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http://issues.apache.org/jira/browse/HADOOP-224?page=comments#action_12412070 ]
Doug Cutting commented on HADOOP-224:
-------------------------------------
I certainly agree that this data should be versioned.
I'm not sure what you mean by, "As we add information to these metadata, the
complexity of the reader increases dramatically". Typically I'd expect we'll
be adding new fields to each entry, and perhaps sometimes removing a field, or
adding a new type of entry. Do you have something else in mind?
The way I'm used to handling this is to put a version byte at the start of each
file and perhaps even each record. I'm not sure that a major/minor versioning
system is required, nor that we need a registry of reader classes, etc. I've
handled lots of such format changes with simple code like:
byte version = in.readByte();
...
if (version > 3) {
newField = in.readXXX();
} else {
newField = YYY;
}
This assumes that one is implementing the readFields() method directly. If
we're instead using generated code to read items, then perhaps an alternate
mechanism is required. Is that what you're assuming?
> 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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