Disable CQL ExecutorService by default

Follow up to #2741

CQL ExecutorPool just adds overhead without good benefits

Reason why it was enabled for 0.6.x version is here: #2545 (reply in thread)

As the next JanusGraph version is a major one, it makes sense to introduce this breaking change

Signed-off-by: Oleksandr Porunov <alexandr.porunov@gmail.com>

Author: porunov

docs/changelog.md

@@ -252,6 +252,16 @@ Notice that `janusgraph-cql` and `janusgraph-scylla` are mutually exclusive. Use
 provide both dependencies in the same classpath. See [ScyllaDB Storage Backend documentation](storage-backend/scylladb.md) for more
 information about how to make `scylla` `storage.backend` options available.
 
+##### Disable CQL ExecutorService by default
+
+Previously CQL ExecutorService was enabled by default mostly for historical reasons.  
+CQL ExecutorService managed by JanusGraph adds additional overhead without bringing any usefulness other than 
+limiting amount of parallel queries which can (and should) be controlled via the underlying CQL driver.  
+
+In case previous behaviour is desired then `storage.cql.executor-service.enabled` configuration option should be set to `true`, 
+but it's recommended to tune CQL queries parallelism using CQL driver configuration options (like `storage.cql.max-requests-per-connection`, 
+`storage.cql.local-max-connections-per-host`) and / or `storage.parallel-backend-ops.*` configuration options.  
+
 ##### Removal of deprecated classes/methods/functionalities
 
 ###### Methods

docs/configs/janusgraph-cfg.md

@@ -451,7 +451,7 @@ Configuration options for CQL executor service which is used to process CQL quer
 | ---- | ---- | ---- | ---- | ---- |
 | storage.cql.executor-service.class | The implementation of `ExecutorService` to use. The full name of the class which extends `ExecutorService` which has either a public constructor with `ExecutorServiceConfiguration` argument (preferred constructor) or a public parameterless constructor. Other accepted options are: `fixed` - fixed thread pool of size `core-pool-size`; `cached` - cached thread pool; | String | fixed | LOCAL |
 | storage.cql.executor-service.core-pool-size | Core pool size for executor service. May be ignored if custom executor service is used (depending on the implementation of the executor service). | Integer | 10 | LOCAL |
-| storage.cql.executor-service.enabled | Whether to use CQL executor service to process queries or not. If not used, the parallelism will be controlled internally by the CQL driver via `storage.cql.max-requests-per-connection` parameter which may be preferable in production environments. Disabling executor service reduces overhead of thread pool but might be more difficult to tune. | Boolean | true | LOCAL |
+| storage.cql.executor-service.enabled | Whether to use CQL executor service to process queries or not. If not used, the parallelism will be controlled internally by the CQL driver via `storage.cql.max-requests-per-connection` parameter which may be preferable in production environments. Disabling executor service reduces overhead of thread pool but might be more difficult to tune. | Boolean | false | LOCAL |
 | storage.cql.executor-service.keep-alive-time | Keep alive time in milliseconds for executor service. When the number of threads is greater than the `core-pool-size`, this is the maximum time that excess idle threads will wait for new tasks before terminating. Ignored for `fixed` executor service and may be ignored if custom executor service is used (depending on the implementation of the executor service). | Long | 60000 | LOCAL |
 | storage.cql.executor-service.max-pool-size | Maximum pool size for executor service. Ignored for `fixed` and `cached` executor services. May be ignored if custom executor service is used (depending on the implementation of the executor service). | Integer | 2147483647 | LOCAL |
 | storage.cql.executor-service.max-shutdown-wait-time | Max shutdown wait time in milliseconds for executor service threads to be finished during shutdown. After this time threads will be interrupted (signalled with interrupt) without any additional wait time. | Long | 60000 | LOCAL |

janusgraph-cql/src/main/java/org/janusgraph/diskstorage/cql/CQLConfigOptions.java

@@ -612,7 +612,7 @@ public interface CQLConfigOptions {
             "Disabling executor service reduces overhead of thread pool but might be more difficult to tune.",
         ConfigOption.Type.LOCAL,
         Boolean.class,
-        true);
+        false);
 
     ConfigOption<Integer> EXECUTOR_SERVICE_CORE_POOL_SIZE = new ConfigOption<>(
         EXECUTOR_SERVICE,

janusgraph-cql/src/test/java/org/janusgraph/diskstorage/cql/CQLConfigTest.java

@@ -62,6 +62,7 @@
 
 import static org.janusgraph.diskstorage.cql.CQLConfigOptions.BASE_PROGRAMMATIC_CONFIGURATION_ENABLED;
 import static org.janusgraph.diskstorage.cql.CQLConfigOptions.EXECUTOR_SERVICE_CLASS;
+import static org.janusgraph.diskstorage.cql.CQLConfigOptions.EXECUTOR_SERVICE_ENABLED;
 import static org.janusgraph.diskstorage.cql.CQLConfigOptions.EXECUTOR_SERVICE_MAX_SHUTDOWN_WAIT_TIME;
 import static org.janusgraph.diskstorage.cql.CQLConfigOptions.FILE_CONFIGURATION;
 import static org.janusgraph.diskstorage.cql.CQLConfigOptions.HEARTBEAT_TIMEOUT;
@@ -326,6 +327,7 @@ public void testRequestLoggerConfigurationSet() {
     public void shouldCreateCachedThreadPool() {
         WriteConfiguration wc = getConfiguration();
         wc.set(ConfigElement.getPath(EXECUTOR_SERVICE_CLASS), ExecutorServiceBuilder.CACHED_THREAD_POOL_CLASS);
+        wc.set(ConfigElement.getPath(EXECUTOR_SERVICE_ENABLED), true);
         graph = (StandardJanusGraph) JanusGraphFactory.open(wc);
         assertDoesNotThrow(() -> {
             graph.traversal().V().hasNext();
@@ -336,6 +338,7 @@ public void shouldCreateCachedThreadPool() {
     @Test
     public void shouldGracefullyCloseGraphWhichLostAConnection(){
         WriteConfiguration wc = getConfiguration();
+        wc.set(ConfigElement.getPath(EXECUTOR_SERVICE_ENABLED), true);
         wc.set(ConfigElement.getPath(EXECUTOR_SERVICE_MAX_SHUTDOWN_WAIT_TIME), 60000);
         wc.set(ConfigElement.getPath(PARALLEL_BACKEND_EXECUTOR_SERVICE_MAX_SHUTDOWN_WAIT_TIME), 60000);
         wc.set(ConfigElement.getPath(IDS_RENEW_TIMEOUT), 10000);