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Modelling

Overarch supports modelling the functional requirements, the architecture and the design of the system under description.

Modelling a system with overarch should provide a value for the project and this guides the selection of model elements and supported abstractions and views.

Overarch currently supports the following kinds of models

Concept Models to model the concepts used for the system and their relations
Use-Case Models to model the scope of the system with the supported use cases
Architecture Models to model the logical architecture of the system with different levels of detail
Class Models to model the design of the system, e.g. the domain model, and the structure of the implementation
State-Machine Models to model relevant states and transitions of the system
Deployment Models to model the physical architecture of the system

The model contains all the elements relevant in the architecture of the system. Models are specified in the Extensible Data Notation (EDN).

EDN Basics

The Extensible Data Notation EDN is a data notation with a rich set of literals for scalar and composite data types. It is also a subset of the Clojure language textual format. Therefore Clojure plugins/extensions for editors or IDEs provide syntax checking/highlighting and code completion.

Compared to JSON, which only supports literals for numbers, strings, vectors (arrays) and maps, EDN provides a richer set of data literals, e.g. integer and floating point numbers, big integers and decimals, strings, symbols, keywords, UUIDs and instants of time. It also provides literals for list, vectors, sets and maps.

The following literals are used in Overarch models and views.

Strings

Strings are used e.g. as names and descriptions of model elements and for the title of views.

"This is a string"

"This is
a multiline
string"

Keywords

Keywords are used as keys in the maps for model elements and views. They are also used as identifiers for model elements and views.

Keywords start with a colon (':'), have an optional namespace followed by a slash ('/') and a mandatory name, e.g. :namespace/name.

Keywords should be prefixed with a namespace to avoid collisions with keywords for other models, which is especially relevant for identifiers or for custom keys in the model elements and views.

:keyword
:namespaced/keyword

Unprefixed keywords and the namespace 'overarch' for map keys are reserved for overarch. Please use your own prefix if you want to add custom information to the maps in the model.

Sets

Sets are unordered collections of elements without duplicates. They are used as top level collections for the model elements and views. They are also used as a container for the children of model elements.

#{"a" "b" "c"}

Maps

Maps are associative collections of key/value pairs. They are used to describe the attributes of model elements and views.

{:firstname "John" :lastname "Doe" :age 42}

Vectors

Vectors are ordered collections of elements which may contain an element multiple times. They are used for the elements as content of a view because the ordering of the elements may be relevant for the rendering of the view (e.g. in PlantUML).

[1 2 3 4]
["John" "Doe"]

As you can see in the example models, all collection literals can be nested.

For more information see the EDN specification.

Examples

The model and diagram descriptions of the C4 model banking example can be found in models/banking folder:

If you have a Clojure environment in some editor or IDE, just use it. If not, try Visual Studio Code with the Calva and PlantUML extensions. With this setup you get an editor for the EDN files with code completion, syntax check and syntax highlighting.

Model editing

Models

You can split your model into separate EDN files, which might be reasonable for big systems. Overarch can recursively read all models from a directory or search path so you are quite free in structuring your model files.

The top level element in each model EDN file is a set which contains the top level model elements. Model elements are denoted as maps in the EDN file.

All model elements have at least two keys, :el for the type of the element and :id for the identifier. The identifiers should be namespaced keywords, so that different models can be composed without collisions of the identifiers.

Model Nodes

Model Nodes describe the elements of the different kind of models for the system.

Common Keys of Model Nodes

key	type	values	description
:el	keyword	see model elements	type of the model node
:id	keyword	namespaced id	id of the model node
:name	string	short name	name of the model node
:desc	string		description of the model node
:tags	set of strings	e.g. #{"critical"}	some tags which can be used in element selection
:ct	set of maps	model nodes	the children of the model node

Model Relations

Relations describe the connections and interactions of the nodes.

Common Keys of Relations

key	type	values	description
:el	keyword	e.g. :rel, :request	type of the relation
:id	keyword	namespaced id	id of the relation
:from	keyword	namespaced id	id of the referrer node
:to	keyword	namespaced id	id of the referred node
:name	string		name of the relation
:desc	string		description of the relation
:tags	set of strings	e.g.	some tags which can be used in element selection

References (:ref)

References refer to a model element with the given id. They are primarily used to refer to the model elements to include in views. They can also be used to refer to model elements in other model elements, e.g. to split a huge hierarchical systems into multiple EDN files.

