Twissandra

Twissandra is an example project, created to learn and demonstrate how to use Cassandra. Running the project will present a website that has similar functionality to Twitter.

You can see a running copy at http://twissandra.com/

Most of the magic happens in twissandra/cass.py, so check that out.

Installation

Installing Twissandra is fairly straightforward. Really it just involves checking out Cassandra and Twissandra, doing a little configuration, and then starting it up. Here's a roadmap of the steps we're going to take to install the project:

1. Check out the latest Cassandra source code
2. Check out the Twissandra source code
3. Install and configure Cassandra
4. Install Thrift
5. Create a virtual Python environment with Twissandra's dependencies
6. Start up the webserver

Check out the latest Cassandra source code

git clone git://git.apache.org/cassandra.git

Check out the Twissandra source code

git clone git://github.com/twissandra/twissandra.git

Install and configure Cassandra

Now build Cassandra:

cd cassandra
ant

Then we need to create our database directories on disk:

sudo mkdir -p /var/log/cassandra
sudo chown -R `whoami` /var/log/cassandra
sudo mkdir -p /var/lib/cassandra
sudo chown -R `whoami` /var/lib/cassandra

Finally we can start Cassandra:

./bin/cassandra -f

Install Thrift

Follow the instructions provided on the Thrift website itself

Create a virtual Python environment with Twissandra's dependencies

First, make sure to have virtualenv installed. If it isn't installed already, this should do the trick:

sudo easy_install -U virtualenv

Now let's create a new virtual environment, and begin using it:

virtualenv twiss
source twiss/bin/activate

We should install pip, so that we can more easily install Twissandra's dependencies into our new virtual environment:

easy_install -U pip

Now let's install all of the dependencies:

pip install -U -r twissandra/requirements.txt

Now that we've got all of our dependencies installed, we're ready to start up the server.

Create the schema

Make sure you're in the Twissandra checkout, and then run the sync_cassandra command to create the proper keyspace in Cassandra:

cd twissandra
python manage.py sync_cassandra

Start up the webserver

This is the fun part! We're done setting everything up, we just need to run it:

python manage.py runserver

Now go to http://127.0.0.1:8000/ and you can play with Twissandra!

Schema Layout

In Cassandra, the way that your data is structured is very closely tied to how how it will be retrieved. Let's start with the user ColumnFamily. The key is a username, and the columns are the properties on the user:

User = {
    'hermes': {
        'password': '****',
        (other properties),
    },
}

Friends and followers are keyed by the username, and then the columns are the friend names and follower names, and we store a timestamp as the value because it's interesting information to have:

Friends = {
    'hermes': {
        # friend id: timestamp of when the friendship was added
        'larry': '1267413962580791',
        'curly': '1267413990076949',
        'moe'  : '1267414008133277',
    },
}

Followers = {
    'hermes': {
        # friend id: timestamp of when the followership was added
        'larry': '1267413962580791',
        'curly': '1267413990076949',
        'moe'  : '1267414008133277',
    },
}

Tweets are stored with a tweet id for the key.

Tweet = {
    '7561a442-24e2-11df-8924-001ff3591711': {
        'username': 'hermes',
        'body': 'Trying out Twissandra. This is awesome!',
    },
}

The Timeline and Userline column families keep track of which tweets should appear, and in what order. To that effect, the key is the username, the column name is a timestamp, and the column value is the tweet id:

Timeline = {
    'hermes': {
        # timestamp of tweet: tweet id
        1267414247561777: '7561a442-24e2-11df-8924-001ff3591711',
        1267414277402340: 'f0c8d718-24e2-11df-8924-001ff3591711',
        1267414305866969: 'f9e6d804-24e2-11df-8924-001ff3591711',
        1267414319522925: '02ccb5ec-24e3-11df-8924-001ff3591711',
    },
}

Userline = {
    'hermes': {
        # timestamp of tweet: tweet id
        1267414247561777: '7561a442-24e2-11df-8924-001ff3591711',
        1267414277402340: 'f0c8d718-24e2-11df-8924-001ff3591711',
        1267414305866969: 'f9e6d804-24e2-11df-8924-001ff3591711',
        1267414319522925: '02ccb5ec-24e3-11df-8924-001ff3591711',
    },
}