deleting keys

Sean Cribbs sean at basho.com
Tue May 31 20:44:30 EDT 2011


Robert,

What Keith said is misleading -- that key cache was solely in the Ruby client driver and not part of Riak itself.

In Riak, deletes have two phases; in the first, so-called "tombstones" are written to the partitions that own replicas of the key.  The tombstone has special metadata marking it as such and an empty value, but has a descendant vector clock from the last known value. In the second phase, the tombstones are read back from the replicas, and iff they all are tombstones (that is, all replicas respond, and all are tombstones), a reaping command is sent such that they will be cleared from the backend.

In your case, what may have occurred is that the replica chosen for key-listing did not receive the tombstone write (only 1/n_val of all partitions are consulted for key-lists), or had not yet received the reaping command. When you read the key again, you obviously get a "not found" because the other replicas will resolve to a tombstone. Eventually your read requests will invoke read-repair, updating the stale partition and causing the value to be reaped.

The moral of the story here is, again, don't rely on key-listings for strong indications of cluster state.

Sean Cribbs <sean at basho.com>
Developer Advocate
Basho Technologies, Inc.
http://basho.com/

On May 31, 2011, at 8:12 PM, Keith Bennett wrote:

> Robert -
> 
> Until a source code change a few days ago, riak would by default cache the keys reported to be in a bucket, so after fetching them once they would not be updated after deletions, additions, etc.  The key is indeed gone, but the keys API did not report the change.
> 
> If you go to the message archive at http://lists.basho.com/pipermail/riak-users_lists.basho.com/2011-May/thread.html, and search for "Riak cleint resources", you'll see the ruckus that I started a week and a half ago about this very subject. ;)
> 
> There is an option to force the reloading of keys but I forget what it is, and anyway it is now gone from the current code base since the strategy was changed.  Be warned that using the keys method is, anyway, as Sean Cribbs pointed out to me, in general an awful idea, and almost always should be avoided.  This is because it's a very expensive operation -- in order to accomplish it, all keys in the data store need to be accessed.
> 
> My guess is that testing for the exception you encountered is probably the best way to test for existence/absence of a key, but hopefully those more knowledgeable than I will enlighten us on that.
> 
> - Keith
> 
> On May 31, 2011, at 7:23 PM, Dingwell, Robert A. wrote:
> 
>> Hi,
>> 
>> When deleting a key from a bucket I'm noticing that the object associated with the key is gone but the key itself is still sticking around.  I loop though all of the keys in a bucket and then call delete on each one, the object for the key is then gone so if I try to get the object for that key I get a 404 as expected.  But if I look at the bucket in the browser with the  keys=true parameter, all of the keys are still there.  Is this normal and if so how do I get rid of the keys?
>> 
>> Thanks_______________________________________________
>> riak-users mailing list
>> riak-users at lists.basho.com
>> http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com
> 
> 
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