zentol commented on code in PR #3:
URL: 
https://github.com/apache/flink-connector-cassandra/pull/3#discussion_r1104371424


##########
flink-connector-cassandra/src/main/java/org/apache/flink/connector/cassandra/source/reader/CassandraSourceReaderFactory.java:
##########
@@ -0,0 +1,53 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.apache.flink.connector.cassandra.source.reader;
+
+import org.apache.flink.api.connector.source.SourceReaderContext;
+import org.apache.flink.streaming.connectors.cassandra.ClusterBuilder;
+import org.apache.flink.streaming.connectors.cassandra.MapperOptions;
+
+import com.datastax.driver.core.Cluster;
+import com.datastax.driver.core.Session;
+import com.datastax.driver.mapping.Mapper;
+import com.datastax.driver.mapping.MappingManager;
+
+/**
+ * Factory to create {@link CassandraSourceReader}s and allow creating the 
cluster and the session
+ * objects.

Review Comment:
   ```suggestion
    * Factory to create {@link CassandraSourceReader}s and allow the sharing of 
cluster and the session
    * objects.
   ```



##########
flink-connector-cassandra/src/test/java/org/apache/flink/connector/cassandra/source/CassandraSourceITCase.java:
##########
@@ -0,0 +1,115 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.apache.flink.connector.cassandra.source;
+
+import org.apache.flink.connector.testframe.environment.ClusterControllable;
+import 
org.apache.flink.connector.testframe.environment.MiniClusterTestEnvironment;
+import org.apache.flink.connector.testframe.environment.TestEnvironment;
+import 
org.apache.flink.connector.testframe.external.source.DataStreamSourceExternalContext;
+import org.apache.flink.connector.testframe.junit.annotations.TestContext;
+import org.apache.flink.connector.testframe.junit.annotations.TestEnv;
+import 
org.apache.flink.connector.testframe.junit.annotations.TestExternalSystem;
+import org.apache.flink.connector.testframe.junit.annotations.TestSemantics;
+import org.apache.flink.connector.testframe.testsuites.SourceTestSuiteBase;
+import org.apache.flink.connector.testframe.utils.CollectIteratorAssertions;
+import org.apache.flink.connectors.cassandra.utils.Pojo;
+import org.apache.flink.streaming.api.CheckpointingMode;
+import org.apache.flink.util.CloseableIterator;
+
+import org.junit.jupiter.api.Disabled;
+
+import java.util.List;
+
+import static java.util.concurrent.CompletableFuture.runAsync;
+import static 
org.apache.flink.connector.cassandra.source.CassandraTestContext.CassandraTestContextFactory;
+import static 
org.apache.flink.connector.testframe.utils.ConnectorTestConstants.DEFAULT_COLLECT_DATA_TIMEOUT;
+import static org.assertj.core.api.Assertions.assertThat;
+
+/** Test for the Cassandra source. */
+public class CassandraSourceITCase extends SourceTestSuiteBase<Pojo> {

Review Comment:
   ```suggestion
   class CassandraSourceITCase extends SourceTestSuiteBase<Pojo> {
   ```



##########
flink-connector-cassandra/src/test/java/org/apache/flink/connector/cassandra/source/reader/CassandraQueryTest.java:
##########
@@ -31,84 +32,35 @@
 /** tests for query generation and query sanity checks. */
 class CassandraQueryTest {
 
+    private static final Pattern PATTERN = 
Pattern.compile(CassandraSplitReader.SELECT_REGEXP);
+
     @Test
     public void testKeySpaceTableExtractionRegexp() {
-        final Pattern pattern = 
Pattern.compile(CassandraSplitReader.SELECT_REGEXP);
-        Matcher matcher;
-        matcher = pattern.matcher("SELECT field FROM keyspace.table where 
field = value;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
-
-        matcher = pattern.matcher("SELECT * FROM keyspace.table;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
 
-        matcher = pattern.matcher("select field1, field2 from 
keyspace.table;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
+        assertQueryFormatCorrect("SELECT field FROM keyspace.table where field 
= value;");
+        assertQueryFormatCorrect("SELECT * FROM keyspace.table;");
+        assertQueryFormatCorrect("select field1, field2 from keyspace.table;");
+        assertQueryFormatCorrect("select field1, field2 from keyspace.table 
LIMIT(1000);");
+        assertQueryFormatCorrect("select field1 from keyspace.table ;");
+        assertQueryFormatCorrect("select field1 from keyspace.table where 
field1=1;");
 
