poorbarcode commented on code in PR #16931:
URL: https://github.com/apache/pulsar/pull/16931#discussion_r997736460


##########
managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/ManagedLedgerFactoryImpl.java:
##########
@@ -420,7 +420,28 @@ public void closeFailed(ManagedLedgerException exception, 
Object ctx) {
         });
     }
 
-
+    @Override
+    public CompletableFuture<ReadOnlyManagedLedgerImpl> 
asyncOpenReadOnlyManagedLedger(String managedLedgerName,
+                              ManagedLedgerConfig config, Object ctx) {
+        CompletableFuture<ReadOnlyManagedLedgerImpl> future = new 
CompletableFuture<>();
+        if (closed) {
+            return FutureUtil.failedFuture(new 
ManagedLedgerException.ManagedLedgerFactoryClosedException());
+        }
+        ReadOnlyManagedLedgerImpl roManagedLedger = new 
ReadOnlyManagedLedgerImpl(this,
+                bookkeeperFactory
+                        .get(new 
EnsemblePlacementPolicyConfig(config.getBookKeeperEnsemblePlacementPolicyClassName(),
+                                
config.getBookKeeperEnsemblePlacementPolicyProperties())),
+                store, config, scheduledExecutor, managedLedgerName);
+        roManagedLedger.initialize().thenRun(() -> {

Review Comment:
   `ReadOnlyManagedLedgerImpl.initialize()` has already return 
`CompletableFuture<ReadOnlyManagedLedgerImpl>` , can we write it like this: 
   
   ```java
   return new ReadOnlyManagedLedgerImpl(this,
                   bookkeeperFactory
                           .get(new 
EnsemblePlacementPolicyConfig(config.getBookKeeperEnsemblePlacementPolicyClassName(),
                                   
config.getBookKeeperEnsemblePlacementPolicyProperties())),
                   store, config, scheduledExecutor, managedLedgerName) 
initialize();
   ```
   
   Or the method `ReadOnlyManagedLedgerImpl.initialize()` can modify the return 
declaration to `CompletableFuture<Void>` ?



##########
pulsar-broker/src/main/java/org/apache/pulsar/broker/systopic/TopicPoliciesSystemTopicClient.java:
##########
@@ -85,42 +85,48 @@ private TopicPolicyWriter(Producer<PulsarEvent> producer, 
SystemTopicClient<Puls
         }
 
         @Override
-        public MessageId write(PulsarEvent event) throws PulsarClientException 
{
-            TypedMessageBuilder<PulsarEvent> builder = 
producer.newMessage().key(getEventKey(event)).value(event);
+        public MessageId write(PulsarEvent event, String key) throws 
PulsarClientException {
+            TypedMessageBuilder<PulsarEvent> builder = 
producer.newMessage().key(key).value(event);
             setReplicateCluster(event, builder);
             return builder.send();
         }
 
         @Override
-        public CompletableFuture<MessageId> writeAsync(PulsarEvent event) {
-            TypedMessageBuilder<PulsarEvent> builder = 
producer.newMessage().key(getEventKey(event)).value(event);
+        public CompletableFuture<MessageId> writeAsync(PulsarEvent event, 
String key) {
+            TypedMessageBuilder<PulsarEvent> builder = 
producer.newMessage().key(key).value(event);
             setReplicateCluster(event, builder);
             return builder.sendAsync();
         }
 
         @Override
-        public MessageId delete(PulsarEvent event) throws 
PulsarClientException {
-            validateActionType(event);
-            TypedMessageBuilder<PulsarEvent> builder = 
producer.newMessage().key(getEventKey(event)).value(null);
+        public MessageId delete(PulsarEvent event, String key) throws 
PulsarClientException {
+            TypedMessageBuilder<PulsarEvent> builder = 
producer.newMessage().key(key).value(null);
             setReplicateCluster(event, builder);
             return builder.send();
         }
 
         @Override
-        public CompletableFuture<MessageId> deleteAsync(PulsarEvent event) {
+        public CompletableFuture<MessageId> deleteAsync(PulsarEvent event, 
String key) {
             validateActionType(event);
             TypedMessageBuilder<PulsarEvent> builder = 
producer.newMessage().key(getEventKey(event)).value(null);

Review Comment:
   should we use `@param key` instead of `getEventKey(event)` here?



