NettyBlockTransferService — Netty-Based BlockTransferService

NettyBlockTransferService is a BlockTransferService that uses Netty for block transport (when uploading or fetching blocks of data).

NettyBlockTransferService is created when SparkEnv is created (and later passed on to create a BlockManager for the driver and executors).

Enable INFO or TRACE logging level for logger to see what happens inside.

Add the following line to conf/

Refer to Logging.

Creating NettyBlockTransferService Instance


fetchBlocks Method

  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 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 called when BlockTransferService fetches one block synchronously and ShuffleBlockFetcherIterator sends a request for blocks (using sendRequest).

Application Id — appId Property


Initializing NettyBlockTransferService — init Method

init(blockDataManager: BlockDataManager): Unit
init is part of the BlockTransferService contract.

init starts a server for…​FIXME

Internally, init creates a NettyBlockRpcServer (using the application id, a JavaSerializer and the input blockDataManager).

FIXME Describe security when authEnabled is enabled.

init creates a TransportContext with the NettyBlockRpcServer created earlier.

FIXME Describe transportConf and 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:

INFO NettyBlockTransferService: 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).

Uploading Block — uploadBlock Method

  hostname: String,
  port: Int,
  execId: String,
  blockId: BlockId,
  blockData: ManagedBuffer,
  level: StorageLevel,
  classTag: ClassTag[_]): Future[Unit]
uploadBlock is part of the BlockTransferService contract.

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 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.

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