References can have other keys, which are merged with the referred element in context of the reference. For example you can mark an internal system as external in the context of a specific view by adding :external true to the reference.

Boundaries

Boundaries group related elements and are normally rendered as a dashed box in a view. There are currently 4 types of boundaries, two of them implicit.

The implicit boundaries are the system boundary and the container boundary. They are not modelled explicitly but are rendered for referenced systems and containers in specific views. A system boundary is rendered, when an internal system with containers as content is referenced in a container view or component view. Likewise a container boundary is rendered for a referenced container in a component view.

The explicit boundaries are enterprise boundary and context boundary. These are explicitly modelled. An enterprise boundary {:el :enterprise-boundary} can be used to group systems by enterprise or company. A context boundary {:el :context-boundary} can be used to group concepts, containers or components by some common context, especially by domain contexts in the sense of domain driven design.

Architecture Model

Overarch supports elements for C4 architecture models.

Logical Data Model for the Architecture Model Elements

Architecture and Deployment Model Elements

Additional Keys for Architecture Model Nodes

key	type	values	description
:subtype	keyword	:database, :queue	specific role of the model node
:external	boolean	true, false	default is false
:tech	string		technology of the model node

Person (:person)

Persons are internal or external actors of the system.

{:el :person
 :id :banking/personal-customer
 :name "Personal Banking Customer"
 :desc "A customer of the bank, with personal banking accounts."}

System (:system)

A System is the top level element of the C4 model an can contain a set of containers. Systems can be internal or external to the project context. The structure of internal systems is modelled with containers.

Container (:container)

A container is a part of a system. It represents a process of the system (e.g. an executable or a service). Containers are composed of a set of components.

Component (:component)

A component is unit of software, which lives in a container of the system.

Relations

Relations describe the connections and interactions of the parts of a view.

kind	sync/async	dependency	description
:request	sync	true	synchrounous request
:response	sync	false	response to a synchronous request
:send	async	true	asynchronous point-to-point message
:publish	async	true	asynchronous broadcast message (via broker, topic, queue)
:subscribe	async	true	subscribtion to an asynchronous broadcast message (via broker, topic, queue)
:dataflow	unspecified	unspecified	flow of data independent of the call semantic
:rel	unspecified	unspecified	unclassified relation

Additional Keys for Architecture Model Relations

key	type	values	description
:tech	string	e.g. "REST"	technology of the relation

Example

Example (exerpt from the banking model containing context and container level elements):

#{{:el :person
  :id :banking/personal-customer
  :name "Personal Banking Customer"
  :desc "A customer of the bank, with personal banking accounts."}
 {:el :system
  :id :banking/internet-banking-system
  :name "Internet Banking System"
  :desc "Allows customers to view information about their bank accounts and make payments."
  :ct #{{:el :container
        :id :banking/web-app
        :name "Web Application"
        :desc "Deliveres the static content and the internet banking single page application."
        :tech "Clojure and Luminus"}
       {:el :container
        :id :banking/single-page-app
        :name "Single-Page Application"
        :desc "Provides all of the internet banking functionality to customers via their web browser."
        :tech "ClojureScript and Re-Frame"}
       {:el :container
        :id :banking/mobile-app
        :name "Mobile App"
        :desc "Provides a limited subset of the internet banking functionality to customers via their mobile device."
        :tech "ClojureScript and Reagent"}
       {:el :container
        :id :banking/api-application
        :name "API Application"
        :desc "Provides internet banking functionality via a JSON/HTTPS API."
        :tech "Clojure and Liberator"}
       {:el :container
        :subtype :database
        :id :banking/database
        :name "Database"
        :desc "Stores the user registration information, hashed authentication credentials, access logs, etc."
        :tech "Datomic"}}}
 {:el :system
  :id :banking/mainframe-banking-system
  :external true
  :name "Mainframe Banking System"
  :desc "Stores all the core banking information about customers, accounts, transactions, etc."}
 {:el :system
  :id :banking/email-system
  :external true
  :name "E-mail System"
  :desc "The internal Microsoft Exchange email system."}

 ; Context level relations 
 {:el :rel
  :id :banking/personal-customer-uses-internet-banking-system
  :from :banking/personal-customer
  :to :banking/internet-banking-system
  :name "Views account balances and makes payments using"}
 {:el :rel
  :id :banking/internet-banking-system-uses-email-system
  :from :banking/internet-banking-system
  :to :banking/email-system
  :name "Sends e-mail using"}
 {:el :rel
  :id :banking/internet-banking-system-using-mainframe-banking-system
  :from :banking/internet-banking-system
  :to :banking/mainframe-banking-system
  :name "Gets account information from, and makes payments using"}
 {:el :rel
  :id :banking/email-system-sends-mail-to-personal-customer
  :from :banking/email-system
  :to :banking/personal-customer
  :name "Sends e-mail to"}}

Deployment Model

Overarch also supports elements for C4 deployment models.

