Datomic
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 at any time. Datomic is also a distributional DBMS, which provides horizontal read scalability.
There are two Datomic products, Datomic Cloud and Datomic On-Prem. Datomic Cloud is built for AWS integration, and Datomic On-Prem (On-Premise) could be built on any infrastructure and storage services.
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 utilized and integrated AWS technologies, for example,
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DynamoDB, EFS, EBS and S3 as storage services;
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CloudFormation for deployment;
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AWS Cloudwatch for logging, monitoring, and metrics.
Since the release of Datomic Cloud, the original Datomic was referred to as Datomic On-Prem (On-Premise) to distinguish from the new release.
Indexes
Datomic indexes are covering indexes. In other words, instead of storing reference to data in the index, Datomic directly reads data from index. The index trees are shallow, with at most 3 levels: root, directories and segment leaf. Datomic maintains four index trees with different sorting orders for efficient access of different queries.
Concurrency Control
Optimistic Concurrency Control (OCC)
Datomic supports optimistic concurrency control, which is made possible by its built-in compare-and-swap :db/cas.
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.
Stored Procedures
Stored procedures are represented as transaction functions in Datomic, which can be invoked during transaction processes.
Storage Model
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. Storage services could also be switched easily by simply changing the connection string.
