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From Ildar Absalyamov <ildar.absalya...@gmail.com>
Subject Re: Metadata changes
Date Tue, 15 Dec 2015 02:19:02 GMT
As for general topic of backwards compatibility I think going “fully open” might be the
best longterm solution.
Once in a while the topic of changing metadata keeps reappearing and there is no guarantee
it will not strike back again. Opening up metadata will release ourselves from burden of producing
migration tools and shipping them with the new version of the binaries with revised catalog.
The performance (mainly storage) impacts of that solution will be tolerable especially considering
how much data is usually stored in metadata.
Moreover, being big proponents of semi-structured data, it does make perfect sense for us
to eat our own dog food here.

> On Dec 14, 2015, at 18:04, Ildar Absalyamov <ildar.absalyamov@gmail.com> wrote:
> 
> I guess the main argument for 2 would be eliminating broken metadata records prior to
backwards compatibility cutoff.
> The last thing what we want to do is to be stuck with wrong implementation for compatibility
reasons. Once the functionality needed for 3 is there we can again introduce those indexes
without building sophisticated migration subsystem.
> 
>> On Dec 14, 2015, at 17:55, Mike Carey <dtabass@gmail.com> wrote:
>> 
>> SO - it seems like 3 is the right long-term answer, but not doable now?
>> (If it was doable now, it would obviously be the ideal choice of the three.)
>> What would be the argument for doing 2 as opposed to 1 for now?
>> As for the question of backwards compatibility, I actually didn't sense a consensus
yet.
>> I would tentatively lean towards "right" over "backwards compatible" for this change.
>> What are others thoughts on that?
>> (Soon we won't have that luxury, but right now maybe we do?)
>> 
>> On 12/14/15 3:43 PM, Steven Jacobs wrote:
>>> We just had a UCR discussion on this topic. The issue is really with the
>>> third "index" here. The code now is using one "index" to go in two
>>> directions:
>>> 1) To find datatypes that use datatype A
>>> 2) To find datatypes that are used by datatype A.
>>> 
>>> The way that it works now is hacked together, but designed for performance.
>>> So we have three choices here:
>>> 
>>> 1) Stick to the status quo, and leave the "indexes" as they are
>>> 2) Remove the Metadata secondary indexes, which will eliminate the hack but
>>> cost some performance on Metadata
>>> 3) Implement the Metadata secondary indexes correctly as Asterix indexes.
>>> For this solution to work with our dataset designs, we will need to have
>>> the ability to index homogeneous lists. In addition, we will have reverse
>>> compatibility issues unless we plan things out for the transition.
>>> 
>>> What are the thoughts?
>>> 
>>> 
>>> Orthogonally, it seems that the consensus for storing the datatype
>>> dataverse in the dataset Metadata is to just add it as an open field at
>>> least for now. Is that correct?
>>> 
>>> Steven
>>> 
>>> 
>>> On Mon, Dec 14, 2015 at 1:23 PM, Mike Carey <dtabass@gmail.com> wrote:
>>> 
>>>> Thoughts inlined:
>>>> 
>>>> On 12/14/15 11:12 AM, Steven Jacobs wrote:
>>>> 
>>>>> Here are the conclusions that Ildar and I have drawn from looking at
the
>>>>> secondary indexes:
>>>>> 
>>>>> First of all it seems that datasets are local to node groups, but
>>>>> dataverses can span node groups, which seems a little odd to me.
>>>>> 
>>>> Node groups are an undocumented but to-be-exploited-someday feature that
>>>> allows datasets to be stored on less than all nodes in a given cluster. 
As
>>>> we face bigger clusters, we'll want to open up that possibility.  We will
>>>> hopefully use them inside w/o having to make users manage them manually
>>>> like parallel DB2 did/does.  Dataverses are really just a namespace thing,
>>>> not a storage thing at all, so they are orthogonal to (and unrelated to)
>>>> node groups.
>>>> 
>>>>> There are three Metadata secondary indexes:  GROUPNAME_ON_DATASET_INDEX,
>>>>> DATATYPENAME_ON_DATASET_INDEX, DATATYPENAME_ON_DATATYPE_INDEX
>>>>> 
>>>>> The first is used in only one case:
>>>>> When dropping a node group, check if there are any datasets using this
>>>>> node
>>>>> group. If so, don't allow the drop
>>>>> BUT, this index has a field called "dataverse" which is not used at all.
>>>>> 
>>>> This one seems like a waste of space since we do this almost never. (Not
>>>> much space, but unnecessary.)  If we keep it it should become a proper
>>>> index.
>>>> 
>>>>> The second is used when dropping a datatype. If there is a dataset using
>>>>> this datatype, don't allow the drop.
>>>>> Similarly, this index has a "dataverse" which is never used.
>>>>> 
>>>> You're about to use the dataverse part, right?  :-)  This index seems like
>>>> it will be useful but should be a proper index.
>>>> 
>>>>> The third index is used to go in two cases, using two different ideas
of
>>>>> "keys"
>>>>> It seems like this should actually be two different indexes.
>>>>> 
>>>> I don't think I understood this comment....
>>>> 
>>>> 
>>>>> This is my understanding so far. It would be good to discuss what the
>>>>> "correct" version should be.
>>>>> Steven
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>> On Mon, Dec 14, 2015 at 10:12 AM, Steven Jacobs <sjaco002@ucr.edu>
wrote:
>>>>> 
>>>>> Hi all,
>>>>>> I'm implementing a change so that datasets can use datatypes from
>>>>>> alternate data verses (previously the type and set had to be from
the
>>>>>> same
>>>>>> dataverse). Unfortunately this means another change for Dataset Metadata
>>>>>> (which will now store the dataverse for its type).
>>>>>> 
>>>>>> As such, I had a couple of questions:
>>>>>> 
>>>>>> 1) Should this change be thrown into the release branch, as it is
another
>>>>>> Metadata change?
>>>>>> 
>>>>>> 2) In implementing this change, I've been looking at the Metadata
>>>>>> secondary indexes. I had a discussion with Ildar, and it seems the
thread
>>>>>> on Metadata secondary indexes being "hacked" has been lost. Is this
also
>>>>>> something that should get into the release? Is there anyone currently
>>>>>> looking at it?
>>>>>> 
>>>>>> Steven
>>>>>> 
>>>>>> 
>> 
> 
> Best regards,
> Ildar
> 

Best regards,
Ildar


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