Client's Request :
=================
Processed in DataXceiver:
+----------------------------------------------+
| Common Header | 1 byte OP == OP_READ_BLOCK |
+----------------------------------------------+
Processed in readBlock() :
+-------------------------------------------------------------------------+
| 8 byte Block ID | 8 byte genstamp | 8 byte start offset | 8 byte length |
+-------------------------------------------------------------------------+
| vInt length | <DFSClient id> |
+-----------------------------------+
Client sends optional response only at the end of receiving data.
DataNode Response :
===================
In readBlock() :
If there is an error while initializing BlockSender :
+---------------------------+
| 2 byte OP_STATUS_ERROR | and connection will be closed.
+---------------------------+
Otherwise
+---------------------------+
| 2 byte OP_STATUS_SUCCESS |
+---------------------------+
Actual data, sent by BlockSender.sendBlock() :
ChecksumHeader :
+--------------------------------------------------+
| 1 byte CHECKSUM_TYPE | 4 byte BYTES_PER_CHECKSUM |
+--------------------------------------------------+
Followed by actual data in the form of PACKETS:
+------------------------------------+
| Sequence of data PACKETs .... |
+------------------------------------+
A "PACKET" is defined further below.
The client reads data until it receives a packet with
"LastPacketInBlock" set to true or with a zero length. If there is
no checksum error, it replies to DataNode with OP_STATUS_CHECKSUM_OK:
Client optional response at the end of data transmission :
+------------------------------+
| 2 byte OP_STATUS_CHECKSUM_OK |
+------------------------------+
PACKET : Contains a packet header, checksum and data. Amount of data
======== carried is set by BUFFER_SIZE.
+-----------------------------------------------------+
| 4 byte packet length (excluding packet header) |
+-----------------------------------------------------+
| 8 byte offset in the block | 8 byte sequence number |
+-----------------------------------------------------+
| 1 byte isLastPacketInBlock |
+-----------------------------------------------------+
| 4 byte Length of actual data |
+-----------------------------------------------------+
| x byte checksum data. x is defined below |
+-----------------------------------------------------+
| actual data ...... |
+-----------------------------------------------------+
x = (length of data + BYTE_PER_CHECKSUM - 1)/BYTES_PER_CHECKSUM *
CHECKSUM_SIZE
CHECKSUM_SIZE depends on CHECKSUM_TYPE (usually, 4 for CRC32)
The above packet format is used while writing data to DFS also.
Not all the fields might be used while reading.
