Composing with conditionals

Reduction strategies are JSON Schema annotations. As such, their applicability at a given document location can be controlled through the use of conditional keywords within the schema, like oneOf or if/then/else. This means Flow’s built-in strategies can be combined with schema conditionals to construct a wider variety of custom reduction behaviors.

For example, here’s a reset-able counter:

collections:
  - name: example/reductions/sum-reset
    schema:
      type: object
      properties:
        key: { type: string }
        value: { type: number }
      required: [key]
      # Use oneOf to express a tagged union over "action".
      oneOf:
        # When action = reset, reduce by taking this document.
        - properties: { action: { const: reset } }
          reduce: { strategy: lastWriteWins }
        # When action = sum, reduce by summing "value". Keep the LHS "action",
        # preserving a LHS "reset", so that resets are properly associative.
        - properties:
            action:
              const: sum
              reduce: { strategy: firstWriteWins }
            value: { reduce: { strategy: sum } }
          reduce: { strategy: merge }
    key: [/key]

tests:
  "Expect we can sum or reset numbers":
    - ingest:
        collection: example/reductions/sum-reset
        documents:
          - { key: "key", action: sum, value: 5 }
          - { key: "key", action: sum, value: -1.2 }
    - verify:
        collection: example/reductions/sum-reset
        documents:
          - { key: "key", value: 3.8 }
    - ingest:
        collection: example/reductions/sum-reset
        documents:
          - { key: "key", action: reset, value: 0 }
          - { key: "key", action: sum, value: 1.3 }
    - verify:
        collection: example/reductions/sum-reset
        documents:
          - { key: "key", value: 1.3 }