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“Varnish seems to be serving an old asset after a deploy. Where should we look to make it serve the new asset we just deployed?”
You should be looking at your web server’s cache settings. If you are cache busting assets on deploy (built in to Rails 2 & 3) then Varnish should always serve the latest assets. You can also use cache headers to determine the TTL Varnish will use, although a “cache forever” + cache bust strategy should be used if possible.