feat: decrease latency when some rows are locally buffered (#17)

* feat: decrease latency when some rows are locally buffered

we instruct Kafka Connect to call us in a reasonable time even
if no new message is delivered. this gives us a chance
to flush whenever we have buffered locally. this means users no
longer have to decrease offset.commit.interval.ms explicitly.

connector/src/main/java/io/questdb/kafka/QuestDBSinkConnectorConfig.java

@@ -64,6 +64,12 @@ public final class QuestDBSinkConnectorConfig extends AbstractConfig {
     public static final String CONFIGURATION_STRING_CONFIG = "client.conf.string";
     public static final String CONFIGURATION_STRING_DOC = "Configuration string for QuestDB client";
 
+    public static final String ALLOWED_LAG_CONFIG = "allowed.lag";
+    public static final String ALLOWED_LAG_DOC = "The maximum lag in milliseconds allowed for the connector to keep buffered data in memory " +
+            "if there are no new records in Kafka topics. Higher lag allows more batching and improves throughput, but increase the time " +
+            "it takes to detect new data in Kafka topics. Low lag reduces the time it takes to detect new data in Kafka topics, but may " +
+            "reduce throughput. The default value is 1000 ms.";
+
     public static final String RETRY_BACKOFF_MS = "retry.backoff.ms";
     private static final String RETRY_BACKOFF_MS_DOC = "The time in milliseconds to wait following an error before a retry attempt is made";
 
@@ -104,13 +110,18 @@ public static ConfigDef conf() {
                 .define(TIMESTAMP_STRING_FIELDS, Type.STRING, null, Importance.MEDIUM, TIMESTAMP_STRING_FIELDS_DOC)
                 .define(DESIGNATED_TIMESTAMP_KAFKA_NATIVE_CONFIG, Type.BOOLEAN, false, Importance.MEDIUM, DESIGNATED_TIMESTAMP_KAFKA_NATIVE_DOC)
                 .define(TLS_VALIDATION_MODE_CONFIG, Type.STRING, "default", ConfigDef.ValidString.in("default", "insecure"), Importance.LOW, TLS_VALIDATION_MODE_DOC)
-                .define(CONFIGURATION_STRING_CONFIG, Type.PASSWORD, null, Importance.HIGH, CONFIGURATION_STRING_DOC);
+                .define(CONFIGURATION_STRING_CONFIG, Type.PASSWORD, null, Importance.HIGH, CONFIGURATION_STRING_DOC)
+                .define(ALLOWED_LAG_CONFIG, Type.INT, 1000, ConfigDef.Range.between(1, Integer.MAX_VALUE), Importance.LOW, ALLOWED_LAG_DOC);
     }
 
     public Password getConfigurationString() {
         return getPassword(CONFIGURATION_STRING_CONFIG);
     }
 
+    public int getAllowedLag() {
+        return getInt(ALLOWED_LAG_CONFIG);
+    }
+
     public String getTlsValidationMode() {
         return getString(TLS_VALIDATION_MODE_CONFIG).toLowerCase(Locale.ENGLISH);
     }

connector/src/main/java/io/questdb/kafka/QuestDBSinkTask.java

@@ -42,6 +42,7 @@ public final class QuestDBSinkTask extends SinkTask {
     private DateFormat dataFormat;
     private boolean kafkaTimestampsEnabled;
     private boolean httpTransport;
+    private int allowedLag;
 
     @Override
     public String version() {
@@ -76,6 +77,7 @@ public void start(Map<String, String> map) {
         this.timestampColumnName = config.getDesignatedTimestampColumnName();
         this.kafkaTimestampsEnabled = config.isDesignatedTimestampKafkaNative();
         this.timestampUnits = config.getTimestampUnitsOrNull();
+        this.allowedLag = config.getAllowedLag();
     }
 
     private Sender createRawSender() {
@@ -123,9 +125,28 @@ private Sender createSender() {
     @Override
     public void put(Collection<SinkRecord> collection) {
         if (collection.isEmpty()) {
-            log.debug("Received empty collection, ignoring");
+            if (httpTransport) {
+                log.debug("Received empty collection, let's flush the buffer");
+                // Ok, there are no new records to send. Let's flush! Why?
+                // We do not want locally buffered row to be stuck in the buffer for too long. Increases latency
+                // between the time the record is produced and the time it is visible in QuestDB.
+                // If the local buffer is empty then flushing is a cheap no-op.
+                try {
+                    sender.flush();
+                } catch (LineSenderException | HttpClientException e) {
+                    onSenderException(e);
+                }
+            } else {
+                log.debug("Received empty collection, nothing to do");
+            }
             return;
+        } if (httpTransport) {
+            // there are some records to send. good.
+            // let's set a timeout so Kafka Connect will call us again in time
+            // even if there are no new records to send. this gives us a chance to flush the buffer.
+            context.timeout(allowedLag);
         }
+
         if (log.isDebugEnabled()) {
             SinkRecord record = collection.iterator().next();
             log.debug("Received {} records. First record kafka coordinates:({}-{}-{}). ",

integration-tests/cp-server/src/test/java/io/questdb/kafka/QuestDBSinkConnectorIT.java

