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From Matthew Jacobs ...@cloudera.com>
Subject Re: Adding some guard rails to Kudu
Date Thu, 01 Dec 2016 00:56:34 GMT
Hm nice catch. I don't think we should do this silently... I filed
https://issues.cloudera.org/browse/IMPALA-4563
I'm not sure if we have similar behavior on db names and table names,
or if there are explicit max lengths there to consider.

On Wed, Nov 30, 2016 at 4:26 PM, Todd Lipcon <todd@cloudera.com> wrote:
> BTW I am doing some testing in Impala, and it seems like Impala silently
> truncates column names to 128 characters.
>
> Even more fun is:
>
> create table todd_test
> (xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> int,
> xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxyy
> int)
>
> (two long names which differ after the 128-character truncation). Results
> in:
>
> ImpalaRuntimeException: Error making 'createTable' RPC to Hive Metastore:
> CAUSED BY: MetaException: Add request failed : INSERT INTO `COLUMNS_V2`
> (`CD_ID`,`COMMENT`,`COLUMN_NAME`,`TYPE_NAME`,`INTEGER_IDX`) VALUES
> (?,?,?,?,?)
>
> So given the 128-character Impala limit, perhaps the Kudu limit should be
> more than 256, since you're creating Kudu table names as
> '<database>::<table>'.
>
>
> On Wed, Nov 30, 2016 at 4:03 PM, Todd Lipcon <todd@cloudera.com> wrote:
>
>> On Wed, Nov 30, 2016 at 3:52 PM, Matthew Jacobs <mj@cloudera.com> wrote:
>>
>>> 1) I think that makes sense, though we need to know what the error
>>> conditions are, when those errors occur (e.g. at table creation, add
>>> column, writing data) and need tests to validate the expected negative
>>> cases. I can certainly guess, though I'd like for the affected API
>>> calls w/ new expected behavior to be documented somewhere so we can
>>> make changes accordingly.
>>>
>>
>> For the schema-related ones, they'll behave the same as any other invalid
>> schema does today (eg trying to use the same column name twice, or using an
>> existing table name, etc).
>>
>> In terms of API docs, I was hoping that the documentation can be general
>> enough about the types of exceptions thrown for general categories rather
>> than having to translate every bit of validation logic into English on the
>> API docs.
>>
>> In other words, we should document that createTable() throws an exception
>> if the schema was invalid, but I don't think we need to re-document all of
>> the ways in which a schema can be invalid in the API docs, do we? I think
>> more general user-facing documentation is probably the appropriate place.
>>
>> Test-wise, I agree that some coverage end-to-end from Impala would be
>> nice. We're adding tests that go through our API to validate it, but
>> validating that the user-exposed error is reasonable too is of course a
>> good idea.
>>
>> 2) Does this mean you'll test up to these limits?
>>>
>>
>> Yea, that's the long-term intention and would be ideal. However, for now,
>> I'm not necessarily guaranteeing that we've got automated testing up to
>> these limits today in all combinations. For example, 300 columns works, and
>> 64kb cells works, but maybe we'd have an issue with a table where all 300
>> columns contain 64kb cells in every row.
>>
>> So, the hope with this patch series isn't to 100% constrain users such
>> that they can never get into a less-tested area. But hopefully we've cut
>> out 95% of the space here and prevented some users from shooting themselves
>> in the foot. For example, we recently had a case where a bug in the user
>> application mis-parsed some input data and tried to insert a 20MB cell,
>> which ended up causing an outage, and this relatively simplistic patch
>> would have prevented that.
>>
>> Put another way, we're telling users "if you go above this range, you may
>> have problems" but not guaranteeing the logical inverse "if you stay below
>> this range, you will never have a problem." That doesn't make it less
>> useful, though, IMO :)
>>
>> -Todd
>>
>>
>>> On Wed, Nov 30, 2016 at 3:33 PM, Todd Lipcon <todd@cloudera.com> wrote:
>>> > Hey Impala folks,
>>> >
>>> > Just FYI, I started the below thread on the Kudu lists about adding some
>>> > limits/guard rails to various dimensions of Kudu data/metadata. Please
>>> take
>>> > a look from the Impala perspective and let us know if you foresee any
>>> > issues with these limits.
