Information Technology Reference
In-Depth Information
To illustrate this, imagine you are building an email system that needs to be able to re-
trieve a message from the message storage system and display it within 300 ms. We will
use the time approximations listed in Figure 1.10 to help us engineer the solution.
Figure 1.10: Numbers every engineer should know
First we follow the transaction from beginning to end. The request comes from a web
browser that may be on another continent. The request must be authenticated, the database
indexisconsultedtodeterminewheretogetthemessagetext,themessagetextisretrieved,
and finally the response is formatted and transmitted back to the user.
Now let's budget for the items we can't control. To send a packet between California
and Europe typically takes 75 ms, and until physics lets us change the speed of light that
won't change. Our 300 ms budget is reduced by 150 ms since we have to account for not
only the time it takes for the request to be transmitted but also the reply. That's half our
budget consumed by something we don't control.
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