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concurrently), spreading the load on the target with a load balancer (so more E-nodes<br />
can participate), and buying a bigger network pipe between clusters (speeding<br />
the delivery).<br />
For more information about Flexible Replication, see the Flexible Replication Guide<br />
and the flexrep functions documentation.<br />
QUERY-BASED FLEXIBLE REPLICATION<br />
Flexible Replication can also be combined with MarkLogic's alerting if you need<br />
fine-grained control over what documents get replicated to what nodes as well as who<br />
gets to see what based on permissions. This feature is known as Query-Based Flexible<br />
Replication (QBFR). QBFR is useful in the case of heterogeneous networks where large<br />
central servers are mixed with smaller systems, such as laptop computers out in the<br />
field. All of the systems run MarkLogic server, but the laptops only require a subset of<br />
the documents that are stored on the main servers. Use cases for QBFR include military<br />
deployments, where field units only need information specific to their location, and<br />
scientific applications, where remote researchers only need data related to their field<br />
of study.<br />
As described previously, alerting lets you define queries that specify which documents in<br />
a data set a user is interested in. When used in QBFR, the queries define the documents<br />
to be replicated from the master clusters to the replica clusters (which could be simple<br />
one-node clusters as in the laptop example). For example, queries in the military<br />
example might define geospatial coordinates, while queries in the scientific example<br />
might define technical keywords. Each replica cluster in the field has a query associated<br />
with it; when a document is inserted or updated on the central master cluster, a reverse<br />
query is run to determine which of the stored queries match the document. The<br />
matching queries determine to which clusters the document is replicated. Each stored<br />
query is also associated with a user, allowing role-based control of replication.<br />
Pull-based replication is typically the preferred method of operation with QBFR. This<br />
allows field clusters that might have times of limited or no connectivity to retrieve<br />
documents when they have network access. Replication can also be bi-directional.<br />
A field cluster can act as a master when it connects and transfer any new data it has<br />
collected back to the home base. For example, military field units might send back<br />
reconnaissance data, with the data's distribution controlled by additional queries.<br />
REBALANCING<br />
Rebalancing in MarkLogic redistributes content in a database so that the forests that<br />
make up the database each have a similar number of documents. Spreading documents<br />
evenly across forests lets you take better advantage of concurrency among hosts. A<br />
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