conformity

A Clojure/Datomic library for idempotently transacting datoms (norms) into your database – be they schema, data, or otherwise.

In the simplest sense, conformity allows you to write migrations and ensure that they run once and only once.

In a more general sense, conformity allows you to declare expectations (in the form of norms) about the state of your database, and enforce those idempotently without repeatedly transacting schema, required data, etc.

Dependency

Conformity is available on clojars, and can be included in your leiningen project.clj by adding the following to :dependencies:

Usage

The easiest way to use conformity is to store your norms in an edn file that lives in your resources/ folder.

;; resources/something.edn
{:my-project/something-schema
  {:txes [[{:db/id #db/id [:db.part/db]
            :db/ident :something/title
            :db/valueType :db.type/string
            :db/cardinality :db.cardinality/one
            :db/index false
            :db.install/_attribute :db.part/db}]]}}

Then in your code:

src/my_project/something.clj

(ns my-project.something
  (:require [io.rkn.conformity :as c]
            [datomic.api :as d]))

(def uri "datomic:mem://my-project")
(d/create-database uri)
(def conn (d/connect uri))

(def norms-map (c/read-resource "something.edn"))

(println (str "Has attribute? " (c/has-attribute? (d/db conn) :something/title)))
(c/ensure-conforms conn norms-map [:my-project/something-schema])
(println (str "Has attribute? " (c/has-attribute? (d/db conn) :something/title)))

; ... Code dependant on the presence of attributes in :my-project/something-schema

You can see this more directly illustrated in a console…

; nREPL 0.1.5

; Setup a in-memory db
(require '[datomic.api :as d])
(def uri "datomic:mem://my-project")
(d/create-database uri)
(def conn (d/connect uri))

; Hook up conformity and your sample datom
(require '[io.rkn.conformity :as c])
(def norms-map (c/read-resource "something.edn"))

(c/has-attribute? (d/db conn) :something/title)
; -> false

(c/ensure-conforms conn norms-map [:my-project/something-schema])
(c/has-attribute? (d/db conn) :something/title)
; -> true

Migrations as code

Instead of using the :txes key to point to an inline transaction, you can also use a :txes-fn key pointing to a symbol reference to a function, as follows...

;; resources/something.edn
{:my-project/something-else-schema
  {:txes-fn 'my-project.migrations.txes/backport-bar-attr-to-entities-with-foo}}

backport-bar-attr-to-entities-with-foo will be passed the Datomic connection and should return transaction data, allowing transactions to be driven by full-fledged inspection of the database.

For example...

(ns my-project.migrations.txes
  (:require [datomic.api :as d])

(def find-eids-with-attr
  '[:find [?e ...]
    :in $ ?attr
    :where
    [?e ?attr]])

(defn backport-bar-attr-to-entities-with-foo
  "Find existing entities bearing the `:some/foo` attribute,
   and apply to them the `:some/bar` attribute and value."
  [conn]
  (let [foo-eids (d/q find-eids-with-attr (d/db conn) :some/foo)
        tx-data  (for [eid foo-eids]
                   {:db/id eid
                    :some/bar :bar-value})]
    tx-data))

Schema dependencies

Norms can also carry a :requires attribute, which points to the keyword/ident of some other such map which it depends on having been already transacted before it can be. This is declarative; Once specified in the map passed to ensure-conforms, confirmity handles the rest.

Caveat: Norms only get conformed-to once!

Once a norm is conformed to that's it! It won't be transacted again. That does mean that you shouldn't edit a norm and expect it to magically get updated the next time ensure-conforms runs.

In the future you may be able to intelligently version norms themselves, but I had to draw the line somewhere for the initial release.

License

Copyright © 2012-2014 Ryan Neufeld

Distributed under the Eclipse Public License, the same as Clojure.