Java master Banzen was reviewing the code of a web
application when he came across a new HTTP filter authored
by his apprentice Satou. The old master called her to his
office.
“You have begun including an ‘ETag’ header with each page
you send,” said Banzen to the nun. “Explain, for I have not
heard of this header.”
“The ETag is a hash of the page contents,” said Satou with
pride, glad to know something her master did not. “I
calculate it just before I return the requested page to the
browser.”
“What is its purpose?” asked Banzen.
“An efficiency hack,” replied Satou. “If the browser asks
for that same page later, it can include the last ETag with
its request, effectively saying: I still possess this
content that you sent me earlier. If the server then
determines that a new response would have the same ETag, it
discards the response and answers only: Nothing has
changed.”
“Ah,” said Banzen, squinting at Satou’s code. “I notice that
you obtain your ‘hash of the page contents’ by loading them
into a byte-array called contents and then invoking
contents.hashCode(). Are you certain that this will be a
good digest value?”
Satou thought a moment. “The hashCode() method returns a
four-byte int, so the chances of collision should be about
four billion to one. I believe these are acceptable odds.”
The Java master congratulated his apprentice on her
reasoning and bade her wait a few minutes while he fetched a
suitable reward. Banzen returned with a box of assorted
chocolates.
“Take one,” he said.
The chocolates were indistinguishable, but they were laid
out in neat rows and columns with a legend on the lid of the
box. Satou found a square labeled Cherry and popped the
corresponding chocolate in her mouth.
Her expression changed from delight to confusion to horror,
but she was too late to stop herself from swallowing the
confection.
“That... wasn’t cherry...” she said, her pale face growing
much paler.
“Of course not!” laughed Banzen. “‘Cherry’ is only the name
of the square it was in. The contents of the chocolate are
anyone’s guess. Here,” he said, moving a different chocolate
to the vacated square. “If you liked that, have another.”
