Add a blog: Introduction to EventWatcher (#5787)

* add a blog: eventwatcher-intro

Signed-off-by: t-kikuc <[email protected]>

* set background color

Signed-off-by: t-kikuc <[email protected]>

* refine title

Signed-off-by: t-kikuc <[email protected]>

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Signed-off-by: t-kikuc <[email protected]>

Author: t-kikuc

docs/content/en/blog/eventwatcher-intro.md

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+---
+date: 2025-04-28
+title: "Introduction to EventWatcher: Connecting CI to PipeCD"
+linkTitle: "Introduction to EventWatcher"
+weight: 983
+author: Tetsuya Kikuchi ([@t-kikuc](https://github.com/t-kikuc))
+categories: ["Introduction"]
+tags: ["Feature Guide"]
+---
+
+This article explains the basics of "**EventWatcher**", a crucial feature in practical PipeCD usage.
+
+This article is intended for those who:
+- "Don't know how to integrate CI with PipeCD"
+- "Have found the EventWatcher feature but can't grasp how it works"
+
+_This article is based on PipeCD v0.51.2 (latest at the time of writing)._
+
+## Background: Why EventWatcher is needed
+
+Basically, PipeCD is a CD tool that continuously deploys specified config/manifests.
+
+![](/images/eventwatcher-only-cd.drawio.png)
+
+Deployment typically requires manifest changes. So, how can we "**deploy using a new image (etc.) after CI completion**"? It's annoying to update the manifest repo manually each time.
+
+![](/images/eventwatcher-problem.drawio.png)
+
+This is where the EventWatcher feature comes into play!
+
+## What is EventWatcher?
+
+EventWatcher is a feature in PipeCD that seamlessly connects CI and CD. It updates the manifest repo based on events from CI, triggering CD.
+
+https://pipecd.dev/docs-v0.51.x/user-guide/event-watcher/
+
+### Mechanism
+
+The overall picture of EventWatcher looks like the diagram below.
+
+![](/images/eventwatcher-overview.drawio.png)
+
+The EventWatcher feature itself handles steps 4-6:
+
+- 1-3. Develop new app code and store the results (container images, etc.) in a container registry
+- **4.** Publish an event in CI to pass the new image URI to PipeCD
+- **5.** Piped detects the event
+- **6.** Update the manifest repo using the data provided by the event (image URI, etc.)
+
+- 7-8. Automatic deployment occurs through the standard deployment flow of PipeCD
+
+## Usage
+
+You need to setup three areas:
+
+#### 1. Piped Configuration
+
+1. Which manifest repo events to handle
+2. Commit permissions for the manifest repo
+
+https://pipecd.dev/docs-v0.51.x/user-guide/managing-piped/configuring-event-watcher/
+
+#### 2. app.pipecd.yaml Configuration
+
+1. Which events to handle for the Application (`matcher`)
+2. What actions to take for matched events (`handler`)
+
+(Example) Configuration to update the `spec.template.spec.containers[0].image` field in `deployment.yaml` when the `helloworld-image-update` event is triggered:
+
+```yaml
+apiVersion: pipecd.dev/v1beta1
+kind: KubernetesApp
+spec:
+  name: helloworld
+  eventWatcher:
+    - matcher: # Which events to handle
+        name: helloworld-image-update
+      handler: # What actions to take
+        type: GIT_UPDATE
+        config:
+          replacements:
+            - file: deployment.yaml
+              yamlField: $.spec.template.spec.containers[0].image
+```
+
+Full configuration reference:
+https://pipecd.dev/docs-v0.51.x/user-guide/configuration-reference/#eventwatcher
+
+#### 3. Event Triggering via pipectl or GitHub Actions
+
+1. Event name (set in `app.pipecd.yaml` as `matcher.name`)
+2. New value (image URI, etc.)
+
+(Example) Command to trigger an event named `helloworld-image-update` to change the image URI to `ghcr.io/xxx/helloworld:v0.2.0`:
+
+```sh
+pipectl event register \
+    --address={CONTROL_PLANE_API_ADDRESS} \
+    --api-key={API_KEY} \
+    --name=helloworld-image-update \
+    --data=ghcr.io/xxx/helloworld:v0.2.0
+```
+
+If you use GitHub Actions for your CI, using PipeCD's official `actions-event-register` is recommended (configurations are the same):
+
+https://github.com/marketplace/actions/pipecd-register-event
+
+For example, triggering an event named `helloworld-image-update` to change the image URI to `ghcr.io/xxx/helloworld:v0.2.0`:
+```yaml
+      - uses: pipe-cd/[email protected]
+        with:
+          api-address: ${{ secrets.API_ADDRESS }}
+          api-key: ${{ secrets.API_KEY }}
+          event-name: helloworld-image-update
+          data: ghcr.io/xxx/helloworld:v0.2.0
+```
+
+You can also handle events differently by environment using the `--labels` option, even for events with the same name.
+
+##### Advanced: `--contexts` Option
+
+Adding `--contexts` to `pipectl event register` allows you to include various information in the manifest repo commit. For example, passing the **"commit hash and URL of the app repo that triggered the event"** makes it easier to track "which app repo changes caused this event" in the manifest repo. This is particularly useful when deployments occur frequently.
+
+https://pipecd.dev/docs-v0.51.x/user-guide/event-watcher/#optional-using-contexts
+
+## Appendix: Code Reading
+
+The EventWatcher code is consolidated in one file (though it's quite long). Remembering that "EventWatcher runs continuously as a goroutine from Piped startup" makes it slightly easier to read.
+
+https://github.com/pipe-cd/pipecd/blob/v0.50.2/pkg/app/piped/eventwatcher/eventwatcher.go
+
+## Conclusion
+
+Using EventWatcher enables seamless integration between CI and CD. There are various advanced applications, and it would be great to see more examples being shared.
+
+Additionally, **Deployment Traces** feature enhances the integration of CI and PipeCD. For more details, please see this article:
+
+https://pipecd.dev/blog/2024/12/26/the-gap-between-ci/cd-and-how-pipecd-appeared-to-fulfill-the-gap/