cqrs & event sourcing

CQRS& EVENT

SOURCINGI - Event sourcing

TRADITIONAL PERSISTENCEStore only the current state

TRADITIONAL PERSISTENCEStore only the current state

TRADITIONAL PERSISTENCESnapshots

EVENT SOURCINGStore the events that have led to every state

1. Created

2. Updated foo

3. Updated bar

4. Deleted

EVENT SOURCINGBenefits

Natural audit log

Ability to reconstruct a previous state

Ability to respond to bugs in state

COMMAND-QUERY SEPARATIONCommand function/method

Produces state change

Has observable side-effects

Either succeeds or fails

Has a "void" return type

Throws exceptions

COMMANDProduces a state change and emits an event

Store only the events

Reproduce the state using the events

RECONSTITUTIONAppend new events

event event event

event

command

EVENTS

Type (class)

Object identifier

Timestamp

Any relevant data

(Playhead)

assignments/01.mdEvent sourcing the `Meetup` aggregate

CQRS& EVENT

SOURCINGII - CQRS

EVENT SOURCINGSlow on the "read" side

We need to:

1. Fetch and recreate all event objects

2. Reconstitute the object based on its events

CQRSHelps with scaling on the read side

COMMAND-QUERY SEPARATIONQuery method

Has no observable side-effects

Either succeeds or fails

Has a specific return type

Throws exceptions

COMMAND/QUERY SEPARATION PRINCIPLE

(CQS)

Every method should be either a command method, or a query method

command(): void query(): type

COMMAND/QUERY RESPONSIBILITY SEGREGATION

(CQRS)

Domain objects should have either command methods, or query methods

query(): typecommand(): void

WE WANT TO PERSIST EVENTSBut we need the current state too

IF ONLY WE COULDTail the event log...

event event event

PROJECTIONSUsing events to update read models

event

READ MODELSAre query objects

query(): type query(): type query(): type

READ MODELSAre use-case specific

READ MODELSAre easy to query

SELECT * FROM ... WHERE ...

READ MODELSCan use polyglot persistence

assignments/02.mdApply CQRS to the `Meetup` aggregate

assignments/03.mdTwitsup: Unfollowing

assignments/04.mdTwitsup: Mentions

assignments/05.mdTwitsup: Mentions

CQRS& EVENT

SOURCINGIII - Potential

CQRS & ESVery useful for

Business insights

Scaling

Application integration

Microservices