What is Error 503, Guru mediation, that led to global Internet outage

At 4 pm on Tuesday, a major Internet outage caused several prominent websites to collapse with the search throwing up ‘Error 503, Unavailable server error”. Among the websites that crashed are those of the UK government and media websites NYT, Quora, guardian, Al Jazeera and e-commerce website amazon.uk.

So, what is Error 503, Guru mediation?

The error 503 means that the Varnish Cache is unable to reach the back end server. The Guru meditation error occurs when the varnish cache makes too many requests and does not get any response from the server.

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503 Service Unavailable server error response code indicates that the server is not ready to handle the request. The most common cause of the 503 error is a breakdown in communication between the server and the website it is supporting, resulting in that website being unable to handle any information requests from a user’s browser. This could have been due to scheduled server maintenance or by some unforeseen technical issue.