66 lines
1.6 KiB
Markdown
66 lines
1.6 KiB
Markdown
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# Haskell Writer for Audit Trails
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This note covers `31-haskell-writer-audit/`, which simulates a rollout while accumulating ordered audit lines with `Writer`.
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---
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## 1. What `Writer` Adds Here
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The example does two things at once:
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- compute a final `RolloutReport`, and
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- collect human-readable audit lines in execution order.
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That is the shape `Writer` is good at:
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```haskell
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simulateRollout :: ReleaseJob -> Writer [String] RolloutReport
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```
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The result and the log are produced together, but the rollout code stays direct and readable.
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---
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## 2. Why the Log Is Worth Teaching
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The audit trail is not decorative. It captures meaningful rollout steps:
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- rollout start,
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- target scaling,
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- schema migration,
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- traffic shifting, and
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- rollout completion.
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For canary rollouts, the example records extra milestones and observation points. That makes the logged structure richer than a toy "hello logger"
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example.
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---
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## 3. Why This Stays Separate from Error Handling
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`Writer` is used here only for logging.
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The example does not mix in failure handling or configuration lookup, because that would blur the concept. Parsing still happens before the writer run,
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and the rollout simulation itself is deterministic.
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That keeps the example focused on one question: how do you accumulate ordered auxiliary output while computing a result?
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---
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## 4. Commands to Try
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```bash
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cd 31-haskell-writer-audit
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nix develop
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cabal run
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cabal run -- api:production:canary:20:3 worker:staging:rolling:0:2
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cabal test
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nix build
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./result/bin/mini-audit api:production:canary:20:3 worker:staging:rolling:0:2
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nix run . -- api:production:canary:20:3 worker:staging:rolling:0:2
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nix flake check
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```
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