poorbarcode commented on code in PR #16931:
URL: https://github.com/apache/pulsar/pull/16931#discussion_r999249164


##########
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:
   This code 
`transactionBufferSnapshotBaseSystemTopicClient.removeWriter(this); ` has 
already been executed once in `closeAsync`. should we remove this line?
   
   Same for `TransactionBufferSnapshotReader`



##########
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:
   There are many System topics under each namespace. I think these resources 
should be released( `gc(SystemTopicClient)` ) on the namespace deleted event. 
Now such an implementation will have a race condition: 
   
   - close the last `writer`
   - registry a new `writer`
   
   | close the last `writer` | registry a new `writer` | 
   |---|---|
   | check `transactionBufferSystemTopicClient.getWriters().size() == 0` : 
`true` |  |
   |  | registry new one |
   | remove(topic) |  |



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