1.
Got the Time?
All we want, two, three, go!
Time, got the time tick tick tickin' in my head!
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2.
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3.
The overdue
invoice
dilemma
At the 10th of every month…
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4.
(manual) batch jobs
a first attempt
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5.
business
evolution
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6.
days spent
checking
invoices
With all sort of issues:
Is the bank statements up to date?
Is the payment from a different bank?
Are there invoices due today?
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7.
What are we looking for?
• An overdue invoices report?
• Or an alert when an invoice is overdue
• and a collateral report?
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8.
We want alerts!
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9.
What’s an invoice?
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10.
What’s an invoice?
• An aggregate: state and behaviors
• Behaviors can be modelled using state machines
• Draft created
• Issued
• Paid
• Overdue
• Credit note issued
(there might be more for partial payments and so on)
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11.
Behavior modelling
•Trigger (in)
•State manipulation
•Events (out)
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12.
Trigger => State Manipulation => Event
Issue
Invoice
Invoice
Issued
When due
Check
payment
?
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13.
messages and sagas you want
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14.
how does it work?
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15.
BUS
Persister
queue
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16.
BUS
SAGA
Persister
create saga
dispatch message
manipulate state
queue
send messages
persist state
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DB

17.
BUS
SAGA
Persister
dispatch message
manipulate state
queue
retrieve state
return saga
if saga completed
delete state
get delete
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DB

18.
show me
the code

19.
Code for your future self
async Task Handle(InvoiceIssued msg, IMessageHandlerContext ctx)
{
Data.InvoiceNumber = msg.InvoiceNumber;
var dueDate = msg.DueDate;
if(msg.CustomerCountry == "Italy")
{
dueDate = dueDate.AddDays(20);
}
await RequestTimeout<CheckPayment>(ctx, dueDate);
}
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20.
Code for your future self
async Task Handle(InvoiceIssued msg, IMessageHandlerContext ctx)
{
Data.InvoiceNumber = msg.InvoiceNumber;
var dueDate = msg.DueDate;
if(msg.CustomerCountry == "Italy")
{
dueDate = dueDate.AddDays(20);
}
await RequestTimeout<CheckPayment>(ctx, dueDate);
}
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21.
Code for your future self
async Task Handle(InvoiceIssued msg, IMessageHandlerContext ctx)
{
Data.InvoiceNumber = msg.InvoiceNumber;
var dueDate = msg.DueDate;
if(msg.CustomerCountry == "Italy")
{
dueDate = dueDate.AddDays(20);
}
await RequestTimeout<CheckPayment>(ctx, dueDate);
}
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22.
Code for your future self
async Task Handle(InvoiceIssued msg, IMessageHandlerContext ctx)
{
Data.InvoiceNumber = msg.InvoiceNumber;
var dueDate = msg.DueDate;
if(msg.CustomerCountry == "Italy")
{
dueDate = dueDate.AddDays(20);
}
await RequestTimeout<CheckPayment>(ctx, dueDate);
}
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Machine learning ;-)

