hive-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Sushanth Sowmyan (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HIVE-5020) HCat reading null-key map entries causes NPE
Date Wed, 07 Aug 2013 20:47:48 GMT
Sushanth Sowmyan created HIVE-5020:
--------------------------------------

             Summary: HCat reading null-key map entries causes NPE
                 Key: HIVE-5020
                 URL: https://issues.apache.org/jira/browse/HIVE-5020
             Project: Hive
          Issue Type: Bug
          Components: HCatalog
            Reporter: Sushanth Sowmyan
            Assignee: Sushanth Sowmyan


Currently, if someone has a null key in a map, HCatInputFormat will terminate with an NPE
while trying to read it.

{noformat}
java.lang.NullPointerException
at java.lang.String.compareTo(String.java:1167)
at java.lang.String.compareTo(String.java:92)
at java.util.TreeMap.put(TreeMap.java:545)
at org.apache.hcatalog.data.HCatRecordSerDe.serializeMap(HCatRecordSerDe.java:222)
at org.apache.hcatalog.data.HCatRecordSerDe.serializeField(HCatRecordSerDe.java:198)
at org.apache.hcatalog.data.LazyHCatRecord.get(LazyHCatRecord.java:53)
at org.apache.hcatalog.data.LazyHCatRecord.get(LazyHCatRecord.java:97)
at org.apache.hcatalog.mapreduce.HCatRecordReader.nextKeyValue(HCatRecordReader.java:203)
{noformat}

This is because we use a TreeMap to preserve order of elements in the map when reading from
the underlying storage/serde.

This problem is easily fixed in a number of ways:

a) Switch to HashMap, which allows null keys. That does not preserve order of keys, which
should not be important for map fields, but if we desire that, we have a solution for that
too - LinkedHashMap, which would both retain order and allow us to insert null keys into the
map.

b) Ignore null keyed entries - check if the field we read is null, and if it is, then ignore
that item in the record altogether. This way, HCat is robust in what it does - it does not
terminate with an NPE, and it does not allow null keys in maps that might be problematic to
layers above us that are not used to seeing nulls as keys in maps.

Why do I bring up the second fix? I bring it up because of the way we discovered this bug.
When reading from an RCFile, we do not notice this bug. If the same query that produced the
RCFile instead produces an Orcfile, and we try reading from it, we see this problem.

RCFile seems to be quietly stripping any null key entries, whereas Orc retains them. This
is why we didn't notice this problem for a long while, and suddenly, now, we are. Now, if
we fix our code to allow nulls in map keys through to layers above, we expose layers above
to this change, which may then cause them to break. (Technically, this is stretching the case
because we already break now if they care) More importantly, though, we have a case now, where
the same data will be exposed differently if it were stored as orc or if it were stored as
rcfile. And as a layer that is supposed to make storage invisible to the end user, HCat should
attempt to provide some consistency in how data behaves to the end user.

That said...

There is another important concern at hand here: nulls in map keys might be due to bad data(corruption
or loading error), and by stripping them, we might be silently hiding that from the user.
This is an important point that does steer me towards the former approach, of passing it on
to layers above, and standardize on an understanding that null keys in maps are acceptable
data that layers above us have to handle. After that, it could be taken on as a further consistency
fix, to fix RCFile so that it allows nulls in map keys.

Having gone through this discussion of standardization, another important question is whether
or not there is actually a use-case for null keys in maps in data. If there isn't, maybe we
shouldn't allow writing that in the first place, and both orc and rcfile must simply error
out to the end user if they try to write a  null map key? Well, it is true that it is possible
that data errors lead to null keys, but it's also possible that the user wants to store a
mapping for value transformations, and they might have a transformation for null as well.
In the case I encountered it, they were writing out an intermediate table after having read
from a sparse table using a custom input format that generated an arbitrary number of columns,
and were using the map to store column name mappings that would eventually be written out
to another table. That seems a valid use, and we shouldn't prevent users from this sort of
usage.

Another reason for not allowing null keys from a java perspective is locking and concurrency
concerns, where locking on a null is a pain, per philosophical disagreements between Joshua
Block and Doug Lea in the design of HashMap and ConcurrentHashMap. However, given that HCatalog
reads are happening in a thread on a drone where there should be no parallel access of that
record, and more importantly, this should strictly be used in a read-only kind of usage, we
should not have to worry about that.

Increasingly, my preference is to change to LinkedHashMaps to allow null keys, and for consistency's
sake, after this is tackled, to see if we should be fixing RCFile to allow null keys(this
might be trickier since RCFile has a lot of other users that are probably currently working.)

Another option is to change to LinkedHashMap, but also add a conf key to hcat to allow the
user to specify whether or not we want to strip nulls. That way, a user can specify what behaviour
they like. That's more cruft though, and I don't want to go down that path unless there is
a user that explicitly wants/needs that.

Anyone have any other thoughts on the matter?

--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators
For more information on JIRA, see: http://www.atlassian.com/software/jira

Mime
View raw message