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From Chad Walters <Chad.Walt...@microsoft.com>
Subject RE: Multi-language serialization discussion
Date Fri, 24 Oct 2008 22:44:50 GMT
Doug,

There has previously been a bunch of discussion on the Thrift list (possibly pre-Incubator)
about self-describing Thrift streams and the like when we talked about providing a superset
of RecordIO functionality. Re-open that discussion and I imagine you might get some interested
parties. Writing an interpreter of Thrift type descriptors for any of the scripting language
doesn't seem like it would be that hard.

Bumping up a level, rather than inventing a whole new set of Hadoop-specific RPC and serialization
mechanisms, I'd suggest that there would be more leverage from adopting Thrift. Thrift is
in the Apache Incubator (as you know ;)) and there is already a fairly significant overlap
in the two communities. A number of Hadoop-related technologies are already using Thrift in
places (HBase, Hive, etc). If there was more involvement in Thrift from core Hadoop development,
I am pretty certain you would get what you wanted out of it pretty quickly.

Chad

-----Original Message-----
From: Doug Cutting [mailto:cutting@gmail.com] On Behalf Of Doug Cutting
Sent: Friday, October 24, 2008 2:40 PM
To: core-dev@hadoop.apache.org
Subject: Re: Multi-language serialization discussion

Bryan Duxbury wrote:
> I've been reading the discussion about what serialization/RPC project to
> use on http://wiki.apache.org/hadoop/Release1.0Requirements, and I
> thought I'd throw in a pro-Thrift vote.

I've been thinking about this, and here's where I've come to:

It's not just RPC.  We need a single, primary object serialization
system that's used for RPC and for most file-based application data.

Scripting languages are primary users of Hadoop.  We must thus make it
easy and natural for scripting languages to process data with Hadoop.

Data should be self-describing.  For example, a script should be able to
read a file without having to first generate code specific to the
records in that file.  Similarly, a script should be able to write
records without having to externally define their schema.

We need an efficient binary file format.  A file of records should not
repeat the record names with each record.  Rather, the record schema
used should be stored in the file once.  Programs should be able to read
the schema and efficiently produce instances from the file.

The schema language should support specification of required and
optional fields, so that class definitions may evolve.

For some languages (e.g., Java & C) one may wish to generate native
classes to represent a schema, and to read & write instances.

So, how well does Thrift meet these needs?  Thrift's IDL is a schema
language, and JSON is a self-describing data format.  But arbitrary JSON
data is not generally readable by any Thrift-based program.  And
Thrift's binary formats are not self-describing: they do not include the
IDL.  Nor does the Thrift runtime in each language permit one to read an
IDL specification and then use it to efficiently read and write compact,
self-describing data.

I wonder if we might instead use use JSON schemas to describe data.

http://groups.google.com/group/json-schema/web/json-schema-proposal---second-draft

We'd implement, in each language, a codec that, given a schema, can
efficiently read and write instances of that schema.  (JSON schemas are
JSON data, so any language that supports JSON can already read and write
a JSON schema.)  The writer could either take a provided schema, or
automatically induce a schema from the records written.  Schemas would
be stored in data files, with the data.

JSON's not perfect.  It doesn't (yet) support binary data: that would
need to be fixed.  But I think Thrift's focus on code-generation makes
it less friendly to scripting languages, which are primary users of
Hadoop.  Code generation is possible given a schema, and may be useful
as an optimization in many cases, but it should be optional, not central.

Folks should be able to process any file without external information or
external compilers.  A small runtime codec is be all that should be
implemented in each language.  Even if that's not present, data could be
transparently and losslessly converted to and from textual JSON by, e.g.
C utility programs, since most languages already have JSON codecs.

Does this make any sense?

Doug


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