Log Last-Modified header in performance logger
rEMMVb3e281237c1d: Log Last-Modified header in performance logger
T78205: Create MediaViewer image varnish hit/miss ratio dashboard
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- P163 Wrong query, but will be useful as a reference later when we have the Date header information
Actually, as @Tgr pointed out, Varnish's X-Timestamp is the same as Last-Modified, and we're already logging that. Assuming that the clocks of varnish servers and eventlogging http (sql?) servers are in sync, "Date" should be the same as the "timestamp" column of the EL table. I'll run some SQL queries to see if that all holds true.
Actually timestamp != Date for one very obvious reason: the EL event will only be recorded after the image load, and will depend on latency. So we do need to log "Date". We don't really need Last-Modified since it should be identical to X-Timestamp, but I'll add it just in case.
I think timestamp is added by Varnish when it receives the EL request, so it would equal the Date header of the EL request, not the Date header of the thumbnail request that's being logged?