Dealing with eventual consistency

Dennis van der Stelt
Dealing with
distributed systems addict
https://bloggingabout.net/
https://particular.net/
Eventual Consistency
@dvdstelt

https://www.pathe.nl/film/22678/game-of-thrones-finale

let’s check history
how did we get to
those issues?

SELLING TICKETS
And when we are too successful for our own good
Hi, how can I help you?
I’d like to order tickets to the
finale of Game of Thrones.
Sure, no problem. How
many would you like?
I have no friends, so I’ll be
alone. Just one ticket, please.

WHEN BUSINESS IS BOOMING
We need to scale out
Game of Thrones finale,
3 tickets. You’ve got them.
Game of Thrones finale,
4 tickets. You’ve got them.

and then…
the internet happened

DIGITIZING COMMERCE
The birth of eCommerce https://movies.com/

DEALING WITH STRONG CONSISTENCY
begin transaction
var quantity = select Quantity from Movies
where Id = @MovieId
if (quantity >= amountOfTicketsCustomerWants)
update Movies
set Quantity = quantity – amountOfTicketsCustomerWants
where Id = @MovieId
commit transaction
Id Name TheaterId Price Quantity
42 Star Wars Rise of Skywalker 1 $ 15.00 400
1337 Game of Thrones 2 $ 0.00 100
Movies table
How every solutionstarts as a very, very small problem

DEALING WITH STRONG CONSISTENCY
Id Name TheaterId Price Quantity
42 Star Wars Rise of Skywalker 1 $ 15.00 400
1337 Game of Thrones 2 $ 0.00 100
Movies table
How every solutionstarts as a very, very small problem
SINGLE ITEM CONGESTION

DIGITIZING COMMERCE
The birth of eCommerce

table index table index table index
Relational Database
ALTERNATIVE SOLUTIONS

Relational Database
Document Database

Relational Database
Document Database
Datacenters
North America Europe
data travels only at the speed of light

Datacenters

Datacenters
Messaging
Searching for data that might not be there yet.

asynchronous
communication

DEALING WITH EVENTUAL CONSISTENCY
What is the problem?
Get a subscription
Enter your details
Full Name:
https://movies.com/subscription/edit/
Address:
Van Zandvlietplein 1
City:
Rotterdam
Country:
The Netherlands
Get subscription
Home Movies Theaters
Subscription
Your subscription details
Full Name
https://movies.com/subscription/details/
Address
City
Country
Edit subscription
Subscription type:
Premium
Payment type:
Monthly
Subscription type
Payment type
: Dennis van der Stelt
: Van Zandvlietplein 1
: Rotterdam
: The Netherlands
Premium
Monthly payment

How to deal with it?
Do we really need strong consistency?

READING
WRITING
what you are

When you’re not reading your own writes…
Add a new movie
Name:
https://movies.com/admin/movies/add
Free Guy
Starring:
Ryan Reynolds, Jody Comer
Rating:
Rated PG
Classification:
Add movie
Now running
https://movies.com/
Price:
$ 10
Hall
Main hall
Action
Comedy
Horror
x
x

When you’re not reading your own writes…
Register for an account
Name:
https://movies.com/users/registration
Email:
dennis@bloggingabout.net
Password:
Register
https://movies.com/users/thanks
Password again:
Check the email we’ve sent you to verify your email address.

When you are reading your own writes
Get a subscription
Enter your details
Full Name:
Address:
City:
Rotterdam
Country:
The Netherlands
Get subscription
Subscription
Your subscription details
Full Name
Address
City
Country
Edit subscription
Subscription type:
Premium
Payment type:
Monthly
Subscription type
Payment type
: Dennis van der Stelt
: Van Zandvlietplein 1
: Rotterdam
: The Netherlands
Premium
Monthly payment

Reading your own writes – the simplest solution
Get a subscription
Enter your details
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Address:
City:
Rotterdam
Country:
The Netherlands
Get subscription
Subscription type:
Premium
Payment type:
Monthly
https://movies.com/subscription/thanks
Thanks for submitting your details
take me back to movies
We are reviewing your changes. This can take a few minutes.

Get a subscription
Enter your details
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Address:
City:
Country:
Get subscription
Subscription type:
<make a selection>
Payment type:
<make a selection>
Reading your own writes – anothersimple solution
Get a subscription
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Address:
City:
Rotterdam
Country:
The Netherlands
Get subscription
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Premium
Payment type:
Monthly
Thanks for submitting your details

Reading your own writes – providing the best user experience, but for who?
<div class="visible">
<textarea name="comment" />
<button type="submit">
Add comment
</button>
</div>
<div class="invisible">
<span class="comment-copy"></div>
<a href="/users/1337">Name</a>
</div>

var comments = repository.GetComments();
foreach (var comment in comments)
{
// display comments
}
var comments = localStorage.GetItem("com
foreach (var comment in comments)
{
if (!commentWasDisplayedAlready)
// display localStorage comment
}

Demo
Using LocalStorage to mimic fast response times.

