1) The document discusses using stateful stream processing to build lightweight microservices that evolve a shared narrative. It outlines various tools from the stream processing toolkit like Kafka, KStreams, KTables, state stores, and transactions that can be used. 2) Various patterns for building stateless, stateful, and joined streaming services are presented, including gates, sidecars and stream-asides. These can be combined to process events and build views. 3) An evolutionary approach is suggested where services start small and stateless, becoming stateful if needed, and layering contexts within contexts. This allows systems to balance sunk costs and future flexibility.

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Putting the ‘Micro’ into
Microservices with Stateful
Stream Processing
Ben Stopford
@benstopford

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Last time: Event Driven Services
• Interact through events, don’t talk to services
• Evolve a Cascading Narrative
• Query views, built inside your bounded context.
Related:
-Event Collaboration
-Service Choreography
-Event storming

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The narrative should contain business
facts
It should read like:
“a day in the life of your business”

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Base services on Event Sourcing
Domain Service
Kafka
Local View of
Customers
Service journals
state changes
(Facts)

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Where stateful, reconstitute from log
Domain Service
Kafka
Local View of
Customers
Reconstitute
state

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Use the narrative to move away from
legacy apps
Domain Service
Kafka
Local View of
Customers

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Retain the Narrative
Kafka scales

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What is this talk about?
Using SSP to build lightweight services that
evolve the shared narrative
1. What’s in the SSP toolbox?
2. Patterns for Streaming Services
3. Sewing together the Big & the Small
4. The tricky parts

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Applying tools
from stateful
stream
processing

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Aside: What is Stateful Stream
Processing?
A machine for converting a set of
streams into something more useful.
Output a stream
or output a table

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Domain Service
Kafka
Local View of
Customers
View
(1) Drive processing from Events
(2) Build views that answer questions

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The tools

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Kafka
Producer
(1) Producer/Consumer APIs
Consumer
• Simple & easy to
understand
• Native in many
languages
• Easy to integrate to
existing services
• No joins, predicates,
groupings or tables
• Includes transactions
in Kafka-11.

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(2) KStreams DSL
• Joins (including
windowing)
• Predicates
• Groupings
• Processor API
Kafka
KTable
Inbound
Stream
(Windowed)
State Store Outbound Stream
Changelog
Stream
DSL

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(3) The KTable
• Recast a stream as a
table via a defined key
• Tends to be backed
by compacted topic.
• Materialized locally in
RocksDB
• Provides guaranteed
join semantics (i.e. no
windowing concerns)
Kafka
KTable
(in RocksDB)
DSL
JVM boundary

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The KTable
• Behaves like a regular stream but
retains the latest value for each key,
from offset 0
• Partitions spread over all available
instances
• Partition counts must match for joins
• Triggers processing
• Initializes lazily.

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(4) Global KTable
• All partitions replicated
to all nodes
• Supports N-way join
• Blocks until initialized
• Doesn’t trigger
processing (reference
resource)
Kafka
Global KTable
(in RocksDB)
DSL

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Global-KTables vs. KTables
P1,3 P6,8 P2,7 P4,5
P1,2,
3,4,5,
6,7,8
P1,2,
3,4,5,
6,7,8
P1,2,
3,4,5,
6,7,8
P1,2,
3,4,5,
6,7,8
KTable
(Sharded)
Global KTable
(Replicated)
Given a service with four instances:

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(5) State Store
• RocksDB instance
• Mutable
• Backed by Kafka
• Integrated into DSL
• Can be instantiated
independently
• Queryable via
Queryable state
• Useful for custom
views or stateful
services.Kafka
State Store
(RocksDB)
Changelog
Stream
DSL

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(6) Hot Replicas
- By default KStreams
keeps state backed up on
a secondary node.
- Single machine failure
doesn’t result in a re-
hydration pause.
=> Highly available, stateful
services.
Service instance 1 Service instance 2

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(7) Transactions
Two problems:
1. Failures create duplicates
• Each message gets a unique ID
• Deduplicate on write
2. You need to write, atomically, to 2
or more topics
• Chandy-Lamport Algorithm
NB: Transactions will be in the next Kafka release

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Snapshot Marker Model
Commit/Abort
Begin
Commit/Abort
Begin
Buffer*
* The buffering is actually done in Kafka no the consumer
Producer Consumer
Topic A
Topic B
Elected Transaction
Coordinator
Consumer only sees
committed messages

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Simple Example
//Orders -> Stock Requests and Payments
orders = ordersTopic.poll()
payments = paymentsFor(orders)
stockUpdates = stockUpdatesFor(orders)
//Send & commit atomically
producer.beginTransaction()
producer.send(payments)
producer.send(stockUpdates)
producer.sendOffsetsToTransaction(offsets(orders))
producer.endTransaction()

