The Web
HTTP client
Web browser
Uses MIME types.
(a) Plug-in - Runs inside browser process.
(b) Helper application - Separate process.
HTTP server
Doesn't make separate
disk access for every file request - too slow.
Instead maintains
cache in memory of frequently accessed files.
Multi-threaded.
Site spread over multiple disks
to help many reads going on at once.
For high-demand sites:
Multiple copies of entire site -
"server farm"
- front end routes requests to different CPUs.
Problem: OK to have all (small size) requests come in through one front end
and get routed to searching nodes.
Not OK to have all (large size) replies go back through one front end - bottleneck.
Solution: TCP handoff
- trick to have the searching node reply directly
in a manner that is invisible to client.
The reply load is therefore distributed over all the nodes.
Some URL formats.
URI schemes listed above:
- http, https
-
ftp (no password) hardly used any more - pre-web system
ftp (with password) still important
now sftp:
-
file:
very useful
(may not need prefix)
-
news:
not as important as used to be:
- read on
Google
- so many other places to talk now - discussion websites, blogs
- gopher - port 70
- not used any more - pre-web system
-
mailto: very useful
- but spammers search for these
-
telnet: (now ssh:) useful - but rarely do it by clicking a link
Others:
Keeping state
Relating one client-server stateless request
with other client-server requests.
Identify user (pay-to-view, register, personalisation).
Shopping carts.
Performance (client-side)
Caching
- Browser maintains cache.
- Site-wide (or ISP-wide) cache
via proxy server.
- wwwproxy.computing.dcu.ie
= 136.206.11.243
(forwards requests through 136.206.11.249)
- proxy.dcu.ie
= alternates between returning a few different IP addresses
(for load balancing)
- port: 8080 or 3128
- alternates between:
- proxy1.dcu.ie = 136.206.1.20
- proxy2.dcu.ie = 136.206.1.49
- proxy3.dcu.ie = 136.206.1.17
To set proxy, something like:
- Firefox - Tools - Options - Advanced - Network - Settings
- IE - Tools - Options - Connections - LAN settings
You may use a
proxy auto-config (PAC) file:
- http://www.computing.dcu.ie/proxy.pac
- http://proxy.dcu.ie/proxy.pac
Test the IP address other sites see: