The Importance of Event Sourcing in Serverless Development

Event sourcing is a way of capturing and persisting the changes happening in a system as a sequence of events.

Figure 3-3 showed a customer account service that emits account created, account updated, and account deleted events. Traditionally, when you store and update data in a table, it records the latest state of each entity. Table 3-1 shows what this might look like for the customer account service. There’s one record (row) per customer, storing the latest information for that customer.

Table 3-1. Sample rows from the Customer Account table Customer ID	First name	Last name	Address	DOB	Status
100-255-8730	Joe	Bloke	99, Edge Lane, London	1966/04/12	ACTIVE
100-735-6729	Biz	Raj	12A, Top Street, Mumbai	1995/06/15	DELETED

While Table 3-1 provides an up-to-date representation of each customer’s data, it does not reveal whether customers’ addresses have changed at any point. Event sourcing helps provide a different perspective on the data by capturing and persisting the domain events as they occur. If you look at the data in Table 3-2, you’ll see that it preserves the domain events related to a customer account. This data store acts as the source for the events if you ever want to reconstruct the activities of an account.

Table 3-2. Event source data store for the customer account service PK	SK	Event ID	First name	Last name	Address	DOB	Status
100-255-8730	2023-04-05T08:47: 30.718Z	Hru343t5-jvcj	Joe	Bloke	99, Edge Lane, London	1966/04/12	UPDATED
100-735-6729	2023-01-15T02:37: 20.545Z	lgojk834sd3-r454	Biz	Raj	12A, Top Street, Mumbai	1995/06/15	DELETED
100-255-8730	2022-10-04T09:27: 20.443Z	Jsd93ebhas-xdfgns	Joe	Bloke	34, Fine Way, Leeds	1966/04/12	UPDATED
100-255-8730	2022-06-15T18:57: 43.148Z	Zxjfie294hfd-kd9e7n	Joe	Bloke	15, Nice Road, Cardiff	1966/04/12	CREATED
100-735-6729	2009-11-29T20:49: 40.003Z	skdj834sd3-j3ns	Biz	Raj	12A, Top Street, Mumbai	1995/06/15	CREATED