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Varnish is a web accelerator designed to speed up web applications and improve website performance. It is a caching HTTP reverse proxy, meaning it sits in front of any web server and accelerates HTTP traffic to and from that server by caching, or storing, any content that is frequently requested by web clients. Varnish Configuration Language, or VCL, is used in conjunction with Varnish to allow developers to customize how Varnish manages web requests.
This is a request sent via HTTP (hypertext transfer protocol) from a client to a server. Browsers translate user actions, such as clicking on a hyperlink or submitting a form, into HTTP requests. The request is then sent on to the server, and the server sends an HTTP response to fulfill the request. An HTTP request also occurs when an application makes an API call.
HTTP caching is when a server or a browser saves a copy of a response to a user's HTTP request in order to produce quicker replies to future requests. A CDN server is an example of an HTTP cache. Some HTTP caching services enable developers to customize how HTTP caching works for their applications.
Service workers can respond to HTTP requests without contacting the web server
Service workers can modify HTTP requests and responses
One of the key differences between using Cloudflare Workers and using service workers is that service workers are run client-side and have to be downloaded by the user's browser, whereas Cloudflare Workers run in the edge of the Cloudflare network in between the user and the rest of the Internet, effectively running within the network itself.