=per #18288 Add docs of leveldb queries

akka-persistence/src/main/scala/akka/persistence/journal/leveldb/LeveldbJournal.scala

@@ -88,6 +88,8 @@ private[persistence] object LeveldbJournal {
    * Subscribe the `sender` to changes (appended events) for a specific `tag`.
    * Used by query-side. The journal will send [[TaggedEventAppended]] messages to
    * the subscriber when `asyncWriteMessages` has been called.
+   * Events are tagged by wrapping in [[akka.persistence.journal.leveldb.Tagged]]
+   * via an [[akka.persistence.journal.EventAdapter]].
    */
   final case class SubscribeTag(tag: String) extends SubscriptionCommand
   final case class TaggedEventAppended(tag: String) extends DeadLetterSuppression

akka-persistence/src/main/scala/akka/persistence/journal/leveldb/LeveldbStore.scala

@@ -63,6 +63,9 @@ private[persistence] trait LeveldbStore extends Actor with WriteJournalBase with
             }
             if (tags.nonEmpty && hasTagSubscribers)
               allTags ++= tags
+
+            require(!p2.persistenceId.startsWith(tagPersistenceIdPrefix),
+              s"persistenceId [${p.persistenceId}] must not start with $tagPersistenceIdPrefix")
             addToMessageBatch(p2, tags, batch)
           }
           if (hasPersistenceIdSubscribers)

akka-persistence/src/main/scala/akka/persistence/journal/leveldb/Tagged.scala

@@ -3,6 +3,9 @@
  */
 package akka.persistence.journal.leveldb
 
+import java.util.ArrayList
+import scala.collection.JavaConverters._
+
 /**
  * The LevelDB journal supports tagging of events that are used
  * by the `EventsByTag` query. To specify the tags you create an
@@ -11,4 +14,12 @@ package akka.persistence.journal.leveldb
  *
  * The journal will unwrap the event and store the `payload`.
  */
-case class Tagged(payload: Any, tags: Set[String])
+case class Tagged(payload: Any, tags: Set[String]) {
+
+  /**
+   * Java API
+   */
+  def this(payload: Any, tags: java.util.Set[String]) = {
+    this(payload, tags.asScala.toSet)
+  }
+}