Slides from Grails coding Dodo (OpenCredo 2011)

1. Grails: Patterns &
Practices
Paul Bowler
Senior Consultant, OpenCredo

2. Who are you?

3. Coding Dojo
• Agile!
• Teams of 2-4 people
• 1 sprint = 20 minutes
• Domain-Drive Design (I’m the Product Owner)
• Test-Driven Development (Maybe!)
• Yes, you can use the user guide and internet!
• User demo at the end of each sprint
• Discussion + Refactoring
• Prize for best app!

4. ?
5 ‘Maturity’ Levels
?
?
?
?

5. Domain-Driven Design
• “Domain-driven design (DDD) is an approach to developing
software for complex needs by deeply connecting the
implementation to an evolving model of the core business
concepts.”
• The premise of domain-driven design is the following:
• Placing the project's primary focus on the core domain and
domain logic
• Basing complex designs on a model
• Initiating a creative collaboration between technical and
domain experts to iteratively cut ever closer to the
conceptual heart of the problem.

7. User Story 1
“As a pomodoro fan, I would like to be able to add
tasks to a uniquely named activity inventory, so that I
can see what work I need to complete over the next
few weeks.”

8. Useful Commands
• grails create-app pomodoro
• grails create-domain-class <domain>
• grails generate-all <domain>
• grails generate-controller <domain> and
add ‘static scaffold = true’ to controller

9. Considerations
• Associations
• One-to-One
• One-to-Many
• Many-to-Many
• Constraints &Validation
• Time-Stamping?
• Default values (and field values inViews?)

10. You did create some
tests first, right?

11. Implementation
class Task {
Inventory inventory
String description
static belongsTo = [Inventory]
static constraints = {
description(nullable: false, blank: false)
}
}
class Inventory {
String name
static hasMany = [tasks: Task]
static constraints = {
name(nullable: false, blank: false, unique: true)
}
}

12. Domain Tests
class InventoryTests extends GrailsUnitTestCase {
void testConstraints() {
def existingInventory = new Inventory(name: "Paul’s Inventory")
mockForConstraintsTests(Inventory, [ existingInventory ])
! ! // Validation should fail if both properties are null.
! ! def inventory = new Inventory()
! ! assertFalse inventory.validate()
! ! assertEquals "nullable", inventory.errors["description"]
! ! // So let's demonstrate the unique constraint.
! ! inventory = new Inventory(name: "Paul’s Inventory")
! ! assertFalse inventory.validate()
! ! assertEquals "unique", inventory.errors["name"]
! ! // Validation should pass!
! ! inventory = new Inventory(name: "John’s Inventory")
! ! assertTrue inventory.validate()
! }
}

13. Gotcha!
• Potential performance issue with mapped
collections:
• Adding to the Set requires loading all
instances from the database to ensure
uniqueness
• Likewise for mapped List
• Works fine in development, but what if
you have 1,000,000+ rows?

14. Implementation (2)
class Task {
Inventory inventory
String description
static constraints = {
description(nullable: false, blank: false)
}
}
class Inventory {
String name
}

15. Side-effects?
• Different syntax for adding Tasks
• No cascading deletes
• Custom finder required to find all Tasks in
an Inventory
• Scaffolding breaks!

16. User Story 2
“As a pomodoro fan, I would like to be able move tasks
onto a ‘To Do Today’ sheet, so that I can see work to be
completed today and view my work history.”

17. Considerations
• Does the ‘Today’ list share any common
attributes with the Inventory?
• How about a more intuitive URL scheme?

18. ?
Level 1
?
?
?
Views

19. Level 1 -Views
Controller
Model
PageView PageViewPageView
Model Model

20. Level 1 ‘Smells’
• Logic built into pages:
• Overuse of Request Parameters
• If-Then tags
• Inline groovy using ${...}
• Poor use of layouts
• Little use of tags
• Domain classes as simple ‘active records’
• Page-based information architecture

21. ?
Level 2
?
?
Controllers
Views

22. Level 1-2 Refactoring
• Move logic out of pages into controllers
• Reduce pages into fragments
• Use layouts to construct device or stakeholder-
centric views from pages and fragments
• Use available tag libraries
• Create your own tag libraries!
• Stylesheets rule - minimise markup

23. User Story 3
“As a pomodoro fan, I would like to have an optimised
workflow for US2, so that I can save time and reduce
input mistakes.”

24. Considerations
• Web Flow plugin?
• Command Objects?
• What changes need to be made to domain
classes?

25. Web Flow
class InventoryController {
…
def inventoryFlow = {
showInventory {
on("done").to "saveInventory"
on("continue").to "addTask"
}
…
addTask {
redirect(controller:"task", action:"create")
}
saveInventory()
}
}
<g:form action="inventory">
<g:submitButton name="continue" value="Add Another"></g:submitButton>
<g:submitButton name="done" value="I’m Done"></g:submitButton>
</g:form>

26. User Story 4
“As a pomodoro fan, I would like to be able update the
number of iterations I’ve completed on each task in my
‘To Do Today’ list, so that I can keep track of my
progress and improve my future estimates.”

