Managing Availability in NoSQL

By Adam Fowler

As with other NoSQL database types, with key-value stores, you can trade some consistency for some availability. Key-value stores typically provide a wide range of consistency and durability models — that is, between availability and partition tolerance and between consistent and partition tolerance.

Some key-value stores go much further on the consistency arm, abandoning BASE for full ACID transactional consistency support. Understanding where to draw the line can help you shorten the list of potential databases to consider for your use case.

Trading consistency

Key-value stores typically trade consistency in the data (that is, the ability to always read the latest copy of a value immediately after an update) in order to improve write times.

Voldemort, Riak, and Oracle NoSQL are all eventually consistent key-value stores. They use a method called read repair. Here are the two steps involved in read repair:

  1. At the time of reading a record, determine which of several available values for a key is the latest and most valid one.

  2. If the most recent value can’t be decided, then the database client is presented with all value options and is left to decide for itself.

Good examples for using eventually consistent key-value stores include sending social media posts and delivering advertisements to targeted users. If a tweet arrives late or a five-minute-old advertisement is shown, there’s no catastrophic loss of data.

Implementing ACID support

Aerospike and Redis are notable exceptions to eventual consistency. Both use shared-nothing clusters, which means each key has the following:

  • A master node: Only the masters provide answers for a single key, which ensures that you have the latest copy.

  • Multiple slave replica nodes: These contain copies of all data on a master node. Aerospike provides full ACID transactional consistency by allowing modifications to be flushed immediately to disk before the transaction is flagged as complete to the database client.

Aerospike manages to do that at very high speeds (which refutes claims that having ACID decreases write speed). Aerospike natively handles raw SSDs for data-writing by bypassing slower operating systems’ file system support.

Of course, more SSDs mean higher server costs. You may decide that using Redis (configured to flush all data to disk as it arrives) is fast enough and guarantees sufficient durability for your needs. The default setting in Redis is to flush data to disk every few seconds, leaving a small window of potential data loss if a server fails.

Here are some examples of when you may need an ACID-compliant key-value store:

  • When receiving sensor data that you need for an experiment.

  • In a messaging system where you must guarantee receipt.

    Redis, for example, provides a Publish/Subscribe mechanism that acts as a messaging server back end. This feature combined with ACID support allows for durable messaging.