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Hodur Engine

Hodur is a descriptive domain modeling approach and related collection of libraries for Clojure.

By using Hodur you can define your domain model as data, parse and validate it, and then either consume your model via an API making your apps respond to the defined model or use one of the many plugins to help you achieve mechanical, repetitive results faster and in a purely functional manner.

This repo is the Hodur core engine that parses your model definitions and exposes a meta-API around it. For a list of what you can do once your model is in Hodur check here.

Motivation

For a deeper insight into the motivations behind Hodur, check the motivation doc.

Getting Started

Hodur has a highly modular architecture. Hodur Engine (this project) is always required as it provides the meta-database functions and APIs consumed by plugins.

Add hodur-engine as a dependency in your deps.edn file:

  {:deps {hodur/engine {:mvn/version "0.1.7"}}}

Either require Hodur as part of your ns definition or directly:

  (require '[hodur-engine.core :as hodur])

In order to initialize an atom representing the meta-database of your model call function hodur/init-schema:

  (def meta-db (hodur/init-schema
                '[Person
                  [^String first-name
                   ^String last-name]]))

In the above example, we are defining a Person entity with a first-name and a last-name both tagged as the scalar type String.

Alternatively, Hodur can be initialized by raw EDN paths or from your classpath using a File (i.e. clojure.java.io/resource):

  (def meta-db (-> "schemas/person.edn"
                   io/resource
                   hodur/init-path))

Hodur's usefulness can be seen when used in conjunction with several plugins that take care of the mechanical aspects of your application. For the sake of getting started, we are also adding hodur-datomic-schema, a plugin that creates Datomic Schemas out of your model to the deps.edn file:

  {:deps {hodur/engine         {:mvn/version "0.1.5"}
          hodur/datomic-schema {:mvn/version "0.1.0"}}}

You should require it any way you see fit:

  (require '[hodur-datomic-schema.core :as hodur-datomic])

Let's expand our Person model above by "tagging" the Person entity for Datomic. You can read more about the concept of tagging for plugins in the sessions below but, in short, this is the way we, model designers, use to specify which entities we want to be exposed to which plugins.

  (def meta-db (hodur/init-schema
                '[^{:datomic/tag-recursive true}
                  Person
                  [^String first-name
                   ^String last-name]]))

The hodur-datomic-schema plugin exposes a function called schema that generates your model as a Datomic schema payload:

  (def datomic-schema (hodur-datomic/schema meta-db))

When you inspect datomic-schema, this is what you have:

  [{:db/ident       :person/first-name
    :db/valueType   :db.type/string
    :db/cardinality :db.cardinality/one}
   {:db/ident       :person/last-name
    :db/valueType   :db.type/string
    :db/cardinality :db.cardinality/one}]

Assuming the Datomic client API is bound to datomic, and your connection to the Database cluster is bound to db-conn, you can simply transact your schema like this:

  (datomic/transact db-conn {:tx-data datomic-schema})

Several other plugins are available and you can also write your own. The following sections detail not only how to model your domain but also these options in further detail.

Hodur Plugins

For visualization/documentation:

hodur-graphviz-schema: generates beautiful GraphViz diagrams of your domain
hodur-visualizer-schema: generates a dynamically, hot-reloaded version of your domain on a web browser

Schemas for persistent systems:

hodur-datomic-schema: generates Datomic Cloud compatible schemas
hodur-contentful-schema: generates Contentful compatible schemas

Schemas for inbound interfaces:

hodur-lacinia-schema: generates Lacinia (GraphQL) schemas

Schemas for validation/data-generation:

hodur-spec-schema: generates Clojure Spec schemas

Experimental adapters:

hodur-lacinia-datomic-adapter: experimental utilities for bridging GraphQL queries and mutations into Datomic

Model Definition

Entities, fields and parameters

In Hodur Entities are the highest level representation of a model. An entity has any number of fields that qualify such entity.

For instance, an employee entity may have an employee-number, a name and a salary as three distinct fields. An entity can have as many fields as you need.

Fields can have any number of parameters. Parameters qualify the field. For instance, a hypothetical height field could have a parameter specifying which unit to use when interpreting this field (CENTIMETERS or FEET for instance).

