MapReduce is a technique for performing aggregate processing on large amounts of data in parallel; it's a particularly common technique in data analytics applications. Cassandra does not offer built-in MapReduce capabilities, but it can be integrated with Hadoop in order to perform MapReduce operations across Cassandra data sets, or Spark for real-time data analysis. The DataStax enterprise product provides integration with both of these tools out of the box.Spark is a fast, distributed, and expressive computational engine used for large-scale data processing similar to MapReduce. It is much more efficient than MapReduce and runs with resource managers such as Mesos and Yarn. It can read data from various sources such as Hadoop or Cassandra or even streams such as Kafka. DataStax provides a Spark-Cassandra connector to load data from Cassandra into Spark and run batch computations on the data.
Cassandra provides an SQL-like syntax to interact with the database. Cassandra Query Language (CQL) presents a familiar row column representation of data. CQL provides a familiar SQL-like table definition with columns and defined data types. Schema is flexible, and new columns can be added while using the existing data. The data model doesn't support features problematic in distributed systems such as joins. On top of this, Cassandra provides other features such as collections to store multiple items in a single column. It also lets you easily define secondary indexes and materialized views for fast lookups on non-primary key columns.
Note
The previous thrift-based interface was closely tied to the internal storage view. This was fairly complex and had a relatively high learning curve to adopt. The CQL interface is much easier to understand because of its similarity to SQL.
As discussed before, Cassandra provides eventual consistency rather than immediate consistency, which means data written will eventually be consistent across multiple replicas of the data. This has implications on the data returned by read queries. There is a possibility that reads could return stale data depending on how writes and reads are configured (the consistency levels at which both queries are performed). Strong consistency, which means reading the most recently written value, can be achieved using quorum reads and writes. But what if strong consistency is not enough? What if we have some operations to perform in sequence that must not be interrupted by others, that is, we must perform them one at a time, or make sure that any that we do run concurrently get the same results as if they really were processed independently. Cassandra provides lightweight transactions with linearizable consistency to ensure a transaction isolation level similar to the serializable level offered by RDBMSs. They are also known as compare and set transactions. You can use lightweight transactions instead of durable transactions with eventual/tunable consistency for situations that require the nodes in the distribution system to agree on changes to the data.
Another interesting feature provided by Cassandra is the ability to replicate data across multiple data centers or geographical zones in near real-time. This is natively supported by Cassandra and doesn't need to be managed at the application level. Cassandra also provides local consistency levels to ensure cross-region latency doesn't impact client queries. A multiregion cluster can sustain disasters or entire data centers going down. Ideally, there is no need for backups or disaster recovery when running multidata center clusters except for cases of data corruption.
Note
On April 21, 2011, Amazon experienced a large outage in AWS US-East. Some websites were impacted, while others were not. For Netflix, their systems are designed explicitly for these sorts of failures. The SimpleDB, S3, and Cassandra services that Netflix depends upon were not affected by the outage because of the cross-region replication that these services provide.
Now that you've got an in-depth understanding of the feature set that Cassandra offers, it's time to figure out which features are most important to you and which database is the best fit. The following table lists a handful of commonly used databases and key features that they do or don't have:
Feature | Cassandra | PostgreSQL | MongoDB | Redis | Riak |
Structured records | Yes | Yes | Yes | Limited | No |
Secondary indexes | Yes | Yes | Yes | No | Yes |
Discretely writable collections | Yes | Yes | Yes | Yes | No |
Relational joins | No | Yes | No | No | No |
Built-in MapReduce | No | No | Yes | No | Yes |
Fast result ordering | Yes | Yes | Yes | Yes | No |
Immediate consistency | Configurable at query level | Yes | Yes | Yes | Configurable at cluster level |
Transparent sharding | Yes | No | Yes | No | Yes |
No single point of failure | Yes | No | No | No | Yes |
High throughput writes | Yes | No | No | Yes | Yes |
As you can see, Cassandra offers a unique combination of scalability, availability, and a rich set of features for modeling and accessing data.