The Memdb package implements a simple in-memory database built on immutable radix trees. The database provides Atomicity, Consistency and Isolation from ACID. Being that it is in-memory, it does not provide durability. The database is instantiated with a schema that specifies the tables and indices that exist and allows transactions to be executed. The database provides the following: - Multi-Version Concurrency Control - By leveraging immutable radix trees the database is able to support any number of concurrent readers without locking, and allows a writer to make progress. - Transaction Support - The database allows for rich transactions, in which multiple objects are inserted, updated or deleted. The transactions can span multiple tables, and are applied atomically. The database provides atomicity and isolation in ACID terminology, such that until commit the updates are not visible. - Rich Indexing - Tables can support any number of indexes, which can be simple like a single field index, or more advanced compound field indexes. Certain types like UUID can be efficiently compressed from strings into byte indexes for reduced storage requirements. - Watches - Callers can populate a watch set as part of a query, which can be used to detect when a modification has been made to the database which affects the query results. This lets callers easily watch for changes in the database in a very general way.