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Tupelo: Making Datomic Even Sweeter

Leiningen coordinates:

The Tupelo-Datomic API Docs are posted on GitHub Pages


Have you ever wanted to jump into using Datomic but wished for a simpler starting point? If so, then Tupelo Datomic is for you! The goal of Tupelo Datomic is to automate all of the detail that rarely changes or is needed when dealing with Datomic, making your job simpler.

Suppose we’re trying to keep track of information for the world’s premiere spy agency. Let’s create a few attributes that will apply to our heroes & villains (see the executable code in the unit test).

(ns bond
  (:require [tupelo.datomic   :as td]
            [tupelo.schema    :as ts]))

; Create some new attributes. Required args are the attribute name (an optionally namespaced
; keyword) and the attribute type (full listing at We wrap
; the new attribute definitions in a transaction and immediately commit them into the DB.
(td/transact *conn* ;   required              required              zero-or-more
                    ;  <attr name>         <attr value type>       <optional specs ...>
  (td/new-attribute   :person/name         :db.type/string         :db.unique/value)      ; each name      is unique
  (td/new-attribute   :person/secret-id    :db.type/long           :db.unique/value)      ; each secret-id is unique
  (td/new-attribute   :weapon/type         :db.type/ref            :db.cardinality/many)  ; one may have many weapons
  (td/new-attribute   :location            :db.type/string)     ; all default values
  (td/new-attribute   :favorite-weapon     :db.type/keyword ))  ; all default values

Notice that, when using (td/transact …​), we don’t need to wrap everything in a vector.

For the :weapon/type attribute, we want to use an enumerated type since there are only a limited number of choices available to our antagonists:

; Create some "enum" values. These are degenerate entities that serve the same purpose as an
; enumerated value in Java (these entities will never have any attributes). Again, we
; wrap our new enum values in a transaction and commit them into the DB.
(td/transact *conn*
  (td/new-enum :weapon/gun)
  (td/new-enum :weapon/knife)
  (td/new-enum :weapon/guile)
  (td/new-enum :weapon/wit))

There are several other naming possibilities for this (or any) attribute:

 original          "pure"          pseudo-namespace  random-namespace   simple
:weapon/gun    :weapon.type/gun     :weapon-gun       :big/gun           :gun
:weapon/knife  :weapon.type/knife   :weapon-knife     :sharp/knife       :knife
:weapon/guile  :weapon.type/guile   :weapon-guile     :effortless/guile  :guile
:weapon/wit    :weapon.type/wit     :weapon-wit       :rapier/wit        :wit

To Datomic, all of these attribute naming styles are equally valid. In particular, Datomic applies no semantic difference between any of these choices. Datomic effectively treats any attribute name as an opaque string. For simplicity, one may wish to omit any attribute namespaces until the project has grown large enough to warrant them. Note that two of our attributes (:location & :favorite-weapon) have no namespace. We also could have omitted the namespace on the weapons themselves, using only :gun, :knife, :guile, and :wit.

Note that the original Datomic docs use the "pure" convention for naming attributes and their values. In the "pure" convention, an attribute like :weapon/type would only have values like :weapon.type/gun, where the name of the attribute becomes the namespace for all of its possible values (with the slash replaced by a dot).

Let’s create a few antagonists and load them into the DB. Note that we are just using plain Clojure values and literals here, and we don’t have to worry about any Datomic specific conversions.

