Mocking datomic.Connection for fast in-memory testing

mock-connection.clj

(ns bs.utils.mock-connection
  "Utilities for using Datomic"
  (:require [datomic.api :as d])
  (:use clojure.repl clojure.pprint)
  (:import (java.util.concurrent BlockingQueue LinkedBlockingDeque)
           (datomic Connection)))

(defrecord MockConnection
  [dbAtom, ^BlockingQueue txQueue]

  Connection
  (db [this] @(.dbAtom this))
  (transact [this tx-data] (doto (datomic.promise/settable-future)
                             (deliver (let [tx-res
                                            (loop []
                                              (let [db-atom (.dbAtom this)
                                                    old-val @db-atom
                                                    tx-res (d/with old-val tx-data)
                                                    new-val (:db-after tx-res)]
                                                (if (compare-and-set! db-atom old-val new-val)
                                                  tx-res
                                                  (recur))
                                                ))]
                                        (.add ^BlockingQueue (.txQueue this) tx-res)
                                        tx-res))
                             ))
  (transactAsync [this tx-data] (.transact this tx-data))

  (gcStorage [this olderThan])
  (requestIndex [this])
  (release [this])
  (sync [this] (doto (datomic.promise/settable-future)
                 (deliver (.db this))))
  (syncExcise [this t] (.sync this))
  (syncIndex [this t] (.sync this))
  (syncSchema [this t] (.sync this))
  (sync [this t] (.sync this))
  (txReportQueue [this] (.txQueue this)))

(defn ^Connection mock-conn
  "Creates a mocked version of datomic.Connection which uses db/with internally.
  Only supports datomic.api/db, datomic.api/transact and datomic.api/transact-async operations.
  Sync and housekeeping methods are implemented as noops. #log() is not supported."
  [db]
  (MockConnection. (atom db) (LinkedBlockingDeque.)))

(comment ;; How fast is it ?
  (def mem-db (bs.dev/local-db)) ;; creates a mem db, loads the schema and some fixture data in it. Takes about 20ms on my dev laptop.
  (time (dotimes [i 1000]
          (let [conn (mock-conn mem-db)]
            @(d/transact conn [{:db/id            (d/tempid :db.part/user)
                                :bouser/firstName (str "Valentinnnnn_" i)
                                :bouser/lastName  (str "Valentinnnnn_" i)
                                :bouser/level     "bandsquare"
                                :bouser/email     (str "val.vvalval" i "@gmail.com")
                                }]))))
  ;; "Elapsed time: 2027.881 msecs". So this is a x10 improvement.
  ;; the schema and fixture data are quite small (35 attributes, 103 datoms), so I expect this will get more compelling as the application grows.
  )

Note that with this implementation Exceptions will not be wrapped, unlike what is described in the Datomic documentation. So be careful of that if you're using this implementation for development.

Now available on Clojars: https://github.com/vvvvalvalval/datomock

nice!