Document how to load your own streaming plugins (#1025)

solr/core/src/java/org/apache/solr/handler/StreamHandler.java

@@ -61,6 +61,19 @@ import org.slf4j.LoggerFactory;
 import static org.apache.solr.common.params.CommonParams.ID;
 
 /**
+ * <p>
+ * Solr Request Handler for streaming data.
+ * </p>
+ * <p>
+ * It loads a default set of mappings via {@link org.apache.solr.handler.SolrDefaultStreamFactory}.
+ * </p>
+ * <p>
+ * To add additional mappings, just define them as plugins in solrconfig.xml via
+ * {@code
+ * &lt;expressible name="count" class="org.apache.solr.client.solrj.io.stream.RecordCountStream" /&gt;
+ * }
+ * </p>
+ *
  * @since 5.1.0
  */
 public class StreamHandler extends RequestHandlerBase implements SolrCoreAware, PermissionNameProvider {
@@ -83,17 +96,6 @@ public class StreamHandler extends RequestHandlerBase implements SolrCoreAware,
   }
 
   public void inform(SolrCore core) {
-
-    /*
-     * The stream factory will always contain the zkUrl for the given collection Adds default streams with their
-     * corresponding function names. These defaults can be overridden or added to in the solrConfig in the stream
-     * RequestHandler def. Example config override
-     * <lst name="streamFunctions">
-     *  <str name="group">org.apache.solr.client.solrj.io.stream.ReducerStream</str>
-     *  <str name="count">org.apache.solr.client.solrj.io.stream.RecordCountStream</str>
-     * </lst>
-     */
-
     String defaultCollection;
     String defaultZkhost;
     CoreContainer coreContainer = core.getCoreContainer();

solr/solr-ref-guide/src/streaming-expressions.adoc

@@ -123,6 +123,16 @@ for the entire expression, it may be faster for the client to send the expressio
 `&streamLocalOnly=true` and handle merging of the results (if required) locally. This is an advanced option, relying
 on a convenient organization of the index, and should only be considered if normal usage poses a performance issue.
 
+=== Adding Custom Expressions
+
+Creating your own custom expressions can be easily done by implementing the {solr-javadocs}/solr-solrj/org/apache/solr/client/solrj/io/stream/expr/Expressible.html[Expressible] interface.   To add a custom expression to the
+list of known mappings for the `/stream` handler, you just need to declare it as a plugin in `solrconfig.xml` via:
+
+[source,xml]
+<expressible name="custom" class="org.example.CustomStreamingExpression"/> 
+
+
+
 == Types of Streaming Expressions
 
 === About Stream Sources