; Create some antagonists and load them into the db.  We can specify some of the
; attribute-value pairs at the time of creation, and add others later. Note that
; whenever we are adding multiple values for an attribute in a single step (e.g.
; :weapon/type), we must wrap all of the values in a set. Note that the set
; implies there can never be duplicate weapons for any one person.  As before,
; we immediately commit the new entities into the DB.
(td/transact *conn*
  (td/new-entity { :person/name "James Bond" :location "London"     :weapon/type #{ :weapon/gun :weapon/wit   } } )
  (td/new-entity { :person/name "M"          :location "London"     :weapon/type #{ :weapon/gun :weapon/guile } } )
  (td/new-entity { :person/name "Dr No"      :location "Caribbean"  :weapon/type    :weapon/gun                 } ))

And, just like that, we have values persisted in the DB! Let’s check that they are really there:

; Verify the antagonists were added to the DB
(let [people (get-people (live-db)) ]
  (is (= people
         #{ {:person/name "James Bond"    :location "London"      :weapon/type #{:weapon/wit    :weapon/gun} }
            {:person/name "M"             :location "London"      :weapon/type #{:weapon/guile  :weapon/gun} }
            {:person/name "Dr No"         :location "Caribbean"   :weapon/type #{:weapon/gun               } } } )))

EntitySpec, EntityID, and LookupRef

Entities in Datomic are specified using an EntitySpec, which is either an EntityID (EID) or a LookupRef.

An EntityID (EID) is a globally unique Long value that uniquely specifies any entity in the Datomic DB. These are always positive for committed entities in Datomic (negative values indicate temporary EIDs used only in building transactions).

A LookupRef is an attribute-value pair (wrapped in a vector), which uniquely specifies an entity. If an entity has an attribute specified as either :db.unique/value or :db.unique/identity, that entity may be specified using a LookupRef.

Here we verify that we can find James Bond and retrieve all of his attr-val pairs using either type of EntitySpec:

; Using James' name, lookup his EntityId (EID). It is a java.lang.Long that is a unique ID across the whole DB.
(let [james-eid   (td/find-value   :let    [$ (live-db)]     ; like Clojure let
                                   :find   [?eid]
                                   :where  {:db/id ?eid :person/name "James Bond"} )
      _ (s/validate ts/Eid james-eid)  ; verify the expected type
      ; Retrieve James' attr-val pairs as a map. An entity can be referenced either by EID or by a
      ; LookupRef, which is a unique attribute-value pair expressed as a vector.
      james-map   (td/entity-map (live-db) james-eid)                       ; lookup by EID
      james-map2  (td/entity-map (live-db) [:person/name "James Bond"] )    ; lookup by LookupRef
  (is (= james-map james-map2
         {:person/name "James Bond" :location "London" :weapon/type #{:weapon/wit :weapon/gun} } ))

We can also use either type of EntitySpec for update

  ; Update the database with more weapons.  If we overwrite some items that are
  ; already present (e.g. :weapon/gun) it is idempotent (no duplicates are
  ; allowed).  The first arg to td/update is an EntitySpec (either EntityId or
  ; LookupRef) and determines the Entity that is updated.
  (td/transact *conn*
    (td/update james-eid   ; update using EID
        { :weapon/type #{ :weapon/gun :weapon/knife }
          :person/secret-id 007 } )   ; Note that James has a secret-id but no one else does

    (td/update [:person/name "Dr No"] ; update using LookupRef
      { :weapon/type #{ :weapon/gun :weapon/knife :weapon/guile } } )))

As expected, our database contains the updated values for Dr No and James Bond. Notice that, since :weapon/type is implemented as a set in Datomic, duplicate values are not allowed and both antagonists have only a single gun:

; Verify current status. Notice there are no duplicate weapons.
(let [people (get-people (live-db)) ]
  (is (= people
    #{ { :person/name "James Bond" :location "London" :weapon/type #{:weapon/wit :weapon/knife :weapon/gun} :person/secret-id 7 }
       { :person/name "M" :location "London"          :weapon/type #{:weapon/guile :weapon/gun} }
       { :person/name "Dr No" :location "Caribbean"   :weapon/type #{:weapon/guile :weapon/knife :weapon/gun} } } )))

Note that James Bond is the only person with an entry for :person/secret-id. This points out an important conceptual point regarding Datomic:

The Datomic Conceptual Model:

Datomic is conceptually structured as a collection of simple maps, each of which has a unique Entity ID and an arbitrary collection of attribute-value pairs.

