Actors or Not: Async Event Architectures

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Actors or Not
Async Event Architectures
Yaroslav Tkachenko
Senior Software Engineer at Demonware (Activision)

Background
• 10 years in the industry
• ~1 year at Demonware/Activision, 5 years at Bench Accounting
• Mostly web, back-en...

Two stories

Context: sync vs async communication
Service A Service B
POST /foo
service-b.example.com
“Easy” way – HTTP (RPC) API

Context: sync vs async communication
• Destination – where to send request?
• Service discovery
• Tight coupling
• Time – ...

You cannot make
synchronous requests
over the network
behave like local ones

Context: async communication styles
• Point-to-Point Channel
• One sender
• One receiver
• Publish-Subscribe Channel (Broa...

Context: Events vs Commands
• Event
• Simply a notification that something happened in the past
• Command
• Request to inv...

• 469+ million gamers
• 3.2+ million concurrent online gamers
• 100+ games
• 300,000 requests per second at peak
• Average...

• Core game services including:
• Auth
• Matchmaking
• Leaderboards
• Marketplace
• Loot & Rewards
• Storage
• Etc.
• Erla...

• Lots of synchronous request/response communication between the monolith
and the services using:
• HTTP
• RPC
• The reque...

• Using Domain Events
• Communication model assumes the following:
• The event may need to be handled by zero or more serv...

Domain Driven Design
Application
Core
Event
Adapter
Events
Commands
Events
HTTP
Adapter
CLI
Adapter
Kafka
Service

Kafka

Kafka
Publish-Subscribe OR Point-to-Point is a decision made by consumers

Kafka
• Service name is used as a topic name in Kafka
• Services have to explicitly subscribe to interested topics on star...

Event Dispatcher
Application
Core
Event
Dispatcher
Kafka topic
Partitions
Kafka Python
Consumer
(librdkafka)
Local
buffer
...

Event Dispatcher
1 @demonata.event.source(
2 name='events_from_service_a'
3 )
4 class ServiceAEventsDispatcher (object):
5...

Publishing Events
The following reliability modes are supported:
• Fire and forget, relying on Kafka producer (acks = 0, 1...

Event Publisher
Application
Core
Event
Publisher
Kafka topic
PartitionsKafka Python
Producer
(librdkafka)
Event
Store Even...

Publishing Events
1 @demonata.coroutine
2 def handle_event_atomically (self, event_to_process):
3 entity_key = self. deter...

Event Framework in Demonware
• Decorator-driven consumers using callbacks
• Reliable producers
• Non-blocking IO using Tor...

But still…
Can we do better?

1 @demonata.event.source(
2 name='events_from_service_a'
3 )
4 class ServiceAEventsDispatcher (object):
5 def __init__(sel...

Can we create
producers and
consumers that support
message-passing
natively?

Actors
• Communicate with asynchronous messages instead of method invocations
• Manage their own state
• When responding t...

Actors

Actors: Erlang
1 loop() ->
2 receive
3 {From, Msg} ->
4 io:format("received ~p~n" , [Msg]),
5
6 From ! "got it";
7 end.

Actors: Akka
1 class MyActor extends Actor with ActorLogging {
2 def receive = {
3 case msg => {
4 log.info(s"received $ms...

Actor-to-Actor communication
• Asynchronous and non-blocking message-passing
• Doesn’t mean senders must wait indefinitely...

Bench Accounting

Bench Accounting Online Services
• Classic SAAS application used by the customers and internal bookkeepers:
• Double-entry...

Bench Accounting Legacy Eventing
• Multiple issues:
• Designed for a few specific use-cases, schema is not extendable
• Wa...

Bench Accounting Eventing System
ActiveMQ
Eventing
service
Service
A
Service
B
queue
queue
or
topic
IntegrationsEvent
store

ActiveMQ
Point-to-Point Publish-Subscribe

ActiveMQ
• Service name is used as a queue or topic name in ActiveMQ, but there is a also a
topic for global events
• Serv...

Secret sauce: Apache Camel
• Integration framework that implements Enterprise Integration Patterns
• akka-camel is an offi...

Event Listener
akka-camel
ActiveMQ
queue or
topic
ActiveMQ
Consumer
prefetch
buffer
Actor

Event Listener
1 class CustomerService extends EventingConsumer {
2 def endpointUri = "activemq:Consumer.CustomerService.V...

Event Sender
akka-camel ActiveMQ
queue or
topic
ActiveMQ
ProducerActor

Event Sender
1 // Broadcast
2 EventingClient
3 .buildSystemEvent (Event.BankError, userId, Component.ServiceA)
4 .send(tru...

Eventing Service
Event
Recorder
Event
Receiver
Event
Forwarder
Event
Reader
HTTP
API
Events
DAO
Event
Store
Integrations
A...

Eventing Service
So, we do we need this “router” service?
• Routing is handled in one place
• Lightweight consumers and pr...

Event framework in Bench Accounting
• Actor-based consumers and producers using Apache Camel
• Producer with ACKs
• Non-bl...

Lessons learned

So, Actors
• Semantics is important! Natural message-passing in Actors is a huge
advantage
• Asynchronous communication an...

Recommendations
• Domain Driven Design and Enterprise Integration Patterns are great!
• Understand your Domain space and c...

Recommendations
• Carefully choose the transport layer. Apache Kafka can handle an impressive
scale, but many messaging fe...

Challenges
• We’re too attached to the synchronous request/response paradigm. It’s
everywhere - in the libraries, framewor...

Challenges
• It’s very straightforward to implement event-based communication for
writes, but harder for reads. You’ll pro...

That signup page...

Thanks
Davide Romani (Demonware)
Pavel Rodionov (Bench Accounting)

Questions?
@sap1ens | sap1ens.com

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