Event-driven Architectures

16 Pages·2007·0.5 MB·English

Programming without a Call Stack - Event-driven Architectures W-JAX 2007
Gregor Hohpe | Google
Programming Without a Call Stack:
Event-driven Architectures
www.eaipatterns.com
Who's Gregor?
(cid:122) Distributed systems, enterprise integration,
service-oriented architectures
(cid:122) MQ, MSMQ, JMS, TIBCO, BizTalk, Web Services
eaipatterns.com
• Patterns
(cid:122) Write code every day. Share knowledge through
• Articles
patterns. • Blog
Enterprise Integration Enterprise Best SOA
Integration Patterns Solution Software Experten-
Patterns Microsoft Patterns Writing I wissen
Addison-Wesley Press Microsoft APress dpunkt Verlag
Press
1
1
Programming without a Call Stack - Event-driven Architectures W-JAX 2007
Agenda
(cid:122)It's All About Coupling
(cid:122)Events Everywhere
(cid:122)Event-driven Architectures
(cid:122)Developing in an EDA
(cid:122)Case Study: Building an EDA in JavaTM
2
In A Connected World It's All About
Coupling
"Measure of dependency between components"
3
2
Programming without a Call Stack - Event-driven Architectures W-JAX 2007
Dynamic Composability
"The ability to build new things from existing pieces."
4
Interaction Takes Center Stage
"The lines are becoming boxes now."
--Ralf Westphal
5
3
Programming without a Call Stack - Event-driven Architectures W-JAX 2007
The World is Full of Events
Ware
Shipping
house
Partner
Inventory Truck
Low Delayed
E-mail
Order New Order
Event Bounced Mail
Entry
Gateway
Cloud
Address Credit Card
Changed Payment Expired
Declined
Web
Financial
Site
System
6
Event-Driven Architecture (EDA)
Channel Node
Event
(cid:122)Distributed processing, no central control.
(cid:122)Nodes respond to incoming events and publish
events in response.
(cid:122)Event channels transport events from one node to the
next, usually asynchronously (sender does not wait).
(cid:122)Composition through channels.
7
4
Programming without a Call Stack - Event-driven Architectures W-JAX 2007
EDA Defining Qualities
(cid:122) Timeliness. Publish events as they occur
AA
instead of waiting for the next batch cycle.
(cid:122) Asynchrony. The publishing system does not Event
wait for the receiving system(s) to process the AA BB
Queue
event.
(cid:122) Fine Grained. Publish single events as
AA
opposed to large aggregated event.
Orders
(cid:122) Ontology. A nomenclature to classify and
On-Line
express interest in certain groups of events.
Updated New
(cid:122) Complex Event Processing. Understanding
the relationships between events, for example
aggregation and causality.
8
Composition via Channels
Channel X
System System
A publish subscribe B
(cid:122)Nodes communicate via Channels
(cid:122)Sender and receiver need to agree to a common
channel. This is a form of coupling.
(cid:122)Sender and receiver have to decide which channel is
“right”. The burden can be shifted between sender
and receiver.
(cid:122)Channel namespace provides some structure.
9
5
Programming without a Call Stack - Event-driven Architectures W-JAX 2007
Channel Naming and Semantics
CreditService
Order Credit
(cid:122)Target component
Entry Service
VerifyCreditCard
Order Credit
(cid:122)Action / Operation
Entry Service
PaymentInfo
(cid:122)Document Order Credit
Entry Service
OrderReceived
(cid:122)Event Order Credit
Entry Service
10
How Do A and B Connect?
Structured
(cid:122) Channel Name / Instance MessageQueue q = new
MessageQueue(“foo");
• Common in message queue systems.
