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http://issues.apache.org/jira/browse/HADOOP-224?page=comments#action_12412075 ]
Doug Cutting commented on HADOOP-224:
-------------------------------------
Yes, it makes sense, but it sounds like a lot of duplicated code. And how will
fields added in V3 be populated correctly by the V2 reader? In other words,
we'll still need code in V3 that knows how to upgrade things from V2. Yes,
this code could be separated from the raw i/o code, but I don't see how it
really saves much complexity.
To continue my example above, under your scheme you'd have something like:
public void readFields(DataInput in) {
byte version = in.readByte();
if (version < 3) {
readFieldsV2(in);
newField = YYY; // version 2 didn't have this field, so we default its
value
} else {
ReadFieldsV3(in)
}
}
public static void readFieldsV3(Foo instance, DataInput in) {
readFieldsV2(in);
newField = in.readYYY();
}
Which doesn't look a lot simpler to me. Or have I missed something?
> 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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