1. Add new setting for Kafka engine, named kafka_thread_per_consumer which...

 1. Add new setting for Kafka engine, named kafka_thread_per_consumer which default value is false. 2. Create separate thread pool for Kafka engine.

src/Common/CurrentMetrics.cpp

@@ -14,6 +14,7 @@
     M(BackgroundSchedulePoolTask, "Number of active tasks in BackgroundSchedulePool. This pool is used for periodic ReplicatedMergeTree tasks, like cleaning old data parts, altering data parts, replica re-initialization, etc.") \
     M(BackgroundBufferFlushSchedulePoolTask, "Number of active tasks in BackgroundBufferFlushSchedulePool. This pool is used for periodic Buffer flushes") \
     M(BackgroundDistributedSchedulePoolTask, "Number of active tasks in BackgroundDistributedSchedulePool. This pool is used for distributed sends that is done in background.") \
+    M(BackgroundMessageBrokerSchedulePoolTask, "Number of active tasks in BackgroundProcessingPool for message streaming") \
     M(CacheDictionaryUpdateQueueBatches, "Number of 'batches' (a set of keys) in update queue in CacheDictionaries.") \
     M(CacheDictionaryUpdateQueueKeys, "Exact number of keys in update queue in CacheDictionaries.") \
     M(DiskSpaceReservedForMerge, "Disk space reserved for currently running background merges. It is slightly more than the total size of currently merging parts.") \
@@ -38,6 +39,7 @@
     M(MemoryTrackingInBackgroundSchedulePool, "Total amount of memory (bytes) allocated in background schedule pool (that is dedicated for bookkeeping tasks of Replicated tables).") \
     M(MemoryTrackingInBackgroundBufferFlushSchedulePool, "Total amount of memory (bytes) allocated in background buffer flushes pool (that is dedicated for background buffer flushes).") \
     M(MemoryTrackingInBackgroundDistributedSchedulePool, "Total amount of memory (bytes) allocated in background distributed schedule pool (that is dedicated for distributed sends).") \
+    M(MemoryTrackingInBackgroundMessageBrokerSchedulePool, "Total amount of memory (bytes) allocated in background message broker pool (that is dedicated for background message streaming).") \
     M(MemoryTrackingForMerges, "Total amount of memory (bytes) allocated for background merges. Included in MemoryTrackingInBackgroundProcessingPool. Note that this value may include a drift when the memory was allocated in a context of background processing pool and freed in other context or vice-versa. This happens naturally due to caches for tables indexes and doesn't indicate memory leaks.") \
     M(EphemeralNode, "Number of ephemeral nodes hold in ZooKeeper.") \
     M(ZooKeeperSession, "Number of sessions (connections) to ZooKeeper. Should be no more than one, because using more than one connection to ZooKeeper may lead to bugs due to lack of linearizability (stale reads) that ZooKeeper consistency model allows.") \

src/Core/Settings.h

@@ -72,6 +72,7 @@ class IColumn;
     M(UInt64, background_pool_size, 16, "Number of threads performing background work for tables (for example, merging in merge tree). Only has meaning at server startup.", 0) \
     M(UInt64, background_move_pool_size, 8, "Number of threads performing background moves for tables. Only has meaning at server startup.", 0) \
     M(UInt64, background_schedule_pool_size, 16, "Number of threads performing background tasks for replicated tables, kafka streaming, dns cache updates. Only has meaning at server startup.", 0) \
+    M(UInt64, background_message_broker_schedule_pool_size, 16, "Number of threads performing background tasks for kafka streaming. Only has meaning at server startup.", 0) \
     M(UInt64, background_distributed_schedule_pool_size, 16, "Number of threads performing background tasks for distributed sends. Only has meaning at server startup.", 0) \
     \
     M(Milliseconds, distributed_directory_monitor_sleep_time_ms, 100, "Sleep time for StorageDistributed DirectoryMonitors, in case of any errors delay grows exponentially.", 0) \

src/Interpreters/Context.cpp

@@ -82,6 +82,9 @@ namespace CurrentMetrics
 
     extern const Metric BackgroundDistributedSchedulePoolTask;
     extern const Metric MemoryTrackingInBackgroundDistributedSchedulePool;
+    
+    extern const Metric BackgroundMessageBrokerSchedulePoolTask;
+    extern const Metric MemoryTrackingInBackgroundMessageBrokerSchedulePool;
 }
 