The "Entity ID" or EID is encoded under the key :db/id in Datomic. A Clojure example equivalent to the above would look like this:

;  <----------------- Maps of Attribute-Value Pairs ----------------------------------------->
   { :db/id 1001  :person/name "James Bond"  :location "London"     ...  :person/secret-id 7 }
   { :db/id 1002  :person/name "M"           :location "London"     ...                      }
   { :db/id 1003  :person/name "Dr No"       :location "Caribbean"  ...                      }

except that the actual EID values are randomly assigned by the Datomic Transactor; we only know that they are of type "positive 64-bit integer". Don’t worry about running out of EIDs. If you created a billion new EIDs each second, it would require 292 years before you ran out of them.

Enum Values

The benefit of using enumerated values in Datomic is that we can restrict the the domain of acceptable values more easily than by using plain keyword values. For example, if we try to give James a non-existent weapon, Datomic will generate an exception:

; Try to add non-existent weapon. This throws an Exception since the
; bogus kw does not match up with an entity.
(is (thrown? Exception @(td/transact *conn*
                          (td/update [:person/name "James Bond"] ; update using a LookupRef
                            { :weapon/type #{ } } ))))
                            ; bogus value for :weapon/type causes exception

If the valueType for the attribute :weapon/type was simply :keyword instead of being an enum, the addition of would have succeeded, since it is a legal keyword.

Query Functions in Tupelo Datomic

When searching for values with Tupelo Datomic, the fundamental result type is a TupleSet (a Clojure set containing unique Clojure vectors). This overcomes a possible problem with the native Datomic return type of datomic.query.EntityMap, which is lazy-loading and may appear to be missing data (unless forced). Here is an example of the Tupelo Datomic find function in action:

; For general queries, use td/find.  It returns a set of tuples (a TupleSet).  Duplicate
; tuples in the result will be discarded.
(let [tuple-set (td/find :let   [$ (live-db)]
                         :find  [?name ?loc] ; <- shape of output tuples
                         :where {:person/name ?name :location ?loc} ) ; <- Clojure map encodes query
  (s/validate  ts/TupleSet  tuple-set)       ; verify expected type using Prismatic Schema
  (s/validate #{ [s/Any] }  tuple-set)       ; literal definition of TupleSet
  (is (= tuple-set #{ ["Dr No"       "Caribbean"]      ; Even though London is repeated, each tuple is
                      ["James Bond"  "London"]         ; still unique. Otherwise, any duplicate tuples
                      ["M"           "London"] } )))   ; will be discarded since output is a clojure set.

Tupelo Datomic uses the find function (& variants) for retrieving values from the database. The find function modifies the original Datomic query syntax of (datomic.api/q …​) in three ways.

  1. For convenience, the Tupelo Datomic find form does not need to be wrapped in a map literal nor is any quoting required.

  2. To clarify the relationship between program symbols and query arguments, the :in keyword has been replaced with the :let keyword, and the syntax has been copied from the Clojure let special form. In this way, each of the query variables is more closely aligned with its actual value. Also, the implicit DB $ must be explicitly tied to its data source in all cases (as shown above).

  3. Most importantly, the Datalog-inspired query syntax has been simplified with an equivalent syntax based on plain Clojure maps.

The above query matches any entity that has both a :person/name and a :location attribute. For each matching entity, the two values corresponding to :person/name and :location will be bound to the ?name and ?loc symbols, respectively, which are used to generate an output tuple of the shape [?name ?loc]. Each output tuple is added to the result set, which is returned to the caller. Since the returned value is a normal Clojure set, duplicate elements are not allowed and any non-unique values will be discarded.