-        matcher = pattern.matcher("select field1, field2 from keyspace.table 
LIMIT(1000);");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
-
-        matcher = pattern.matcher("select field1 from keyspace.table ;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
-
-        matcher = pattern.matcher("select field1 from keyspace.table where 
field1=1;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
-
-        matcher = pattern.matcher("select field1 from table;"); // missing 
keyspace
-        assertThat(matcher.matches()).isFalse();
-
-        matcher = pattern.matcher("select field1 from keyspace.table"); // 
missing ";"
-        assertThat(matcher.matches()).isFalse();
+        assertQueryFormatIncorrect("select field1 from table;"); // missing 
keyspace
+        assertQueryFormatIncorrect("select field1 from keyspace.table"); // 
missing ";"
     }
 
     @Test
     public void testProhibitedClauses() {
-        assertThatThrownBy(
-                        () ->
-                                CassandraSource.checkQueryValidity(
-                                        "SELECT COUNT(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () -> CassandraSource.checkQueryValidity("SELECT 
AVG(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-
-        assertThatThrownBy(
-                        () -> CassandraSource.checkQueryValidity("SELECT 
MIN(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () -> CassandraSource.checkQueryValidity("SELECT 
MAX(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () -> CassandraSource.checkQueryValidity("SELECT 
SUM(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () ->
-                                CassandraSource.checkQueryValidity(
-                                        "SELECT field1, field2 from 
flink.table ORDER BY field1;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () ->
-                                CassandraSource.checkQueryValidity(
-                                        "SELECT field1, field2 from 
flink.table GROUP BY field1;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
+        Arrays.stream(
+                        new String[] {

Review Comment:
   `Arrays.asList()` would allow you to avoid `new String[] {}`



##########
flink-connector-cassandra/src/main/java/org/apache/flink/connector/cassandra/source/enumerator/CassandraEnumeratorStateSerializer.java:
##########
@@ -0,0 +1,81 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.apache.flink.connector.cassandra.source.enumerator;
+
+import org.apache.flink.connector.cassandra.source.split.CassandraSplit;
+import 
org.apache.flink.connector.cassandra.source.split.CassandraSplitSerializer;
+import org.apache.flink.core.io.SimpleVersionedSerializer;
+import org.apache.flink.core.memory.DataInputDeserializer;
+import org.apache.flink.core.memory.DataOutputSerializer;
+
+import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.Queue;
+
+/** Serializer for {@link CassandraEnumeratorState}. */
+public class CassandraEnumeratorStateSerializer
+        implements SimpleVersionedSerializer<CassandraEnumeratorState> {
+
+    public static final CassandraEnumeratorStateSerializer INSTANCE =
+            new CassandraEnumeratorStateSerializer();
+    private static final ThreadLocal<DataOutputSerializer> SERIALIZER_CACHE =
+            ThreadLocal.withInitial(() -> new DataOutputSerializer(64));

Review Comment:
   This is overkill for the enumerator state.
   In contrast to splits (of which there can be thousands...[hmm]()) there's 
only 1 state at all times.
   
   Fun fact about thread locals: They can easily leak the user-code 
classloader. If any thread that out-lives the task (like say, a shared thread 
pool) ever accesses the ThreadLocal, then the thread retains a reference to the 
thread local, due to which it can't be GC'd. Just thought I'd throw this out 
there.



