What it does is basically to cache incomming requests in order to reply rapidly to repeated requests, thus taking a load of your servers and possibly reduce the need for a clustered solution.
Again there are many aspects on solutions like this, but if you “need more power” (as Cpt. Kirk always said to Scotty in Star-Trek then this may be a possible road to go down).
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Varnish Cache is a web application accelerator also known as a caching HTTP reverse proxy. You install it in front of any server that speaks HTTP and configure it to cache the contents. Varnish Cache is really, really fast. It typically speeds up delivery with a factor of 300 - 1000x, depending on your architecture. A high level overview of what Varnish does can be seen in the video attached to this web page.
Link to video that explain what it’s all about (in VERY general terms 😀 but still)..
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