We are uncovering better ways of developing software by doing it and helping others to do it. Through this work, we have come to value:
– individuals and interactions over processes and tools;
– working software over comprehensive documentation;
– customer collaboration over contract negotiation;
– responding to change over following a plan.
While there is value on the items on the right, we value the items on the left more.
Activity Description
Choose features to be included in an
Increment Using the list of features in the planned product, select those features that can be implemented in the next product increment.
Refine feature descriptions Add detail to the feature descriptions so that the team members have a common understanding of each feature and there is sufficient detail to begin implementation.
Implement and test Implement the feature and develop automated tests for that feature that show that its behavior is consistent with its description. I explain automated testing in Chapter 9.
Table 2.2 Incremental development activities (1 of 2)
Activity Description
Integrate feature and test Integrate the developed feature with the existing system and test it to check that it works in conjunction with other features.
Deliver system increment Deliver the system increment to the customer or product manager for checking and comments. If enough features have been implemented, release a version of the system
for customer use.
Table 2.2 Incremental development activities (2 of 2)
Principle Description
Involve the customer Involve customers closely with the software development team. Their role is to provide and prioritize new system requirements and to evaluate each increment of the system.
Embrace change Expect the features of the product and the details of these features to change as the development team and the product manager learn more about the product. Adapt the software to cope with changes as they are made.
Develop and deliver
Incrementally Always develop software products in increments. Test and evaluate each increment as it is developed and feed back required changes to the development team.
Principle Description
Maintain simplicity Focus on simplicity in both the software being
developed and the development process. Wherever possible, do what you can to eliminate complexity from the system.
Focus on people, not the
development process Trust the development team and do not expect
everyone to always do things in the same way.
Team members should be left to develop their
own ways of working without being limited by
prescriptive software processes.
Description
Practice Description
Incremental planning/ user Stories There is no “grand plan” for the system. Instead,what needs to be implemented (the requirements) in each increment are established in discussions with a customer representative. The requirements are written as user stories. The stories to be included in a release are determined by the time available and their relative priority.
Small releases The minimal useful set of functionality that provides business value is developed first. Releases of the system are frequent and incrementally add functionality to the previous release.
Test-driven development Instead of writing code and then tests for that code, developers write the tests first. This helps clarify what the code should actually do and that there is always a “tested” version of the code available. An automated unit test framework is used to run the tests after every change. New code should not “break” code that has already been implemented.
Practice Description
Continuous integration As soon as the work on a task is complete, it is integrated into the whole system and a new version of the system is created. All unit tests from all developers are run automatically and must be successful before the new version of the system is accepted.
Refactoring Refactoring means improving the structure, readability, efficiency, and security of a program. All developers are expected to refactor the code as soon as potential code improvements are found. This keeps the code simple and maintainable.
Scrum term Explanation
Product The software product that is being developed by the
Scrum team.
Product Owner A team member who is responsible for identifying product features and attributes. The Product Owner reviews work done and helps to test the product.
Product backlog A to-do list of items such as bugs, features, and product
improvements that the Scrum team has not yet completed.
Development team A small self-organizing team of five to eight people who
are responsible for developing the product.
Sprint A short period, typically two to four weeks, when a product increment is developed.
Scrum A daily team meeting where progress is reviewed and work to be done that day is discussed and agreed.
Scrum term Explanation
ScrumMaster A team coach who guides the team in the effective use of Scrum.
Potentially shippable
product increment The output of a sprint that is of high enough quality to be deployed for customer use.
Velocity An estimate of how much work a team can do in a single sprint.
1. As a teacher, I want to be able to configure the group of tools that are
available to individual classes. (feature)
2. As a parent, I want to be able to view my children’s work and the
assessments made by their teachers. (feature)
3. As a teacher of young children, I want a pictorial interface for children with
limited reading ability. (user request)
4. Establish criteria for the assessment of open source software that might be
used as a basis for parts of this system. (development activity)
5. Refactor user interface code to improve understandability and performance.
(engineering improvement)
6. Implement encryption for all personal user data. (engineering improvement)
Heading Description
Ready for consideration These are high-level ideas and feature descriptions that will be considered for inclusion in the product. They are tentative so may radically change or may not be included
in the final product.