##########
pulsar-broker/src/main/java/org/apache/pulsar/broker/service/TransactionBufferSnapshotServiceFactory.java:
##########
@@ -0,0 +1,74 @@
+/**
+ * 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.pulsar.broker.service;
+
+import 
org.apache.pulsar.broker.transaction.buffer.matadata.TransactionBufferSnapshot;
+import 
org.apache.pulsar.broker.transaction.buffer.matadata.v2.TransactionBufferSnapshotIndexes;
+import org.apache.pulsar.client.api.PulsarClient;
+import org.apache.pulsar.common.events.EventType;
+
+public class TransactionBufferSnapshotServiceFactory {
+
+    private SystemTopicTxnBufferSnapshotService<TransactionBufferSnapshot> 
txnBufferSnapshotService;
+
+    private 
SystemTopicTxnBufferSnapshotService<TransactionBufferSnapshotIndexes.TransactionBufferSnapshot>
+            txnBufferSnapshotSegmentService;
+
+    private 
SystemTopicTxnBufferSnapshotService<TransactionBufferSnapshotIndexes> 
txnBufferSnapshotIndexService;
+
+    public TransactionBufferSnapshotServiceFactory(PulsarClient pulsarClient,
+                                                boolean 
transactionBufferSegmentedSnapshotEnabled) {

Review Comment:
   Now `@param transactionBufferSegmentedSnapshotEnabled ` is not unused, how 
do we turn the feature on or off?



##########
pulsar-broker/src/main/java/org/apache/pulsar/broker/service/SystemTopicTxnBufferSnapshotService.java:
##########
@@ -23,64 +23,63 @@
 import java.util.concurrent.ConcurrentHashMap;
 import org.apache.pulsar.broker.systopic.NamespaceEventsSystemTopicFactory;
 import org.apache.pulsar.broker.systopic.SystemTopicClient;
-import org.apache.pulsar.broker.systopic.SystemTopicClient.Reader;
-import org.apache.pulsar.broker.systopic.SystemTopicClient.Writer;
-import org.apache.pulsar.broker.systopic.TransactionBufferSystemTopicClient;
-import 
org.apache.pulsar.broker.transaction.buffer.matadata.TransactionBufferSnapshot;
+import org.apache.pulsar.broker.systopic.SystemTopicClientBase;
 import org.apache.pulsar.client.api.PulsarClient;
-import 
org.apache.pulsar.client.api.PulsarClientException.InvalidTopicNameException;
+import org.apache.pulsar.client.api.PulsarClientException;
 import org.apache.pulsar.common.events.EventType;
 import org.apache.pulsar.common.naming.TopicName;
 import org.apache.pulsar.common.util.FutureUtil;
 
-public class SystemTopicBaseTxnBufferSnapshotService implements 
TransactionBufferSnapshotService {
+public class SystemTopicTxnBufferSnapshotService<T> {
 
-    private final Map<TopicName, SystemTopicClient<TransactionBufferSnapshot>> 
clients;
+    protected final Map<TopicName, SystemTopicClient<T>> clients;
+    protected final NamespaceEventsSystemTopicFactory 
namespaceEventsSystemTopicFactory;
 
-    private final NamespaceEventsSystemTopicFactory 
namespaceEventsSystemTopicFactory;
+    protected final Class<T> schemaType;
+    protected final EventType systemTopicType;
 
-    public SystemTopicBaseTxnBufferSnapshotService(PulsarClient client) {
+    public SystemTopicTxnBufferSnapshotService(PulsarClient client, EventType 
systemTopicType,
+                                               Class<T> schemaType) {
         this.namespaceEventsSystemTopicFactory = new 
NamespaceEventsSystemTopicFactory(client);
+        this.systemTopicType = systemTopicType;
+        this.schemaType = schemaType;
         this.clients = new ConcurrentHashMap<>();
     }
 
