Cassandra 101 : Understanding what Cassandra Is
As some of you may know, in my current role at Pythian I am tackling OSDB, and Cassandra is on my radar. One of the things I have been trying to do is learn what Cassandra is, so in this post, I'm going to share a bit of what I have been able to learn. According to the whitepaper "
Solving Big Data Challenges for Enterprise Application Performance Management" , Cassandra is a "
distributed key value store developed at Facebook. It was designed to handle very large amounts of data spread out across many commodity servers while providing a highly available service without single point of failure allowing replication even across multiple data centers as well as for choosing between synchronous or asynchronous replication for each update." Cassandra, in layman's terms, is a NoSQL database developed in JavaOne. One of Cassandra's many benefits is that it's an open source DB with deep developer support. It is also a fully distributed DB, meaning that there is no master DB (unlike Oracle or MySQL) so this allows this database to have no point of failure. It also touts being linearly scalable, meaning that if you have 2 nodes and a throughput of 100,000 transactions per second, and you added 2 more nodes, you would now get 200,000 transactions per second, and so forth. Cassandra is based on 2 core technologies, Google's Big Table and Amazon's Dynamo. Facebook uses the latter to power their Inbox Search feature. It was released as an open source project on Google Code and then incubated at Apache, nowadays, Cassandra is considered a Top-Level-Project. Currently there are 2 versions of Cassandra:
- Community Edition.- This is distributed under the Apache™ License
- Enterprise Edition .- This is distributed by Datastax
- Consistency.- A read is guaranteed to return the most recent write for a given client.
- Availability.-A non-failing node will return a reasonable response within a reasonable amount of time (no error or timeout).
- Partition Tolerance.-The system will continue to function when network partitions occur.
- Keyspace is the container for your application data, similar to a schema in a relational database. Keyspaces are used to group column families together. Typically, a cluster has one keyspace per application. It also defines the replication strategy and data objects belong to a single keyspace.
- Column Family is a set of one, two, or more individual rows with a similar structure.
- Row is a collection of sorted columns, it is the the smallest unit that stores related data in Cassandra, and any component of a Row can store data or metadata.
- Row Key uniquely identifies a row in a column family.
- Column key uniquely identifies a column value in a row.
- Column value stores one value or a collection of values.
- Row Key uniquely identifies a row in a column family.
- Node is one Cassandra instance and is the basic infrastructure component in Cassandra. Cassandra assigns data to nodes in the cluster; each node is assigned a part of the database based on the Row Key. Usually corresponds to a host, but not necessarily, especially in Dev or Test environments.
- Rack is a logical set of nodes.
- Data Center is a logical set of Racks, a data center can be a physical data center or virtual data center. Replication is set by data center.
- Cluster contains one or more data centers and is the full set of nodes which map to a single complete token ring.
