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Figure 1.3: A server tree
To query the entire dataset, the root receives the original query and forwards it to the
parents. The parents forward the query to the leaf servers, which search their parts of the
corpus. Each leaf sends its findings to the parents, which sort and filter the results before
forwarding them up to the root. The root then takes the response from all the parents, com-
bines the results, and replies with the full answer.
Imagine you wanted to find out how many times George Washington was mentioned in
anencyclopedia. Youcouldreadeachvolumeinsequenceandarriveattheanswer.Altern-
atively, you could give each volume to a different person and have the various individuals
search their volumes in parallel. The latter approach would complete the task much faster.
The primary benefit of this pattern is that it permits parallel searching of a large corpus.
Not only are the leaves searching their share of the corpus in parallel, but the sorting and
ranking performed by the parents are also done in parallel.
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