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Andrew Cairns

@acairns.co.uk

2.1K followers 2K following 300 posts

Using metaphors and analogies to explain Software Engineering in fun ways: https://youtube.com/@metaphoricallyspeaking Staff Software Engineer. Passionate about DDD, CQRS, Event Sourcing and Distributed Systems. Kayaking, too.

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Andrew Cairns @acairns.co.uk · Oct 4

Composition over Inheritance: explained by retro games!

www.youtube.com/watch?v=HNzP...

Composition over Inheritance Explained by Games! #programming

YouTube video by Metaphorically Speaking

www.youtube.com

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Andrew Cairns @acairns.co.uk · 9h

Your projection crashes halfway through 1M events.

When it restarts, where does it begin?

Checkpoints solve this: periodically save "processed up to event 458,295"

Restart resumes from the checkpoint.

No checkpoint? You replay from the beginning.

Trade-off: checkpoint frequency vs replay cost.

Andrew Cairns @acairns.co.uk · 4d

Medical records are append-only.

Doctors don't UPDATE past entries, they add corrections.

March 15: "Patient reports knee pain"
March 16: "Diagnosis: sprain"
March 20: "Corrected diagnosis: torn meniscus"

All three facts are true. All three stay in the record.

Just like Event Sourcing.

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Andrew Cairns @acairns.co.uk · 5d

Internal Events:
- CustomerAgeCalculated
- InventoryReserved
Private domain implementation. Can change anytime

External Events:
- OrderPlaced
- PaymentReceived
Published API. Version instead.

Exposing internals creates distributed coupling.

Keep them separate.

Reposted by Andrew Cairns

Duncan Jones (aka Merrion) (he/him) @duncanjonesmerrion.bsky.social · 6d

It is on YouTube but it was in the time of COVID so is an on-line teams type presentation thing...

youtu.be/eD7ls4KynRo

What do you mean by “is”? - Duncan Jones - DDD Europe 2021

YouTube video by Domain-Driven Design Europe

youtu.be

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Andrew Cairns @acairns.co.uk · 6d

I like the idea, but I've never practiced it in production.

I have a local app which displays me some metrics which is aggregate-less. But the complexity is low. I don't know how I'd handle enforcing a constraint, without duplicating it's source of truth, required by multiple commands.

wdyd?

Andrew Cairns @acairns.co.uk · 6d

Common Event Sourcing mistake: multiple aggregates in one stream.

One stream = One aggregate

Customer[123] stream → CustomerRegistered, EmailChanged, AddressUpdated
Order[456] stream → OrderPlaced, ItemAdded, OrderShipped

Different streams. Different things.

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Reposted by Andrew Cairns

Duncan Jones (aka Merrion) (he/him) @duncanjonesmerrion.bsky.social · 7d

You can have the event chain organised as a workflow (based on an event stream) identified by an unique identifier and pass that identifier to the individual processes as a correlation id...and have them write back their doings to that event stream.

Thus your workflow is its own log.

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Andrew Cairns @acairns.co.uk · 7d

Correlation IDs allow you to:
- Trace an entire business flow across services
- Subscribe to "everything about order ABC123"
- Debug distributed systems without questioning your life choices

It is a massive debuggability improvement.

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Andrew Cairns @acairns.co.uk · 7d

I'm an advocate for giving every event a Correlation ID copied from the command that caused it.

OrderAccepted → OrderPaid → OrderShipped
All share CorrelationId: "ABC123"

Now you can... 👇

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Andrew Cairns @acairns.co.uk · 8d

I know these sound like negatives, but in many ways, these are the positives!

I’ll expand more, soon :)

Andrew Cairns @acairns.co.uk · 8d

🥁 🤣

Andrew Cairns @acairns.co.uk · 8d

It can be designed FOR their queries and everything can be denormalised.

It's read-only and can optimised for the specific use case.

This way, you can give them EXACTLY what they need!

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Andrew Cairns @acairns.co.uk · 8d

Your data team wants to build reports and dashboards but your events live in an append-only log...

They want a relational schema.

Solution: project events into a dedicated reporting database!

Why? 👇

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Andrew Cairns @acairns.co.uk · 9d

The event store checks: "Is current version still 67?"
- Yes: Write succeeds
- No: Reject with a `ConcurrencyException`

Retrying the command here is quite common.

The check happens atomically during the write with no separate locking step needed.

Andrew Cairns @acairns.co.uk · 9d

How do you handle two processes acting on the same thing in Event Sourcing?

Optimistic concurrency with expected version:

1. Load events from stream (version 67)
2. Process command, generate new events
3. Write with expectedVersion=67

And then... 👇

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Andrew Cairns @acairns.co.uk · 9d

Really does. Currently advocating for a shift towards workflows, too! 🤞

Andrew Cairns @acairns.co.uk · 9d

"First Attempt In Learning"

That's incredible 👏