Cleanup README and documentation. Add docs for Prometheus Metrics. #422
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docs/clustering.md
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| cache configuration used to drive low-latency event routing. The instance local cache is built asynchronously based on | ||
| events from backing DB. | ||
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| The Event Gateway is a horizontally scalable system. It can be scaled by adding instances to the cluster. A cluster is |
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I feel this paragraph that introduces the notion of clustering should come first.
docs/clustering.md
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| │ │ │ └─────────────┘ │ │ | ||
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| │ Cloud Region 2───────┐ │ │ │ Cloud Regio│ 3───────┐ │ |
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typo on Region 3 at the far right
| # Prometheus Metrics | ||
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| Both Events and Configuration API exposes Prometheus metrics. The metrics are accesible via `/v1/metrics` endpoint. | ||
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For future, writing an prometheus exporter for EG metrics might be really useful for those who want to integrate with EG.
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Is it possible to add some screenshots of the charts that we have been doing via Grafana and EG data?
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The next step is to add Grafana dashboard JSON file. I will do that In separate PR.
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| ## Events are delivered _at most once_ | ||
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| Event Gateway attempts delivery fulfillment for an event only once and consequently any event received successfully by the Event Gateway is guaranteed to be received by the subscriber _at most once_. That said, the nature of Event Gateway provider implementation could result in retries under specific circumstances, but these should not cause delivering the same event multiple times. For example, Providers for AWS Services that use the AWS SDK are subject to auto retry logic that's built into the SDK ([AWS documentation on API retries](https://docs.aws.amazon.com/general/latest/gr/api-retries.html)). |
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"and consequently any event received successfully by the Event Gateway is guaranteed to be received by the subscriber at most once." -> is it guaranteed? We say in the previous section that "Events are not durable".
After reading the two sections, I am really confused. Let's clarify the message.
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It's "at most once" so basically anything guarantee that.
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| Event Gateway attempts delivery fulfillment for an event only once and consequently any event received successfully by the Event Gateway is guaranteed to be received by the subscriber _at most once_. That said, the nature of Event Gateway provider implementation could result in retries under specific circumstances, but these should not cause delivering the same event multiple times. For example, Providers for AWS Services that use the AWS SDK are subject to auto retry logic that's built into the SDK ([AWS documentation on API retries](https://docs.aws.amazon.com/general/latest/gr/api-retries.html)). | ||
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| AWS Lambda provider uses `RequestResponse` invocation type which means that retry logic for asynchronous AWS events doesn't apply here. Among others it means, that failed deliveries of custom events are not sent to DLQ. Please find more information in [Understanding Retry Behavior](https://docs.aws.amazon.com/lambda/latest/dg/retries-on-errors.html), "Synchronous invocation" section. |
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This paragraph seems out of place and only addresses AWS. How about other cloud providers since our focus is for all of them?
Are we talking about "retry behavior" in EG?
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Right now we natively support only Lambda which has that behaviour. Other providers exposing HTTP endpoint doesn't have that AFAIK.
| _Current implementation supports plugins written only in Golang. We plan to support other languages in the future._ | ||
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| Plugin system is based on [go-plugin](https://github.com/hashicorp/go-plugin). A plugin needs to implement the following | ||
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To make it easier, I think we should do a post on "Writing a plugin for EG". That way we can show how to write a plugin and also drive some traffic.
docsdir