Designing in a Distributed World - The Practice of Cloud System Administration - page 14

Figure 1.6: State updates using cached data lead to an inconsistent view.
In the most simple pattern, a root server receives requests to store or retrieve state. It
determines which shard contains that part of the state and forwards the request to the ap-
propriate leaf server. The reply then flows up the tree. This looks similar to the server tree
pattern described in the previous section but there are two differences. First, queries go to
a single leaf instead of all leaves. Second, requests can be update (write) requests, not just
read requests. Updates are more complex when a shard is stored on many replicas. When
one shard is updated, all of the replicas must be updated, too. This may be done by having
the root update all leaves or by the leaves communicating updates among themselves.
A variation of that pattern is more appropriate when large amounts of data are being
transferred. Inthiscase, therootreplies withinstructions onhowtogetthedata rather than
the data itself. The requestor then requests the data from the source directly.