Treat one router as valid in routing table

Previously driver considered routing table with single router to be stale
and had to perform rediscovery before read/write transaction. Requirement
to have more than 1 router is quite strict and can easily be violated by
partially unavailable clusters. Additional rediscoveries in such cases
add more load on the available core server.

This commit makes driver tread routing table with single router as not
stale, given that other non-staleness requirements are satisfied as well.

Author: lutovich

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 {}