$('#submitReviewButton').click(function(e) {
var comment = $('#comment').val();
var newReview = {
Identifier: guid,
ReviewedAt: new Date(),
Description: comment
};
localStorage.setItem('popcornStoredReviews', JSON.stringify(newReview));
$('#comment').val('');
$('#reviews').append('<li>').append(
$('<div>').attr('class', 'comment').text(newReview)
);
});

browser
SessionStorage LocalStorage IndexedDB
(web)server
Cache Redis Database
BENEFITS
on
server

(web)server
browser
SessionStorage LocalStorage IndexedDB
database
Cache Redis Database

Another simple solution – waiting for something happening in the background

ASYNCHRONOUS EXECUTION
Keeps a callback thread open on the client and frequently polls the server
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ASYNCHRONOUS COMMUNICATION
Using asynchronous messaging and SignalR
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SignalR
SubmitOrder
OrderSubmission
You can already order movie tickets for free.*
After we’ve received the first payment, your snacks in our
theater will also be free. The payment will be fully automatic.
All you need to do is sit back and enjoy your favorite movies!

Demo
Ordering movie tickets using NServiceBus & SignalR

$('#submit_button').click(function (e) {
var connection = new
signalR.HubConnectionBuilder().withUrl('/ticketHub').build();
connection.on('OrderSubmission', function (message) {
$('#ackonwledgement').html(message);
});
var order = $('#ticketOrderingForm').serialize();
connection.invoke("SubmitOrder", order);
});
ASYNCHRONOUSLY SUBMITTING A FORM
Set up a SignalR connection and submit the form to the backend

public class TicketHub : Hub
{
IMessageSession messageSession;
public TicketHub(IMessageSession messageSession) {
this.messageSession = messageSession;
}
public async Task SubmitOrder(var order)
{
await messageSession.Send(new SubmitOrder(order));
await this.ReplyToSenderIfLottery(order);
}
}
Accept SignalR message and use NServiceBus to send message to queue

The message is now in the queue
Website / SignalR Hub Server Process

public class SubmitOrderHandler : IHandleMessages<SubmitOrder>
{
public async Task Handle(SubmitOrder message, IMessageHandlerContext context)
{
var result = await RegisterOrder(message);
return await context.Reply(new OrderSubmission(result));
}
}
NServiceBus handler processing message and replying with answer.

public class OrderSubmissionHandler : IHandleMessages<OrderSubmission>
{
IHubContext<TicketHub> ticketHubContext;
public OrderSubmissionHandler(IHubContext<TicketHub> ticketHubContext) {
this.ticketHubContext = ticketHubContext;
}
public Task Handle(OrderSubmission message, IMessageHandlerContext context)
{
return ticketHubContext.Clients.User(user).SendAsync("OrderSubmission",
ticket);
}
}
NServiceBus handler in website using SignalR to reply to browser

What about the business?
Dealing with eventual consistency in the business

Id Name Price Quantity
42 i can has cheezburger? $ 15.00 10
1337 The Winds of Winter $ 25.00 1000
HOW THE BUSINESS DEALS WITH EVENTUAL CONSISTENCY
Inventory table
Eventual consistency is baked into the business
ProductId Delta Timestamp
1337 -5 09:03:22 17-1-2016
1337 -3 09:03:24 17-1-2016
1337 -4 09:03:25 17-1-2016
1337 -1 09:03:27 17-1-2016
1337 +250 09:03:28 17-1-2016
1337 -4 09:03:30 17-1-2016
Inventory table
EventSourcing done simple
No locking, append only
Inventory is eventually consistent
No transactional lock to discover the current inventory left

STEAM
CONTROLLER

Fake consistency in the user interface
Patterns of Enterprise Application Architecture
42 customer reviews
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Kindle Hardcover
This item ships to Rotterdam, The Netherlands
Others
with more on the way
https://www.amazon.com/peaa-martin/
how important is it
that all numbers
are 100% accurate?

When you’re faking it, please do it well…

A hybrid solution, optimized for customer satisfaction and consistency

Putting good user experience before occasional money loss
Tom Cruise, 57
DETAILS
https://happypancake.com/profile/tom_cruise/
Location
BeverlyHills,California,USA
Searching for
Women,45-55
Favorite activity
Base-jumpingfrombuildings
Tom Cruise, 37
DETAILS
https://happypancake.com/profile/tom_cruise/
Location
BeverlyHills,California,USA
Searching for
Women,25-35
Favorite activity
Base-jumpingfrombuildings
strong consistency VS user experience

WHAT PROBLEM ARE WE SOLVING?
Strive for high customer satisfaction

dennis@bloggingabout.net
https://bloggingabout.net
@dvdstelt
calendly.com/dvdstelt
https://cutt.ly/io10
Solution Architect specialized in distributed business software.

Dealing with eventual consistency

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