Mocking and forking Datomic Peer connections in-memory.
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Valentin Waeselynck
Valentin Waeselynck v0.2.2
Latest commit 3ab532f Jul 3, 2018


Mocking and forking Datomic connections in-memory.

Clojars Project


  • This library is not an in-memory re-implementation of Datomic - just a thin wrapper on top of the Datomic Peer Library. All the heavy lifting is done by Datomic's 'speculative writes' (a.k.a db.with(tx)) and Clojure's managed references (atoms)
  • Only for Peers, not Clients.

Project maturity: beta quality. Note that you will probably not need to use this library in production.


(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 essentially the same as: 
(def mocked-conn (dm/fork-conn my-conn))

;; dm/fork-conn is likely what you'll use most.

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 hold 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.


  • Write expressive tests: write tests as a tree of scenarios exploring various alternatives. In particular, this makes it very easy to write system-level tests that run fast. Forget about setup and teardown phases: they are respectively replaced by forking and garbage-collection.
  • Cheap, safe debugging: instantly reproduce your production environment on your local machine. Save and re-use as many checkpoints of your state as you need as you debug. Dry-run data patches and migrations safely before committing them to production.
  • Explore new database schemas: in particular, you can experiment with changes to your database schema without committing to them.
  • Staging environments / QA / CI: want one staging environment (Peer) for each pull-request on your app? Just have each of them use an in-memory fork of a shared database (or even your production database).
  • Ephemeral demos: want to let people experiment with your app without accumulating their manual changes? Just have them work on a fork, and discard it afterwards.
  • Ephemeral dev environments: similarly, it's usually better to always work on the same data when developing, and have the manual changes you've made while experimented be discarded at the end of the session.

Useful links:

How it works

Essentially, by putting a Datomic Database value in a Clojure Managed Reference (currently an Atom, may evolve to use an Agent instead) and using db.with() to update the state of that reference.

That's it, you now know how to re-implement Datomock yourself!

Actually, there are a few additional complications to make this work smoothly:

  • Log: the reference needs to hold not only a Database Value, but also a Log Value (for some strange reason, in Datomic, Log Values are not part of Database values).
  • Futures-based API: to match the interface of Datomic Connections, the library needs to provide a Futures-based API, which requires some additional work on top of Clojure references.
  • txReportQueue: the library needs to provide an implementation of that as well.

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.


Copyright © 2016 Valentin Waeselynck and contributors.

Distributed under the MIT License.