Ready for refinement The team has agreed that this is an important item that should be implemented as part of the current development. There is a reasonably clear definition of what is required. However, work is needed to understand and refine the item.
Ready for implementation The PBI has enough detail for the team to estimate the effort involved and to implement the item. Dependencies on other items have been identified.
A scrum is a short, daily meeting that is usually held at the beginning of the day. During a scrum, all team members share information, describe their progress since the previous day’s scrum, and present problems that have arisen and plans for the coming day. This means that everyone on the team knows what is going on and, if problems arise, can re-plan short-term work to cope with them.
Scrum meetings should be short and focused. To dissuade team members from getting involved in long discussions, scrums are sometimes organized as “stand-up” meetings where there are no chairs in the meeting room.
During a scrum, the sprint backlog is reviewed. Completed items are removed from it.New items may be added to the backlog as new information emerges. The team then decides who should work on sprint backlog items that day.
State Description
Reviewed The code has been reviewed by another team member who has checked that it meets agreed coding standards, is understandable, includes appropriate comments, and has been refactored if necessary.
Unit tested All unit tests have been run automatically and all tests have
executed successfully.
Integrated The code has been integrated with the project codebase and no integration errors have been reported.
Integration tested All integration tests have been run automatically and all tests have been executed successfully.
Accepted Acceptance tests have been run if appropriate and the Product Owner or the development team has confirmed that the product backlog item has been completed.
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Scrum (1 of 2)
Begin the project with a high-level plan that can be changed on the fly
Success of the project is most important
Individual success is secondary
Project leader has some (not much) influence on the detail
Systems team works within a strict time frame (two to four weeks for development)
Roles Played in Scrum
There are three roles in Scrum:
Product owner
Scrum Master
Team member
Scrum is a high-intensity methodology. It is just one of the approaches that adopts the philosophy of agile modeling.
Product backlog—in which a list is derived from product specifications.
Sprint backlog—a dynamically changing list of tasks to be completed in the next sprint.
Sprint—a 30-day period in which the development team transforms the backlog into software that can be demonstrated.
Daily scrum—a brief meeting in which communication is the number-one rule.
Demo—working software that can be demonstrated to the customer.
4
Product Backlog
Features and other deliverables designers intend for the product based on user stories
The list of user stories is reorganized so that the most important user stories appear on the top
Figure 6.5 A Product Backlog Registry
Sprint Cycle
Stories are deliverables the team accomplishes
Tasks are parts of the story or units of work that each team members does
The sprint cycle can vary in length, but the usual is two weeks
At the end of the cycle, determine whether the product should be released
Two-Week Release Advantages
Team spirit remains high
Completing the product is more real to the team
Continual feedback from customers
Figure 6.6 Teams Work Together to Accomplish a Sprint Cycle
Lessons Learned from Agile Modeling (1 of 2)
Short releases allow the system to evolve
Pair programming enhances the overall quality
Onsite customers are mutually beneficial to the business and the agile development team
Short releases allow the system to evolve—through the use of short releases, the development team compresses the time between releases of their product, improving the product later as the dynamic situation demands.
Pair programming enhances overall quality—fosters good communication, identifying with the customer, focusing on the most valuable aspects of the project first, testing all code as it is developed, and integrating the new code after it successfully passes its tests.
Onsite customers are mutually beneficial to the business and the agile development team—customers serve as a ready reference and reality check, and the focus of the system design will always be maintained via their presence.