 
@@ -341,6 +344,7 @@ struct ContextShared
     std::optional<BackgroundProcessingPool> background_move_pool; /// The thread pool for the background moves performed by the tables.
     std::optional<BackgroundSchedulePool> schedule_pool;    /// A thread pool that can run different jobs in background (used in replicated tables)
     std::optional<BackgroundSchedulePool> distributed_schedule_pool; /// A thread pool that can run different jobs in background (used for distributed sends)
+    std::optional<BackgroundSchedulePool> message_broker_schedule_pool;    /// A thread pool that can run different jobs in background (used in kafka streaming)
     MultiVersion<Macros> macros;                            /// Substitutions extracted from config.
     std::unique_ptr<DDLWorker> ddl_worker;                  /// Process ddl commands from zk.
     /// Rules for selecting the compression settings, depending on the size of the part.
@@ -441,6 +445,7 @@ struct ContextShared
         schedule_pool.reset();
         distributed_schedule_pool.reset();
         ddl_worker.reset();
+        message_broker_schedule_pool.reset();
 
         /// Stop trace collector if any
         trace_collector.reset();
@@ -1421,6 +1426,18 @@ BackgroundSchedulePool & Context::getDistributedSchedulePool()
     return *shared->distributed_schedule_pool;
 }
 
+BackgroundSchedulePool & Context::getMessageBrokerSchedulePool()
+{
+    auto lock = getLock();
+    if (!shared->message_broker_schedule_pool)
+        shared->message_broker_schedule_pool.emplace(
+            settings.background_message_broker_schedule_pool_size,
+            CurrentMetrics::BackgroundMessageBrokerSchedulePoolTask,
+            CurrentMetrics::MemoryTrackingInBackgroundMessageBrokerSchedulePool,
+            "BgMBSchPool");
+    return *shared->message_broker_schedule_pool;
+}
+
 void Context::setDDLWorker(std::unique_ptr<DDLWorker> ddl_worker)
 {
     auto lock = getLock();

src/Interpreters/Context.h

@@ -503,6 +503,7 @@ public:
     BackgroundProcessingPool & getBackgroundPool();
     BackgroundProcessingPool & getBackgroundMovePool();
     BackgroundSchedulePool & getSchedulePool();
+    BackgroundSchedulePool & getMessageBrokerSchedulePool();
     BackgroundSchedulePool & getDistributedSchedulePool();
 
     void setDDLWorker(std::unique_ptr<DDLWorker> ddl_worker);

src/Storages/Kafka/KafkaSettings.h

@@ -28,7 +28,8 @@ class ASTStorage;
     M(String, kafka_format, "", "The message format for Kafka engine.", 0) \
     M(Char, kafka_row_delimiter, '\0', "The character to be considered as a delimiter in Kafka message.", 0) \
     M(String, kafka_schema, "", "Schema identifier (used by schema-based formats) for Kafka engine", 0) \
-    M(UInt64, kafka_skip_broken_messages, 0, "Skip at least this number of broken messages from Kafka topic per block", 0)
+    M(UInt64, kafka_skip_broken_messages, 0, "Skip at least this number of broken messages from Kafka topic per block", 0) \
+    M(Bool, kafka_thread_per_consumer, false, "Provide independent thread for each consumer", 0)
 
     /** TODO: */
     /* https://github.com/edenhill/librdkafka/blob/master/CONFIGURATION.md */

src/Storages/Kafka/StorageKafka.cpp

@@ -138,11 +138,13 @@ StorageKafka::StorageKafka(
     , semaphore(0, num_consumers)
     , intermediate_commit(kafka_settings->kafka_commit_every_batch.value)
     , settings_adjustments(createSettingsAdjustments())
+    , thread_per_consumer(kafka_settings->kafka_thread_per_consumer.value)
 {
     StorageInMemoryMetadata storage_metadata;
     storage_metadata.setColumns(columns_);
     setInMemoryMetadata(storage_metadata);
-    for (size_t i = 0; i < num_consumers; i++)
+    auto task_count = thread_per_consumer ? num_consumers : 1;
+    for (size_t i = 0; i < task_count; ++i)
     {
         auto task = global_context.getSchedulePool().createTask(log->name(), [this, i]{ threadFunc(i); });
         task->deactivate();
@@ -258,20 +260,24 @@ void StorageKafka::startup()
         {
             tryLogCurrentException(log);
         }
+    }
+
     // Start the reader thread
+    for (size_t i = 0; i < tasks.size(); ++i)
+    {
         tasks[i]->holder->activateAndSchedule();
     }
-
 }
 
 
 void StorageKafka::shutdown()
 {
-    // Interrupt streaming thread
-    LOG_TRACE(log, "Waiting for cleanup");
-    for (size_t i = 0; i < num_consumers; i++)
+    for (size_t i = 0; i < tasks.size(); ++i)
     {
+        // Interrupt streaming thread
         tasks[i]->stream_cancelled = true;
+
+        LOG_TRACE(log, "Waiting for cleanup");
         tasks[i]->holder->deactivate();
     }
 
