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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4403) DFSClient can infer checksum type when not provided by reading first byte
Date Tue, 15 Jan 2013 23:44:12 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4403?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13554527#comment-13554527
] 

Todd Lipcon commented on HDFS-4403:
-----------------------------------

bq. new client talks to old server: new clients infers the checksum type by reading first
byte

If you mark it 'required', then the new client will fail to deserialize a response from an
old server, due to the missing field. To verify this, I changed my test protobuf above to
make the field required, and tried to deserialize a byte array which didn't include the field
using MyTestProto.parseFrom. It threw "InvalidProtocolBufferException: Message missing required
fields: testField"

BTW, the protobuf guide also talks about this specific case of changing the default:
{quote}
Changing a default value is generally OK, as long as you remember that default values are
never sent over the wire. Thus, if a program receives a message in which a particular field
isn't set, the program will see the default value as it was defined in that program's version
of the protocol. It will NOT see the default value that was defined in the sender's code.
{quote}
                
> DFSClient can infer checksum type when not provided by reading first byte
> -------------------------------------------------------------------------
>
>                 Key: HDFS-4403
>                 URL: https://issues.apache.org/jira/browse/HDFS-4403
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs-client
>    Affects Versions: 2.0.2-alpha
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>            Priority: Minor
>         Attachments: hdfs-4403.txt
>
>
> HDFS-3177 added the checksum type to OpBlockChecksumResponseProto, but the new protobuf
field is optional, with a default of CRC32. This means that this API, when used against an
older cluster (like earlier 0.23 releases) will falsely return CRC32 even if that cluster
has written files with CRC32C. This can cause issues for distcp, for example.
> Instead of defaulting the protobuf field to CRC32, we can leave it with no default, and
if the OpBlockChecksumResponseProto has no checksum type set, the client can send OP_READ_BLOCK
to read the first byte of the block, then grab the checksum type out of that response (which
has always been present)

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