NettyBlockTransferService is a BlockTransferService that uses Netty for uploading or fetching blocks of data.

Figure 1. NettyBlockTransferService, SparkEnv and BlockManager

Creating Instance

NettyBlockTransferService takes the following to be created:

  • SparkConf

  • SecurityManager

  • Bind address

  • Host name

  • Port number

  • Number of CPU cores

NettyBlockTransferService is created when SparkEnv is created for the driver and executors (and creates the BlockManager).

TransportConf, TransportContext

NettyBlockTransferService creates a TransportConf for shuffle module (using SparkTransportConf utility) when created.

NettyBlockTransferService uses the TransportConf for the following:

NettyBlockTransferService uses the TransportContext to create the TransportClientFactory and the TransportServer.


NettyBlockTransferService creates a TransportClientFactory when requested to initialize.

NettyBlockTransferService uses the TransportClientFactory for the following:

NettyBlockTransferService requests the TransportClientFactory to close when requested to close.


NettyBlockTransferService creates a TransportServer when requested to initialize.

NettyBlockTransferService uses the TransportServer for the following:

NettyBlockTransferService requests the TransportServer to close when requested to close.


NettyBlockTransferService simply requests the TransportServer for the port.

Shuffle Metrics

shuffleMetrics(): MetricSet


shuffleMetrics is part of the ShuffleClient abstraction.

Fetching Blocks

  host: String,
  port: Int,
  execId: String,
  blockIds: Array[String],
  listener: BlockFetchingListener): Unit

When executed, fetchBlocks prints out the following TRACE message in the logs:

TRACE Fetch blocks from [host]:[port] (executor id [execId])

fetchBlocks then creates a RetryingBlockFetcher.BlockFetchStarter where createAndStart method…​FIXME

Depending on the maximum number of acceptable IO exceptions (such as connection timeouts) per request, if the number is greater than 0, fetchBlocks creates a RetryingBlockFetcher and starts it immediately.

RetryingBlockFetcher is created with the RetryingBlockFetcher.BlockFetchStarter created earlier, the input blockIds and listener.

If however the number of retries is not greater than 0 (it could be 0 or less), the RetryingBlockFetcher.BlockFetchStarter created earlier is started (with the input blockIds and listener).

In case of any Exception, you should see the following ERROR message in the logs and the input BlockFetchingListener gets notified (using onBlockFetchFailure for every block id).

ERROR Exception while beginning fetchBlocks

fetchBlocks is part of BlockTransferService abstraction.

Application Id

Closing NettyBlockTransferService

close(): Unit


close is part of the BlockTransferService abstraction.

Initializing NettyBlockTransferService

  blockDataManager: BlockDataManager): Unit

init creates a NettyBlockRpcServer (for the application id, a JavaSerializer and the given BlockDataManager) that is used to create a TransportContext.

init creates the internal clientFactory and a server.

FIXME What’s the "a server"?

In the end, you should see the INFO message in the logs:

Server created on [hostName]:[port]
hostname is given when NettyBlockTransferService is created and is controlled by Spark property for the driver and differs per deployment environment for executors (as controlled by --hostname for CoarseGrainedExecutorBackend).

init is part of the BlockTransferService abstraction.

Uploading Block

  hostname: String,
  port: Int,
  execId: String,
  blockId: BlockId,
  blockData: ManagedBuffer,
  level: StorageLevel,
  classTag: ClassTag[_]): Future[Unit]

Internally, uploadBlock creates a TransportClient client to send a UploadBlock message (to the input hostname and port).

UploadBlock message is processed by NettyBlockRpcServer.

The UploadBlock message holds the application id, the input execId and blockId. It also holds the serialized bytes for block metadata with level and classTag serialized (using the internal JavaSerializer) as well as the serialized bytes for the input blockData itself (this time however the serialization uses ManagedBuffer.nioByteBuffer method).

The entire UploadBlock message is further serialized before sending (using TransportClient.sendRpc).

FIXME Describe TransportClient and clientFactory.createClient.

When blockId block was successfully uploaded, you should see the following TRACE message in the logs:

TRACE NettyBlockTransferService: Successfully uploaded block [blockId]

When an upload failed, you should see the following ERROR message in the logs:

ERROR Error while uploading block [blockId]

uploadBlock is part of the BlockTransferService abstraction.

UploadBlock Message

UploadBlock is a BlockTransferMessage that describes a block being uploaded, i.e. send over the wire from a NettyBlockTransferService to a NettyBlockRpcServer.

Table 1. UploadBlock Attributes
Attribute Description


The application id (the block belongs to)


The executor id


The block id



The block data as an array of bytes

As an Encodable, UploadBlock can calculate the encoded size and do encoding and decoding itself to or from a ByteBuf, respectively.

createServer Internal Method

  bootstraps: List[TransportServerBootstrap]): TransportServer


createServer is used when NettyBlockTransferService is requested to initialize.


Enable ALL logging level for logger to see what happens inside.

Add the following line to conf/

Refer to Logging.