Apache Cassandra: Iterate over all columns in a row

Recently I have been using Cassandra for one of my projects, and one of the needs is to iterate over all columns of a row. Each column represents an individual data, of type identified by row id, and keeps changing. So I can’t simply use a set of known column names. Using the setRange call on a SliceQuery and setting a large count is also not an option, since Cassandra will try to load the entire set of columns into memory. Instead I’ve written this iterator which takes a query on which row key and column family has been set, and will load columns as they are requested. By default it loads a 100 columns at a time. You could make it take the count as a parameter and all, but this works for me for now.

The one ‘problem’ with this is the removal of the last column to ensure that there are no duplicates, but still having a start point for the next query. This is because each column is independent, so you cannot ask a column who it’s next neighbour is and start the next query from there. If anybody has a tip to make it more elegant, I’d love to hear it.

Trending

Twitter usually syncs with the (developed) world pretty fast in hashtags. But I'm surprised Copenhagen is not on it.

To add some semblance of code: I've been mucking with node.js and some of the effect is simple TagLib bindings with that being used for something more important soon. Much improved ( or rather simplified ) kwin-tiling too with a few regressions like moving and resizing.

Macros for Observers

Here are two little macros too make implementing the Observer design pattern less repetitive. This is C++

File: observermacros.h

#ifndef DDM_OBSERVERMACRO_H
#define DDM_OBSERVERMACRO_H

#define MAKE_OBSERVABLE( obscls ) \
    private:\
        std::list< obscls *> m_observers;\
    public:\
        void addObserver( obscls *obs ) {\
            m_observers.push_back( obs );\
        }\
        void delObserver( obscls *obs ) {\
            m_observers.remove( obs );\
        }

#define NOTIFY( obscls, obsmethod, argument ) \
        for( std::list::const_iterator it = m_observers.begin();\
                it != m_observers.end(); it++ )\
            (*it)->obsmethod( argument )
    
#endif

Now if you want a class to be observable do

#include "observermacros.h"

class NeedsObserving
{
    MAKE_OBSERVABLE( Observes );
};

Observes is the name of the class which will observe NeedsObserving.

// add/del observers
NeedsObserving no;
Observes o = new Observes;
no.addObserver( o );
no.delObserver( o );

// To notify observers of changes

void NeedsObserving::didSomething()
{
    // CoolThing *coolThing is modified/used here
    // ...
    NOTIFY( Observes, coolThingDone, coolThing );
    // Calls Observes::coolThingDone( CoolThing * );
}

The code is pretty naive, but it might help. Cheers

Rounded Corners

Rounded corners have become a staple of web 2.0. So I decided to write my own version. So whats special in this among all the others. Well mine is relatively customisable pertaining to the elements to round. Further it allows elements to have variable width and scales on resizing. The images used use the mountain top technique so that the same image is required, whatever the background color of the elements. Of course if you have a non-white background you will have to edit the images. To check them out see Rounded Corners on 22bits.