What if you want to query for a person with two types of weapons? Suppose you wish to find all of the operatives with both :weapon/guile and :weapon/gun attributes (or more)? Then just list both desired traits, and Datomic will perform in implicit join operation in the query (logical "and"):

; Search for people that match both {:weapon/type :weapon/guile} and {:weapon/type :weapon/gun}
(let [tuple-set   (td/find :let    [$ (live-db)]
                           :find   [?name]
                           :where  {:person/name ?name :weapon/type :weapon/guile }
                                   {:person/name ?name :weapon/type :weapon/gun } ) ]
  (is (= #{["Dr No"] ["M"]} tuple-set )))

Receiving a TupleSet result is the most general case, but in many instances we can save some effort. If we are retrieving the value for a single attribute per entity, we don’t need to wrap that result in a tuple. In this case, we can use the function td/find-attr, which returns a set of scalars as output rather than a set of tuples of scalars:

; If you want just a single attribute as output, you can get a set of values (rather than a set of
; tuples) using td/find-attr.  As usual, any duplicate values will be discarded.
(let [names     (td/find-attr  :let     [$ (live-db)]
                               :find   [?name] ; <- a single attr-val output allows use of td/find-attr
                               :where  {:person/name ?name} )
      cities    (td/find-attr  :let     [$ (live-db)]
                               :find   [?loc]  ; <- a single attr-val output allows use of td/find-attr
                               :where  {:location ?loc} )
  (is (= names    #{"Dr No" "James Bond" "M"} ))  ; all names are present, since unique
  (is (= cities   #{"Caribbean" "London"} )))     ; duplicate "London" discarded

A parallel case is when we want results for just a single entity, but multiple values are needed. In this case, we don’t need to wrap the resulting tuple in a set and we can use the function td/find-entity, which returns just a single tuple as output rather than a set of tuples:

; If you want just a single tuple as output, you can get it (rather than a set of
; tuples) using td/find-entity.  It is an error if more than one tuple is found.
(let [beachy    (td/find-entity    :let    [$    (live-db)      ; assign multiple find variables
                                            ?loc "Caribbean"]   ;   just like clojure 'let' special form
                                   :find   [?eid ?name] ; <- output tuple shape
                                   :where  {:db/id ?eid :person/name ?name :location ?loc} )
      busy      (try ; error - both James & M are in London
                  (td/find-entity  :let    [$     (live-db)
                                            ?loc  "London"]
                                   :find   [?eid ?name] ; <- output tuple shape
                                   :where  {:db/id ?eid :person/name ?name :location ?loc} )
                  (catch Exception ex (.toString ex)))
  (is (matches? [_ "Dr No"] beachy ))           ; found 1 match as expected
  (is (re-find #"Exception" busy)))  ; Exception thrown/caught since 2 people in London

Note that, in the first the call to find-entity, the symbol ?loc is bound to the string "Caribbean", while the symbols ?eid and ?name are left free. This means the query map in the :where clause will match any entity that posseses all three attributes :db/id, :location, and :person/name (note that every entity has the :db/id attribute by definition). In addition, only entities whose :location attribute has the value "Caribbean" will be selected. Once an entity is selected, its values for the attributes :db/id and :location are bound to the symbols ?eid and ?name, respectively, and the output tuple [?eid ?name] is added to the result set. Similar processing happens for the second call to find-entity when ?loc is bound to the string "London".

Of course, in some instances you may only want the value of a single attribute for a single entity. In this case, we may use the function td/find-value, which returns a single scalar result instead of a set of tuples of scalars:

; If you know there is (or should be) only a single scalar answer, you can get the scalar value as
; output using td/find-value. It is an error if more than one tuple or value is present.
(let [beachy    (td/find-value   :let    [$    (live-db)     ; Find the name of the
                                          ?loc "Caribbean"]  ; only person in the Caribbean
                                 :find   [?name]
                                 :where  {:person/name ?name :location ?loc} )
      busy      (try ; error - multiple results for London
                  (td/find-value   :let    [$    (live-db)
                                            ?loc "London"]
                                   :find   [?eid]
                                   :where  {:db/id ?eid :person/name ?name :location ?loc} )
                  (catch Exception ex (.toString ex)))
      multi     (try ; error - result tuple [?eid ?name] is not scalar
                  (td/find-value   :let    [$    (live-db)
                                            ?loc "Caribbean"]
                                   :find   [?eid ?name]
                                   :where  {:db/id ?eid :person/name  ?name :location ?loc} )
                  (catch Exception ex (.toString ex)))
  (is (= beachy "Dr No"))                       ; found 1 match as expected
  (is (re-find #"Exception" busy))   ; Exception thrown/caught since 2 people in London
  (is (re-find #"Exception" multi))) ; Exception thrown/caught since 2-vector is not scalar

