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Date: Mon, 18 Jul 2016 17:46:20 +0000 (UTC)
From: "Doug Cutting (JIRA)" <jira@apache.org>
To: dev@avro.apache.org
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Subject: [jira] [Commented] (AVRO-1704) Standardized format for encoding
 messages with Avro
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archived-at: Mon, 18 Jul 2016 17:46:22 -0000


    [ https://issues.apache.org/jira/browse/AVRO-1704?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15382696#comment-15382696 ] 

Doug Cutting commented on AVRO-1704:
------------------------------------

I doubt we'll ever need this abstracted, and having it so might encourage a proliferation of message formats, but it might also prove useful someday, so I can live with that.  However I still don't see the need for multiple levels of abstraction (interface + abstract base class).  That still seems like vast overkill to me, but is probably not worth fighting about.

As far as terminology, "datum" is used to refer to the in-memory data structure (generic, specific, reflect, Thrift, protobuf) while "encode/decode" refer to specific serialized formats (binary, json).  A reader/writer translates between the in-memory structure and the abstract encoding API.  So where does "message" fit into this taxonomy?  I suppose it's a new serialized format, an extension of "binary", so "encode/decode" are probably more appropriate than "read/write".

Not sure what you mean about the builder.

> Standardized format for encoding messages with Avro
> ---------------------------------------------------
>
>                 Key: AVRO-1704
>                 URL: https://issues.apache.org/jira/browse/AVRO-1704
>             Project: Avro
>          Issue Type: Improvement
>            Reporter: Daniel Schierbeck
>            Assignee: Niels Basjes
>             Fix For: 1.9.0, 1.8.3
>
>         Attachments: AVRO-1704-2016-05-03-Unfinished.patch, AVRO-1704-20160410.patch
>
>
> I'm currently using the Datafile format for encoding messages that are written to Kafka and Cassandra. This seems rather wasteful:
> 1. I only encode a single record at a time, so there's no need for sync markers and other metadata related to multi-record files.
> 2. The entire schema is inlined every time.
> However, the Datafile format is the only one that has been standardized, meaning that I can read and write data with minimal effort across the various languages in use in my organization. If there was a standardized format for encoding single values that was optimized for out-of-band schema transfer, I would much rather use that.
> I think the necessary pieces of the format would be:
> 1. A format version number.
> 2. A schema fingerprint type identifier, i.e. Rabin, MD5, SHA256, etc.
> 3. The actual schema fingerprint (according to the type.)
> 4. Optional metadata map.
> 5. The encoded datum.
> The language libraries would implement a MessageWriter that would encode datums in this format, as well as a MessageReader that, given a SchemaStore, would be able to decode datums. The reader would decode the fingerprint and ask its SchemaStore to return the corresponding writer's schema.
> The idea is that SchemaStore would be an abstract interface that allowed library users to inject custom backends. A simple, file system based one could be provided out of the box.


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