[jira] Commented: (HADOOP-558) Adding versioning to Writable classes

["Doug Cutting (JIRA)" <ji...@apache.org>]

    [ http://issues.apache.org/jira/browse/HADOOP-558?page=comments#action_12437681 ] 
            
Doug Cutting commented on HADOOP-558:
-------------------------------------

Part of this, I assume, will include:

1.In SequenceFile, adding to the header the version map for key and value classes (& all classes referenced by their non-transient fields), and passing this map to keys and values as they're read.

2. In RPC, Transmitting the version map for all parameter & return classes (& those refernced by their fields) from the client to the server as a handshake on connect.



> 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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