Using the Datomic Pull API

If one wishes to use queries returning possibly duplicate result items, then the Datomic Pull API is required. Searching for data via find-pull returns results in a List (a Clojure vector), rather than a Set, so that duplicate result items are not discarded. As an example, let’s find the location of all of our entities:

; If you wish to retain duplicate results on output, you must use td/find-pull and the Datomic
; Pull API to return a list of results (instead of a set).
(let [result-pull     (td/find-pull   :let    [$ (live-db)]                 ; $ is the implicit db name
                                      :find   [ (pull ?eid [:location]) ]   ; output :location for each ?eid found
                                      :where  [ [?eid :location] ] )        ; find any ?eid with a :location attr
      result-sort     (sort-by #(-> % first :location) result-pull)
  (s/validate [ts/TupleMap] result-pull)    ; a list of tuples of maps
  (is (= result-sort  [ [ {:location "Caribbean"} ]
                        [ {:location "London"   } ]
                        [ {:location "London"   } ] ] )))

Retracting Data from Datomic

Suppose James throws his knife at a villan. We need to remove it from the DB.

(td/transact *conn*
  (td/retract-value james-eid :weapon/type :weapon/knife))
(is (= (td/entity-map (live-db) james-eid)  ; lookup by EID
       {:person/name "James Bond" :location "London" :weapon/type #{:weapon/wit :weapon/gun} :person/secret-id 7 } ))

Once James has defeated Dr No, we need to remove him (& everything he possesses) from the database.

; We see that Dr No is in the DB...
(let [tuple-set   (td/find  :let    [$ (live-db)]
                            :find   [?name ?loc] ; <- shape of output tuples
                            :where  {:person/name ?name :location ?loc} ) ]
  (is (= tuple-set #{ ["James Bond"     "London"]
                      ["M"              "London"]
                      ["Dr No"          "Caribbean"]
                      ["Honey Rider"    "Caribbean"] } )))
; we do the retraction...
(td/transact *conn*
  (td/retract-entity [:person/name "Dr No"] ))
; ...and now he's gone!
(let [tuple-set   (td/find  :let    [$ (live-db)]
                            :find   [?name ?loc]
                            :where  {:person/name ?name :location ?loc} ) ]
  (is (= tuple-set #{ ["James Bond"     "London"]
                      ["M"              "London"]
                      ["Honey Rider"    "Caribbean"] } )))

Using Datomic Partitions

Datomic allows the user to create partitions within the DB. Datomic partitions serve solely as a structural optimization, and do not control or limit how or by whom datoms may be accessed. The effect of a partition in Datomic is to effectively "pre-group" all entities in that partition so that they are adjacent in storage, which may (or may not) improve access times for related entities that are often accessed together.

In Tupelo Datomic, we may easily create and use partitions:

; Create a partition named :people (we could namespace it like :db.part/people if we wished)
(td/transact *conn*
  (td/new-partition :people ))

; Create Honey Rider and add her to the :people partition
(let [tx-result   @(td/transact *conn*
                      (td/new-entity :people ; <- partition is first arg (optional) to td/new-entity
                        { :person/name "Honey Rider" :location "Caribbean" :weapon/type #{:weapon/knife} } ))
      [honey-eid]  (td/eids tx-result)  ; retrieve Honey Rider's EID from the seq (destructuring)
  (s/validate ts/Eid honey-eid)  ; verify the expected type
  (is (= :people ; verify the partition name for Honey's EID
         (td/partition-name (live-db) honey-eid))))

