event sourcing as the foundation of traceability · event sourcing for about a year. @tbfangel...
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IDA Driving IT Århus 2020Thomas Bøgh Fangel @tbfangel
Event Sourcing asthe Foundation of Traceability
@tbfangel
MeThomas Bøgh Fangel
@tbfangel
Architect at Lunar since 2016
Distributed systems since 2004
Java, Scala, Typescript, Go
Event Sourcing for about a year
@tbfangel
Agenda● Building a bank from scratch○ Tech vision
● Event Sourcing○ Why & How○ PatternsAgenda
@tbfangel
100+Employees
25%Engineers
1M+Tx pr month
~100μ services
150kUsers
3K8S clusters
LunarFounded
2015Live2016
Smartphone onlyChallenger Bank
@tbfangel
Building a bank from scratch
@tbfangel
CardProcessor
NationalClearing
What does it mean to be a bank?
Pay with card
Transfer money
Keep money safe
@tbfangel
Trustworthy Secure
Correct
Tech Vision
@tbfangel
Traceability at all levels
nothing should happen without us knowing and our system should never be in a state
we cannot explain
Tech Vision
@tbfangel
≠Traceability
Distributed Tracing
@tbfangel
func DoSomeBusiness(s *State) (Result, error) {
if s.InImpossibleState() {//this should never happen - what to do?panic(“current state is impossible”)
}//happy cases below
}
@tbfangel
@tbfangel
Production=
The place where the impossible happens
@tbfangel
Traceability=
Explain the impossible
@tbfangel
“Something really awful happened to your money - but
we know exactly where the money is and we will fix it”
@tbfangel
Event Sourcing
@tbfangel
Shift of Focus
From state…...to events
@tbfangel
Event Sourcing Components
Storage
Event stream
Projection
Side effects
Handler
Event stream
Projection
+
+Business logic
(pure functions)
Aggregate root
@tbfangel
Implementation● Apogee: ES & CQRS* library in Go
(Open Source in 2020)● Postgres as event storage● In-memory views for AR● SQL backed views
* CQRS: Command Query Responsibility Segregation
@tbfangel
Patterns
@tbfangel
Adapter NationalClearing
Public APIs
Bank Coreasync sync
@tbfangel
External Event Streams● Public API● Same guarantees and properties
as internal event stream● Derived from internal event stream● Essentially a handler writing to an
event stream ● May keep track of source events
@tbfangel
External Event Streams
Storage
InternalEvent stream
Projection
Handler External Event stream A
External Event stream B
@tbfangel
Distributed flows● Distributed transactions● Represented as aggregate roots
(state machines)● State determines handler action● Side effects in handlers● Public event streams as API
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
1.
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
2. Initiated
2.
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
2. Initiated
3.
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
2. Initiated 4. Initiated
4.
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
2. Initiated 4. Initiated
5. TimedOut
5.
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
2. Initiated 4. Initiated
5. TimedOut
6. Succeeded
6.
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
2. Initiated 4. Initiated
5. TimedOut
6. Succeeded
7.
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
2. Initiated 4. Initiated
8. Succeeded 5. TimedOut
6. Succeeded
8.
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
2. Initiated 4. Initiated
9. TransferCompleted ReservationRemoved TransactionPosted
8. Succeeded 5. TimedOut
6. Succeeded
9.
@tbfangel
TRANSFER
Bank Core
TRANSFER
DK Adapter
CLEARING
ACCOUNT
TRANSFER
Handler Aggregate
Request
9.
1.
2.
3. 4.5.+ 6.
7.8.
Account Bank Transfer Adapter Transfer
1. TransferInitiated FundsReserved
2. Initiated 4. Initiated
9. TransferCompleted ReservationRemoved TransactionPosted
8. Succeeded 5. TimedOut
6. Succeeded
@tbfangel
Error handling● Failures are 1st class domain citizens● Idempotency* crucial both internally and
externally ● Only measure against timeouts and
crashes
* Idempotency: system state remains the same after one or several calls
@tbfangel
Tech VisionRevisited
@tbfangel
What about Correctness...?● 100% correctness is impossible● Understand your errors, then fix● Traceability leads to correctness by
explaining errors● Error correction is just another event
@tbfangel
… and Consistency?● Ordered event streams● Empowers consumers ● Eventually is better than maybe
@tbfangel
Thank You!https://tech.lunarway.com/blog/
https://tech.lunarway.com/talks/
https://tech.lunarway.com/opensource/