##########
flink-connector-cassandra/src/main/java/org/apache/flink/connector/cassandra/source/reader/CassandraSplitReader.java:
##########
@@ -0,0 +1,280 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.apache.flink.connector.cassandra.source.reader;
+
+import org.apache.flink.annotation.VisibleForTesting;
+import org.apache.flink.connector.base.source.reader.RecordsBySplits;
+import org.apache.flink.connector.base.source.reader.RecordsWithSplitIds;
+import org.apache.flink.connector.base.source.reader.splitreader.SplitReader;
+import org.apache.flink.connector.base.source.reader.splitreader.SplitsChange;
+import org.apache.flink.connector.cassandra.source.split.CassandraSplit;
+import org.apache.flink.connector.cassandra.source.split.CassandraSplitState;
+import org.apache.flink.connector.cassandra.source.split.RingRange;
+import org.apache.flink.streaming.connectors.cassandra.ClusterBuilder;
+
+import com.datastax.driver.core.Cluster;
+import com.datastax.driver.core.ColumnMetadata;
+import com.datastax.driver.core.Metadata;
+import com.datastax.driver.core.PreparedStatement;
+import com.datastax.driver.core.ResultSet;
+import com.datastax.driver.core.Session;
+import com.datastax.driver.core.Token;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+
+import java.math.BigInteger;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import java.util.stream.Collectors;
+
+/**
+ * {@link SplitReader} for Cassandra source. This class is responsible for 
fetching the records as
+ * {@link CassandraRow}. For that, it executes a range query (query that 
outputs records belonging
+ * to a {@link RingRange}) based on the user specified query. This class 
manages the Cassandra
+ * cluster and session.
+ */
+public class CassandraSplitReader implements SplitReader<CassandraRow, 
CassandraSplit> {
+
+    private static final Logger LOG = 
LoggerFactory.getLogger(CassandraSplitReader.class);
+    public static final String SELECT_REGEXP = "(?i)select .+ from 
(\\w+)\\.(\\w+).*;$";
+
+    private final Cluster cluster;
+    private final Session session;
+    private final Set<CassandraSplitState> unprocessedSplits;
+    private final AtomicBoolean wakeup = new AtomicBoolean(false);
+    private final String query;
+
+    public CassandraSplitReader(ClusterBuilder clusterBuilder, String query) {
+        // need a thread safe set
+        this.unprocessedSplits = ConcurrentHashMap.newKeySet();
+        this.query = query;
+        cluster = clusterBuilder.getCluster();
+        session = cluster.connect();
+    }
+
+    @Override
+    public RecordsWithSplitIds<CassandraRow> fetch() {
+        Map<String, Collection<CassandraRow>> recordsBySplit = new HashMap<>();
+        Set<String> finishedSplits = new HashSet<>();
+        Metadata clusterMetadata = cluster.getMetadata();
+
+        String partitionKey = getPartitionKey(clusterMetadata);
+        String finalQuery = generateRangeQuery(query, partitionKey);
+        PreparedStatement preparedStatement = session.prepare(finalQuery);
+        // Set wakeup to false to start consuming.
+        wakeup.compareAndSet(true, false);
+        for (CassandraSplitState cassandraSplitState : unprocessedSplits) {

Review Comment:
   > I'd prefer that the user conf parameter is in bytes.
   
   I agree. (well it should be a `MemorySize`, but the same idea)
   
   > I can get my size estimates from Beam 2017 code so that the SplitGenerator 
provides splits of a given size. Then all the records of each split will be 
stored in memory to create the needed RecordsBySplits.
   
   Sounds good. We do something similar in the [mongodb 
connector](https://github.com/apache/flink-connector-mongodb/blob/main/flink-connector-mongodb/src/main/java/org/apache/flink/connector/mongodb/source/enumerator/splitter/MongoSampleSplitter.java).



##########
flink-connector-cassandra/src/main/java/org/apache/flink/connector/cassandra/source/enumerator/CassandraEnumeratorState.java:
##########
@@ -0,0 +1,86 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.apache.flink.connector.cassandra.source.enumerator;
+
+import org.apache.flink.connector.cassandra.source.split.CassandraSplit;
+
+import javax.annotation.Nullable;
+
+import java.util.ArrayDeque;
+import java.util.Collection;
+import java.util.Objects;
+import java.util.Queue;
+
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/** State for {@link CassandraSplitEnumerator} to track the splits yet to 
assign. */
+public class CassandraEnumeratorState {
+
+    private final Queue<CassandraSplit> unassignedSplits;
+
+    CassandraEnumeratorState() {
+        this.unassignedSplits = new ArrayDeque<>();
+    }
+
+    CassandraEnumeratorState(Queue<CassandraSplit> unassignedSplits) {
+        checkNotNull(unassignedSplits);
+        this.unassignedSplits = unassignedSplits;
+    }
+
+    public Queue<CassandraSplit> getUnassignedSplits() {
+        return unassignedSplits;
+    }
+
+    public void addNewSplits(Collection<CassandraSplit> newSplits) {
+        unassignedSplits.addAll(newSplits);
+    }
+
+    public @Nullable CassandraSplit getASplit() {
+        return unassignedSplits.poll();
+    }
+
+    boolean hasMoreSplits() {
+        return unassignedSplits.size() != 0;

Review Comment:
   ```suggestion
           return !unassignedSplits.isEmpty();
   ```



