Document plan execution across nodes (elastic#125422)

Author: idegtiarenko

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plugin/ComputeService.java

@@ -68,7 +68,45 @@
 import static org.elasticsearch.xpack.esql.plugin.EsqlPlugin.ESQL_WORKER_THREAD_POOL_NAME;
 
 /**
- * Computes the result of a {@link PhysicalPlan}.
+ * Once query is parsed and validated it is scheduled for execution by {@code org.elasticsearch.xpack.esql.plugin.ComputeService#execute}
+ * This method is responsible for splitting physical plan into coordinator and data node plans.
+ *
+ * Coordinator plan is immediately executed locally (using {@code org.elasticsearch.xpack.esql.plugin.ComputeService#runCompute})
+ * and is prepared to collect and merge pages from data nodes into the final query result.
+ *
+ * Data node plan is passed to {@code org.elasticsearch.xpack.esql.plugin.DataNodeComputeHandler#startComputeOnDataNodes}
+ * that is responsible for
+ * <ul>
+ * <li>
+ *     Determining list of nodes that contain shards referenced by the query with
+ *     {@code org.elasticsearch.xpack.esql.plugin.DataNodeRequestSender#searchShards}
+ * </li>
+ * <li>
+ *     Each node in the list processed in
+ *     {@code org.elasticsearch.xpack.esql.plugin.DataNodeComputeHandler#startComputeOnDataNodes}
+ *     in order to
+ *     <ul>
+ *     <li>
+ *         Open ExchangeSink on the target data node and link it with local ExchangeSource for the query
+ *         using `internal:data/read/esql/open_exchange` transport request.
+ *         {@see org.elasticsearch.compute.operator.exchange.ExchangeService#openExchange}
+ *     </li>
+ *     <li>
+ *         Start data node plan execution on the target data node
+ *         using `indices:data/read/esql/data` transport request
+ *         {@see org.elasticsearch.xpack.esql.plugin.DataNodeComputeHandler#messageReceived}
+ *         {@see org.elasticsearch.xpack.esql.plugin.DataNodeComputeHandler#runComputeOnDataNode}
+ *     </li>
+ *     <li>
+ *         While coordinator plan executor is running it will read data from ExchangeSource that will poll pages
+ *         from linked ExchangeSink on target data nodes or notify them that data set is already completed
+ *         (for example when running FROM * | LIMIT 10 type of query) or query is canceled
+ *         using `internal:data/read/esql/exchange` transport requests.
+ *         {@see org.elasticsearch.compute.operator.exchange.ExchangeService.ExchangeTransportAction#messageReceived}
+ *     </li>
+ *     </ul>
+ * </li>
+ * </ul>
  */
 public class ComputeService {
     public static final String DATA_ACTION_NAME = EsqlQueryAction.NAME + "/data";