ADR for deployment slots

adr/023-deployment-slots.md

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+# 23. Deployment Slots for Zero Downtime Deploys
+
+Date: 2024-09-03
+
+## Decision
+We will use Azure Web App Deployment Slots to facilitate zero-downtime deploys of the TI app
+
+## Status
+
+Accepted.
+
+## Context
+Because TI is driven by web traffic from ReportStream, we can receive http calls at any time.
+If TI fails to respond, ReportStream will have to try sending the data again later, causing delays.
+By implementing zero-downtime deploys, our service can remain available to any incoming calls.
+
+Even though there are some significant downsides to Deployment Slots, they're Azure's recommended
+approach to zero-downtime deploys (ZDD), and they're lower effort and lower risk than the alternatives.
+Other options to achieve ZDD are Kubernetes (significantly more complexity and effort), creating
+our own custom deploy system (significantly more complexity, effort, and risk), or switching to
+a cloud service provider that makes this easier, like AWS (not currently in scope as an option).
+
+## Impact
+### Positive
+- **Zero-downtime deploys**: Zero-downtime deploys keep us from dropping incoming calls during deployment.
+- **Easy rollback**: Deployment slots make it easy to roll back to the previous version of the
+app if we find errors after deploy.
+- **Consistency**: Deployment Slots are an Azure feature specifically designed to enable
+zero-down time deployment. We use deployment slots in all TI environments and
+in the SFTP Ingestion Service.
+
+### Negative
+- **Incomplete support for Linux**: The auto-swap feature is not available for Linux-based web apps like ours.
+ so we had to include an explicit swapping step in our updated deployment process.
+- **Opaque responses from `az webapp deployment slot swap` CLI**: When there are issues swapping slots, the CLI doesn't
+return any details about the issue. The swapping operation can also take as much as 20 minutes
+to time out if there's a silent failure, which slows down deploy and validation.
+- **Steep learning curve**: Most of the official docs and unofficial resources
+(such as blogs and tutorials) for deployment slots are written for people using Windows
+servers and Microsoft-published programing languages. This lack of support for other platforms
+and languages means a lot more trial and error is involved.
+
+### Risks
+- Because of the incomplete support for and documentation of our usecase, we may not have
+chosen the optimal implementation of this feature. It may also be time-consuming to
+troubleshoot if we run into future issues.