##########
flink-connector-cassandra/src/main/java/org/apache/flink/connector/cassandra/source/enumerator/CassandraSplitEnumerator.java:
##########
@@ -0,0 +1,128 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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.apache.flink.connector.cassandra.source.enumerator;
+
+import org.apache.flink.api.connector.source.SplitEnumerator;
+import org.apache.flink.api.connector.source.SplitEnumeratorContext;
+import org.apache.flink.connector.cassandra.source.split.CassandraSplit;
+import org.apache.flink.connector.cassandra.source.split.SplitsGenerator;
+import org.apache.flink.streaming.connectors.cassandra.ClusterBuilder;
+
+import com.datastax.driver.core.Cluster;
+import com.datastax.driver.core.Metadata;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+
+import java.io.IOException;
+import java.util.List;
+
+import static 
org.apache.flink.connector.cassandra.source.split.SplitsGenerator.CassandraPartitioner.MURMUR3PARTITIONER;
+import static 
org.apache.flink.connector.cassandra.source.split.SplitsGenerator.CassandraPartitioner.RANDOMPARTITIONER;
+
+/** {@link SplitEnumerator} that splits Cassandra cluster into {@link 
CassandraSplit}s. */
+public final class CassandraSplitEnumerator
+        implements SplitEnumerator<CassandraSplit, CassandraEnumeratorState> {
+    private static final Logger LOG = 
LoggerFactory.getLogger(CassandraSplitEnumerator.class);
+
+    private final SplitEnumeratorContext<CassandraSplit> enumeratorContext;
+    private final CassandraEnumeratorState state;
+    private final Cluster cluster;
+
+    public CassandraSplitEnumerator(
+            SplitEnumeratorContext<CassandraSplit> enumeratorContext,
+            CassandraEnumeratorState state,
+            ClusterBuilder clusterBuilder) {
+        this.enumeratorContext = enumeratorContext;
+        this.state = state == null ? new CassandraEnumeratorState() : state /* 
snapshot restore*/;
+        this.cluster = clusterBuilder.getCluster();
+    }
+
+    @Override
+    public void handleSplitRequest(int subtaskId, @Nullable String 
requesterHostname) {
+        checkReaderRegistered(subtaskId);
+        final CassandraSplit cassandraSplit = state.getASplit();
+        if (cassandraSplit != null) {
+            LOG.info("Assigning splits to reader {}", subtaskId);
+            enumeratorContext.assignSplit(cassandraSplit, subtaskId);
+        } else {
+            LOG.info(
+                    "No split assigned to reader {} because the enumerator has 
no unassigned split left",
+                    subtaskId);
+        }
+        if (!state.hasMoreSplits()) {
+            LOG.info(
+                    "No more CassandraSplits to assign. Sending 
NoMoreSplitsEvent to reader {}.",
+                    subtaskId);
+            enumeratorContext.signalNoMoreSplits(subtaskId);
+        }
+    }
+
+    @Override
+    public void start() {
+        // discover the splits and update unprocessed splits and then assign 
them.
+        // There is only an initial splits discovery, no periodic discovery.
+        enumeratorContext.callAsync(
+                this::discoverSplits,
+                (splits, throwable) -> {
+                    LOG.info("Add {} splits to CassandraSplitEnumerator.", 
splits.size());
+                    state.addNewSplits(splits);
+                });
+    }
+
+    private List<CassandraSplit> discoverSplits() {
+        final int numberOfSplits = enumeratorContext.currentParallelism();
+        final Metadata clusterMetadata = cluster.getMetadata();
+        final String partitionerName = clusterMetadata.getPartitioner();
+        final SplitsGenerator.CassandraPartitioner partitioner =
+                partitionerName.contains(MURMUR3PARTITIONER.className())
+                        ? MURMUR3PARTITIONER
+                        : RANDOMPARTITIONER;
+        return new SplitsGenerator(partitioner).generateSplits(numberOfSplits);

Review Comment:
   I'm wondering if splits could be generated lazily. Slightly worried about us 
eagerly creating millions of splits when a user configures a small split size.



