Persistence that tells the truthRyan Platteryan@burnbush.net

Your model lies

Your model lies

“Customer 123 ordered the inexpensive product”

The truth is what actually happens

The truth is what actually happens

“Customer 123 ordered the expensive product and the cheaper one, then removed the expensive one after checking shipping”

So why not persist the whole truth?

So why not persist the whole truth?

Added Product

Customer 123

Product 1

Computed Shipping

Added Product

Customer 123

Product 2

Removed Product

Customer 123

Product 2

Past history is immutable

Ask your accountant whether it’s OK to change historical records

Append-only writes scalehttps://flic.kr/p/obgzhM

This is how many databases work internally anyway

Working from the whole truth supports future features

How to create event-sourced software

Command-event separation

Imperative vs. past tense

Command is an attempt or request

Event is a change that definitely registered

Persisting events in a database

Minimal implementation

Events

Aggregate_id (GUID)Data (blob or JSON)Version (integer)

Aggregates, or Event ProviderAggregate_id (GUID)Type (string)Version (integer)

Persisting events in a database

Events

Aggregate_id (GUID)Data (blob or JSON)Version (integer)TimestampIP addressPermission level

Aggregates, or Event ProviderAggregate_id (GUID)Type (string)Version (integer)

Read model: go nuts

Order openedItem addedItem added

Item removedOrder placed

Order approvedOrder shipped

Order delivered

Objects in memory

Mutable DB model

Graph DB

Work queues