23.
Code for your future self
async Task Handle(InvoiceIssued msg, IMessageHandlerContext ctx)
{
Data.InvoiceNumber = msg.InvoiceNumber;
var dueDate = msg.DueDate;
if(msg.CustomerCountry == "Italy")
{
dueDate = dueDate.AddDays(20);
}
await RequestTimeout<CheckPayment>(ctx, dueDate);
}
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24.
Code for your future self
async Task Handle(InvoiceIssued msg, IMessageHandlerContext ctx)
{
Data.InvoiceNumber = msg.InvoiceNumber;
var dueDate = msg.DueDate;
if(msg.CustomerCountry == "Italy")
{
dueDate = dueDate.AddDays(20);
}
await RequestTimeout<CheckPayment>(ctx, dueDate);
}
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25.
Code for your future self
async Task Timeout(CheckPayment msg, IMessageHandlerContext ctx)
{
var invoiceNumber = Data.InvoiceNumber;
await ctx.Publish(new InvoiceOverdue()
{
InvoiceNumber = invoiceNumber;
});
MarkAsComplete();
}
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26.
Code for your future self
async Task Timeout(CheckPayment msg, IMessageHandlerContext ctx)
{
var invoiceNumber = Data.InvoiceNumber;
await ctx.Publish(new InvoiceOverdue()
{
InvoiceNumber = invoiceNumber;
});
MarkAsComplete();
}
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27.
Code for your future self
async Task Timeout(CheckPayment msg, IMessageHandlerContext ctx)
{
var invoiceNumber = Data.InvoiceNumber;
await ctx.Publish(new InvoiceOverdue()
{
InvoiceNumber = invoiceNumber;
});
MarkAsComplete();
}
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28.
Code for your future self
async Task Timeout(CheckPayment msg, IMessageHandlerContext ctx)
{
var invoiceNumber = Data.InvoiceNumber;
await ctx.Publish(new InvoiceOverdue()
{
InvoiceNumber = invoiceNumber;
});
MarkAsComplete();
}
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29.
Code for your future self
async Task Timeout(CheckPayment msg, IMessageHandlerContext ctx)
{
var invoiceNumber = Data.InvoiceNumber;
await ctx.Publish(new InvoiceOverdue()
{
InvoiceNumber = invoiceNumber;
});
MarkAsComplete();
}
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30.
Hold on!
async Task Timeout(CheckPayment msg, IMessageHandlerContext ctx)
{
var invoiceNumber = Data.InvoiceNumber;
await ctx.Publish(new InvoiceOverdue()
{
InvoiceNumber = invoiceNumber;
});
MarkAsComplete();
}
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31.
That’s fine!
Task Handle(InvoicePaid msg, IMessageHandlerContext ctx)
{
MarkAsComplete();
await Task.CompletedTask;
}
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32.
That’s fine!
Task Handle(InvoicePaid msg, IMessageHandlerContext ctx)
{
MarkAsComplete();
await Task.CompletedTask;
}
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33.
Trigger => State Manipulation => Event
Issue
Invoice
Invoice
Issued
When due
Check
payment
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34.
Trigger => State Manipulation => Event
Issue
Invoice
Invoice
Issued
Check
payment Delayed Delivery
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35.
The premium customer
•If monthly running total is > $300
•Customer get a 10% discount
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36.
A naïve approach
int CalculateDiscount(int customerId)
{
var lastMonth = DateTimeNow.AddMonths(-1);
var lastMonthTotal = (from order in db.Orders
where order.CustomerId = customerId
and order.Timestamp >= lastMonth
select order.Amount).Sum();
return lastMonthTotal > 300 ? 10 : 0;
}
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37.
business
evolution
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38.
Concurrency
1. Order is placed for customer 123
• Total amount: $100
2. Discount is calculated
• Running total is $250, less than 300 => no discount
Concurrently
1. Order is placed for customer 123
• Total amount: $60
2. Discount is calculated
• Running total is still $250, less than 300 => no discount
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39.
The problem with a naïve approach
int CalculateDiscount(int customerId)
{
var lastMonth = DateTimeNow.AddMonths(-1);
var lastMonthTotal = (from order in db.Orders
where order.CustomerId = customerId
and order.Timestamp >= lastMonth
select order.Amount).Sum();
return lastMonthTotal > 300 ? 10 : 0;
}
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40.
A naïve approach to concurrency
using var tx = conn.BeginTransaction(IsolationLevel.Serializable);
using var db = new OrdersContex(conn, contextOwnsConnection: false);
db.Database.UseTransaction(tx);
var lastMonthTotal = (from order in db.Orders
where order.CustomerId = customerId
and order.Timestamp >= lastMonth
select order.Amount).Sum();
tx.Commit();
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41.
pessimistic locking? Thanks, no thanks.
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42.
batch jobs
shall we try again?
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43.
Batch jobs
• The batch job is run on a schedule
• Runs over all customers
• Sums orders total amount for the last 30 days
• If the policy is matched creates a discount coupon
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44.
Batch jobs
• The batch job is run on a schedule
• Runs over all customers
• Sums orders total amount for the last 30 days
• If the policy is matched creates a discount coupon
However…
• Good luck running the batch job during Black Friday
• Discount coupons are generated only when/if the batch job runs
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45.
timeline
death to the batch job
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46.
Behavior modelling
•Trigger (in)
•State manipulation
•Events (out)
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47.
Trigger => State Manipulation => Event
Place
Order
Order
Placed
Calculate
Discount
Deduct from
Running Total
Add to
Running Total
Process Order
30-day Delayed Delivery
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48.
Place
Order
Order
Placed
Calculate
Discount
Deduct from
Running Total
Add to
Running Total
Process Order
Arrow of time
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t0 t1 t1 + 30 days