Logical Data Model for the Deployment Model Elements

Deployment Model Elements

Node (:node)

A node is a unit in a deployment view. Nodes represent parts of the infrastructure in which the containers of the system are deployed. They can contain a set of other nodes or containers.

Deployment model relations

relation type	description
:link	A link between two nodes of the deployment model, e.g. two virtual networks
:deployed-to	A deployment relation between a container and a node in the deployment model

:rel

Concept Model

A concept model captures relevant concepts of the domain(s) of the system. The concepts could be part of the ubiquous language of the systems domain.

A concept model can contain the concepts of the domain and the high level elements of the architecture model, e.g. the persons (actors), external systems and the system itself with it's containers.

Logical Data Model for the Concept Model Elements

Concept Model Elements

Concepts (:concept)

A concept which is relevant for the domain of the system. The description should document the meaning of the concept.

Concept Model Relations (:is-a, :has, :rel)

Concepts can be related with other concepts.

relation type	description
:is-a	the :from node is a specialization of the :to node
:has	the :from node is a part or attribute of the :to node
:rel	unclassified relation between the nodes

Example

See example concept model.

Use Case Model

A use case model captures the functionality a system is suposed to deliver. High level use cases provide an overview of this functionality and may link to business processes, domain stories and arcitectural elements.

As such they provide a pivot for the traceability from business processes into the design of the system.

The elements of the use case model are mainly borrowed from the UML use case model so prior knowledge of UML modelling applies here.

Logical Data Model for the Use Case Model Elements

Use Case Model Elements

Example

Example Use Case Model

Use Cases (:use-case)

A use case describes the goal of an actor in the context of the system described. The goal can be a concrete user goal, a high level summary of user goals or a subfunction of a user goal. This is captured by the :level key.

key	type	values	description
:level	keyword	:summary :user-goal :subfunction	specific role of the element
:ext-points	string		extension points of a use case

Actors (:person, :system, :container, :actor)

Persons, systems and containers from the architecture model should be used as actors in the use case model to provide a connection between the architecture model and the use case model.

You can use the :actor element to model actors not present as persons or systems in the architectural model, but this should be avoided if possible. A reason for an :actor element might be the introduction of a time actor to model the scheduling of use cases.

Relations (:uses :include :extends :generalizes)

Relations connect actors to the use cases or use cases with other use cases. Use case models support different kinds of relations.

kind	description
:uses	a use case element uses another use case element (e.g. an actor uses a use case or a use case uses an external system)
:include	a use case includes the functionality of another use cases
:extends	a use case extends the functionality of another use case
:generalizes

State Machine Model

A state model describes a state machine which can be used to model the states a system component can be in and the transition from one state to the next state based on the events the system receives as input.

The elements of the class model are mainly borrowed from the UML class model so prior knowledge of UML modelling applies here.

Logical Data Model for the State Machine Model Elements

State Machine Elements

Example

Example State Model

State Machine (:state-machine)

A state machine is the root element for a state machine view. It contains the set of states and transistions as value of the :ct key.

States (:state, :start-state, :end-state)

A simple state machine has at least one start state, some normal states to model the different states a system can be in, and at least one end state.

A start state starts the state machine and an end state terminates the state machine.

States can be compound, they can have an internal state machine. This is modelled as a set of states and transitions in the :ct key, analog to the state machine itself.

Transitions (:transition)

A transition connects two states and models the input that leads to the transition from the current state (:from) to the next state (:to).

Forks and Joins (:fork-state, :join-state)

You can split a transition to trigger multiple new states with a fork state. A fork has a single input transition and multiple output transitions.

To join multiple transitions after a fork a join state is used. A join has multiple input transitions and a single output transition.

Class Model

A class model captures the static structure of the code.

The abstraction level of a class model is not very high compared to the actual implementation. Therfore modelling and updating a complete class model is not of much value. But class models of the domain can be very valuable as a means of communication between domain experts and developers to shape and document the domain model for a bounded context.

The elements of the class model are mainly borrowed from the UML class model so prior knowledge of UML modelling applies here.

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