1) The document discusses the art of building event streaming applications using various techniques like bounded contexts, stream processors, and architectural pillars. 2) Key aspects include modeling the application as a collection of loosely coupled bounded contexts, handling state using Kafka Streams, and building reusable stream processing patterns for instrumentation. 3) Composition patterns involve choreographing and orchestrating interactions between bounded contexts to capture business workflows and functions as event-driven data flows.

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The art of the event streaming application.
streams, stream processors and scale
Neil Avery,
Office of the CTO,
@avery_neil

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3. 55
Out of the tar pit, 2006

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“We believe that the major
contributor to this complexity in
many systems is the handling of
state and the burden that this adds
when trying to analyse and reason
about the system.”
Out of the tar pit, 2006

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What about Microservices?

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What are microservices?
Microservices are a software development
technique - a variant of the service-oriented
architecture (SOA) architectural style that
structures an application as a collection of
loosely coupled services.
https://en.wikipedia.org/wiki/Microservices

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structures an application as a collection of
loosely coupled services.
this is new!

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So what went wrong?

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Making changes is risky

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Handling state is hard Cache?
Embedded?
Route to right instance?

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● Scaling is hard
● Handling state is hard
● Sharing, coordinating is hard
● Run a database in each microservice - is hard
What have we learned about microservices?

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We had it all wrong

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We actually had some of it right

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Immutability

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What’s the big idea?

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Event driven architectures

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aren’t new…
...but...

18. 2020
the world has changed

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New technology, requirements and expectations

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Events
FACT!
SOMETHING
HAPPENED!

21. Ad placement
Examples...
User signed up Item was sold Payment

22. Events
Why do you care?
Loose coupling, autonomy, evolvability, scalability, resilience, traceability, replayability
EVENT-FIRST CHANGES HOW YOU
THINK ABOUT WHAT YOU ARE BUILDING
...more importantly...

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Store events in
..a stream..

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Different types of event models
● Change Data Capture - CDC (database txn log)
● Time series (IoT, metrics)
● Microservices (domain events)

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Capture behavior

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Time travel user experience?
how many users
affected?has it happened
before?
Ask many questions of the same data, again and again
time

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old world : event-driven architectures
new world: event-streaming architectures

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Stream processing
Kafka
Streams
processor
input events
output events
...temporal reasoning...
event-driven microservice

29. {
user: 100
type: bid
item: 389
cat: bikes/mtb
region: dc-east
}
Partitions give you horizontal scale
/bikes/ by item-id
key#
Key
space
{...}
{...}
{...}
ConsumerTopic
Partition
Partition
assignment

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Stream processors are uniquely
convergent.
Data + Processing
(sorry dba’s)

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All of your data
is
a stream of events

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stop...where is my database?
(you said scaling data was hard)

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Streams are your persistence model
They are also
your local
database

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The atomic unit for tackling complexity
Stream
processor
input events
output events
...or microservice or whatever...

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It’s pretty powerful
Stream
processor
Stream
processor
Stream
processor
Topic: click-stream
Interactive query
CDC events from KTable
CDC Stream
partition
partition
partition
CQRS
Elastic

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Stream processor == Single atomic unit
It does one thing
Like

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We think in terms of function
“Bounded Context”
(dataflow - choreography)

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Let’s build something….
A simple dataflow series of processors
“Payment processing”

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KPay looks like this:
https://github.com/confluentinc/demo-scene/tree/master/scalable-payment-processing

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Bounded context
“Payments”
1. Payments inflight
2. Account processing [debit/credit]
3. Payments confirmed

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Payments bounded context
choreography

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Payments system: bounded context
[1] How much is being processed?
Expressed as:
- Count of payments inflight
- Total $ value processed
[2&3] Update the account balance
Expressed as:
- Debit
- Credit [4] Confirm successful payment
Expressed as:
- Total volume today
- Total $ amount today