Basic structure

Hodur can be initialized by either a series of EDN files (using function init-path) or vectors (using function init-schema).

A domain model is a vector of tuples of symbols and sub-vectors. The symbols represent entity names and the sub-vectors represent fields.

An Employee entity with name and salary as fields could be defined as:

  [Employee
   [name
    salary]]

With this setup we are not specifying what name and salary are. It might be a good idea to do something like this:

  [Employee
   [^String name
    ^Float  salary]]

Types are defined using a meta payload to the symbol that represents the field or the parameter. You can read more about scalar types below.

Types can also be represented by the more explicit meta object:

  [Employee
   [^{:type String} name
    ^{:type Float}  salary]]

Entities are also considered types therefore, if an Employee has a supervisor who's also an Employee you might write:

  [Employee
   [^String   name
    ^Float    salary
    ^Employee supervisor]]

You could want a height field that can return the employee's height in a particular unit:

  [Employee
   [^String   name
    ^Float    salary
    ^Employee supervisor
    ^Integer  height [^Unit unit]]

   ^{:enum true}
   Unit
   [CENTIMETERS FEET]]

There's quite a bit going on here that you can explore in detail in the sections below. But here's a summary. First we've added the field height to the Employee entity. It returns an Integer and it also expects a parameter called unit of the type Unit.

We've defined Unit separately as an enum (you can see more details in the sections below). Unit can be either CENTIMETER or FEET.

Scalar types

Hodur has five primitive scalar types that can be composed with your own entities to design your model. Four of them are quite self-explanatory: String, Float, Integer and Boolean.

The last two are highly opinionated and are DateTime and ID.

Hodur's plugins must have reasonable defaults to represent each one of these scalar types. Plugins may also expose finer grained controls to manage type precision (for instance 32bit integers vs 64bit integers).

Cardinalities

One employee may have a series of reportees. This kind of cardinality is defined with the :cardinality meta marker:

  [Employee
   [^{:type String}       name
    ^{:type Float}        salary
    ^{:type Employee
      :cardinality [0 n]} reportees]]

In this example we are telling Hodur that reportees can be anywhere from 0 employees to n employees.

You can be as specific as you want. A cardinality of [4] means exactly 4 entries; [3 5] means 3 to 5. If :cardinality is unspecified, it's assumed as [1].

Optional fields and parameters

Fields and parameters are required by default. In other words, plugins must implement mechanisms to avoid null problems if a field or parameter is mandatory.

If you want to make a field optional, use the :optional meta marker on the field:

  [Employee
   [^{:type String}    first-name
    ^{:type String
      :optional true}  middle-name
    ^{:type String}    last-name]]

If you want to make a parameter optional, use the :optional meta marker on the parameter:

  [QueryRoot
   [employees [^{:type String
                 :optional true} search-term]]]

A common pattern is to make a parameter optional while also assigning a default value to it with :default:

  [QueryRoot
   [employees-by-location [^{:type String
                             :optional true
                             :default "HQ"} location]]]

Special entity markers

Interfaces and Implementations

Entities can be marked as :interface which can be used by plugins that explore such a concept. Entities that implement an interface use the :implements marker to indicate which interface(s) they implement:

  [^{:interface true}
   Pet
   [^String name]

   ^{:implements Pet}
   Dog
   [^String bark]

   ^{:implements Pet}
   Cat
   [^String mewow]]

The :implements marker also accepts a vector with a series of interfaces that the entity implements.

Enums

Enums are special kind of entities. They can assume one of the values defined as fields. Enum fields do not support parameters.

Enums are marked with :enum:

  [Employee
   [^String   name
    ^Float    salary
    ^Employee supervisor
    ^Integer  height [^Unit unit]]

   ^{:enum true}
   Unit
   [CENTIMETERS FEET]]

Unions

Unions are very similar to interfaces, but they don't get to specify any common fields between the types. They are useful when a certain field or parameter can be any one of the specified entities within the union.