10
Other Unique Scrum Features
Scrum planning meeting
Planning poker
Daily meetings
Sprint burndown chart
Sprint review
Scrum Planning Meeting
Two parts to a Scrum planning meeting:
Product owner presents the list of features on the wish list of user stories
Estimate the resources needed to complete all of the features
Common way to do this is to play planning poker
Sprint Burndown Chart
Way to keep track of performance
Horizontal axis tracks the time that has elapsed
Vertical axis may track the number of tasks remaining or the number of hours to complete the remaining tasks
Red line shows hours of work remaining, and the yellow bars show the number of tasks remaining
Figure 6.8 A Burndown Chart
Sprint Review
Team gets together in a meeting to review the work done
Note any tasks that were not completed
User stories completed are prominently documented
User stories the team committed to that were not finished are noted
Kanban
Key elements of the Kanban system as applied to software development are:
Visualize the workflow
Keep work-in-process (W I P) as small as possible
Reevaluate the workflow, reassigning priorities if need be
Strive for continual improvement
Figure 6.9 A Kanban Board
Scrum Advantages (1 of 2)
Advantages:
Quick product development
Exercises a user-oriented approach
Encourages teamwork
Less confusing than more formal approaches
Flexibility
Scrum Advantages (2 of 2)
Advantages:
Satisfying to team members
Rewards smaller but meaningful accomplishments
Provides feedback
Adaptability
Scrum Disadvantages
Disadvantages:
Documenting features improperly
Releasing products with errors
Releasing products too soon for the user
Completing the sprint backlog under pressure
Working as a geographically dispersed team may be difficult
Working as a team when special skills are required may be challenging
Replacing team members who leave the team is difficult
Characteristic Explanation
Self-contained Microservices do not have external dependencies. They manage their own data and implement their own user interface.
Lightweight Microservices communicate using lightweight protocols, so that service communication overheads are low.
Implementation
independent Microservices may be implemented using different
programming languages and may use different
technologies (e.g., different types of database) in their
implementation.
Independently
Deployable Each microservice runs in its own process and is
independently deployable, using automated systems.
Business-oriented Microservices should implement business capabilities and needs, rather than simply provide a technical service.
Imagine that you are developing a photo-printing service for mobile devices. Users can upload photos to your server from their phone or specify photos from their Instagram account that they would like to be printed. Prints can be made at different sizes and on different media.
Users can choose print size and print medium. For example, they may decide to print a picture onto a mug or a T-shirt. The prints or other media are prepared and then posted to their home. They pay for prints either using a payment service such as Android or Apple Pay or by registering a credit card with the printing service provider.
Table 6.2 A photo-printing system for mobile devices
Failure type Explanation
Internal service failure These are conditions that are detected by the service
and can be reported to the service requestor in an error
message. An example of this type of failure is a service
that takes a URL as an input and discovers that this is
an invalid link.
External service failure These failures have an external cause that affects the
availability of a service. Failure may cause the service to become unresponsive and actions have to be taken to restart the service.
Service performance failure The performance of the service degrades to an
unacceptable level. This may be due to a heavy load or
an internal problem with the service. External service
monitoring can be used to detect performance failures
and unresponsive services.
Principle Explanation
Use HTTP verbs The basic methods defined in the HTTP protocol (GET, PUT, POST, DELETE) must be used to access the operations made available by the service.
Stateless services Services must never maintain internal state. As I have already explained, microservices are stateless, so fit with this principle.
URI addressable All resources must have a URI, with a hierarchical structure, that is used to access subresources.
Use XML or JSON Resources should normally be represented in JSON or XML or both. Other representations, such as audio and video representations, may be used if appropriate.
Action Implementation
Create Implemented using HTTP POST, which creates the resource with the given URI. If the resource has already been created, an error is returned.
Read Implemented using HTTP GET, which reads the resource and returns its value. GET operations should never update a resource so that successive GET operations with no intervening PUT operations always return the same value.
Update Implemented using HTTP PUT, which modifies an existing resource. PUT should not be used for resource creation.
Delete Implemented using HTTP DELETE, which makes the resource inaccessible using the specified URI. The resource may or may not be physically deleted.
Incident ID: A90N17061714391
Date: 17 June 2017
Time reported: 1439
Severity: Significant
Description: Broken-down bus on north carriageway. One lane closed. Expect delays of up to 30 minutes.
