make entries with legacy class names backwards compatible

README.md

@@ -64,6 +64,12 @@ If previously the `akka-persistence-mongo` library was used together with Akka a
 complete stack should be migrated to Pekko, Pekko-Persistence and `pekko-persistence-mongo` as a replacement, the
 following section describes migration steps to take.
 
+If you are using a feature of Akka that uses persistence under the hood and potentially persists the full Akka class name as a part
+of the persistence data (classic PersistentFSM is a known example), then this module will implicitly translate that data when reading
+to the Pekko class name to be able to deserialize it and keep compatibility. The opposite is true if you migrate and find an issue with
+your Pekko migration and need to roll back, but your application has already persisted entries with the new Pekko class name. The same functionality
+has been backported to allow rolling back your application, but you must be on `akka-persistence-mongo` version **4.0.0** to have this translation.
+
 The root configuration key changed from `akka` to `pekko`, so adjust your configuration accordingly, e.g.  
 from:
 ```hocon

scala/src/main/scala/pekko/contrib/persistence/mongodb/MongoDataModel.scala

@@ -8,7 +8,7 @@ import org.apache.pekko.actor.ActorRef
 import org.apache.pekko.persistence.journal.Tagged
 import org.apache.pekko.persistence.query.{EventEnvelope, Offset}
 import org.apache.pekko.persistence.{AtomicWrite, PersistentRepr}
-import org.apache.pekko.serialization.{Serialization, SerializerWithStringManifest}
+import org.apache.pekko.serialization.{Serialization, Serializer, SerializerWithStringManifest}
 
 import scala.collection.immutable.{Seq => ISeq}
 import scala.util.{Failure, Success, Try}
@@ -48,17 +48,30 @@ case class Serialized[C <: AnyRef](bytes: Array[Byte],
 
   val hint = "ser"
   lazy val content: C = {
-
     val clazz = loadClass.getClassFor[X forSome { type X <: AnyRef }](className)
 
-    val tried = (serializedManifest,serializerId,clazz.flatMap(c => Try(ser.serializerFor(c)))) match {
+    Try(tryDeserialize(clazz, clazz.flatMap(c => Try(ser.serializerFor(c)))))
+      .recover({
+        case _ if className.startsWith("akka.") =>
+          val backwardsCompatClazz = loadClass.getClassFor[X forSome { type X <: AnyRef }](className.replaceFirst("akka", "org.apache.pekko"))
+          tryDeserialize(backwardsCompatClazz, backwardsCompatClazz.flatMap(c => Try(ser.serializerFor(c))))
+        case x => throw x
+      }) match {
+      case Failure(x) => throw x
+      case Success(deser) => deser
+    }
+  }
+
+  private def tryDeserialize(clazz: Try[Class[_ <: X forSome {type X <: AnyRef}]],
+                             serializer: Try[Serializer]): C = {
+    val tried = (serializedManifest, serializerId, serializer) match {
       // Manifest was serialized, class exists ~ prefer read-time configuration
       case (Some(manifest), _, Success(clazzSer)) =>
         ser.deserialize(bytes, clazzSer.identifier, manifest)
 
       // No manifest id serialized, prefer read-time configuration
       case (None, _, Success(clazzSer)) =>
-        ser.deserialize[X forSome { type X <: AnyRef }](bytes, clazzSer.identifier, clazz.toOption)
+        ser.deserialize[X forSome {type X <: AnyRef}](bytes, clazzSer.identifier, clazz.toOption)
 
       // Manifest, id were serialized, class doesn't exist - use write-time configuration
       case (Some(manifest), Some(id), Failure(_)) =>
@@ -67,11 +80,11 @@ case class Serialized[C <: AnyRef](bytes: Array[Byte],
       // Below cases very unlikely to succeed
 
       // No manifest id serialized, class doesn't exist - use write-time configuration
-      case (None, Some(id),Failure(_)) =>
-        ser.deserialize[X forSome { type X <: AnyRef }](bytes, id, clazz.toOption)
+      case (None, Some(id), Failure(_)) =>
+        ser.deserialize[X forSome {type X <: AnyRef}](bytes, id, clazz.toOption)
 
       // fall back
-      case (_,None, Failure(_)) =>
+      case (_, None, Failure(_)) =>
         ser.deserialize(bytes, clazz.get)
     }