-    @Override
-    public CompletableFuture<Writer<TransactionBufferSnapshot>> 
createWriter(TopicName topicName) {
+    public CompletableFuture<SystemTopicClient.Writer<T>> 
createWriter(TopicName topicName) {
         return 
getTransactionBufferSystemTopicClient(topicName).thenCompose(SystemTopicClient::newWriterAsync);
     }
 
-    private CompletableFuture<SystemTopicClient<TransactionBufferSnapshot>> 
getTransactionBufferSystemTopicClient(
-            TopicName topicName) {
-        TopicName systemTopicName = NamespaceEventsSystemTopicFactory
-                .getSystemTopicName(topicName.getNamespaceObject(), 
EventType.TRANSACTION_BUFFER_SNAPSHOT);
-        if (systemTopicName == null) {
-            return FutureUtil.failedFuture(
-                    new InvalidTopicNameException("Can't create 
SystemTopicBaseTxnBufferSnapshotService, "
-                            + "because the topicName is null!"));
-        }
-        return 
CompletableFuture.completedFuture(clients.computeIfAbsent(systemTopicName,
-                (v) -> namespaceEventsSystemTopicFactory
-                        
.createTransactionBufferSystemTopicClient(topicName.getNamespaceObject(), 
this)));
-    }
-
-    @Override
-    public CompletableFuture<Reader<TransactionBufferSnapshot>> 
createReader(TopicName topicName) {
+    public CompletableFuture<SystemTopicClient.Reader<T>> 
createReader(TopicName topicName) {
         return 
getTransactionBufferSystemTopicClient(topicName).thenCompose(SystemTopicClient::newReaderAsync);
     }
 
-    @Override
-    public void removeClient(TopicName topicName,
-                                          TransactionBufferSystemTopicClient 
transactionBufferSystemTopicClient) {
+    public void removeClient(TopicName topicName, SystemTopicClientBase<T> 
transactionBufferSystemTopicClient) {
         if (transactionBufferSystemTopicClient.getReaders().size() == 0
                 && transactionBufferSystemTopicClient.getWriters().size() == 
0) {
             clients.remove(topicName);

Review Comment:
   How do we prevent concurrency between `createReader | createWriter` and 
`removeClient`? Now that we have a fixed number of system topics, maybe we 
should not recycle `SystemTopicClient` objects



##########
managed-ledger/src/main/java/org/apache/bookkeeper/mledger/ManagedLedgerFactory.java:
##########
@@ -117,6 +118,15 @@ ReadOnlyCursor openReadOnlyCursor(String 
managedLedgerName, Position startPositi
     void asyncOpenReadOnlyCursor(String managedLedgerName, Position 
startPosition, ManagedLedgerConfig config,
             OpenReadOnlyCursorCallback callback, Object ctx);
 
+    /**
+     * Asynchronous open a Read-only managedLedger.
+     * @param managedLedgerName the unique name that identifies the managed 
ledger
+     * @param config the managed ledegr configuratiion.
+     * @param ctx opaque context
+     */
+    CompletableFuture<ReadOnlyManagedLedgerImpl> 
asyncOpenReadOnlyManagedLedger(String managedLedgerName,

Review Comment:
   Why do we need this method when we can't seem to find anywhere else to use 
it but test?



##########
managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/ManagedLedgerFactoryImpl.java:
##########
@@ -420,7 +420,28 @@ public void closeFailed(ManagedLedgerException exception, 
Object ctx) {
         });
     }
 
