Passing bookmark to rediscover service

For bolt v4, when bookmark is passed into a session, if getRT call is invoked for that session, then the bookmark is passed down to the procedure call.
Changed bookmark to be an object. The content of the object is transparent to driver users.

driver/src/main/java/org/neo4j/driver/Session.java

@@ -21,6 +21,7 @@
 import java.util.Map;
 
 import org.neo4j.driver.async.AsyncSession;
+import org.neo4j.driver.internal.Bookmark;
 import org.neo4j.driver.util.Resource;
 
 /**
@@ -212,7 +213,7 @@ public interface Session extends Resource, StatementRunner
      *
      * @return a reference to a previous transaction
      */
-    String lastBookmark();
+    Bookmark lastBookmark();
 
     /**
      * Reset the current session. This sends an immediate RESET signal to the server which both interrupts

driver/src/main/java/org/neo4j/driver/SessionConfig.java

@@ -19,11 +19,11 @@
 package org.neo4j.driver;
 
 import java.util.Arrays;
-import java.util.List;
 import java.util.Objects;
 import java.util.Optional;
 
 import org.neo4j.driver.async.AsyncSession;
+import org.neo4j.driver.internal.Bookmark;
 import org.neo4j.driver.reactive.RxSession;
 
 import static java.util.Objects.requireNonNull;
@@ -36,7 +36,7 @@ public class SessionConfig
 {
     private static final SessionConfig EMPTY = builder().build();
 
-    private final List<String> bookmarks;
+    private final Iterable<Bookmark> bookmarks;
     private final AccessMode defaultAccessMode;
     private final String database;
 
@@ -85,7 +85,7 @@ public static SessionConfig forDatabase( String database )
      *
      * @return the initial bookmarks.
      */
-    public List<String> bookmarks()
+    public Iterable<Bookmark> bookmarks()
     {
         return bookmarks;
     }
@@ -143,7 +143,7 @@ public String toString()
      */
     public static class Builder
     {
-        private List<String> bookmarks = null;
+        private Iterable<Bookmark> bookmarks = null;
         private AccessMode defaultAccessMode = AccessMode.WRITE;
         private String database = null;
 
@@ -164,11 +164,10 @@ private Builder()
          * are permitted, and indicate that the bookmarks do not exist or are unknown.
          * @return this builder.
          */
-        public Builder withBookmarks( String... bookmarks )
+        public Builder withBookmarks( Bookmark... bookmarks )
         {
             if ( bookmarks == null )
             {
-                // TODO bookmarks should not be null
                 this.bookmarks = null;
             }
             else
@@ -189,7 +188,7 @@ public Builder withBookmarks( String... bookmarks )
          * are permitted, and indicate that the bookmarks do not exist or are unknown.
          * @return this builder
          */
-        public Builder withBookmarks( List<String> bookmarks )
+        public Builder withBookmarks( Iterable<Bookmark> bookmarks )
         {
             this.bookmarks = bookmarks;
             return this;

driver/src/main/java/org/neo4j/driver/async/AsyncSession.java

@@ -29,6 +29,7 @@
 import org.neo4j.driver.Transaction;
 import org.neo4j.driver.TransactionConfig;
 import org.neo4j.driver.Values;
+import org.neo4j.driver.internal.Bookmark;
 
 /**
  * Provides a context of work for database interactions.
@@ -301,7 +302,7 @@ public interface AsyncSession extends AsyncStatementRunner
      *
      * @return a reference to a previous transaction
      */
-    String lastBookmark();
+    Bookmark lastBookmark();
 
     /**
      * Signal that you are done using this session. In the default driver usage, closing and accessing sessions is

driver/src/main/java/org/neo4j/driver/internal/Bookmark.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2002-2019 "Neo4j,"
+ * Neo4j Sweden AB [http://neo4j.com]
+ *
+ * This file is part of Neo4j.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.neo4j.driver.internal;
+
+/**
+ * Causal chaining is carried out by passing bookmarks between transactions.
+ *
+ * When starting a session with initial bookmarks, the first transaction will be ensured to run at least after
+ * the database is as up-to-date as the latest transaction referenced by the supplied bookmarks.
+ *
+ * Within a session, bookmark propagation is carried out automatically.
+ * Thus all transactions in a session including explicit and implicit transactions are ensured to be carried out one after another.
+ *
+ * To opt out of this mechanism for unrelated units of work, applications can use multiple sessions.
+ */
+public interface Bookmark
+{
+}

driver/src/main/java/org/neo4j/driver/internal/BookmarksHolder.java → driver/src/main/java/org/neo4j/driver/internal/BookmarkHolder.java

@@ -18,31 +18,23 @@
  */
 package org.neo4j.driver.internal;
 
-public interface BookmarksHolder
+public interface BookmarkHolder
 {
-    Bookmarks getBookmarks();
+    InternalBookmark getBookmark();
 
-    void setBookmarks( Bookmarks bookmarks );
+    void setBookmark( InternalBookmark bookmark );
 
-    String lastBookmark();
-
-    BookmarksHolder NO_OP = new BookmarksHolder()
+    BookmarkHolder NO_OP = new BookmarkHolder()
     {
         @Override
-        public Bookmarks getBookmarks()
-        {
-            return Bookmarks.empty();
-        }
-
-        @Override
-        public void setBookmarks( Bookmarks bookmarks )
+        public InternalBookmark getBookmark()
         {
+            return InternalBookmark.empty();
         }
 
         @Override
-        public String lastBookmark()
+        public void setBookmark( InternalBookmark bookmark )
         {
-            return null;
         }
     };
 }