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From Julien Le Dem <jul...@ledem.net>
Subject Re: Thinking about Drill 2.0
Date Wed, 07 Jun 2017 20:53:46 GMT
Hi Paul,
My 2ct regarding Arrow:
The goal of Arrow is to be a standard representation that does not break compatibility in
the future.
If moving to Arrow is a breaking change, It don’t think it makes sense to abstract it out
to present a row oriented representation to the client. I defeats the purpose.
You can still use Arrow as your standard representation to the client and allow for custom
vectors on the server side that get converted before sending. This sounds like it could be
part of the smaller API you are taking about.
As for backward compatibility with the Drill ValueVectors it is possible to make a compatibility
layer that patches the few differences (byte instead of bits for nullability, some type width
difference) with little code.
For changing the offset vectors it would be great to have this discussion on the Arrow mailing
list so that we don’t diverge. (one simple workaround seems to use 64K-1 batches?)
Some work has been done on the arrow side regarding json support (for example maps are now
nullable: ARROW-274)

> On Jun 5, 2017, at 11:59 AM, Paul Rogers <progers@mapr.com> wrote:
> Hi All,
> A while back there was a discussion about the scope of Drill 2.0. Got me thinking about
possible topics. My two cents:
> Drill 2.0 should focus on making Drill’s external APIs production ready. This means
five things:
> * Clearly identify and define each API.
> * (Re)design each API to ensure it fully isolates the client from Drill internals.
> * Ensure the API allows full version compatibility: Allow mixing of old/new clients and
servers with some limits.
> * Fully test each API.
> * Fully document each API.
> Once client code is isolated from Drill internals, we are free to evolve the internals
in either Drill 2.0 or a later release.
> In my mind, the top APIs to revisit are:
> * The drill client API.
> * The storage plugin API.
> (Explanation below.)
> What other APIs should we consider? Here are some examples, please suggest items you
know about:
> * Command line scripts and arguments
> * Names and contents of system tables
> * Structure of the storage plugin configuration JSON
> * Structure of the query profile
> * Structure of the EXPLAIN PLAN output.
> * Semantics of Drill functions, such as the date functions recently partially fixed by
adding “ANSI” alternatives.
> * Naming of config and system/session options.
> * (Your suggestions here…)
> I’ve taken the liberty of moving some API-breaking tickets in the Apache Drill JIRA
to 2.0. Perhaps we can add others so that we have a good inventory of 2.0 candidates.
> Here are the reasons for my two suggestions.
> Today, we expose Drill value vectors to the client. This means if we want to enhance
anything about Drill’s internal memory format (i.e. value vectors, such as a possible move
to Arrow), we break compatibility with old clients. Using value vectors also means we need
a very large percentage of Drill’s internal code on the client in Java or C++. We are learning
that doing so is a challenge.
> A new client API should follow established SQL database tradition: a synchronous, row-based
API designed for versioning, for forward and backward compatibility, and to support ODBC and
JDBC users.
> We can certainly maintain the existing full, async, heavy-weight client for our tests
and for applications that would benefit from it.
> Once we define a new API, we are free to alter Drill’s value vectors to, say, add the
needed null states to fully support JSON, to change offset vectors to not need n+1 values
(which doubles vector size in 64K batches), and so on. Since vectors become private to Drill
(or Arrow) after the new client API, we are free to innovate to improve them.
> Similarly, the storage plugin API exposes details of Calcite (which seems to evolve with
each new version), exposes value vector implementations, and so on. A cleaner, simpler, more
isolated API will allow storage plugins to be built faster, but will also isolate them from
Drill internals changes. Without isolation, each change to Drill internals would require plugin
authors to update their plugin before Drill can be released.
> Thoughts? Suggestions?
> Thanks,
> - Paul

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