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Payments system: AccountProcessor
accountBalanceKTable = inflight.groupByKey()
.aggregate(
AccountBalance::new,
(key, value, aggregate) -> aggregate.handle(key, value), accountStore);
KStream<String, Payment>[] branch = inflight
.map((KeyValueMapper<String, Payment, KeyValue<String, Payment>>) (key, value) -> {
if (value.getState() == Payment.State.debit) {
value.setStateAndId(Payment.State.credit);
} else if (value.getState() == Payment.State.credit) {
value.setStateAndId(Payment.State.complete);
}
return new KeyValue<>(value.getId(), value);
})
.branch(isCreditRecord, isCompleteRecord);
branch[0].to(paymentsInflightTopic);
branch[1].to(paymentsCompleteTopic);
https://github.com/confluentinc/demo-scene/blob/master/scalable-payment-processing/.../AccountProcessor.java
KTable state
(Kafka Streams)

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Payments system: AccountBalance
public AccountBalance handle(String key, Payment value) {
this.name = value.getId();
if (value.getState() == Payment.State.debit) {
this.amount = this.amount.subtract(value.getAmount());
} else if (value.getState() == Payment.State.credit) {
this.amount = this.amount.add(value.getAmount());
} else {
// report to dead letter queue via exception handler
throw new RuntimeException("Invalid payment received:" + value);
}
this.lastPayment = value;
return this;
}
https://github.com/confluentinc/demo-scene/.../scalable-payment-processing/.../model/AccountBalance.java

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Payments system: event model
https://github.com/confluentinc/demo-scene/.../scalable-payment-processing/.../io/confluent/kpay/payments

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Bounded context
“Payments”
Is it enough?
no

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“It’s asynchronous, I don’t trust it”
(some developer, 2018)

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We only have one part of the picture
○ What about failures?
○ Upgrades?
○ How fast is it going?
○ What is happening - is it working?

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Event-streaming provides
● Evolution
● Decoupling
● Bounded context modelling
● Composition
(because of SoC)

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Composition

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Event-streaming pillars:
1. Business function (payment)
2. Instrumentation plane (trust)
3. …
4. ...

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Instrumentation Plane (trust)
Goal: Prove the application is meeting business requirements
Metrics:
- Payments Inflight, Count and Dollar value
- Payment Complete, Count and Dollar value

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Instrumentation Plane
KStream<String, Payment> complete = builder.stream(paymentsCompleteTopic);
statsKTable = complete
.groupBy((key, value) -> "all-payments")
.windowedBy(TimeWindows.of(ONE_MINUTE))
.aggregate(
ThroughputStats::new,
(key, value, aggregate) -> aggregate.update(value),
completeWindowStore
);

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Instrumentation Plane
public ThroughputStats update(Payment payment) {
totalPayments++;
totalDollarAmount = totalDollarAmount.add(payment.getAmount());
maxLatency = Math.max(maxLatency, payment.getElapsedMillis());
minLatency = Math.min(minLatency, payment.getElapsedMillis());
if payment.getAmount().doubleValue() > largestPayment.getAmount().doubleValue()) {
largestPayment = payment;
}
timestamp = System.currentTimeMillis();
return this;
}

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Instrumentation Plane: Using IQ
https://github.com/confluentinc/demo-scene/blob/master/scalable-payment-processing/../ThroughputStats.java

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Event-streaming pillars:
1. Business function (payment)
2. Instrumentation plane (trust)
3. Control plane (coordinate)
4. ...