>>> >
>>> > Just to repeat one thing: I know many SQL workloads require more than
>>> 300
>>> > columns in a table, but right now Kudu isn't great in that realm, so
>>> we're
>>> > setting the limits conservatively. The idea is that over time as we
>>> improve
>>> > test coverage we'll raise the limits.
>>> >
>>> > -Todd
>>> >
>>> > ---------- Forwarded message ----------
>>> > From: Todd Lipcon <todd@cloudera.com>
>>> > Date: Wed, Nov 30, 2016 at 3:30 PM
>>> > Subject: Re: Adding some guard rails to Kudu
>>> > To: user@kudu.apache.org, dev <dev@kudu.apache.org>
>>> >
>>> >
>>> > BTW I filed a JIRA here and started linking related issues to it:
>>> > https://issues.apache.org/jira/browse/KUDU-1775
>>> >
>>> >
>>> > On Wed, Nov 30, 2016 at 3:25 PM, Todd Lipcon <todd@cloudera.com> wrote:
>>> >
>>> >> Hey folks,
>>> >>
>>> >> I've started working on a few patches to add "guard rails" to various
>>> >> user-specified dimensions in Kudu. In particular, I'm planning to add
>>> >> limits to the following:
>>> >>
>>> >> - max number of columns in a table (proposal: 300)
>>> >> - max replication factor (proposal: 7)
>>> >> - max table name or column name length (proposal: 256)
>>> >> - max size of a binary/string column cell value (proposal: 64kb)
>>> >>
>>> >> The reasoning is that, even though in some cases we don't know a
>>> specific
>>> >> issue that will happen outside these limits, we've done very little
>>> testing
>>> >> (and have no automated testing) outside of these ranges. In some
>>> cases, we
>>> >> do know that there is a certain threshold that will cause a big
>>> problem (eg
>>> >> large cell sizes can cause tablet servers to crash). In other cases,
>>> it's
>>> >> just "unknown territory".
>>> >>
>>> >> In all cases, I'm planning on making the limits overridable via an
>>> >> "unsafe" configuration flag. That means that a user can run with
>>> >> "--unlock_unsafe_flags --max_identifier_length=1000" if they want to,
>>> but
>>> >> they're explicitly accepting some risk that they're entering untested
>>> >> territory.
>>> >>
>>> >> Of course, in all cases, if we hear that there are people who are
>>> bumping
>>> >> the maxes higher than the defaults and having good results, we can
>>> consider
>>> >> raising the maximum, but I think it's smarter to start conservatively
>>> low
>>> >> and raise later as we increase test coverage. Also, I'm sure down the
>>> road
>>> >> we'll add features such as BLOB support or sparse column support, and
>>> at
>>> >> that time we can remove the corresponding guard rails.
>>> >>
>>> >> I'm sending this note to both user@ and dev@ to solicit feedback. Are
>>> >> there any other dimensions people can think of where we should
>>> probably add
>>> >> guard-rails? Is anyone out there already outside of the above ranges
>>> and
>>> >> can make a case that we're being too conservative?
>>> >>
>>> >> Thanks
>>> >> -Todd
>>> >> --
>>> >> Todd Lipcon
>>> >> Software Engineer, Cloudera
>>> >>
>>> >
>>> >
>>> >
>>> > --
>>> > Todd Lipcon
>>> > Software Engineer, Cloudera
>>> >
>>> >
>>> >
>>> > --
>>> > Todd Lipcon
>>> > Software Engineer, Cloudera
>>>
>>
>>
>>
>> --
>> Todd Lipcon
>> Software Engineer, Cloudera
>>
>
>
>
> --
> Todd Lipcon
> Software Engineer, Cloudera

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