I'm not really familiar with pelops code, but I found two implementations (~ line 454 and ~ line 559) of getColumnsFromRows in Selector.java in pelops trunk.
The first uses a HashMap so it clearly isn't ordered, the second uses a LinkedHashMap but it inserts the keys in the order returned by C* which we already know isn't ordered.
See http://bit.ly/egZaXi for relevant code.
Like I said, I'm not really familiar with pelops so I could be completely off on this, but it looks like if pelops was intending to preserve the order of the requested keys that it's not actually doing it...
Hi Matthew,As you mention the map returned from multiget_slice is not order preserving, Pelops is doing this on the client side...Cheers,Dan
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On Wednesday, 23 February 2011 at 8:38 PM, Matthew Dennis wrote:The map returned by multiget_slice (what I suspect is the underlying thrift call for getColumnsFromRows) is not a order preserving map, it's a HashMap so the order of the returned results cannot be depended on. Even if it was a order preserving map, not all languages would be able to make use of the results since not all languages have ordered maps (though many, including Java, certainly do).
That being said, it would be fairly easy to change this on the C* side to preserve the order the keys were requested in, though as mentioned not all clients could take advantage of it.On Mon, Feb 21, 2011 at 4:09 PM, email@example.com <firstname.lastname@example.org> wrote:
As Jonathan mentions the compareWith on a column family def. is defines the order for the columns *within* a row... In order to control the ordering of rows you'll need to use the OrderPreservingPartitioner (http://www.datastax.com/docs/0.7/operations/clustering#tokens-partitioners-ring).
As for getColumnsFromRows; it should be returning you a map of lists. The map is insertion-order-preserving and populated based on the provided list of row keys (so if you iterate over the entries in the map they should be in the same order as the list of row keys).Thanks for your answer and for your time, I will take a look at this.
mmm ... well it didn't happen like this. In my code I had a CF named comments and also a CF called usercomments. UserComments use an uuid as row-key to keep, TimeUUID sorted, the "pointers" to the comments of the user. When I get the sorted list of keys from the UserComments and I use this list as row-keys-list in the GetColumnsFromRows I don't get back the data sorted as I expect them to be.
It looks like if Cassandra/Pelops does not care on how I provide the row-keys-list. I am sure about that cause I did something different: I iterate over my row-keys-list and made many GetColumnFromRow instead of one GetColumnsFromRows and when I iterate data are correctly sorted. But this can not be a solution ...
I am using Cassandra 0.6.9
I profit of your knownledge of Pelops to ask you something: I am evaluating the migration to Cassandra 0.7 ... as far as you know, in terms of written code, is it an heavy job?
On Saturday, 19 February 2011 at 8:16 AM, email@example.com wrote:Hi all,
I created a CF in which i need to get, sorted by time, the Rows inside. Each
Row represents a comment.
<ColumnFamily name="Comments" compareWith="TimeUUIDType" / >
I've created a few rows using as Row Key a generated TimeUUID but when I call
the Pelops method "GetColumnsFromRows" I don't get the data back as I expect:
rows are not sorted by TimeUUID.
I though it was probably cause of the random-part of the TimeUUID so I create
a new CF ...
<ColumnFamily name="Comments2" compareWith="LongType" / >
This time I created a few rows using the java System.CurrentTimeMillis() that
retrieve a long. I call again the "GetColumnsFromRows" and again the same
results: data are not sorted!
I've read many times that Rows are sorted as specified in the compareWith but
I can't see it.
To solve this problem for the moment I've used a SuperColumnFamily with an
UNIQUE ROW ... but I think this is just a workaround and not the solution.
<ColumnFamily name="Comments" type="Super" compareWith="TimeUUIDType"
Now when I call the "GetSuperColumnsFromRow" I get all the SuperColumns as I
expected: sorted by TimeUUID. Why it does not happen the same with the Rows?
TIA for any help.