-
+    @Override
+    public CompletableFuture<ReadOnlyManagedLedgerImpl> 
asyncOpenReadOnlyManagedLedger(String managedLedgerName,
+                              ManagedLedgerConfig config, Object ctx) {
+        CompletableFuture<ReadOnlyManagedLedgerImpl> future = new 
CompletableFuture<>();
+        if (closed) {
+            return FutureUtil.failedFuture(new 
ManagedLedgerException.ManagedLedgerFactoryClosedException());
+        }
+        ReadOnlyManagedLedgerImpl roManagedLedger = new 
ReadOnlyManagedLedgerImpl(this,
+                bookkeeperFactory
+                        .get(new 
EnsemblePlacementPolicyConfig(config.getBookKeeperEnsemblePlacementPolicyClassName(),
+                                
config.getBookKeeperEnsemblePlacementPolicyProperties())),
+                store, config, scheduledExecutor, managedLedgerName);
+        roManagedLedger.initialize().thenRun(() -> {
+            log.info("[{}] Successfully initialize Read-only managed ledger", 
managedLedgerName);
+            future.complete(roManagedLedger);
+        }).exceptionally(e -> {
+            log.error("[{}] Failed to initialize Read-only managed ledger", 
managedLedgerName, e);
+            future.completeExceptionally(new 
ManagedLedgerException.ManagedLedgerFactoryClosedException());

Review Comment:
   Why wrap it as a `ManagedLedgerFactoryClosedException `, and would it be 
better for the caller to handle it?



##########
pulsar-broker/src/main/java/org/apache/pulsar/broker/systopic/TransactionBufferSnapshotBaseSystemTopicClient.java:
##########
@@ -0,0 +1,218 @@
+/**
+ * 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.pulsar.broker.systopic;
+
+import java.io.IOException;
+import java.util.concurrent.CompletableFuture;
+import lombok.extern.slf4j.Slf4j;
+import org.apache.pulsar.broker.service.SystemTopicTxnBufferSnapshotService;
+import org.apache.pulsar.client.api.Message;
+import org.apache.pulsar.client.api.MessageId;
+import org.apache.pulsar.client.api.Producer;
+import org.apache.pulsar.client.api.PulsarClient;
+import org.apache.pulsar.client.api.PulsarClientException;
+import org.apache.pulsar.client.api.Schema;
+import org.apache.pulsar.common.naming.TopicName;
+
+@Slf4j
+public class  TransactionBufferSnapshotBaseSystemTopicClient<T> extends 
SystemTopicClientBase<T> {
+
+    protected final SystemTopicTxnBufferSnapshotService<T> 
systemTopicTxnBufferSnapshotService;
+    protected final Class<T> schemaType;
+
+    public TransactionBufferSnapshotBaseSystemTopicClient(PulsarClient client,
+                                                          TopicName topicName,
+                                                          
SystemTopicTxnBufferSnapshotService<T>
+                                                                  
systemTopicTxnBufferSnapshotService,
+                                                          Class<T> schemaType) 
{
+        super(client, topicName);
+        this.systemTopicTxnBufferSnapshotService = 
systemTopicTxnBufferSnapshotService;
+        this.schemaType = schemaType;
+    }
+
+    protected void removeWriter(Writer<T> writer) {
+        writers.remove(writer);
+        this.systemTopicTxnBufferSnapshotService.removeClient(topicName, this);
+    }
+
+    protected void removeReader(Reader<T> reader) {
+        readers.remove(reader);
+        this.systemTopicTxnBufferSnapshotService.removeClient(topicName, this);
+    }
+
+    protected static class TransactionBufferSnapshotWriter<T> implements 
Writer<T> {
+
+        protected final Producer<T> producer;
+        protected final TransactionBufferSnapshotBaseSystemTopicClient<T>
+                transactionBufferSnapshotBaseSystemTopicClient;
+
+        protected TransactionBufferSnapshotWriter(Producer<T> producer,
+                                                  
TransactionBufferSnapshotBaseSystemTopicClient<T>
+                                                    
transactionBufferSnapshotBaseSystemTopicClient) {
+            this.producer = producer;
+            this.transactionBufferSnapshotBaseSystemTopicClient = 
transactionBufferSnapshotBaseSystemTopicClient;
+        }
+
+        @Override
+        public MessageId write(T t, String key)
+                throws PulsarClientException {
+            return producer.newMessage().key(key)
+                    .value(t).send();
+        }
+
+        @Override
+        public CompletableFuture<MessageId> writeAsync(T t, String key) {
+            return producer.newMessage()
+                    .key(key)
+                    .value(t).sendAsync();
+        }
+
+        @Override
+        public MessageId delete(T t, String key)
+                throws PulsarClientException {
+            return producer.newMessage()
+                    .key(key)
+                    .value(null)
+                    .send();
+        }
+
+        @Override
+        public CompletableFuture<MessageId> deleteAsync(T t, String key) {
+            return producer.newMessage()
+                    .key(key)
+                    .value(null)
+                    .sendAsync();
+        }
+
+        @Override
+        public void close() throws IOException {
+            this.producer.close();
+            transactionBufferSnapshotBaseSystemTopicClient.removeWriter(this);

