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NameNode源码注释
/**********************************************************
* NameNode serves as both directory namespace manager and
* "inode table" for the Hadoop DFS. There is a single NameNode
* running in any DFS deployment. (Well, except when there
* is a second backup/failover NameNode, or when using federated NameNodes.)
*
* The NameNode controls two critical tables:
* 1) filename->blocksequence (namespace)
* 2) block->machinelist ("inodes")
*
* The first table is stored on disk and is very precious.
* The second table is rebuilt every time the NameNode comes up.
*
* 'NameNode' refers to both this class as well as the 'NameNode server'.
* The 'FSNamesystem' class actually performs most of the filesystem
* management. The majority of the 'NameNode' class itself is concerned
* with exposing the IPC interface and the HTTP server to the outside world,
* plus some configuration management.
*
* NameNode implements the
* {@link org.apache.hadoop.hdfs.protocol.ClientProtocol} interface, which
* allows clients to ask for DFS services.
* {@link org.apache.hadoop.hdfs.protocol.ClientProtocol} is not designed for
* direct use by authors of DFS client code. End-users should instead use the
* {@link org.apache.hadoop.fs.FileSystem} class.
*
* NameNode also implements the
* {@link org.apache.hadoop.hdfs.server.protocol.DatanodeProtocol} interface,
* used by DataNodes that actually store DFS data blocks. These
* methods are invoked repeatedly and automatically by all the
* DataNodes in a DFS deployment.
*
* NameNode also implements the
* {@link org.apache.hadoop.hdfs.server.protocol.NamenodeProtocol} interface,
* used by secondary namenodes or rebalancing processes to get partial
* NameNode state, for example partial blocksMap etc.
**********************************************************/
DataNode源码注释
/**********************************************************
* DataNode is a class (and program) that stores a set of
* blocks for a DFS deployment. A single deployment can
* have one or many DataNodes. Each DataNode communicates
* regularly with a single NameNode. It also communicates
* with client code and other DataNodes from time to time.
*
* DataNodes store a series of named blocks. The DataNode
* allows client code to read these blocks, or to write new
* block data. The DataNode may also, in response to instructions
* from its NameNode, delete blocks or copy blocks to/from other
* DataNodes.
*
* The DataNode maintains just one critical table:
* block-> stream of bytes (of BLOCK_SIZE or less)
*
* This info is stored on a local disk. The DataNode
* reports the table's contents to the NameNode upon startup
* and every so often afterwards.
*
* DataNodes spend their lives in an endless loop of asking
* the NameNode for something to do. A NameNode cannot connect
* to a DataNode directly; a NameNode simply returns values from
* functions invoked by a DataNode.
*
* DataNodes maintain an open server socket so that client code
* or other DataNodes can read/write data. The host/port for
* this server is reported to the NameNode, which then sends that
* information to clients or other DataNodes that might be interested.
*
**********************************************************/
SecondaryNameNode源码注释
/**********************************************************
* The Secondary NameNode is a helper to the primary NameNode.
* The Secondary is responsible for supporting periodic checkpoints
* of the HDFS metadata. The current design allows only one Secondary
* NameNode per HDFs cluster.
*
* The Secondary NameNode is a daemon that periodically wakes
* up (determined by the schedule specified in the configuration),
* triggers a periodic checkpoint and then goes back to sleep.
* The Secondary NameNode uses the NamenodeProtocol to talk to the
* primary NameNode.
*
**********************************************************/ 