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Solr Caches - solrconfig.xml

Another useful parameter in solrconfig.xml is the updateHandler.

Some important options for the same:
  1. Transaction log used to ensure reliability of updates.
  2. Auto-commit options: maxDocs, maxTime, openSearcher etc.
  3. Difference between hard and soft commits.
  4. Event listeners: postCommit, postOptimize.

<!-- The default high-performance update handler -->
  <updateHandler class="solr.DirectUpdateHandler2">

    <!-- Enables a transaction log, used for real-time get, durability, and           
         solr cloud replica recovery.  The log can grow as big as
         uncommitted changes to the index, so use of a hard autoCommit
         is recommended (see below).
         "dir" - the target directory for transaction logs, defaults to the
                solr data directory.  --> 
      <str name="dir">${solr.ulog.dir:}</str>
    <!-- AutoCommit

         Perform a hard commit automatically under certain conditions.
         Instead of enabling autoCommit, consider using "commitWithin"
         when adding documents. 

         maxDocs - Maximum number of documents to add since the last
                   commit before automatically triggering a new commit.

         maxTime - Maximum amount of time in ms that is allowed to pass
                   since a document was added before automatically
                   triggering a new commit. 
         openSearcher - if false, the commit causes recent index changes
           to be flushed to stable storage, but does not cause a new
           searcher to be opened to make those changes visible.

         If the updateLog is enabled, then it's highly recommended to
         have some sort of hard autoCommit to limit the log size.

    <!-- softAutoCommit is like hard-commit except the changes committed in
         soft commit are not persisted to the disk. Due to this, soft commit
         is faster and more near-real-time (NRT) friendly than a hard commit.


    <!-- Update Related Event Listeners
         Various IndexWriter related events can trigger Listeners to
         take actions.

         postCommit - fired after every commit or optimize command
         postOptimize - fired after every optimize command
    <!-- The RunExecutableListener executes an external command from a
         hook such as postCommit or postOptimize.
         exe - the name of the executable to run
         dir - dir to use as the current working directory. (default=".")
         wait - the calling thread waits until the executable returns. 
         args - the arguments to pass to the program.  (default is none)
         env - environment variables to set.  (default is none)
    <!-- This example shows how RunExecutableListener could be used
         with the script based replication...
       <listener event="postCommit" class="solr.RunExecutableListener">
         <str name="exe">solr/bin/snapshooter</str>
         <str name="dir">.</str>
         <bool name="wait">true</bool>
         <arr name="args"> <str>arg1</str> <str>arg2</str> </arr>
         <arr name="env"> <str>MYVAR=val1</str> </arr>


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