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From Doug Cutting <cutt...@apache.org>
Subject Re: Multi-language serialization discussion
Date Fri, 24 Oct 2008 21:39:59 GMT
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.


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?


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