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From "Paulo Motta (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-11500) Obsolete MV entry may not be properly deleted
Date Mon, 18 Sep 2017 14:12:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-11500?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16170065#comment-16170065
] 

Paulo Motta commented on CASSANDRA-11500:
-----------------------------------------

Committed {{afc55e8fe103597ef2a663be21828861a4832be7}} and {{6220394e84c79e6ef94651fc5e0aa03c12ddd307}}
to cassandra-dtest with a minor fix to {{test_base_column_in_view_pk_complex_timestamp}} flakiness
and minor change to {{test_base_column_in_view_pk_complex_timestamp}} to make it work on 3.0.

> Obsolete MV entry may not be properly deleted
> ---------------------------------------------
>
>                 Key: CASSANDRA-11500
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11500
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Materialized Views
>            Reporter: Sylvain Lebresne
>            Assignee: ZhaoYang
>             Fix For: 3.0.15, 3.11.1, 4.0
>
>
> When a Materialized View uses a non-PK base table column in its PK, if an update changes
that column value, we add the new view entry and remove the old one. When doing that removal,
the current code uses the same timestamp than for the liveness info of the new entry, which
is the max timestamp for any columns participating to the view PK. This is not correct for
the deletion as the old view entry could have other columns with higher timestamp which won't
be deleted as can easily shown by the failing of the following test:
> {noformat}
> CREATE TABLE t (k int PRIMARY KEY, a int, b int);
> CREATE MATERIALIZED VIEW mv AS SELECT * FROM t WHERE k IS NOT NULL AND a IS NOT NULL
PRIMARY KEY (k, a);
> INSERT INTO t(k, a, b) VALUES (1, 1, 1) USING TIMESTAMP 0;
> UPDATE t USING TIMESTAMP 4 SET b = 2 WHERE k = 1;
> UPDATE t USING TIMESTAMP 2 SET a = 2 WHERE k = 1;
> SELECT * FROM mv WHERE k = 1; // This currently return 2 entries, the old (invalid) and
the new one
> {noformat}
> So the correct timestamp to use for the deletion is the biggest timestamp in the old
view entry (which we know since we read the pre-existing base row), and that is what CASSANDRA-11475
does (the test above thus doesn't fail on that branch).
> Unfortunately, even then we can still have problems if further updates requires us to
overide the old entry. Consider the following case:
> {noformat}
> CREATE TABLE t (k int PRIMARY KEY, a int, b int);
> CREATE MATERIALIZED VIEW mv AS SELECT * FROM t WHERE k IS NOT NULL AND a IS NOT NULL
PRIMARY KEY (k, a);
> INSERT INTO t(k, a, b) VALUES (1, 1, 1) USING TIMESTAMP 0;
> UPDATE t USING TIMESTAMP 10 SET b = 2 WHERE k = 1;
> UPDATE t USING TIMESTAMP 2 SET a = 2 WHERE k = 1; // This will delete the entry for a=1
with timestamp 10
> UPDATE t USING TIMESTAMP 3 SET a = 1 WHERE k = 1; // This needs to re-insert an entry
for a=1 but shouldn't be deleted by the prior deletion
> UPDATE t USING TIMESTAMP 4 SET a = 2 WHERE k = 1; // ... and we can play this game more
than once
> UPDATE t USING TIMESTAMP 5 SET a = 1 WHERE k = 1;
> ...
> {noformat}
> In a way, this is saying that the "shadowable" deletion mechanism is not general enough:
we need to be able to re-insert an entry when a prior one had been deleted before, but we
can't rely on timestamps being strictly bigger on the re-insert. In that sense, this can be
though as a similar problem than CASSANDRA-10965, though the solution there of a single flag
is not enough since we can have to replace more than once.
> I think the proper solution would be to ship enough information to always be able to
decide when a view deletion is shadowed. Which means that both liveness info (for updates)
and shadowable deletion would need to ship the timestamp of any base table column that is
part the view PK (so {{a}} in the example below).  It's doable (and not that hard really),
but it does require a change to the sstable and intra-node protocol, which makes this a bit
painful right now.
> But I'll also note that as CASSANDRA-1096 shows, the timestamp is not even enough since
on equal timestamp the value can be the deciding factor. So in theory we'd have to ship the
value of those columns (in the case of a deletion at least since we have it in the view PK
for updates). That said, on that last problem, my preference would be that we start prioritizing
CASSANDRA-6123 seriously so we don't have to care about conflicting timestamp anymore, which
would make this problem go away.



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