Datomic

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Datomic is a proprietary database management system. It is an operational DBMS, in other words, it allows updates in real time. Instead of assigning and overwriting values into named attributes, Datomic keeps track of all immutable facts over time, which sets Datomic apart because previous states can be accessed easily. Datomic is also a distributional DBMS, which provides horizontal read scalability. Therefore, Datomic is a good fit for storing a history of valuable records that requires operational flexibility and read scalability.

In addition, Datomic Cloud is built for AWS integration and is a quicker and easier way to access Datomic on AWS.

History

Early March 2012, the Relevance team (later joined with Metadata to form Cognitect) around Rich Hickey released Datomic, which they have started to work on since 2010.

To make better use of recent advancements in cloud computing, Datomic Cloud was release in early 2018, which is better tailored to AWS.

Data Model

Relational Key/Value Entity-Attribute-Value

Datomic stores immutable facts as datoms over time. A datom follows the form of (entity, attribute, value, time). Although Datomic doesn't require a table schema that specifies attribute columns in advance, it requires to specify properties of individual attributes. This is called universal schema.

Concurrency Control

Multi-version Concurrency Control (MVCC) Deterministic Concurrency Control

Datomic stores the entire history of transaction assertions and retractions, which

Compression

Naïve (Record-Level)

Index trees contain "segments," arrays of records that are serialized and then compressed with zip. Compression allows more efficient caching and storage.

Foreign keys can be defined using :db.type/ref attribute, but no foreign key constraints are enforced on them automatically. User needs to specify their own database functions to impose those constraints.

Storage Model

Custom

Datomic treats storage as a service, which means that Datomic only provides the ways to access underlying storage, but doesn't provide the actual storage.

Query Compilation

JIT Compilation Stored Procedure Compilation

Isolation Levels

Serializable

There is only one thread responsible for writing transactions, so transactions are always serializable.

Query Interface

Datalog

Datomic's query interface is an extension from Datalog. The main difference is that Datalog systems usually have a global fact database and a set of rules, but Datomic Datalog could take multiple databases and sets of rules.