49.
show me
the code

50.
Code for your future self
async Task Handle(OrderPlaced msg, IMessageHandlerContext ctx)
{
var discount = Data.MonthlyRunningTotal > 300 ? 10 : 0;
Data.MonthlyRunningTotal += msg.OrderTotalAmount;
await ctx.Send(new ProcessOrder()
{
OrderId = msd.OrderId,
Discount = discount
});
var delay = DateTime.Now.AddMonths(1);
await RequestTimeout(ctx, delay, new DeductFromRunningTotal()
{
OrderTotalAmount = msg.OrderTotalAmount
});
}
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51.
Code for your future self
async Task Handle(OrderPlaced msg, IMessageHandlerContext ctx)
{
var discount = Data.MonthlyRunningTotal > 300 ? 10 : 0;
Data.MonthlyRunningTotal += msg.OrderTotalAmount;
await ctx.Send(new ProcessOrder()
{
OrderId = msd.OrderId,
Discount = discount
});
var delay = DateTime.Now.AddMonths(1);
await RequestTimeout(ctx, delay, new DeductFromRunningTotal()
{
OrderTotalAmount = msg.OrderTotalAmount
});
}
mauroservienti

52.
Code for your future self
async Task Handle(OrderPlaced msg, IMessageHandlerContext ctx)
{
var discount = Data.MonthlyRunningTotal > 300 ? 10 : 0;
Data.MonthlyRunningTotal += msg.OrderTotalAmount;
await ctx.Send(new ProcessOrder()
{
OrderId = msd.OrderId,
Discount = discount
});
var delay = DateTime.Now.AddMonths(1);
await RequestTimeout(ctx, delay, new DeductFromRunningTotal()
{
OrderTotalAmount = msg.OrderTotalAmount
});
}
mauroservienti

53.
Code for your future self
async Task Handle(OrderPlaced msg, IMessageHandlerContext ctx)
{
var discount = Data.MonthlyRunningTotal > 300 ? 10 : 0;
Data.MonthlyRunningTotal += msg.OrderTotalAmount;
await ctx.Send(new ProcessOrder()
{
OrderId = msd.OrderId,
Discount = discount
});
var delay = DateTime.Now.AddMonths(1);
await RequestTimeout(ctx, delay, new DeductFromRunningTotal()
{
OrderTotalAmount = msg.OrderTotalAmount
});
}
mauroservienti
Request to process order

54.
Code for your future self
async Task Handle(OrderPlaced msg, IMessageHandlerContext ctx)
{
var discount = Data.MonthlyRunningTotal > 300 ? 10 : 0;
Data.MonthlyRunningTotal += msg.OrderTotalAmount;
await ctx.Send(new ProcessOrder()
{
OrderId = msd.OrderId,
Discount = discount
});
var delay = DateTime.Now.AddMonths(1);
await RequestTimeout(ctx, delay, new DeductFromRunningTotal()
{
OrderTotalAmount = msg.OrderTotalAmount
});
}
mauroservienti

55.
Code for your future self
async Task Handle(OrderPlaced msg, IMessageHandlerContext ctx)
{
var discount = Data.MonthlyRunningTotal > 300 ? 10 : 0;
Data.MonthlyRunningTotal += msg.OrderTotalAmount;
await ctx.Send(new ProcessOrder()
{
OrderId = msd.OrderId,
Discount = discount
});
var delay = DateTime.Now.AddMonths(1);
await RequestTimeout(ctx, delay, new DeductFromRunningTotal()
{
OrderTotalAmount = msg.OrderTotalAmount
});
}
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56.
Code for your future self
Task Timeout(DeductFromRunningTotal msg, IMessageHandlerContext ctx)
{
Data.MonthlyRunningTotal -= msg.OrderTotalAmount;
return Task.CompletedTask;
}
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57.
Code for your future self
Task Timeout(DeductFromRunningTotal msg, IMessageHandlerContext ctx)
{
Data.MonthlyRunningTotal -= msg.OrderTotalAmount;
return Task.CompletedTask;
}
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58.
What about
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59.
DB
queue
BUS
SAGA
Persister
create saga
dispatch message
send messages
persist saga
BUS
SAGA
Persister
create saga
dispatch message
send messages
persist saga
persist saga
BusinessId = 1337
persist saga
BusinessId = 1337
Concurrency Control
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60.
Take away
• Avoid the technical solution trap
• Understand the real business needs
• Model time by sending messages to your future self
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61.
Mauro Servienti
Solution Architect @ Particular Software
the makers of NServiceBus
mauro.servienti@particular.net
@mauroservienti
//github.com/mauroservienti
//milestone.topics.it
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62.
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Demos: bit.ly/particular-time-demos
Slides: bit.ly/particular-time-slides
Tutorial: go.particular.net/particular-time