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https://issues.apache.org/jira/browse/HBASE-2893?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12894418#action_12894418
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Andrew Purtell commented on HBASE-2893:
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bq. I'm a little concerned that this would be rather disruptive to the code
but used by a very small portion of users.
We anticipate that access control will be a widely used feature if available.
Impetus for this issue started with considerations for implementing access
control.
However, the adjustable TTL case comes for free if metacolumns are implemented
in a more general manner, and is something that would make life easier for some
dev groups I am working with.
One way to address concerns regarding disruption would be to build this --
therefore perhaps much of security (HBASE-1697 and subtasks) -- on top of
coprocessor-style server side extensions (HBASE-2000 and subtasks). I have been
considering this approach. It is compelling to consider pulling up all of the
functional and performance impact to an extension which can be dynamically
loaded per table. The core code is only touched by coprocessor framework
changes and the user has full choice in the matter when taking on anything
else. On the other hand, support is more challenging, perhaps a lot more. First
question: "What extensions do you have loaded, in what combination?" So on
balance I recommend _if we agree_ that HBASE-1697 is a core concern, then it
and related changes such as this issue should be in core, not an extension.
bq. this would break with the pattern of each family able to be processed in
isolation
Ack.
bq. So, if reading from a 5 family table (+1 for meta), you'd end up reading
the metacolumn 5 times, once for each user family?
No, only one time, anything in the metacolumn for the row retrieved in one read.
bq. Things like the bloom filter check would have to happen during the read, so
at a different level than it's currently done.
At the Region level, yes, for the metacolumn case. So an access to a row in the
Region would trigger a read of the metacolumn and then caching of the result to
be passed around. Exactly how this would be passed around is unsettled. One
option is thread locals.
This is part of a larger issue related to the security work, that of creating a
context (for access control) and then referencing it wherever an authoritative
decision must be made. We have been debating if to use JAAS or instead sprinkle
around access checks by hand. The issue of building context and passing it
around must be dealt with to implement security. If we have it, then passing
around KVs read from metacolumns is straightforward.
bq. Or if you have these very specific and fine-grained settings like variable
TTL you would implement them in your application.
... and then lose a feature -- automatic TTL based expiration and garbage
collection with single-table scale out properties -- that makes use of HBase
compelling as opposed to something else. (Not sure what, if anything, that
something else would be.)
bq. I guess it's less clear why you couldn't break stuff up into separate
tables for varied TTLs or multi-tenancy.
Yes, that's the problem. Single table multitenancy has better scale out
properties than per-user tables, and in the HBase case, 1M+ tables for 1M+
users is not tenable.
For the variable TTL case, consider an event logging application designed to
archive data for long periods of time, but the different event types have
different lifetimes, and lifetimes may be adjusted over time (updated system
design). If a bunch of tables, this requires a join, which HBase does not
support. So what you would do is set up column families to each serve as a TTL
bucket ("join" over column families). Events could only have the TTL of one of
the buckets. Application would store into appropriate column family according
to TTL. But this then results in a wide schema, with resulting unnecessary
reduction in I/O locality in access patterns, more store files than otherwise,
and so on. Design changes require adding or modifying column families, taking
the table offline, at least for now. Not necessarily a fatal problem if we can
avoid taking the table offline _ever_ after the master rewrite, but if we
already have per-row overrides for ACLs then this straightforwardly extends to
the TTL case (at least) and that's enough I think to make this problem go away.
bq. Would Put be extended with per-row setTTL, setACL methods now?
I would recommend that, yes. The metacolumn is a column family like any other;
to set stuff, put values as KVs into the Put to be stored directly. Convenience
functions on Put are desirable so the user doesn't have to learn about the
value formatting for various overrides.
So add to this:
bq. It would be accessible like any other column family, but we expect a
default ACL that only allows access by the system and operator principals
and any principal the table creator adds to the ACL.
bq. Out of curiosity, which BT-like systems support per-value ACLs?
It's a rumor. I'll try to find out more.
> Table metacolumns
> -----------------
>
> Key: HBASE-2893
> URL: https://issues.apache.org/jira/browse/HBASE-2893
> Project: HBase
> Issue Type: New Feature
> Reporter: Andrew Purtell
>
> Some features like TTLs or access control lists have use cases that call for
> per-value configurability.
> Currently in HBase TTLs are set per column family. This leads to potentially
> awkward "bucketing" of values into column families set up to accommodate the
> common desired TTLs for all values within -- an unnecessarily wide schema,
> with resulting unnecessary reduction in I/O locality in access patterns, more
> store files than otherwise, and so on.
> Over in HBASE-1697 we're considering setting ACLs on column families.
> However, we are aware of other BT-like systems which support per-value ACLs.
> This allows for multitenancy in a single table as opposed to really requiring
> tables for each customer (or, at least column families). The scale out
> properties for a single table are better than alternatives. I think
> supporting per-row ACLs would be generally sufficient: customer ID could be
> part of the row key. We can still plan to maintain column-family level ACLs.
> We would therefore not have to bloat the store with per-row ACLs for the
> normal case -- but it would be highly useful to support overrides for
> particular rows. So how to do that?
> I propose to introduce _metacolumns_.
> A _metacolumn_ would be a column family intrinsic to every table, created by
> the system at table create time. It would be accessible like any other
> column family, but we expect a default ACL that only allows access by the
> system and operator principals, and would function like any other, except
> administrative actions such as renaming or deletion would not be allowed.
> Into the metacolumn would be stored per-row overrides for such things as ACLs
> and TTLs. The metacolumn therefore would be as sparse as possible; no storage
> would required for any overrides if a value is committed with defaults. A
> reasonably sparse metacolumn for a region may fit entirely within blockcache.
> It may be possible for all metacolumns on a RS to fit within blockcache
> without undue pressure on other users. We can aim design effort at this
> target.
> The scope of changes required to support this is:
> - Introduce metacolumn concept in the code and into the security model
> (default ACL): A flag in HCD, a default ACL, and a few additional checks for
> rejecting disallowed administrative actions.
> - Automatically create metacolumns at table create time.
> - Consult metacolumn as part of processing reads or mutations, perhaps using
> a bloom filter to shortcut lookups for rows with no metaentries, and apply
> configuration or security policy overrides if found.
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