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From "Milind Bhandarkar (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-224) Allow simplified versioning for namenode and datanode metadata.
Date Tue, 16 May 2006 21:56:05 GMT
Allow simplified versioning for namenode and datanode metadata.

         Key: HADOOP-224
         URL: http://issues.apache.org/jira/browse/HADOOP-224
     Project: Hadoop
        Type: Improvement

  Components: dfs  
 Environment: All
    Reporter: Milind Bhandarkar

Currently namenode has two types of metadata: The FSImage, and FSEdits. FSImage contains information
abut Inodes, and FSEdits contains a list of operations that were not saved to FSImage. Datanode
currently does not have any metadata, but would have it some day. 

The file formats used for storing these metadata will evolve over time. It is important for
the file-system to be backward compatible. That is, the metadata readers need to be able to
identify which version of the file-format we are using, and need to be able to read information
therein. As we add information to these metadata, the complexity of the reader increases dramatically.

I propose a versioning scheme with a major and minor version number, where a different reader
class is associated with a major number, and that class interprets the minor number internally.
The readers essentially form a chain starting with the latest version. Each version-reader
looks at the file and if it does not recognize the version number, passes it to the version
reader next to it by calling the parse method, returnng the results of the parse method up
the chain (In case of the namenode, the parse result is an array of Inodes.

This scheme has an advantage that every time a new major version is added, the new reader
only needs to know about the reader for its immediately previous version, and every reader
needs to know only about which major version numbers it can read.

The writer is not so versioned, because metadata is always written in the most current version

One more change that is needed for simplified versioning is that the "struct-surping" of dfs.Block
needs to be removed. Block's contents will change in later versions, and older versions should
still be able to readFields properly. This is more general than Block of course, and in general
only basic datatypes should be used as Writables in DFS metadata.

For edits, the reader should return <opcode, ArrayWritable> pairs' array. This will
also remove the limitation of two operands for very opcodes, and will be more extensible.

Even with this new versioning scheme, the last Reader in the reader-chain would recognize
current format, thus maintaining full backward compatibility.

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