Event-Driven Architecture Traps

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JakubPilimon
EVENT-DRIVEN ARCHITECTURE
TRAPS
… AND HOW TO AVOID THEM

JakubPilimon
DISCLAIMERS
• Traps based on private experiences
• Solutions worked in my contexts
• Haven’t pushed anything ...

JakubPilimon
WHOAMI
SPRING DEVELOPER ADVOCATE
AND PROGRAMMER
Loves to tackle complex enterprises with:
• Domain-Driven Des...

JakubPilimon
PATTERN THAT PROMOTES DETECTION
AND REACTION TO STATE CHANGES
REPRESENTED BY DOMAIN EVENTS
ME
EVENT-DRIVEN TO...

JakubPilimon
EVENT-DRIVEN TO ME
• EDA != “I have microservices”
• EDA != “I have a message broker”
• Events might be trans...

JakubPilimon
EVENT-DRIVEN TO ME
EVENT 1
EVENT 2
EVENT 3
JMS/AMQP
HTTP
IN MEMORY
DB #1 DB #1 or not
DB #2

TRAP #1
MISSING AN EVENT

JakubPilimon
BACKEND2. Start transaction
1. Customer plugged in
4. Stop transaction
3. Customer ﬁnished charging
BACKEND
5...

JakubPilimon

JakubPilimon
TRAP #1: MISSING AN EVENT
REASONS:
▸ Treating boundary of software as equals to boundary of business process
...

TRAP #2
VERSIONING OF BEHAVIOR

JakubPilimon
BACKEND1. Stop session
BACKEND TAXES
2. ChargingSessionFinished
TAXES
3. Calculate and insert tax
EVENT 1
EVE...

JakubPilimon
TRAP #2: VERSIONING OF BEHAVIORS
REASONS:
▸ Hard-coded values are not temporal
▸ Occurred vs Arrived mismatch...

TRAP #3
EXTERNAL CALLS

JakubPilimon
TAXES
ChargingSessionFinished
TAX RATES

JakubPilimon
TRAP #3: EXTERNAL CALLS
REASONS:
▸ Thinking that external systems are always temporal
▸ Relying on single poi...

TRAP #4
IGNORING CAP THEOREM

TRAP #5
ANEMIC EVENTS

JakubPilimon
TRANSACTION LOG
MYSQL
session.finish();
T1 T2 T3 T4 T…
1
2
3
4
5
6
7
8
9
10
3
7
9
1
4
5
6
“charging_sessions”...

JakubPilimon
TRAP #5: ANEMIC-EVENTS
REASONS:
▸ Focusing on data instead of behaviors
SOLUTIONS:
▸ Domain events (with beha...

JakubPilimon
TRANSACTION
eventPublisher.publish(…);
proxy.commit(…);
chargingSessionRepository
.findById(…)
session.finish...

JakubPilimon
TRAP #4: IGNORING CAP THEOREM
REASONS:
▸ Ignoring the fact that every now and then things will not work
▸ Try...

TRAP #6
VERSIONING OF EVENTS

JakubPilimon
I WILL JUST FIND IT AND
UPDATE IT. WHAT CAN
POSSIBLY GO WRONG?

JakubPilimon
1
2
3
“sessions”
BACKEND
4
4’
ChargingSessionFinished
Charging session has ﬁnished
ChargingSessionFinishedV2
...

JakubPilimon
1
2
3
1’
2’
3’
“sessions” “sessions-v2”
TRANSLAT
OR

JakubPilimon
TRAP #6: VERSIONING OF EVENTS
REASONS:
▸ Using strong schema
▸ Canonical shared model of events - coupling
SO...

JakubPilimon
QUESTIONS?
https://github.com/pilloPl/edatraps
https://github.com/ddd-by-examples

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