• Limited expressiveness. q.Send(“Msg”);
(cid:122) Topic Hierarchy Orders
• Allows wildcard subscription
On-Line
• Requires mapping of topic space onto a
tree. Forces prioritization. Updated New
(cid:122) Content-based
Channel.Subscribe(
• Flexible, but difficult to implement “/element/foo=‘bar’”
efficiently in widely distributed systems. );
Unstructured
11
6
Programming without a Call Stack - Event-driven Architectures W-JAX 2007
Composition Strategies
(cid:122) Implicit Composition (cid:122) Explicit Composition
cchhaannnneell == CChhaannnneell..bbyyNNaammee((““oorrddeerrss””));; AA((CChhaannnneell cchh)) {{ tthhiiss..cchhaannnneell == cchh;; }}
cchhaannnneell..ppuubblliisshh((mmeessssaaggee));; ……
cchhaannnneell..ppuubblliisshh((mmeessssaaggee));;
A A
Channel
<<create>>
orders “Composer”
Channel
B B
cchhaannnneell == CChhaannnneell..bbyyNNaammee((““oorrddeerrss””));; BB((CChhaannnneell cchh)) {{ tthhiiss..cchhaannnneell == cchh;; }}
cchhaannnneell..ssuubbssccrriibbee((eevveennttHHaannddlleerr));; ……
cchhaannnneell..ssuubbssccrriibbee((eevveennttHHaannddlleerr));;
12
Event Collaboration
Multiple Components work together by communicating with each other by
sending events when their internal state changes.
(Fowler)
Order Customer
Shipping
Management Management
Request Order RequestFor
Address
Collaboration
Address
SendShipment
Order Customer
Shipping
Management Management
Event Order Address
Changed
Collaboration
13
7
Programming without a Call Stack - Event-driven Architectures W-JAX 2007
Event Collaboration
(cid:122)Adding Consumers is Side-Effect Free
• Debugging / logging / sniffing
• Parallel implementations
(cid:122)Simple components, more complex interactions
(cid:122)Robust against unstable connections
(cid:122)Can be difficult to manage and /or debug
• Need to understand relationship between events
• Shift burden from design-time to run-time
(cid:122)Components may operate on stale data
14
Event-sourced Systems
Capture all changes to an application state as a
sequence of events.
(Fowler)
Persisted
State
(Snapshot)
Local Domain
State Objects
State
Event Bus
Changes
Event
Log
15
8
Programming without a Call Stack - Event-driven Architectures W-JAX 2007
Event-sourced Systems
(cid:122)More than an event log or "temporal database"
(cid:122)Rebuild state based on events by re-executing
behavior
(cid:122)Temporal Query
• Determine application state at specific point in time
(cid:122)Event replay
• Run "what if" scenarios or corrections to past events
(cid:122)Limitation: code changes
16
Composite Events Processing
(cid:122)Understand causality
(cid:122)Some events are the result a
of a sequence of events Causality
(cid:122)CEP = Complex Event Processing
(cid:122)Pattern matching languages
can be challenging
Aggregation
17
9
Programming without a Call Stack - Event-driven Architectures W-JAX 2007
Case Study – Existing System
OOnn--lliinnee CCooppyy RReeppoorrtt CCoommppuuttee
DDBB PPrroocc DDBB PPrroocc
UI Client
A
B FFrroonnttEEnndd
C
(cid:122) Compute statistics based on responses to on-line
questionnaires
(cid:122) Responses stored in database
(cid:122) At the end, stored procedure computes “scores” based on
user responses
• Load on RDBMS
• Single thread, monolithic, synchronous
• Poor response time at end of user session
(cid:122) Goal: scalable, extensible architecture
18
Case Study – New Architecture
Event-driven
LLoogg
UI Client Calculators
A
B FFrroonnttEEnndd PPeerrssiisstteennccee DDBB
C
(cid:122) Decompose logic into individual “calculators”
(cid:122) Calculators precompute results as response events
arrive
(cid:122) Channels connect calculators
(cid:122) Calculators do not updates database
(cid:122) Persist results into database once all scores computed
(cid:122) Pure Java (1.4) implementation
19
10