Event Sourcing • Greg Young • GOTO 2014

This presentation was recorded at GOTO Aarhus 2014. #gotocon #gotoaar

Greg Young - Coined the term CQRS (Command Query Responsibility Segregation)

ABSTRACT
Event Sourcing has been becoming more and more popular over the last few years. Greg has been talking, teaching, and innovating in the Event Sourcing space since 2006. This talk will look in depth at what Event Sourcing is, when it may be valuable (more importantly when its not), and how problems can change when looked at through a different lens. There are many trade offs, misconceptions, and of course FUD associated around Event Sourcing, hopefully we can sort them all out.

#EventSourcing #cqrs #GregYoung

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[-andymel]

00:00 Introduction (it is not a new idea)
01:00 What brought them towards Event Sourcing
01:25 Mature businesses like finance, banking, gambling, insurance are all naturally event sourced

03:00 *Definition* of event sourcing - all state is transient, only store facts
06:46 We *never update or delete facts* in event sourced systems
11:52 Event Sourcing is the only model that you can possibly use that does not loose information

14:05 *Projections* - Deriving state off of an event stream
15:04 Projections must start at event 0
16:16 Projections can be done for every point in time since the start of the system
17:09 "You can not change what happened in the past, but you can have a new perception of what happened in the past"

17:39 *Advantages of event sourcing from a developers point of view*
17:52 It's sequential writing -> fast (~50.000-100.000 requests per sec on a fairly naive system on one node)
18:39 Go back in the past
19:19 Easy smoke tests (for me he even describes full regression tests not just smoke tests)
20:16 Prevent super user attacks

24:45 *Snapshots*
26:42 Store snapshots off the side and point back to the version of the event log
27:11 You can also have multiple different folds with different perception of the same version of the event log in parallel
27:26 Avoid snapshots if you can (state is hard to version)

29:53 How do I query a series of events? You don't. You use *read models* .
36:22 Consider a graph database for read models
37:36 You can have as many read models as you want - they just have to subscribe to your event system

*Wrong statements you hear about event sourcing:*

38:01 *Event sourcing needs a service bus*

39:12 Problem: if you want to add a new read model you need to replay the former events. With a service bus you would need an additional control channel to ask for events.
Use a consumer driven subscription system like Kafka

41:55 *Event sourcing is more complex (than CRUD systems)*
It seems more complex to people that are used to the CRUD way just because it's another way of dealing with the domain and have to learn it.

42:55 *What big companies are using Event Sourcing?*
Don't decide what to use because of other companies.

43:29 *Event sourced systems must be slow*
They can be really fast and it's used for latency sensitive systems eg for trading

44:25 *Event Sourced systems must be object oriented*
No, it is an inherently functional system. You have an immutable series of events and "left fold" your state out of this series

45:12 *What is the "bestest" Event Sourcing framework ever?*
Probably none.

45:51 *But what about all my data? With Event Sourcing it must be huge!*
Don't think about the amount of data you are going to have but at which current and which future rate you will be retaining data and compare that to moore's law.
If you are slower than moore's law ( < ~5.000 req/sec) your data will get cheaper over time.

47:40 *CQRS is just a teaching pattern*
"CQRS is the dumbest pattern ever written". It was never meant to be a pattern. The main idea is to make people see the benefits of separating the reading and the writing part of the process.

49:34 *Event Sourcing is not 'enterprisy'*
True, thanks.

*Questions*

50:33 *How to know what details to put in an event?*
That's an use case analysis problem

51:59 *How to avoid the big bang release when switching to event sourcing?*
Two ways of dealing with migrating data to an event sourced system
- Either migrate all data and transactions of all accounts to the new system
- or just bring in the initial balance
Decide which one to use aggregate by aggregate. Either you can reverse engineer the history or save a snapshot as init-event.
This allows to run the systems side by side. The old system can already raise the events.

[deyan.todorov]

As people saying, old talk but precious as a gold!

[steveroger]

I've actually once come out this idea my own, where instead of C R U d (with soft delete), I can just use C R in database such that all data is logged as transaction, essentially like bank account balance and no 'act' is ever deleted.
But my uni lecturer discarded my idea and said my implementation is the 'Wrong' way to do it, plus homework required implementing C R U D. Since then, I always thought I was wrong and not doing correctly.
my life is filled with lies.

[brianevans]

This guy seems a bit arrogant, but you've gotta give it to him that he has a good sense of humor.

"If I can fit your entire database on a micro SD, you're not big data" 😂😂😭😭

That is so funny. Every company thinks their database is massive, until you put it in perspective like that

[RodrigoBranas]

Is Martin Fowler in the front row, near Greg?

[itihas]

All his talks has the same information. It's a great talk but I am looking for a talk with inclination towards implementation details.

[jfaquinojr]

21:03 criminal mastermind i stopped here and tried to search to no avail. Chris Horne or Harn?

[dr.bnagarajan]

the entire speech is full of information ... which is useful for research scholars

[FenelHyacinthe]

Event sourcing have the same principal as blockchain. Once a transaction in you cannot delete or update it, you have to do a reversal if mistake happen.

[CosasCotidianas]

35:15 I think we all had to deal with that. Weird.

[flippy]

Within 2 years we already have 4TB SSD's ^^

[dgaston]

8 years later a 2 TB SSD is $100. Double the size for 1/5th of the price.

[willl]

The argument that storing information is some how not a big cost it is false in my experience. As a cloud based Saas, cloud storage for thousands of clients it is expensive

[tarikkhan]

Looks like 2GD from dota 2 went in to system designing after being fired by Gaben

[crazyheinz]

what's up with his breathing tho. Doesn't sound too healthy.

[sidekickrida]

His talks would take half the time if he didn't ask the audience silly questions… "how many of you have talked to a lawyer before?", "how many of you have been to the doctor before?", "how many of you have heard of XYZ?" REALLY ANNOYING

[tknayak]

Many times he is very boring to listen to