##########
flink-connector-cassandra/src/test/java/org/apache/flink/connector/cassandra/source/reader/CassandraQueryTest.java:
##########
@@ -31,84 +32,35 @@
 /** tests for query generation and query sanity checks. */
 class CassandraQueryTest {
 
+    private static final Pattern PATTERN = 
Pattern.compile(CassandraSplitReader.SELECT_REGEXP);
+
     @Test
     public void testKeySpaceTableExtractionRegexp() {
-        final Pattern pattern = 
Pattern.compile(CassandraSplitReader.SELECT_REGEXP);
-        Matcher matcher;
-        matcher = pattern.matcher("SELECT field FROM keyspace.table where 
field = value;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
-
-        matcher = pattern.matcher("SELECT * FROM keyspace.table;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
 
-        matcher = pattern.matcher("select field1, field2 from 
keyspace.table;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
+        assertQueryFormatCorrect("SELECT field FROM keyspace.table where field 
= value;");
+        assertQueryFormatCorrect("SELECT * FROM keyspace.table;");
+        assertQueryFormatCorrect("select field1, field2 from keyspace.table;");
+        assertQueryFormatCorrect("select field1, field2 from keyspace.table 
LIMIT(1000);");
+        assertQueryFormatCorrect("select field1 from keyspace.table ;");
+        assertQueryFormatCorrect("select field1 from keyspace.table where 
field1=1;");
 
-        matcher = pattern.matcher("select field1, field2 from keyspace.table 
LIMIT(1000);");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
-
-        matcher = pattern.matcher("select field1 from keyspace.table ;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
-
-        matcher = pattern.matcher("select field1 from keyspace.table where 
field1=1;");
-        assertThat(matcher.matches()).isTrue();
-        assertThat(matcher.group(1)).isEqualTo("keyspace");
-        assertThat(matcher.group(2)).isEqualTo("table");
-
-        matcher = pattern.matcher("select field1 from table;"); // missing 
keyspace
-        assertThat(matcher.matches()).isFalse();
-
-        matcher = pattern.matcher("select field1 from keyspace.table"); // 
missing ";"
-        assertThat(matcher.matches()).isFalse();
+        assertQueryFormatIncorrect("select field1 from table;"); // missing 
keyspace
+        assertQueryFormatIncorrect("select field1 from keyspace.table"); // 
missing ";"
     }
 
     @Test
     public void testProhibitedClauses() {
-        assertThatThrownBy(
-                        () ->
-                                CassandraSource.checkQueryValidity(
-                                        "SELECT COUNT(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () -> CassandraSource.checkQueryValidity("SELECT 
AVG(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-
-        assertThatThrownBy(
-                        () -> CassandraSource.checkQueryValidity("SELECT 
MIN(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () -> CassandraSource.checkQueryValidity("SELECT 
MAX(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () -> CassandraSource.checkQueryValidity("SELECT 
SUM(*) from flink.table;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () ->
-                                CassandraSource.checkQueryValidity(
-                                        "SELECT field1, field2 from 
flink.table ORDER BY field1;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
-        assertThatThrownBy(
-                        () ->
-                                CassandraSource.checkQueryValidity(
-                                        "SELECT field1, field2 from 
flink.table GROUP BY field1;"))
-                .isInstanceOf(IllegalStateException.class)
-                .hasMessageContaining("Aggregations/OrderBy are not 
supported");
+        Arrays.stream(
+                        new String[] {

Review Comment:
   Why is this one using `forEach` unlike `testKeySpaceTableExtractionRegexp`?



##########
flink-connector-cassandra/src/main/java/org/apache/flink/connector/cassandra/source/enumerator/CassandraSplitEnumerator.java:
##########
@@ -57,7 +57,22 @@ public CassandraSplitEnumerator(
 
     @Override
     public void handleSplitRequest(int subtaskId, @Nullable String 
requesterHostname) {
-        assignUnprocessedSplitToReader(subtaskId);
+        checkReaderRegistered(subtaskId);
+        final CassandraSplit cassandraSplit = state.getASplit();
+        if (cassandraSplit != null) {
+            LOG.info("Assigning splits to reader {}", subtaskId);
+            enumeratorContext.assignSplit(cassandraSplit, subtaskId);
+        } else {
+            LOG.info(
+                    "No split assigned to reader {} because the enumerator has 
no unassigned split left",
+                    subtaskId);
+        }
+        if (!state.hasMoreSplits()) {
+            LOG.info(
+                    "No more CassandraSplits to assign. Sending 
NoMoreSplitsEvent to reader {}.",
+                    subtaskId);
+            enumeratorContext.signalNoMoreSplits(subtaskId);
+        }

Review Comment:
   Isn't the else branch and this branch always executed together?
   



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