HTTP Accelerator/Reverse Proxy

Type of proxy server that retrieves resources on behalf of a client. Resources are returned to the client as if they originated from the proxy server rather than its upstreams.

Forward proxies act as an intermediary for the client, a Reverse proxy acts as an intermediary for groups of servers.

Varnish

Designed for content heavy dynamic web sites, focused exclusively on HTTP. Unlike others, varnish was designed as an HTTP accelerator.

• supports load balancing, with Saint mode for handling failing upstream servers by blacklisting them for quarantine periods
• configuration mechanism is Varnish Configuration Language, a DSL used to write hooks that are called when handlign requests
• VCL is compiled to C
• supports Gzip compression
• If the server malfunctions and returns 50x errors, Grace mode will ignore expiry headers and continue to return cached versions

Squid

Can be used as a reverse proxy, however the technology is slightly older than Varnish, and it was originally built as a forward proxy. It has builtin ssl which Varnish does not.