ACID

Transactions are ACID: Atomic, Consistent, Isolated, and Durable. This document defines the four components of ACID and explains how Datomic Cloud delivers them.

Atomicity

Atomicity requires that each transaction is "all or nothing". If one part of a transaction fails, the entire transaction fails, and the database is left unchanged.

A Datomic transaction is written to the log in a single atomic write, so there is no possibility of partial work. Inside peer processes, the in-memory representation of a database is a pointer to a persistent data structure, and is also updated via a single atomic operation.

Consistency

Consistency ensures that any transaction will take the database from one valid state to another. Datomic makes the following consistency guarantees:

  • Every peer sees completed transactions as of a particular point in time, called a time basis.
  • The time basis of transactions is a global ordering of transactions for a particular database. Peers always see all transactions up to their time basis, in order, with no gaps.

Datomic also provides first class support for accessing the time basis of information:

  • Every fact in the database knows its time basis via the transaction component of a datom.
  • A database value knows its time basis.

To correctly implement domain models, transactions need to be able to derive new facts based on existing facts (e.g. adding to a bank balance). Datomic transaction functions support arbitrary transformations of transaction data based on the current value of the database, enabling both optimistic and pessimistic concurrency strategies.

Transactions often need to enforce domain-specific functional constraints (e.g. a valid account must include name, email, and password hash). Datomic supports functional constraints at both the attribute and entity level:

  • attribute predicates support functional constraints on individual attribute values
  • entity specs support required attributes and functional constraints on individual entities

Isolation

The Isolation property ensures serializability: concurrent transactions result in the same system state that would result if the transactions were executed serially.

Datomic transaction are not just serializable, they are fully serialized. Every transaction uses DynamoDB conditional put to ensure that it is the sole successor of the preceding transaction. To increase throughput, Datomic can automatically batch more than one transaction into a single conditional put, amortizing the latency of storage writes across transactions.

Durability

Durability means that once a transaction has been committed, it has been recorded in the log. Datomic is fully durable–it always awaits acknowledgment from DynamoDB before reporting that a transaction is complete.