Review Comment:
   If `this.producer.close()` fails, should we delete `reader`? Maybe we can 
directly use `closeAsync().join`?



##########
pulsar-broker/src/main/java/org/apache/pulsar/broker/systopic/TransactionBufferSnapshotBaseSystemTopicClient.java:
##########
@@ -0,0 +1,218 @@
+/**
+ * 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.pulsar.broker.systopic;
+
+import java.io.IOException;
+import java.util.concurrent.CompletableFuture;
+import lombok.extern.slf4j.Slf4j;
+import org.apache.pulsar.broker.service.SystemTopicTxnBufferSnapshotService;
+import org.apache.pulsar.client.api.Message;
+import org.apache.pulsar.client.api.MessageId;
+import org.apache.pulsar.client.api.Producer;
+import org.apache.pulsar.client.api.PulsarClient;
+import org.apache.pulsar.client.api.PulsarClientException;
+import org.apache.pulsar.client.api.Schema;
+import org.apache.pulsar.common.naming.TopicName;
+
+@Slf4j
+public class  TransactionBufferSnapshotBaseSystemTopicClient<T> extends 
SystemTopicClientBase<T> {
+
+    protected final SystemTopicTxnBufferSnapshotService<T> 
systemTopicTxnBufferSnapshotService;
+    protected final Class<T> schemaType;
+
+    public TransactionBufferSnapshotBaseSystemTopicClient(PulsarClient client,
+                                                          TopicName topicName,
+                                                          
SystemTopicTxnBufferSnapshotService<T>
+                                                                  
systemTopicTxnBufferSnapshotService,
+                                                          Class<T> schemaType) 
{
+        super(client, topicName);
+        this.systemTopicTxnBufferSnapshotService = 
systemTopicTxnBufferSnapshotService;
+        this.schemaType = schemaType;
+    }
+
+    protected void removeWriter(Writer<T> writer) {
+        writers.remove(writer);
+        this.systemTopicTxnBufferSnapshotService.removeClient(topicName, this);
+    }
+
+    protected void removeReader(Reader<T> reader) {
+        readers.remove(reader);
+        this.systemTopicTxnBufferSnapshotService.removeClient(topicName, this);
+    }
+
+    protected static class TransactionBufferSnapshotWriter<T> implements 
Writer<T> {
+
+        protected final Producer<T> producer;
+        protected final TransactionBufferSnapshotBaseSystemTopicClient<T>
+                transactionBufferSnapshotBaseSystemTopicClient;
+
+        protected TransactionBufferSnapshotWriter(Producer<T> producer,
+                                                  
TransactionBufferSnapshotBaseSystemTopicClient<T>
+                                                    
transactionBufferSnapshotBaseSystemTopicClient) {
+            this.producer = producer;
+            this.transactionBufferSnapshotBaseSystemTopicClient = 
transactionBufferSnapshotBaseSystemTopicClient;
+        }
+
+        @Override
+        public MessageId write(T t, String key)
+                throws PulsarClientException {
+            return producer.newMessage().key(key)
+                    .value(t).send();
+        }
+
+        @Override
+        public CompletableFuture<MessageId> writeAsync(T t, String key) {
+            return producer.newMessage()
+                    .key(key)
+                    .value(t).sendAsync();
+        }
+
+        @Override
+        public MessageId delete(T t, String key)
+                throws PulsarClientException {
+            return producer.newMessage()
+                    .key(key)
+                    .value(null)
+                    .send();
+        }
+
+        @Override
+        public CompletableFuture<MessageId> deleteAsync(T t, String key) {
+            return producer.newMessage()
+                    .key(key)
+                    .value(null)
+                    .sendAsync();
+        }
+
+        @Override
+        public void close() throws IOException {
+            this.producer.close();
