Adding versioning to Writable classes
-------------------------------------
Key: HADOOP-558
URL: http://issues.apache.org/jira/browse/HADOOP-558
Project: Hadoop
Issue Type: Improvement
Components: io
Affects Versions: 0.6.2
Reporter: Owen O'Malley
Assigned To: Owen O'Malley
We currently don't have a way to support versioned Writables, which among other
problems means that it is very difficult to change the serialization of any
types. (A recent example is that we can't change any of the Writables that
currently use UTF8.writeString to use Text.writeString with out breaking
everything.) Just changing the file version doesn't work because you can't read
the old versions.
Therefore, I propose adding a new interface:
public interface VersionMapWritable extends Writable {
/**
* Destructively read into this object from in based on the version map.
* @param versionMap a map from each class to its version in the DataInput
(version 0 classes are omitted)
*/
public void readFields(DataInput in, Map<Class<VersionMapWritable>, int>
versionMap) throws IOException;
/**
* Classes with non-zero versions should register themselves in static blocks.
*/
public static void registerCurrentVersion(Class<VersionMapWritable> class,
int version) {...}
/**
* If a version map includes the parent type, always include the child type
as well.
*/
public static void addDependence(Class<VersionMapWritable> parent,
Class<VersionMapWritable> child) { ... }
/**
* Build a version map for a given list of classes, including any dependent
types.
*/
public static Map<Class<VersionMapWritable>, int>
buildVersionMap(Set<Class<VersionMapWritable>> classes) {...}
/**
* Add the types in the parameter/result types to the list of classes.
* Recursively adds the field types for any new types that are added to the
set.
*/
public static void addMethodTypes(Set<Class<VersionMapWritable>> result,
Class<VersionedProtocol> protocol) {...}
/**
* Add the non-transient fields to the list of classes.
*/
public static void addFieldTypes(Set<Class<VersionMapWritable>> result,
Class<Writable> writable) {...}
public static Map<Class<VersionMapWritable>, int> readVersionMap(DataInput
in) throws IOException { ... }
public static void writeVersionMap(DataOutput out,
Map<Class<VersionMapWritable> versionMap) throws IOException
{...}
}
VersionedWritable, which stored a version byte within each object, will be
depriciated.
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