MARKLOGIC SERVER

database allowed to have most of its documents in one forest would do most of the
work on that forest's host, whereas redistributing the documents will spread the work
more evenly across all of the hosts. In this way, rebalancing results in more efficient use
of hardware and improved cluster performance.
Rebalancing is triggered by any reconfiguration of a database, such as when forests are
added or retired. 18 If you plan to detach and delete a forest, you'll want to retire it first.
This ensures that any content in that forest first gets redistributed among the remaining
forests. If you just detach and delete a forest without retiring it, any content in that
deleted forest is lost.
UNREBALANCED
REBALANCED
REBALANCED
AFTER ADDITION
REBALANCED
AFTER RETIREMENT
DATABASE
DATABASE
DATABASE
DATABASE
Forest-1
700
Forest-1
450
Forest-1
300
Forest-1
450
Forest-2
200
REBALANCING
ENABLED
Forest-2
450
FOREST
ADDED
Forest-2
300
FOREST
RETIRED
Forest-2
0
Forest-3
300
Forest-3
450
Figure 23: Rebalancing keeps documents evenly distributed among a database's forests.
ASSIGNMENT POLICIES
Rebalancing works based on an assignment policy, which is a set of rules that determine
what document goes into what forest. A database's assignment policy applies to
rebalancing as well as how documents are distributed to forests in the first place
during ingest.
An assignment policy defines how data can be distributed horizontally across multiple
hosts to improve performance. This is what MarkLogic is doing when it allocates
documents in a database across multiple forests and those forests exist on different hosts.
But even if you're running MarkLogic on a single host, spreading documents across
more than one forest can improve performance because it takes advantage of parallel
processing on a host's multiple cores.
There are four assignment policies: bucket, legacy, statistical, and range.
The bucket policy (the default) uses an algorithm to map a document's URI to one of
16,000 "buckets," with each bucket being associated with a forest. (A table mapping
18 You can control how aggressively rebalancing occurs by setting a throttle value, which establishes the rebalancer's
priority when it comes to the use of system resources.
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