+            transactionBufferSnapshotBaseSystemTopicClient.removeWriter(this);
+        }
+
+        @Override
+        public CompletableFuture<Void> closeAsync() {
+            CompletableFuture<Void> completableFuture = new 
CompletableFuture<>();
+            producer.closeAsync().whenComplete((v, e) -> {
+                // if close fail, also need remove the producer
+                
transactionBufferSnapshotBaseSystemTopicClient.removeWriter(this);
+                if (e != null) {
+                    completableFuture.completeExceptionally(e);
+                    return;
+                }
+                completableFuture.complete(null);
+            });
+            return completableFuture;
+        }
+
+        @Override
+        public SystemTopicClient<T> getSystemTopicClient() {
+            return transactionBufferSnapshotBaseSystemTopicClient;
+        }
+    }
+
+    protected static class TransactionBufferSnapshotReader<T> implements 
Reader<T> {
+
+        private final org.apache.pulsar.client.api.Reader<T> reader;
+        private final TransactionBufferSnapshotBaseSystemTopicClient<T> 
transactionBufferSnapshotBaseSystemTopicClient;
+
+        protected TransactionBufferSnapshotReader(
+                org.apache.pulsar.client.api.Reader<T> reader,
+                TransactionBufferSnapshotBaseSystemTopicClient<T> 
transactionBufferSnapshotBaseSystemTopicClient) {
+            this.reader = reader;
+            this.transactionBufferSnapshotBaseSystemTopicClient = 
transactionBufferSnapshotBaseSystemTopicClient;
+        }
+
+        @Override
+        public Message<T> readNext() throws PulsarClientException {
+            return reader.readNext();
+        }
+
+        @Override
+        public CompletableFuture<Message<T>> readNextAsync() {
+            return reader.readNextAsync();
+        }
+
+        @Override
+        public boolean hasMoreEvents() throws PulsarClientException {
+            return reader.hasMessageAvailable();
+        }
+
+        @Override
+        public CompletableFuture<Boolean> hasMoreEventsAsync() {
+            return reader.hasMessageAvailableAsync();
+        }
+
+        @Override
+        public void close() throws IOException {
+            this.reader.close();
+            transactionBufferSnapshotBaseSystemTopicClient.removeReader(this);
+        }
+
+        @Override
+        public CompletableFuture<Void> closeAsync() {
+            CompletableFuture<Void> completableFuture = new 
CompletableFuture<>();
+            reader.closeAsync().whenComplete((v, e) -> {
+                // if close fail, also need remove the reader
+                
transactionBufferSnapshotBaseSystemTopicClient.removeReader(this);
+                if (e != null) {
+                    completableFuture.completeExceptionally(e);
+                    return;
+                }
+                completableFuture.complete(null);
+            });
+            return completableFuture;
+        }
+
+        @Override
+        public SystemTopicClient<T> getSystemTopic() {
+            return transactionBufferSnapshotBaseSystemTopicClient;
+        }
+    }
+
+    @Override
+    protected CompletableFuture<Writer<T>> newWriterAsyncInternal() {
+        return client.newProducer(Schema.AVRO(schemaType))
+                .topic(topicName.toString())
+                .createAsync().thenCompose(producer -> {

Review Comment:
   Should we use `thenApply`, which would save a `CompletableFuture` object? 
   
   Same for `newReaderAsyncInternal`.



##########
pulsar-client-api/src/main/java/org/apache/pulsar/client/api/transaction/TxnID.java:
##########
@@ -39,14 +43,14 @@ public class TxnID implements Serializable {
      *
      * @serial
      */
-    private final long mostSigBits;
+    private long mostSigBits;

Review Comment:
   Why remove the final modifier?
   
   Same for `leastSigBits`



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