27. Considerations
• Can we do this without page refreshes?
• How can we test this?
• Domain changes?

28. Level 2 - Controllers
Controller
Domain
Controller Controller
Domain Domain Domain
Layout
Fragments Fragments
Layout
Fragments Fragments
Layout
Fragments Fragments

29. Level 2 ‘Smells’
• Large, complex Controllers
• Different scenarios driven by ‘If/Then’ logic
• Content negotiation increases complexity
further
• Many similar controller methods (not
DRY!)
• Poorly handled Transactions

30. ?
Level 3 - Services
?
Services
Controllers
Views

31. Level 2-3 Refactoring
• Move domain transaction logic out of
controllers into services
• Controllers should be ‘glue’ that binds
business services to UI
• Service methods should reflect business
scenarios
• Make use of transactional capability of
services

32. User Story 5
“As a pomodoro partner, I would like a simple REST
API over your daily task view, so I can integrate your
data into my application.”

33. Considerations
• Don’t clutter your Controllers!
• REST-ful URLs
• Content negotiation?
• Custom XML/JSON formats?

34. RESTful URL Mappings
static mappings = {
"/task/$id?"(resource:"task")
}
static mappings = {
"/task/$id"(controller:"task") {
action = [GET:"show", PUT:"update", DELETE:"delete", POST:"save"]
}
}
static mappings = {
"/task/$id"(controller:"task", parseRequest:true) {
action = [GET:"show", PUT:"update", DELETE:"delete", POST:"save"]
}
}

35. Content Negotiation
class InventoryController {
def inventory
def list = {
this.inventory = Inventory.list()
withFormat {
html inventoryList:inventory
json { render inventory as JSON }
xml { render inventory as XML }
}
}
}

36. Custom Formats?
def listAsXML = {
def inventory = Inventory.get(params.id)
def tasks = inventory.tasks
render(contentType:"text/xml") {
inventory(name:inventory.name) {
tasks {
for(t in tasks) {
task(title:t.title)
! }
}!
}
}
}

37. User Story 6
“As a pomodoro fan, I’d like to be able to add
unplanned and urgent tasks to the bottom of my daily
list, so that I can track and manage interruptions.”

38. Level 3 - Services
Controller
Domain
Controller Controller
Domain Domain Domain
Services
Layout
Fragments Fragments
Layout
Fragments Fragments
Layout
Fragments Fragments

39. Level 3 ‘Smells’
• Large, complex Services
• Services acting as proxies for domain
behaviour
• ‘Cut-and-paste’ methods

40. ?
Level 4 - Libraries
Libraries
Services
Controllers
Views

41. Level 3-4 Refactoring
• Move common code out of services into
POGO’s (or POJO’s)
• Enrich our domain model to simplify services:
• Named Queries
• Derived Properties
• Criteria: Conjunctions, Disjunctions,
Projections, Restrictions

42. User Story 7
“As a pomodoro fan, I would like to be able to search
for tasks on my inventory through a simple interface, so
I can find and modify them easily.”

43. Level 4 - Libraries
Controller
Domain
Controller Controller
Domain Domain Domain
Services
Libraries Libraries
Layout
Fragments Fragments
Layout
Fragments Fragments
Layout
Fragments Fragments

44. Level 4 ‘Smells’
• Large, monolithic application
• Increased cognitive overhead
• New starters struggle
• Components ‘cut and pasted’ into similar
projects

45. Plugins
Level 5 - Plugins
Libraries
Services
Controllers
Views

46. Level 4-5 Refactoring
• Componentise the application into plugins
• Construct applications by combining plugins
• Could your application itself be constructed
as a plugin for an organisation’s product
suite?
• Writing plugins that modify the Grails/Spring
context is beyond the scope of this
workshop!

47. User Story 8
“As a pomodoro fan, I would like a simplified version of
my Inventory, so I can view it on my iPhone.”

48. Useful Commands
• grails create-plugin <plugin>
• grails package-plugin
• grails install-plugin /path/to/plugin/grails-
example-0.1.zip

49. Layouts and Fragments
<g:include action="show" id="1" />
<g:include action="show" id="${currentTask.id}" />
<g:include controller="task" />
<g:include controller="task" action="list" />
<g:include action="list" params="[sort:'title', order:'asc'] />
<html>
<head>
<title><g:layoutTitle default="An example decorator" /></title>
<g:layoutHead />
</head>
<body>
<div class="menu"><!--my common menu goes here--></menu>
<div class="body">
<g:layoutBody />
</div>
</div>
</body>
</html>

50. Layout Options
• In your views:
<meta name="layout" content="main"></meta>
• In your controller:
static layout = 'task'
static layout = 'custom/task'
• By Convention:
grails-app/views/layouts/task.gsp
grails-app/views/layouts/task/list.gsp
• Inline:
<g:applyLayout name="myLayout" template="taskTemplate" collection="${tasks}" />
<g:applyLayout name="myLayout" url="http://www.google.com" />
<g:applyLayout name="myLayout">The content to apply a layout to</g:applyLayout>

51. Level 5 - Plugins
Controller
Domain
Controller
Domain Domain
Plugins
Page
View
Page
View
Controller
Services
Libraries / Plugins
Domain Domain
Layout
Fragments Fragments
Layout
Fragments Fragments
Services
Libraries Libraries

52. Plugins
Libraries
Services
Controllers
The Full Picture
Views

53. Phew!
Well Done.