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Event Sourcing and CQRS in a Microservices/Serverless architecture

Fredrik Liljegren, VOYD, 2022

Note:

    • 3:00
  • 2009 Textalk
  • Copying files, no history, no rollback
  • Vanished code, failing code panic
  • Git - change, who, when, purpose
commit 1999fb5fc37bd5265c84f8f5e95d01012987786b
Author: Fredrik Liljegren <fredrik@liljegren.org>
Date:   Tue Sep 22 10:36:48 2020 +0200

    Updating The File

diff --git a/theFile.md b/theFileWithANewFilename.md
index 040f798..2040db2 100644
--- a/theFile.md
+++ b/theFileWithANewFilename.md
@@ -76,7 +76,6 @@ Note:
 An untouched line for context
-Some line to remove 😲
+Added a line 😏

Note:

    • 4:00
  • Aggregate current state
  • Annotate
3dc71318 (Fredrik 2019-03-31 09:59:12 +0200  55) Event characteristics
3dc71318 (Fredrik 2019-03-31 09:59:12 +0200  56) ---------------------
3dc71318 (Fredrik 2019-03-31 09:59:12 +0200  57) 
7c23a00d (Fredrik 2019-04-02 09:58:58 +0100  58) * Semantic
7c23a00d (Fredrik 2019-04-02 09:58:58 +0100  59) * Atomic

Note:

    • 5:00
  • DST 2 lines
  • Surprised if anyone updates the current state, losing all history
  • Still see state changing in backend!
  • UPDATE row -> loose data
  • Some practices. Basics:
   stream: Comment-abc123

eventType: Commented
 revision: 0
       at: 2022-03-10T09:58:50.123Z
       by: user-sdf234
       on: Product-ghj567
     body: This is my comment body.

eventType: EditedComment
 revision: 1
       at: 2022-03-10T10:58:50.987Z
       by: user-sdf234
     body: THIS is MY comment body!!!

Note:

  • -...
  • Want change - add event
  • Streams & categories
  • All events: type, at, by, revision
  • Data specific to event type
eventType: Deleted
 revision: 3
       at: 2022-03-10T11:58:50.555Z
       by: user-000001

Note:

  • Admin?
  • How to use this data

Making projections

Note:

  • ..:00
  • User sends command
  • Command handler check permissions and possibility.
  • -> eventstore -> any listeners
  • git checkouts, pulling - es pushing
const comment = events.reduce(
  reduceComment, { id: streamId }
)

const reduceComment = (comment, event) => {
  switch (event.type) {
    case 'Commented': return {
      ...comment,
      createdAt: event.at,
      author: event.by,
      body: event.body,
    }
    case 'EditedComment': return {
      ...comment,
      updatedAt: event.at,
      body: event.body,
    }
    case 'Deleted': return {
      ...comment,
      deletedAt: event.at,
    }
  }
}

Note:

  • Many GB? Store it!

Storing projections

Note:

  • Projection store can never deny what happened!
    • Checks are already done

Read sides

Note:

  • ..:00
  • Make a query interface for the store
  • Want a list of comments by Product?
    • Read side.

CQRS

Command/Query Responsibility Segregation

Micro Services & Serverless

Note:

  • Splitting up
  • Separation of concerns
  • Deploying separately (crashing separately)
  • Scaling separately
  • serverless - scaling functions separately
PUT /api/comments/abc123/EditComment
If-Match: 6
{
  "body": "A new comment body"
}

Note:

  • Optimistic
await eventstore.appendToStream(
  `Comment-${id}`,
  jsonEvent<EditedComment>({
    type: EditedComment',
    data: { at, by, body },
  }),
  { expectedRevision: 6 },
)

412 Precondition Failed
Etag: 8
  • Event routing and storing in the same service
  • Subscriptions and shared subscriptions
  • Optimistic concurrency
  • Category projection streams

Note:

The event store named EventStore 🙄

As long as you UPDATE your main source of truth, you are loosing valuable data.

Fredrik Liljegren / @fiddur