In the following example the search field of the QueryRoot entity returns a collection of SearchItem which are unions of Employee and Company:

  [Employee
   [^String name
    ^Float  salary]

   Company
   [^String address]

   ^{:union true}
   SearchItem
   [Employee Company]
   
   QueryRoot
   [^{:type SearchItem
      :cardinality [0 n]}
    search [^String term]]]

Documentation and deprecation

Entities, fields, and parameters can all be documented by using marker :doc.

  [^{:doc "A representation of an Employee"}
   Employee
   [^{:type String
      :doc "The employee's name"}   name
    ^{:type Float
      :doc "The employee's salary"} salary]]

Entities, fields, and parameters can additionally be marked for deprecation by using the marker :deprecation. Deprecation is a string that describes the reasons for the deprecation as well as points to alternatives.

  [^{:doc "A representation of an Employee"}
   Employee
   [^{:type String
      :doc "The employee's name"}
    name
    ^{:type Float
      :doc "The employee's salary"}
    salary
    ^{:type Float
      :deprecation "This field will be fully removed by December. Please use `name` instead."}
    first-name]]

Tagging

In general, plugins should only process entities, fields, and parameters that have been tagged for them. I.e. a datomic plugin will have a particular tagging marker such as :datomic/tag that needs to be added to each symbol you want the plugin to process.

The following example tags Employee and its fields first-name and last-name for the datomic plugin.

  [^{:datomic/tag true}
   Employee
   [^{:type String
      :datomic/tag true} first-name
    ^{:type String
      :datomic/tag} last-name]

   Project
   [^{:type String} name]]

Recursive tagging

Tagging can be very repetitive so Hodur provides features for tagging in a recursive fashion. The example above could be rewritten with:

  [^{:datomic/tag-recursive true}
   Employee
   [^{:type String} first-name
    ^{:type String} last-name]

   Project
   [^{:type String} name]]

This kind of scenario is ideal for entities that have several fields and/or parameters.

The marker :<plugin>/tag-recursive can also have filters such as :only and :except.

The following example will only tag the Employee entity and the fields first-name and last-name:

  [^{:datomic/tag-recursive {:only [Employee first-name last-name]}}
   Employee
   [^{:type String} first-name
    ^{:type String} middle-name
    ^{:type String} last-name]]

The following example would achieve the same result as above but by tagging everything but middle-name:

  [^{:datomic/tag-recursive {:except [middle-name]}}
   Employee
   [^{:type String} first-name
    ^{:type String} middle-name
    ^{:type String} last-name]]

Default tagging

Some times you just want to tag everything you are sending as part of a group of entities. In these scenarios you need to first name the very first symbol of your group default and then mark it. Hodur will apply whatever you mark on default to all items in the group.

In the following example, Hodur will tag everything for the datomic plugin:

  [^{:datomic/tag true}
   default
   
   Employee
   [^{:type String} first-name
    ^{:type String} last-name]

   Project
   [^{:type String} name]]

The special default symbol can also be used to carry other markers down into the group's items but the general usage is for tagging.

Naming conventions

Hodur does not care about naming conventions. However, it does delegate naming choices fully to plugins. The way Hodur achieves this is by internally converting whatever naming convention was used in the symbols into several options. This is done by leveraging [[https://github.com/qerub/camel-snake-kebab][camel-snake-kebab]].

Meta API

Once your model gets parsed, Hodur will retain an in-memory meta-database that can be queried by either plugins or your implementation proper.

The API is exposed as a DataScript API atom and DataScript proper is a dependency of Hodur. Therefore, you can require DataScript and use its query directly.

The example below uses both pull and a Datalog query to return all the items which are marked with a :datomic/tag.

  (require '[datascript.core :as d])

  (d/q '[:find [(pull ?e [*]) ...]
         :where
         [?e :datomic/tag true]]
       @c)

Attributes are named with qualified keywords in four different categories:

:type/...: all entities (AKA types)
:field/...: all fields
:param/...: all parameters
<plugin>/...: plugin names should qualify keywords (see :datomic/tag above)

Naming

For entities, fields, and parameters the provided name in the model is exposed as either :type/name, :field/name, and :param/name. Additionally, Hodur generates indexes with:

/kebab-case-name
/PascalCaseName
/camelCaseName
/snake_case_name

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