Information Technology Reference
In-Depth Information
There are a few solutions to this problem. First, usually each user's email is stored in
two different datacenters. That way, if one datacenter goes down, the data is still available.
Now there are twice as many chances that Mary will be directed to a datacenter with her
email, but there is still a chance that her HTTP requests will reach the wrong datacenter.
To resolve this dilemma, the frontend communicates with the email service to find out
where Mary's email is and, from then on, accesses the application servers in the correct
datacenter. The web frontends are generic, but they pull email from the specific datacenter.
Todothisthecompanymusthaveconnectionsbetweeneachdatacentersothatthefron-
tendscantalktoanyapplicationserver.Theycouldcommunicatebetweendatacentersover
the internet, but typically a company in this situation owns private, dedicated wide area
network (WAN) connections between data-centers. A dedicated WAN gives the company
more control and more reliable performance.
4.5.4 Internal Backbone
The private WAN links that connect datacenters form an internal backbone. An internal
backbone is not visible to the internet at large. It is a private network.
This internal network connects to the internet in many places. Wherever there is a data-
center,thedatacenterwillgenerallyconnecttomanyISPsinthearea.ConnectingtoanISP
directly has speed and cost benefits. If you do not have a direct connection to a particular
ISP, then sending data to users of that ISP involves sending the data through another ISP
thatconnectsyourISPandtheirs.ThisISPinthemiddleiscalleda transit ISP .Thetransit
ISPchargestheotherISPsfortheprivilegeofpermittingpacketstotraveloveritsnetwork.
There are often multiple transit ISPs between you and your customers. The more transits,
the slower, less reliable, and more expensive the connections become.
POPs
A point of presence (POP) is a small, remote facility used for connection to local ISPs.
It is advantageous to connect to many ISPs but they cannot always connect to your data-
center. For example, your datacenter may not be in the state or country they operate in.
Since they cannot go to you, you must extend your network to someplace near them. For
example, you might create a POP in Berlin to connect to many different German ISPs.
A POP is usually a rack of equipment in a colocation center or a small space that re-
semblesacloset.Itcontainsnetworkequipmentandconnectionsfromvarioustelecompro-
viders, but no general-purpose computers.
A POP plus a small number of computers is called a satellite . The computers are used
for frontend and content distribution services. The frontends terminate HTTP connections
and proxy to application servers in other datacenters. Content distribution servers are
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