datomock

Mocking and forking Datomic connections in a local context.

Usage

(require '[datomic.api :as d])
(require '[datomock.core :as dm])

(def my-conn (d/connect "datomic:mem://hello-world"))

;; ... create a mock connection from a Database value:
(def starting-point-db (d/db my-conn))
(def mocked-conn (dm/mock-conn starting-point-db))

;; which is the same as: 
(def mocked-conn (dm/fork-conn my-conn))

Rationale and semantics

Mocked connections use Datomic's speculative writes (db.with()) and Clojure's managed references to emulate a Datomic connection locally.

The main benefit is the ability to 'fork' Datomic connections. More precisely, if conn1 is forked from conn2:

• at the time of forking, conn1 and conn2 have the same database value;
• subsequent writes to conn1 will leave conn2 unaffected
• subsequent writes to conn2 will leave conn1 unaffected

Because Datomic database values are persistent data structures, forking is extremely cheap in both space and time.

Here are some applications:

• write your tests as walking a tree of possibilities. Just like Nicolas Cage in the Next movie.
• eliminate the need for teardown phases in your tests
• make your test faster by installing your schema and test data only once for your whole test suite.
• speculatively run some code on your production database

See this blog post for a more in-depth analysis of forkability and its applications.

Mocked connections vs datomic:mem

How is this different than using Datomic memory databases, as in (d/connect "datomic:mem://my-db") ?

Mocked connections differ from Datomic's memory connections in several ways:

• you create a memory connection from scratch, whereas you create a mocked connection from a starting-point database value
• a mocked connection is not accessible via a global URI

Compatibility notes

This library requires Datomic 0.9.4470 or higher, in order to provide an implementation of the most recent methods of datomic.Connection.

However, if you need to work with a lower version, forking this library and removing the implementation of the syncSchema(), syncExcise() and syncIndex() should work just fine.

License

Copyright © 2016 Valentin Waeselynck and contributors.

Distributed under the MIT License.