@@ -374,8 +380,12 @@ ConsumerBufferPtr StorageKafka::createReadBuffer(const size_t consumer_number)
     consumer->set_destroy_flags(RD_KAFKA_DESTROY_F_NO_CONSUMER_CLOSE);
 
     /// NOTE: we pass |stream_cancelled| by reference here, so the buffers should not outlive the storage.
+    if (thread_per_consumer)
+    {
         auto& stream_cancelled = tasks[consumer_number]->stream_cancelled;
         return std::make_shared<ReadBufferFromKafkaConsumer>(consumer, log, getPollMaxBatchSize(), getPollTimeoutMillisecond(), intermediate_commit, stream_cancelled, topics);
+    }
+    return std::make_shared<ReadBufferFromKafkaConsumer>(consumer, log, getPollMaxBatchSize(), getPollTimeoutMillisecond(), intermediate_commit, tasks.back()->stream_cancelled, topics);
 }
 
 size_t StorageKafka::getMaxBlockSize() const
@@ -473,7 +483,8 @@ bool StorageKafka::checkDependencies(const StorageID & table_id)
 
 void StorageKafka::threadFunc(size_t idx)
 {
-    auto& stream_cancelled = tasks[idx]->stream_cancelled;
+    assert(idx < tasks.size());
+    auto task = tasks[idx];
     try
     {
         auto table_id = getStorageID();
@@ -484,7 +495,7 @@ void StorageKafka::threadFunc(size_t idx)
             auto start_time = std::chrono::steady_clock::now();
 
             // Keep streaming as long as there are attached views and streaming is not cancelled
-            while (!stream_cancelled && num_created_consumers > 0)
+            while (!task->stream_cancelled && num_created_consumers > 0)
             {
                 if (!checkDependencies(table_id))
                     break;
@@ -515,10 +526,8 @@ void StorageKafka::threadFunc(size_t idx)
     }
 
     // Wait for attached views
-    if (!stream_cancelled)
-    {
-        tasks[idx]->holder->scheduleAfter(RESCHEDULE_MS);
-    }
+    if (!task->stream_cancelled)
+        task->holder->scheduleAfter(RESCHEDULE_MS);
 }
 
 
@@ -545,7 +554,15 @@ bool StorageKafka::streamToViews()
     InterpreterInsertQuery interpreter(insert, *kafka_context, false, true, true);
     auto block_io = interpreter.execute();
 
+    // Create a stream for each consumer and join them in a union stream
+    BlockInputStreams streams;
+  
+    auto stream_count = thread_per_consumer ? 1 : num_created_consumers;
+    streams.reserve(stream_count);
+    for (size_t i = 0; i < stream_count; ++i)
+    {
         auto stream = std::make_shared<KafkaBlockInputStream>(*this, metadata_snapshot, kafka_context, block_io.out->getHeader().getNames(), log, block_size, false);
+        streams.emplace_back(stream);
 
         // Limit read batch to maximum block size to allow DDL
         IBlockInputStream::LocalLimits limits;
@@ -556,16 +573,26 @@ bool StorageKafka::streamToViews()
 
         limits.timeout_overflow_mode = OverflowMode::BREAK;
         stream->setLimits(limits);
+    }
 
+    // Join multiple streams if necessary
+    BlockInputStreamPtr in;
+    if (streams.size() > 1)
+        in = std::make_shared<UnionBlockInputStream>(streams, nullptr, streams.size());
+    else
+        in = streams[0];
 
     // We can't cancel during copyData, as it's not aware of commits and other kafka-related stuff.
     // It will be cancelled on underlying layer (kafka buffer)
     std::atomic<bool> stub = {false};
-    copyData(*stream, *block_io.out, &stub);
+    copyData(*in, *block_io.out, &stub);
 
     bool some_stream_is_stalled = false;
-    some_stream_is_stalled = some_stream_is_stalled || stream->isStalled();
-    stream->commit();
+    for (auto & stream : streams)
+    {
+        some_stream_is_stalled = some_stream_is_stalled || stream->as<KafkaBlockInputStream>()->isStalled();
+        stream->as<KafkaBlockInputStream>()->commit();
+    }
 
     return some_stream_is_stalled;
 }

src/Storages/Kafka/StorageKafka.h

@@ -103,6 +103,7 @@ private:
         } 
     };
     std::vector<std::shared_ptr<TaskContext>> tasks;
+    bool thread_per_consumer = false;
 
     SettingsChanges createSettingsAdjustments();
     ConsumerBufferPtr createReadBuffer(const size_t consumer_number);