Treat one router as valid in routing table

driver/src/main/java/org/neo4j/driver/internal/cluster/ClusterRoutingTable.java

@@ -60,7 +60,7 @@ private ClusterRoutingTable( Clock clock )
     public boolean isStaleFor( AccessMode mode )
     {
         return expirationTimeout < clock.millis() ||
-               routers.size() <= MIN_ROUTERS ||
+               routers.size() < MIN_ROUTERS ||
                mode == AccessMode.READ && readers.size() == 0 ||
                mode == AccessMode.WRITE && writers.size() == 0;
     }

driver/src/test/java/org/neo4j/driver/internal/RoutingDriverBoltKitTest.java

@@ -963,22 +963,7 @@ public void shouldAcceptRoutingTableWithoutWritersAndThenRediscover() throws Exc
             // start another router which knows about writes, use same address as the initial router
             router2 = StubServer.start( "acquire_endpoints.script", 9010 );
 
-            List<String> names = session.readTransaction( new TransactionWork<List<String>>()
-            {
-                @Override
-                public List<String> execute( Transaction tx )
-                {
-                    List<Record> records = tx.run( "MATCH (n) RETURN n.name" ).list();
-                    List<String> names = new ArrayList<>( records.size() );
-                    for ( Record record : records )
-                    {
-                        names.add( record.get( 0 ).asString() );
-                    }
-                    return names;
-                }
-            } );
-
-            assertEquals( asList( "Bob", "Alice", "Tina" ), names );
+            assertEquals( asList( "Bob", "Alice", "Tina" ), readStrings( "MATCH (n) RETURN n.name", session ) );
 
             StatementResult createResult = session.run( "CREATE (n {name:'Bob'})" );
             assertFalse( createResult.hasNext() );
@@ -993,6 +978,35 @@ public List<String> execute( Transaction tx )
         }
     }
 
+    @Test
+    public void shouldTreatRoutingTableWithSingleRouterAsValid() throws Exception
+    {
+        StubServer router = StubServer.start( "discover_one_router.script", 9010 );
+        StubServer reader1 = StubServer.start( "read_server.script", 9003 );
+        StubServer reader2 = StubServer.start( "read_server.script", 9004 );
+
+        try ( Driver driver = GraphDatabase.driver( "bolt+routing://127.0.0.1:9010", config );
+              Session session = driver.session( AccessMode.READ ) )
+        {
+            // returned routing table contains only one router, this should be fine and we should be able to
+            // read multiple times without additional rediscovery
+
+            StatementResult readResult1 = session.run( "MATCH (n) RETURN n.name" );
+            assertEquals( "127.0.0.1:9003", readResult1.summary().server().address() );
+            assertEquals( 3, readResult1.list().size() );
+
+            StatementResult readResult2 = session.run( "MATCH (n) RETURN n.name" );
+            assertEquals( "127.0.0.1:9004", readResult2.summary().server().address() );
+            assertEquals( 3, readResult2.list().size() );
+        }
+        finally
+        {
+            assertEquals( 0, router.exitStatus() );
+            assertEquals( 0, reader1.exitStatus() );
+            assertEquals( 0, reader2.exitStatus() );
+        }
+    }
+
     private static Driver newDriverWithSleeplessClock( String uriString )
     {
         DriverFactory driverFactory = new DriverFactoryWithClock( new SleeplessClock() );

driver/src/test/java/org/neo4j/driver/internal/cluster/ClusterRoutingTableTest.java

@@ -26,6 +26,7 @@
 import org.neo4j.driver.internal.util.FakeClock;
 
 import static java.util.Arrays.asList;
+import static java.util.Collections.singletonList;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
@@ -181,4 +182,19 @@ public void shouldPreserveOrderingOfReaders()
         assertEquals( D, routingTable.readers().next() );
         assertEquals( B, routingTable.readers().next() );
     }
+
+    @Test
+    public void shouldTreatOneRouterAsValid()
+    {
+        ClusterRoutingTable routingTable = new ClusterRoutingTable( new FakeClock() );
+
+        List<BoltServerAddress> routers = singletonList( A );
+        List<BoltServerAddress> writers = asList( B, C );
+        List<BoltServerAddress> readers = asList( D, E );
+
+        routingTable.update( createClusterComposition( routers, writers, readers ) );
+
+        assertFalse( routingTable.isStaleFor( READ ) );
+        assertFalse( routingTable.isStaleFor( WRITE ) );
+    }
 }

driver/src/test/resources/discover_one_router.script

@@ -0,0 +1,9 @@
+!: AUTO INIT
+!: AUTO RESET
+!: AUTO PULL_ALL
+
+C: RUN "CALL dbms.cluster.routing.getServers" {}
+   PULL_ALL
+S: SUCCESS {"fields": ["ttl", "servers"]}
+   RECORD [9223372036854775807, [{"addresses": ["127.0.0.1:9001","127.0.0.1:9002"],"role": "WRITE"}, {"addresses": ["127.0.0.1:9003","127.0.0.1:9004"], "role": "READ"},{"addresses": ["127.0.0.1:9005"], "role": "ROUTE"}]]
+   SUCCESS {}