@@ -66,7 +66,7 @@ public class QuestDBSinkConnectorIT {
             .withEnv("CONNECT_CONFIG_STORAGE_REPLICATION_FACTOR", "1")
             .withEnv("CONNECT_OFFSET_STORAGE_REPLICATION_FACTOR", "1")
             .withEnv("CONNECT_STATUS_STORAGE_REPLICATION_FACTOR", "1")
-            .withEnv("QDB_CLIENT_CONF", "http::addr=questdb;auto_flush_rows=1;")
+            .withEnv("QDB_CLIENT_CONF", "http::addr=questdb;auto_flush=off;") // intentionally disabled auto-flush
             .withNetwork(network)
             .withExposedPorts(8083)
             .withCopyFileToContainer(MountableFile.forHostPath(connectorJarResolver.getJarPath()), "/usr/share/java/kafka/questdb-connector.jar")
@@ -97,6 +97,7 @@ public void test() throws Exception {
                 .with("key.converter", "org.apache.kafka.connect.storage.StringConverter")
                 .with("value.converter", "org.apache.kafka.connect.storage.StringConverter")
                 .with("topics", topicName);
+        // ilp client conf string set via environment variable
 
         connectContainer.registerConnector("my-connector", connector);
 

integration-tests/debezium/src/test/java/kafka/DebeziumIT.java

@@ -70,13 +70,11 @@ public class DebeziumIT {
     private final GenericContainer<?> questDBContainer = new GenericContainer<>("questdb/questdb:7.4.0")
             .withNetwork(network)
             .withExposedPorts(QuestDBUtils.QUESTDB_HTTP_PORT)
-            .withLogConsumer(new Slf4jLogConsumer(LoggerFactory.getLogger("questdb")))
-            .withEnv("QDB_CAIRO_COMMIT_LAG", "100")
-            .withEnv("JAVA_OPTS", "-Djava.locale.providers=JRE,SPI");
+            .withLogConsumer(new Slf4jLogConsumer(LoggerFactory.getLogger("questdb")));
 
 
     private ConnectorConfiguration newQuestSinkBaseConfig(String questTableName) {
-        String confString = "http::addr=" + questDBContainer.getNetworkAliases().get(0) + ":9000;auto_flush_rows=1000;";
+        String confString = "http::addr=" + questDBContainer.getNetworkAliases().get(0) + ":9000;";
         return ConnectorConfiguration.create()
                 .with("connector.class", QuestDBSinkConnector.class.getName())
                 .with("client.conf.string", confString)

integration-tests/exactlyonce/src/test/java/io/questdb/kafka/ExactlyOnceIT.java

@@ -172,7 +172,6 @@ private static GenericContainer<?> newConnectContainer(int id) {
                 .withEnv("CONNECT_BOOTSTRAP_SERVERS", "kafka0:9092")
                 .withEnv("CONNECT_GROUP_ID", "test")
                 .withEnv("CONNECT_OFFSET_FLUSH_INTERVAL_MS", "5000")
-                .withEnv("CONNECT_CONNECTOR_CLIENT_CONFIG_OVERRIDE_POLICY", "All")
                 .withEnv("CONNECT_OFFSET_STORAGE_TOPIC", "connect-storage-topic")
                 .withEnv("CONNECT_CONFIG_STORAGE_TOPIC", "connect-config-topic")
                 .withEnv("CONNECT_STATUS_STORAGE_TOPIC", "connect-status-topic")
@@ -298,7 +297,7 @@ private static void startKillingRandomContainers(CyclicBarrier barrier) {
     }
 
     private static void startConnector() throws IOException, InterruptedException, URISyntaxException {
-        String confString = "http::addr=questdb:9000;auto_flush_rows=10000;auto_flush_interval=1200000;retry_timeout=60000;";
+        String confString = "http::addr=questdb:9000;auto_flush_rows=10000;auto_flush_interval=" + Integer.MAX_VALUE + ";retry_timeout=60000;";
 
         String payload = "{\"name\":\"my-connector\",\"config\":{" +
                 "\"tasks.max\":\"4\"," +