Apache Solr Reference Guide Covering Apache Solr 6.0

2.
required between requests. The factory class must be thread-safe.
Every update request processor chain is constructed during loading of a Solr core and cached until the core is
unloaded. Each UpdateRequestProcessorFactory specified in the chain is also instantiated and initialized with
configuration that may have been specified in solrconfig.xml.
When an update request is received by Solr, it looks up the update chain to be used for this request. A new
instance of each UpdateRequestProcessor specified in the chain is created using the corresponding factory. The
update request is parsed into corresponding UpdateCommand objects which are run through the chain. Each
UpdateRequestProcessor instance is responsible for invoking the next plugin in the chain. It can choose to short
circuit the chain by not invoking the next processor and even abort further processing by throwing an exception.
A single update request may contain a batch of multiple new documents or deletes and therefore the
corresponding processXXX methods of an UpdateRequestProcessor will be invoked multiple times for
every individual update. However, it is guaranteed that a single thread will serially invoke these methods.
Configuration
Update request processors chains can be created by either creating the whole chain directly in solrconfig.xml or
by creating individual update processors in solrconfig.xml and then dynamically creating the chain at run-time by
specifying all processors via request parameters.
However, before we understand how to configure update processor chains, we must learn about the default
update processor chain because it provides essential features which are needed in most custom request
processor chains as well.
The default update request processor chain
In case no update processor chains are configured in solrconfig.xml, Solr will automatically create a default
update processor chain which will be used for all update requests. This default update processor chain consists
of the following processors (in order):
1.
2.
3.
LogUpdateProcessorFactory - Tracks the commands processed during this request and logs them
DistributedUpdateProcessorFactory - Responsible for distributing update requests to the right node e.g.
routing requests to the leader of the right shard and distributing updates from the leader to each replica. T
his processor is activated only in SolrCloud mode.
RunUpdateProcessorFactory - Executes the update using internal Solr APIs.
Each of these perform an essential function and as such any custom chain usually contain all of these
processors. The RunUpdateProcessorFactory is usually the last update processor in any custom chain.
Custom update request processor chain
The following example demonstrates how a custom chain can be configured inside solrconfig.xml.
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