A pictorial introduction to the concepts of Event Sourcing and CQRS, as a persistence model in modern, distributed and scalable applications.

1. A visual introduction to
Event Sourcing and CQRS
1
@nicusX
https://opencredo.com/author/lorenzo/
Lorenzo Nicora
Senior Consultant @ OpenCredo

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A couple of concepts from DDD
Aggregate
(Current) State
of the Aggregate
Lorenzo Nicora Intro to Event Sourcing and CQRS

3. 3
Once upon a time…
Everything
is synchronous
Request - Response
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Scaling up…
Updates —> Locks —> Contention!
<— Block <—
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Let’s go Asynchronous
Pwd —> “secret”
Pwd —> “12345”
Pwd —> “54321”
Pwd —> “secret”
Pwd —> “54321”
Pwd —> “12345”
===>
Out of
Order
Asynchronous, Message-driven
Request/Response
ACID Transaction
Distributed, Message-based
—> No order guaranteed
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Command Sourcing
💡
• Append Only —> No Contention
• Build State from Command history
Write fast,
Think later
K/V Store
Distributed
Lorenzo Nicora Intro to Event Sourcing and CQRS

7. Command
“Submit Order!”
—> A request (imperative sentence)
—> May fail
—> May affect multiple Aggregates
7
Commands vs Events
✉
Rebuild Aggregate State
from Commands
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Event
“Order submitted”
—> Statement of facts (past tense)
—> Never fails
—> May affect a single Aggregate
✉
Rebuild Aggregate State
from Events
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Commands to Events
(DDD patterns: Aggregate / Process Manager)
X
Y
Z
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Command > Event Sourcing
💡
Think a little,
Write,
Think later
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Event Sourcing
In many domains
Commands Events
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Additional Benefits
Easy
Eventual Business Consistency
—> Corrective Events
Robust to data corruption
(bugs, fat fingers…)
—> Rebuild state ignoring wrong events
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Additional Benefits
History
(for free)
Rebuild State
at a point in Time
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Main Benefit
Scalable
—> Append only
-> Fits distributed k/v stores
—> Low-latency writes
—> Allows asynchronous processing
Lorenzo Nicora Intro to Event Sourcing and CQRS

15. What about reads?
15
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Retrieving the State
How do I retrieve the State?
“Get details of Order ‘AB123’”
❔
not very efficient, but…
…may work
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Querying (Searching) the State
❓
❓ How do query the State?
“Get all Orders delivered to ‘SE1 0NZ’”
❓
❓
Lorenzo Nicora Intro to Event Sourcing and CQRS

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CQRS
Command
Query
Responsibility
Segregation
💡 Separate
• Code
• muService
• Datastore
-> Update
-—> Retrieve
“Query”
datastore is
downstream
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Not a new idea
Specialised
Downstream
Lorenzo Nicora Intro to Event Sourcing and CQRS

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CQRS and Event Sourcing
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Materialised Views
(of current State)
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Materialised Views (of State)
Latest (known) State
(Persistent)
Rebuildable
from Events
In Memory
K/V Store
Graph
…
RDBMS
Delayed
💡
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Materialised View of State
Query a RDBMS?!?
Wasn’t it the old way?
❓
RDBMS is just one of our options:
easy to use, easily become a bottleneck
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Materialised Views of State
* Views are optimised for
specific query use cases
—> multiple Views from same Events
* Updated asynchronously, delayed
—> to Scale
-> may reorder Events
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Materialised Views of State
* Views can be rebuilt from Events
Event Log
is our Source of Truth
* Easy to evolve or fix
—> change or fix logic;
rebuild view from events
(not the View)
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Indexes
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Indexes
Search Engines
K/V Stores
• Optimised for querying (less for retrieving)
• Latest State; rebuild on the fly
💡
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Hybrid solutions
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Hybrid solutions: e.g. Snapshots
• Speed up rebuilding the current State
• Use recent Events to rebuild up-to-date
💡 Long delayed
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Eventual (Business) Consistency
Guess —> Compensate—> Apologies
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Eventual Consistency: Sagas
StatefulOut of band
Corrective Command
or Event
Saga
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Lesson
from the Trenches
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Lesson Learned #1
If you put data in…
…you will eventually
have to get them out!
The “Query” side
is not secondary
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Lessons Learned #2
In old days:
normalising one DB
to support as many queries as possible
With CQRS:
multiple denormalised “data stores”
optimised for different queries
No single “Q” implementation
for all your queries
Lorenzo Nicora Intro to Event Sourcing and CQRS

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Lessons Learned #3
A central, shared Event Store
may not be the best option
No Event-sourced Monolith
Prefer persistence
per Bounded-Context
Lorenzo Nicora Intro to Event Sourcing and CQRS

36. +++ Summing up +++
36
Lorenzo Nicora Intro to Event Sourcing and CQRS

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ES/CQRS Optimal Use Cases
High Volume
Low Latency writes
(big data)
Event Sourcing + CQRS
😋
Out-of-order Commands/Events
(IoT)
Lorenzo Nicora Intro to Event Sourcing and CQRS

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ES/CQRS Drawbacks
x No “One-Size-Fits-All”
—> Multiple “Q” implementations
x Delayed reads
x No ACID Transactions
x Additional complexity (!)
🙁
Lorenzo Nicora Intro to Event Sourcing and CQRS

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ES/CQRS Benefits
+ No “One-Size-Fits-All”
—> “Q” are optimised for use cases
+ Eventual (Business) Consistency
+ History, Temporal queries
+ Robust to data corruption
😀
Lorenzo Nicora Intro to Event Sourcing and CQRS

40. That’s all, Folks!
40
Lorenzo Nicora Intro to Event Sourcing and CQRS

41. ??? Questions ???
41
Thanks.
⏳
Lorenzo Nicora Intro to Event Sourcing and CQRS