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Add Reliablity Guarantees section in README #403

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Mar 29, 2018
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Add Reliablity Guarantees section in README #403

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Add Reliablity Guarantees section in README
Mar 26, 2018
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update
Mar 28, 2018
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rewrite the sentence
Mar 28, 2018
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add documentation about error responses from Events API
Mar 29, 2018
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13 changes: 13 additions & 0 deletions README.md
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Expand Up @@ -42,6 +42,7 @@ yet ready for production applications._
1. [Comparison](#comparison)
1. [Architecture](#architecture)
1. [System Overview](#system-overview)
1. [Reliability Guarantees](#reliability-guarantees)
1. [Clustering](#clustering)
1. [Background](#background)

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└────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
```

## Reliability Guarantees

### Events are not durable

The event received by Event Gateway is stored only in memory, it's not persisted to disk during processing. This means that in case of hardware failure or software crash the event may not be delivered to the subscriber. For a synchronous subscription (`http` or `invoke` event) it can manifest by error message returned to the requester. If there is multiple subscribes to the same custom event type, in case of failure the event may not be delivered to all of them.
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What is the error code and error message returned. Are they the same under all failure conditions? Are these error codes and messages documented as such in our API documentation?

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If there is hardware failure or software crash nothing is returned from EG because of the obvious reason. What I meant here was that client will get some kind of error which depends on the setup e.g. for locally hosted EG it will be simple connection error, for self hosted EG behind AWS ELB there will be error from ELB etc.

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I understand, but we should have the codes and messages clearly documented - Is that in the API docs?


### Events are delivered _at most once_

Event Gateway tries to deliver an event only once, there is no retry mechanism. This and lack of durability implicates that event will be delivered to the subscriber _at most once_. Even though Event Gateway itself doesn't retry failed function invocations, AWS SDK used by few providers does that internally by default. The retry logic there happens only for very specific reasons and should not cause delivering the same event multiple times. Please find more information in [AWS documentation on API retries](https://docs.aws.amazon.com/general/latest/gr/api-retries.html).
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Would suggest rephrasing

"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 thats built into the SDK AWS documentation on API retries."


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.

## Background

SOA came along with a new set of challenges. In monolithic architectures, it was simple to call a built-in library or
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