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State-Stores & Transactions
• In KStreams Transactions
span stream and
changelog
• Processing of stream &
state is atomic
• Example: counting orders.
Kafka
State Store Stream
DSL
Atomic Transaction
Changelog
Stream

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All these tools scale horizontally and avoid
single points of failure
Sources Kafka KafkaService Layer Service Layer Kafka

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Service patterns
(building blocks)

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(1) Stateless Processor
e.g.
- Filter orders by region
- Convert to local domain model
- Simple rule

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(2) Stateless, Data Enabled Processor
Similar to star schema
• Facts are stream
• Dimensions are GKTables
e.g. Enrich Orders with Customer Info & Account details
Stream
GKTable
GKTable
Stream-Table
Join

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(3) Gates
e.g. rules engines or event correlation
When Order & Payment then …
Stream 1
Window
Window Stream-Stream
Join
Stream 2

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(4) Stateful Processor
e.g.
- Average order amount by region
- Posting Engine (reverse replace of previous position)
State store
backed up to Kafka

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(5) Stream-Aside Pattern
State store
backed up to Kafka
public static
void main(…){
…
}
JVM

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(7) Sidecar Pattern
State store
backed up to Kafka

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Combine them:
1. Stateless
2. Data enriched
3. Gates
4. Stateful processors
5. Stream-aside
6. Sidecar

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Query patterns covered in last talk

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Building ON the
narrative
The big and the small

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Evolutionary approach
Services should:
• Fit in your head
• Should be quick to stand up & get going
But sometimes bigger is the right answer

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Event Driven First
• Build on an event driven backbone
• Add Request-Driven interfaces where
needed.
• Prefer intra-context
REST
Kafka

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Putting the Micro into Microservices
- Combine simple services that develop
the shared narrative
- Start stateless, make stateful if
necessary

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Contexts within Contexts
Where services are
tiny, they will typically
be grouped together.
• Shared domain?
• Shared deployment?
• Shared streams?
Several small
services
Single bounded
context

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Layering within multi-service contexts
Kafka
Data layer:
-Stream enrichment
-View creation
-Streams maintenance
Business layer
encapsulates logic,
integrates with legacy
code etc.
Request-Driven layer
provides REST / RPC /
websotcket etc for UIs
etc

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Kafka
Different levels of data
visibility within Kafka
Kafka
Contexts within Contexts
Accounting
Fulfillment
Online Services

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Bringing it
together

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Good architectures have little to
do with this:

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It’s more about how systems evolves
over time

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Sunk Cost vs Future Cost
Architecture: Sunk Cost Change: Future Cost
Balance

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Part 1: Data dichotomy
Request-Driven Services don’t handle
shared data well
- God service problem
- The data erosion problem
Request-Driven Services struggle with
complex flows
- the service entanglement problem
- the batch processing problem.

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Part 2: Building Event Driven Services
Solution is to part 1:
- Event source facts into a Cascading
Narrative
- Communication & State transfer
converge
- Decentralize the role of turning facts
into queryable views
- Keep these views as lightweight and
disposable as possible.

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Putting the Micro in Microservices
- Start lightweight, stateless functions
which compose and evolve
- Leverage the stateful stream processing
toolkit to blend streams, state, views,
transactions.
- Use layered contexts that segregate
groups of streams and services.

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Highly
scalable
architecture

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All order logic,
stateless service
Orders-by-Customer view
(KStreams + Queryable State API)
UI Service
Stream
Maintenance &
Monitoring
Highly available, stateful service
UI uses Orders-by-
Customer View directly
via Queryable State
History Service pushes data to
a local Elastic Search Instance
Orders
Service
Derived
View
UI joins data
Tables & Streams
Fulfillment Service
Analytics
OrdersProduct Custo-
mers KTables
KTables KTables
Schemas

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Tips & Tricky parts
• Reasoning about service choreography
• Debugging multi-stage topologies
• Monitoring/debug via the narrative
• Segregate facts from intermediary streams
• Retaining data comes at a cost
• Schema migration etc
• Accuracy over non trivial topologies
• Often requires transactions
• Transactions, causality & the flow of events
• Avoid side effects

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Event Driven Services
Create a cascading narrative of events,
then sew them together with stream
processing.

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Discount code: kafcom17
‪Use the Apache Kafka community discount
code to get $50 off
‪www.kafka-summit.org
Kafka Summit New York: May 8
Kafka Summit San Francisco: August 28
Presented by

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Thank You!
@benstopford