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From "Milind Bhandarkar (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-224) Allow simplified versioning for namenode and datanode metadata.
Date Tue, 16 May 2006 23:05:06 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-224?page=comments#action_12412072 ] 

Milind Bhandarkar commented on HADOOP-224:
------------------------------------------

I am referring to exactly this kind of code:

if (version > 3) { 
  newField = in.readXXX(); 
} else { 
  newField = YYY; 
} 

As more and more of such code gets added to the load and save method, it becomes complicated
to follow. Such code is currently in the FSDirectory class, thus tightly coupling the abstraction
of directory to the format used to store this data in the FSImage.

If we instead separate this into a separate class, say FSImageReader, and as we make major
changes to file format, introduce new classes such as FSImageReaderV2, FSImaageReaderV3 etc.
A member in FSDirectory will hold a reference to current file format reader, of which, the
"parse" method will be called by FSDirectory. If FSImageReaderV3 is the current version, and
it is trying to read an image that it cannot read, it will call FSImageReaderV2's parse method,
and so on. Since the readers will not have any real state, the parse methods will be static
methods, to avoid overhead of chaining.

So, as a first step, I could separate the loadFSImage and loadFSEdits into two different classes.

Does it make sense ?

> 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 format.
> 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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