Added event patterns

Concepts/Event Sourcing Pattern.md

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+---
+Aliases: []
+Tags: [seedling]
+publish: true
+---
+
+The event sourcing pattern captures all changes to an application as a sequence of event objects. This pattern creates a ledger of changes that can be used to replay events and view application state at any point in time.
+
+![Event Sourcing|500](event-sourcing.png)
+*Image source: [Event Sourcing and Event-Driven Architecture - In Under 5 Minutes!](https://ercin.medium.com/event-sourcing-and-event-driven-architecture-in-under-5-minutes-927f4e88351a)*
+
+## Event Sourcing Pattern Advantages
+
+- Provides an audit trail which is useful for auditing and compliance.
+- You can rebuild the state of any entity to any point in time by replaying events.
+- Aligns well with event-driven architectures and microservices where services communicate via events.
+
+## Event Sourcing Pattern Disadvantages
+
+- Managing the event store can be complex as the number of events grow and schema changes happen.
+- Leads to an eventually consistent system (which is fine if that's acceptable) but should be noted it can introduce latency.
+- Rebuilding an entity's current state by replaying all past events can resource intensive. Snapshotting can help mitigate this.
+
+[^1]: https://martinfowler.com/eaaDev/EventSourcing.html
+
+%% wiki footer: Please don't edit anything below this line %%
+
+## This note in GitHub
+
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+
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Concepts/Publisher-Subscriber Pattern.md

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+---
+Aliases: [Pub/sub messaging]
+Tags: [seedling]
+publish: true
+---
+
+The Publisher-Subscriber pattern is an asynchronous messaging pattern that decouples sending events to one or more subscribers/consumers of those events. [^1]
+
+```mermaid
+%%{init: { "flowchart": { "useMaxWidth": true } } }%%
+
+graph LR
+
+A((Publisher))-->|Event| B[[Message Broker]]
+B -->|Event| C((Subscriber))
+B -->|Event| D((Subscriber))
+```
+
+## Publisher-Subscriber Pattern Advantages
+
+- Decouples software and improves scalability due to async sending and ability to scale different parts of the system independently. [^2]
+- Push-based delivery reduces latency and eliminates the need for polling.
+- Simplifies integration code between systems.
+
+## Publisher-Subscriber Pattern Disadvantages
+
+- Message ordering isn't guaranteed and messages may be duplicated. Must design system to be [[Idempotence|idempotent]].
+- Subscribers may need the ability to autoscale to keep up with the rate of messages.
+
+## Popular Publisher-Subscriber Tools
+
+- [[Apache Kafka]]
+- [[Redis]]
+- [AWS EventBridge](https://aws.amazon.com/eventbridge/)
+- [Azure Event Grid](https://azure.microsoft.com/en-us/products/event-grid/)
+- [GCP Eventarc](https://cloud.google.com/eventarc/)
+
+[^1]: https://learn.microsoft.com/en-us/azure/architecture/patterns/publisher-subscriber
+[^2]: https://aws.amazon.com/what-is/pub-sub-messaging/
+
+%% wiki footer: Please don't edit anything below this line %%
+
+## This note in GitHub
+
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+
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