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Control Plane
Goal: Provide mechanisms to coordinate system behavior
Why? Recover from outage, DR, overload etc
Applied: Flow control, start, pause, bootstrap, scale, gate and rate limit
Model:
- Status [pause, resume)
- Gate processor [Status]
- etc

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Event-streaming pillars:
1. Business function (payment)
2. Instrumentation plane (trust)
3. Control plane (coordinate)
4. Operational plane (run)

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Dependent on Control and Instrumentation planes
Dataflow patterns
● Application logs
● Error/Warning logs
● Audit logs
● Lineage
● Dead-letter-queues
/dead-letter/bid/region/processor
/ops/logs/ca
/ops/metric
Stream
processor
Operational Plane

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Architectural pillars
/payments/incoming
PAY
/payments/confirmed
Core dataflow
Control plane
/control/state
START
STOP
/control/status
stream.filter()
Instrumentation plane
/payments/confirmed
BIZ
METRIC
IQ
IQ
IQ
/payments/dlq
ERROR
WARN
IQ
Operational plane

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Payment system

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Composition Patterns

63. Bounded context (dataflow)
Choreography:
- Capture business function as a bounded context
- Events as API
2.
Accounts
[from]
payment.incoming
3.
Accounts
[to]
4.
Payment
Conf’d
1.
Payment
Inflight
payment.confirmed
payment.inflight
payment.inflight
payment.complete
payment.complete

64. Multiple Bounded contexts
Choreography:
- Chaining
- Layering
2.
Logistics
payment.incoming 1.
Payment
payment.complete

65. Multiple Bounded contexts
Orchestration
○ Captures workflow
○ Controls bounded context interaction
○ Business Process Model and Notation 2.0 (BPMN)
(Zeebe, Apache Airflow)
Source: https://docs.zeebe.io/bpmn-workflows/README.html

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Composition patterns at scale
Flickr: Dave DeGobbi

67. {faas}
events as a backbone
appappappapp
Payments Department 2
{faas}appappappapp
Department 3 Department 4
Pattern: Events as a backbone

68. {faas}
What is going on here?
appappappapp
Payments Department 2
Patterns: Topic naming
bikeshedding (uncountable)
1. Futile investment of time and energy in
discussion of marginal technical issues.
2. Procrastination.
https://en.wiktionary.org/wiki/bikeshedding
Parkinson observed that a committee whose
job is to approve plans for a nuclear power
plant may spend the majority of its time on
relatively unimportant but easy-to-grasp
issues, such as what materials to use for the
staff bikeshed, while neglecting the design
of the power plant itself, which is far more
important but also far more difficult to
criticize constructively.

69. Patterns: Topic conventions
Don’t
1. Use fields that change
2. Use fields if data is available elsewhere
3. Tie topic names to consumers or producers
Do
<message type>.<dataset name>.<data name>
<app-context>.<message type>.<dataset name>.<data name>
Source: Chris Riccomini
https://riccomini.name/how-paint-bike-shed-kafka-topic-naming-conventions
● Logging
● Queuing
● Tracking
● etl/db
● Streaming
● Push
● user

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What about that software crisis that started in
1968?
“We believe that the major contributor to this
complexity in many systems is the handling of state
and the burden that this adds when trying to analyse
and reason about the system.”
Out of the tar pit, 2006

71. Our mental model: Abstraction as an Art
Chained/Orchestrated
Bounded contexts
Stream processor
Stream
Event
Pillars
Business function Control plane Instrumentation Operations
Bounded context

72. Key takeaway (state)
Event streamingdriven microservices are the new atomic unit:
1. Provide simplicity (and time travel)
2. Handle state (via Kafka Streams)
3. Provide a new paradigm: convergent data and logic processing
Stream
processor

73. Key takeaway (complexity)
● Event-Streaming apps: model as bounded-context dataflows, handle
state & scaling
● Patterns: Build reusable dataflow patterns (instrumentation)
● Composition: Bounded contexts chaining and layering
● Composition: Choreography and Orchestration

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This is just the beginning

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Questions?
@avery_neil
“Journey to event driven” blog
1. Event-first thinking
2. Programming models
3. Serverless
4. Pillars of event-streaming ms’s
https://bit.ly/2tFfU84
or @avery_neil twitter profile

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@avery_neil