. . .
Broken-down bus on north carriageway.One lane closed. Expect delays of up to
30 minutes. {
id: “A90N17061714391”,
“date”: “20170617”,
“time”: “1437”,
“road_id”: “A90”,
“place”: “Stonehaven”,
“direction”: “north”,
“severity”: “significant”,
“description”: “Broken-down bus on north carriageway. One lane closed. Expect
delays of up to 30 minutes.”
}
Figure 6.7 Synchronous and asynchronous microservice interaction
Calls
Returns
Requests (B)
Synchronous – A waits for B
Asynchronous – A and B execute concurrently
Queue B Queue A
Requests (A)
Service B
Service A
Processing Waiting Processing
Processing Processing
Processing Processing
ProcessingProcessing
Service B
Figure 6.8 Direct and indirect service communication
Direct communication – A and B send messages to each other
Indirect communication – A and B communicate through a message broker
Message broker
Service A Service B
Service A Service B
Pending transactions log
A1/DB update 1
A1/DB update 2
A2/DB update 1
Figure 6.9 Using a pending transactions log
Service A1
Database A
Service A2
Database A
End
Retry
login
Get login
Start
End
Check
login
Get
password
Check
password
Indicate
failure
login OK
login invalid
password OK
password
invalid
attempts > 3
attempts = 1
authfail = F
authfail=T
Figure 6.10 Authentication workflow
authfail = F
Increment
attempts
attempts <= 3
authfail = T
authfail=F
Authentication
controller
Service orchestration Service choreography
Figure 6.11 Orchestration and choreography
Authentication eventsLogin
service
Password
service
Login
service
Password
service
Circuit breaker
Check S2
availability
retries>3
retries<=3
timeout ok
timeout fail
S2 available
S2 unavailable
Figure 6.12 Using a circuit breaker to cope with service failure
Service S1 Service S2
Set timeout Route service
request
Respond S2
unavailable
Set S2
unavailable
Route service
response
Increment
retries
Check
timeout
HTTP
request
HTTP
response
Service actions
Microservice
Figure 6.13 HTTP request and response processing
Request
processing
Response
generation
[Request header]
[Request body]
REQUEST
[HTTP verb] [URI] [HTTP version]
Figure 6.14 HTTP request and response message organisation
[Response header]
[Response body]
RESPONSE
[Response code][HTTP version]
REQUEST
GET HTTP/1.1
...
Content-Length: 461
Content-Type: text/json
RESPONSE
HTTP/1.1
Figure 6.15 A GET request and the associated response
200incidents/A90/stonehaven/
Host: trafficinfo.net
...
Accept: text/json, text/xml, text/plain
Content-Length: 0
{
“number”: “A90N17061714391”,
“date”: “20170617”,
“time”: “1437”,
“road_id”: “A90”,
“place”: “Stonehaven”,
“direction”: “north”,
“severity”: “significant”,
“description”: “Broken-down bus on north
carriageway. One lane closed. Expect delays
of up to 30 minutes.”
}
{
“number”: “A90S17061713001”,
“date”: “20170617”,
“time”: “1300”,
“road_id”: “A90”,
“place”: “Stonehaven”,
“direction”: “south”,
“severity”: “minor”,
“description”: “Grass cutting on verge. Minor
delays”
}
Commit change to
version manage-
ment
Triggers
pass
Reject change Reject change Reject change
Reject change
pass
pass
fail
fail
fail
Figure 6.16 A continuous deployment pipeline
Run unit tests
Containerize
service
Run integration
tests
Build test
system
Replace current
service
Deploy service
container
Run acceptance
tests
fail
pass
API
gateway cameras
service request
for cameras service
Figure 6.17 Versioned services
current version
link
service
response
cameras service
response
monitor
response
Service
monitor
cameras 001
cameras 002
.MsftOfcThm_Text1_Fill {
fill:#000000;
}
.MsftOfcThm_MainDark1_Stroke {
stroke:#000000;
}
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