In addition to keeping related entities adjacent in storage, one may also look up all entities in a given partition by using the (td/partition-eids …​) function:

; Show that only Honey is in the people partition
(let [people-eids           (td/partition-eids (live-db) :people)
      people-entity-maps    (map #(td/entity-map (live-db) %) people-eids) ]
                                 ; td/entity-map returns a map of attr-vals given an EntitySpec
  (is (= people-entity-maps [
           {:person/name "Honey Rider", :weapon/type #{:weapon/knife},
            :location "Caribbean"} ] )))

Processing Transaction Results

We may wish on occasion to inspect the results of a particular transaction for transaction ID, EID, etc. We can easily gain access to the datoms created during a transaction by using the tx-datoms function. Using the example for Honey Rider above,

  ; Create Honey Rider and add her to the :people partition
  (let [tx-result   @(td/transact *conn*
                        (td/new-entity :people ; <- partition is first arg (optional) to td/new-entity
                          { :person/name "Honey Rider" :location "Caribbean" :weapon/type #{:weapon/knife} } ))
        tx-datoms   (td/tx-datoms (live-db) tx-result)
    ; tx-datoms looks like:
    ;    [ {:e 13194139534328,
    ;       :a :db/txInstant,
    ;       :v #inst "2016-10-02T21:45:44.689-00:00",
    ;       :tx 13194139534328,
    ;       :added true}
    ;      {:e 299067162756089,
    ;       :a :person/name,
    ;       :v "Honey Rider",
    ;       :tx 13194139534328,
    ;       :added true}
    ;      {:e 299067162756089,
    ;       :a :location,
    ;       :v "Caribbean",
    ;       :tx 13194139534328,
    ;       :added true}
    ;      {:e 299067162756089,
    ;       :a :weapon/type,
    ;       :v 17592186045419,
    ;       :tx 13194139534328,
    ;       :added true} ]
    (is (= "Honey Rider" (:v (only (keep-if #(= :person/name  (:a %)) tx-datoms)))))
    (is (= "Caribbean"   (:v (only (keep-if #(= :location     (:a %)) tx-datoms)))))
    (is (= 1                (count (keep-if #(= :weapon/type  (:a %)) tx-datoms))))
    (is (= 1                (count (keep-if #(= :db/txInstant (:a %)) tx-datoms))))
    (is (apply = (map :tx tx-datoms)))  ; All datoms have the same :tx value

Other informational functions (please see the Tupelo-Datomic API docs for details).

(is-transaction?  db-val entity-spec)
  "Returns true if an entity is a transaction (i.e. it is in the :db.part/tx partition)"

(transactions db-val)
  "Returns a lazy sequence of entity-maps for all DB transactions"

(eids tx-result)
  "Returns a collection of the EIDs created in a transaction."

(txid tx-result)
  "Returns the EID of a transaction"

Future Work

Lots more to come!


  • Clojure 1.7.0

  • Java 1.8


Copyright © 2015-2017 Alan Thompson

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

Developed using IntelliJ IDEA with the Cursive Clojure plugin.

IntelliJ Cursive

ToDo List (#todo)

add example for (d/entity-db ...)
add comparison from conj 2015 talk
add example linking entities
add docs for new-attribute optional specs (& default values)
seattle tutorial using tupelo datomic
mbrainz tutorial using tupelo datomic
general datamoic tutorial using tupelo
  including details & gotchas
data import semantics
maybe rename td/transact -> td/transact! or td/tx!
think about copying semantics from atoms (swap! reset! etc)
rename new-attribute -> create-attribute, etc:   new-*  ->  create-*
test/document if re-create a given attr, enum, partition etc. Also, does it matter if do same
  thing twice or 2nd one is different


Tupelo-Datomic - Datomic With A Spoonful of Honey




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