As presented at droidcon Tel Aviv 2019 http://il.droidcon.com

1. Mobile at Scale
Dor Samet | 19.12.2019
Lessons from taking a startup approach at a big company

2. ● Developer for 6+ years
● Co-founder of the KotlinTLV
community
● Scotch enthusiast
● Android Developer at Booking.com
Machine Learning site
Quick Introduction
Dor Samet

3. Azrieli
sarona
50+
people
and
growing
opened
2017
only centre
outside of
AMS
ML center
P13N
TLV Center.

4. Startup
Structured chaos
● Informal
● Small group to coordinate with
● One time zone, one-to-one communication

5. Startup == “One-Table” Decision Process

7. 1996
Founded in
Amsterdam
in 220+
countries
& territories
29+ million
listings
including
6+ million
listing in homes,
apartments and other
unique places to stay.
43
supported
languages
24/7
customer
service
200+offices
in over
70countries
17,500+
dedicated
employees
1,550,000
room nights
booked
every 24 hours
187
million+
verified
guest reviews
online
17,500+
affiliate partners
145+ million
destination
recommendations
Company
highlights.
30
different
property
types
2005
Became a part of
Booking Holdings
Inc. (then Priceline
Group)

8. Stakeholders
70+ People
20+ countries

9. Stakeholders
● Amsterdam
● London
● Shanghai
● New York
● Tel Aviv

12. Lessons learned.

13. What will we discuss today?
1. Modules
2. Automatic Checks
3. Experimentation
4. Soft Skills

14. Modularize

15. Build time
> 260K methods
Cold start (after clean): ~14 minutes
Incremental build (changes in a few modules up the chain): ~ 3
minutes
Incremental build (changes in one modules): ~700 milliseconds

16. ● Module scopes a certain entity in
the app
● Prepare for the future (instant apps,
fast iterations)
● Makes your gradle build FAST
● Gradle caches what is already built
Why modularization?

17. Layers - Grouping of module
Current structure:
Presentation
screens
UI Components
AppServices
data, models, etc.
CoreServices
monitoring, localization, network, icons, experiment management, etc.

18. ● Place org.gradle.caching=true
In your gradle.properties file*
● Create a new module
● Develop in the new module
● Include like another dependency
(gradle)
○ implementation
project(path:`yourModuleName`)
How modularization?

19. ● Module scopes a certain entity in
the app
● Prepare for the future (instant apps,
fast iterations)
● Makes your gradle build FAST
● Scale through fast iterations, and
decoupled logic
Why modularization?

20. Automatic Checks

21. ● Avoid common bugs through
known patterns
○ Syntactically correct code - but isn’t
● Code reviews
○ Only occur once the checks and tests
allow
● Run on CI
○ Asynchronous testing
● Synchronize teams to standards
○ Teams in London, Shanghai, Amsterdam
and Tel Aviv
Why Automatic Checks?

22. How Automatic Checks?
● Static Analysis tools
○ e.g. Lint
○ Avoid common bugs
○ Push to use company tools
○ Run on CI for asynchronicity
○ Super easy to implement:

23. Time for some code

24. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
}

25. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
}

26. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
}
}

27. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
if (node.isStatic
&& evaluator.isVariableExtendClass(node, ClassNameConstants.Android.ASYNC_TASK, false)
) {
}
}
}

28. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
if (node.isStatic
&& evaluator.isVariableExtendClass(node, ClassNameConstants.Android.ASYNC_TASK, false)
) {
}
}
}

29. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
if (node.isStatic
&& evaluator.isVariableExtendClass(node, ClassNameConstants.Android.ASYNC_TASK, false)
) {
}
}
}

30. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
if (node.isStatic
&& evaluator.isVariableExtendClass(node, ClassNameConstants.Android.ASYNC_TASK, false)
) {
}
}
}

31. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
if (node.isStatic
&& evaluator.isVariableExtendClass(node, ClassNameConstants.Android.ASYNC_TASK, false)
) {
context.report(
asyncTaskStaticReference, node, context.getLocation(node),
)
}
}
}

32. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
if (node.isStatic
&& evaluator.isVariableExtendClass(node, ClassNameConstants.Android.ASYNC_TASK, false)
) {
context.report(
asyncTaskStaticReference, node, context.getLocation(node),
)
}
}
}

33. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
if (node.isStatic
&& evaluator.isVariableExtendClass(node, ClassNameConstants.Android.ASYNC_TASK, false)
) {
context.report(
asyncTaskStaticReference, node, context.getLocation(node),
)
}
}
}

34. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
if (node.isStatic
&& evaluator.isVariableExtendClass(node, ClassNameConstants.Android.ASYNC_TASK, false)
) {
context.report(
asyncTaskStaticReference, node, context.getLocation(node),
"Do not reference `AsyncTask` in a `static` context, use an object with lifecycle instead"
)
}
}
}

35. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
override fun createUastHandler(context: JavaContext): UElementHandler? = object : UElementHandler() {
override fun visitField(node: UField) {
val evaluator = context.evaluator
if (node.isStatic
&& evaluator.isVariableExtendClass(node, ClassNameConstants.Android.ASYNC_TASK, false)
) {
context.report(
asyncTaskStaticReference, node, context.getLocation(node),
"Do not reference `AsyncTask` in a `static` context, use an object with lifecycle instead"
)
}
}
}

36. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
// ..
// ..
}

37. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
)
}
// ..
// ..
}

38. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
)
}
// ..
// ..
}

39. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
)
}
// ..
// ..
}

40. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
"async-task-static-reference",
)
}
// ..
// ..
}

41. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
"async-task-static-reference",
"ERROR_STRING",
)
}
// ..
// ..
}

42. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
"async-task-static-reference",
"ERROR_STRING",
"MORE_DETAILED_ERROR_GOES_HERE"
)
}
// ..
// ..
}

43. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
"async-task-static-reference",
"ERROR_STRING",
"MORE_DETAILED_ERROR_GOES_HERE",
Category.CORRECTNESS,
)
}
// ..
// ..
}

44. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
"async-task-static-reference",
"ERROR_STRING",
"MORE_DETAILED_ERROR_GOES_HERE",
Category.CORRECTNESS,
5,
)
}
// ..
// ..
}

45. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
"async-task-static-reference",
"ERROR_STRING",
"MORE_DETAILED_ERROR_GOES_HERE",
Category.CORRECTNESS,
5,
Severity.ERROR,
)
}
// ..
// ..
}

46. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
"async-task-static-reference",
"ERROR_STRING",
"MORE_DETAILED_ERROR_GOES_HERE",
Category.CORRECTNESS,
5,
Severity.ERROR,
Implementation(AsyncTaskStaticReferenceCheck::class.java, Scope.JAVA_FILE_SCOPE)
)
}
// ..
// ..
}

47. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
"async-task-static-reference",
" `AsyncTask` in a `static` context might leak memory",
"""`AsyncTask` usually have a reference to one or more `Context` instances
|(e.g. `Activity`) which might leak if a reference is held without a lifecycle.""".trimMargin(),
Category.CORRECTNESS,
5,
Severity.ERROR,
Implementation(AsyncTaskStaticReferenceCheck::class.java, Scope.JAVA_FILE_SCOPE)
)
}
// ..
// ..
}

48. class AsyncTaskStaticReferenceCheck : Detector(), Detector.UastScanner {
companion object Issues {
@JvmField
val asyncTaskStaticReference = Issue.create(
"async-task-static-reference",
" `AsyncTask` in a `static` context might leak memory",
"""`AsyncTask` usually have a reference to one or more `Context` instances
|(e.g. `Activity`) which might leak if a reference is held without a lifecycle.""".trimMargin(),
Category.CORRECTNESS,
5,
Severity.ERROR,
Implementation(AsyncTaskStaticReferenceCheck::class.java, Scope.JAVA_FILE_SCOPE)
)
}
// ..
// ..
}
Wait a minute…

50. How Automatic Checks?
● Tests
○ Unit
○ UI automation
○ Manual Regression for critical flows

51. ● Automatic checks are easy to
implement
● Write once
● Runs asynchronously
● Scale through independence
Conclusion Automatic
Checks

52. Experimentation

56. Why experimentation?
● Safety net for feature release
● Basis for “Data-Driven” decisions
● Commercial improvement through
metrics

57. A/B Tests!

58. Scientific Method!

59. Hypothesis!

60. Typical approaches to hypotheses
I have a feeling that placing our app in the
Amazon store is a good idea.
Our CEO wants us to work on a chatbot for
better engagement.
Based on what I saw in our analytics, our
users tend to be younger, so we should
put our app on ads in Instagram.

61. Build
Create a thing that answers a
hypothesis
Measure
Does it answer my original hypothesis?
Is my target audience correctly
organized?
Learn
Was my hypothesis answered? How can
I improve upon it?
Experimentation
definition
Build
M
easure
Learn

62. Hypothesis + Success
metrics
I have a feeling that placing our app in the
Amazon store is a good idea. We will
increase user acquisition by at least 10%.
We’ll learn if it’s a good idea.
Our CEO wants us to work on a chatbot for
better engagement. User retention
increased by at least a week. Let’s see if
that’s a good use of our time.
Based on what I saw in our analytics, our
users tend to be younger, so we should
put our app on ads in Instagram.
Segmented audience increased by at least
23%. We’ll see if young users like the app.
Build
Create a thing that answers a
hypothesis
Measure
Does it answer my original hypothesis?
Is my target audience correctly
organized?
Learn
Was my hypothesis answered? How can
I improve upon it?

63. How experimentation?
if (newFeatureEnabled) {
// new feature code
} else {
// old solution
}

64. How experimentation?
● Pick a metric
● Segment your audience
● Run the experiment
● Measure results
● Learn and Repeat

65. Conclusion
experimentation
● Safety net for feature release
● Basis for “Data-Driven” decisions
● Scientific method
● Scale
○ Release controlled features
○ Knowledge

66. Soft Skills

67. Soft, but critical skills.
● Build more capable products
● Be humble
● Celebrate differences

68. ● Take ownership
● Take initiative
Soft, but critical skills.

69. Recap

70. What did we discuss today?
1. Modules
2. Automated Checks
3. Experimentation
4. Soft Skills

71. Lessons learned.

72. Dor Samet
dor.samet@booking.com

73. Questions?
Dor Samet
dor.samet@booking.com