Dealing with eventual consistency

Dennis van der Stelt
Dealing with
distributed systems addict
http://dennis.bloggingabout.net/
http://compilesoftware.nl/
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?
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Address:
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City:
Rotterdam
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The Netherlands
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Home Movies Theaters
Subscription
Your subscription details
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Address
City
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: 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
Zombieland: Double tap
Description:
they face off against evolved zombies,
Rating:
Rated R
Classification:
Add movie
Now running
https://movies.com/
Price:
$ 10
Hall
Main hall
Action
Comedy
Horror
x
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
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Address:
City:
Rotterdam
Country:
The Netherlands
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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
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Address:
City:
Rotterdam
Country:
The Netherlands
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Premium
Payment type:
Monthly
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Thanks for submitting your details
take me back to movies
We are reviewing your changes. This can take a few minutes.

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City:
Country:
Get subscription
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<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(comment)
);
});

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

(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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City:
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ASYNCHRONOUS COMMUNICATION
Using asynchronous messaging and SignalR
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Address:
City:
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VIP
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Thanks for applying for a VIP subscription.
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!
SignalR
SubmitOrder
OrderSubmission

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 Task SubmitOrder(var order)
{
return messageSession.Send(new SubmitOrder(order));
}
}
Accept SignalR message and use NServiceBus to send message to queue

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",
screenMessage);
}
}
NServiceBus handler in website using SignalR to reply to browser

LET’S DISCUSS THE DESIGN
So is this the way to approach this, Dennis?

Reading from multiple datastores
Add a new movie
Name:
https://movies.com/admin/movies/add
Zombieland: Double tap
Description:
they face off against evolved zombies,
Rating:
Rated R
Classification:
Add movie
Now running
https://movies.com/
Price:
$ 10
Hall
Main hall
Action
Comedy
Horror
x
x
x
Movie prices
Movie description
remember this slide?

MULTI-PAGE CHECKOUT PROCESS
How to trickthe user into thinking everything is immediately processed
Shopping Cart
https://www.amazon.com/peaa-martin/
Patterns of Enterprise
Application Architecture
by Martin Fowler
Proceed to checkout
Subtotal: $42.00
QuantityItem Price
$42.00 1
This list contains a gift.
or
Sign in for 1 click ordering.
1
start
checkout

Where should we deliver your order?
Enter a shipping address
Full Name:
https://www.amazon.com/buy/shippingaddress/
AddressLine 1:
City:
Rotterdam
Country:
The Netherlands
Ship to this Address
2
shipping
address

Select a payment option
Your credit and debit cards
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Amazon accepts all major credit and debit cards
> Add a card
Continue
MasterCard ending in 2464 Dennis van der Stelt 09/2026
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4
payment
method

Review your order
https://www.amazon.com/buy/payment/
Shipping address
Rotterdam, The Netherlands
Payment method
ending in 2464
Billing Address
Same as shipping address
Estimated delivery: Aug 17, 2018 – Aug 21, 2018
Patterns of Enterprise
Application Architecture
by Martin Fowler
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change
Place your order
Order Summary
Items:
Shipping:
Total before tax:
Estimated tax:
Order Total
$42.00
$7.98
$49.98
$0.00
$49.98
5
summary

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
Add to cart
$42.00
save $13.37
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
dennis.bloggingabout.net
@dvdstelt
linkedin.com/in/dvdstelt/
http://bit.ly/eventualconsistency
Solution Architect at Particular Software, makers of NServiceBus

Dealing with eventual consistency

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Dealing with eventual consistency