Exo was designed to facilitate database migrations in environments where a single schema must be maintained across an unbounded number of databases. The primary motivation for its development is multi-tenant applications in which each tenant has a dedicated database that must stay up to date with application development. The library offers a programmatic means of defining and executing migrations without making assumptions about application architecture.
Operations are the core primitive used by the library and offer an abstract representation of a change to the database schema. Operations represent the creation, modification and removal of database tables, columns and indexes. Prior to being transformed into SQL statements, several operations may be reduced into a single operation in order to avoid redundant database queries.
Migrations offer a developer-friendly means of defining changes to the schema and act as factories for table operations.
Histories are versioned collections of migrations that provide facilities for playing and rewinding sets of operations.
Handlers are used to execute histories against database connections. Specific source and target versions are used to determine which sets of operations are applied.