Subflow Recipes

This page collects common reusable subflow patterns. It is meant as a design cookbook: when you know the behavior you want, find the closest pattern and adapt it.

For YAML field definitions such as inputs_from and outputs_from, see Flow YAML Authoring.

Choosing a Pattern

Goal	Typical pattern
Transform each item into another item	map
Keep only items that satisfy a condition	filter
Return whether any item satisfies a condition	some
Return whether all items satisfy a condition	every
Accumulate items into one result	reduce / collector
Split empty and non-empty branches early	switch-by-length + multi-source outputs_from

Map

Use map when each input item should produce one output item.

Good fit:

• Renaming files
• Formatting strings
• Converting objects into another schema

Key rule:

• The slot should return the transformed item, not a boolean predicate.

Filter

Use filter when each item produces a yes/no decision.

Good fit:

• Keeping only valid records
• Removing blocked domains
• Selecting images that pass a test

Key rule:

• The slot should return a boolean-like decision for each item.

Some

Use some when the result should be one boolean saying whether at least one item matches.

Good fit:

• Detect whether any file is oversized
• Detect whether any message contains a keyword
• Detect whether any step failed a check

Key rule:

• The reducer initial value is usually false.

Every

Use every when the result should be one boolean saying whether all items match.

Good fit:

• Confirming every required file exists
• Confirming every output is valid
• Confirming every item passed moderation

Key rule:

• The reducer initial value is usually true.

Reduce / Collector

Use reduce when many items should accumulate into one structured result.

Good fit:

• Building a summary object
• Counting categories
• Grouping items by key
• Producing a report from many records

Key rules:

• Use a dedicated reducer init Node.
• Feed the reducer output back into the reducer input.
• Choose an initial value that matches the final output type.

Empty-Array Short-Circuit

Many reusable array subflows should handle empty arrays explicitly.

Why this helps:

• It makes the empty case predictable.
• It avoids unnecessary work.
• It keeps reducers and slots from receiving invalid assumptions about array length.

Passing Extra Parameters into Slots

Sometimes the per-item behavior needs shared arguments, such as a prefix, threshold, or API setting.

Good fit:

• Prefix text for map
• Threshold value for filter
• Category rules for reduce

Key rules:

• Declare the extra slot inputs.
• Wire the extra slot inputs from the caller Flow.
• Keep the slot contract stable when possible.

Recipe Selection Heuristics

Use these shortcuts when deciding quickly:

• If the output length should equal the input length, start from map.
• If the output length may shrink, start from filter.
• If the output is a single boolean, start from some or every.
• If the output is one object, array, or summary value, start from reduce.

Common Failure Modes

Symptom	Likely recipe issue
Empty arrays fail unexpectedly	No short-circuit branch
Slot output never arrives	Slot and slotflow contracts do not match
Final type is inconsistent	Reducer initial value does not match the intended result type
Logic is hard to update	Too much behavior is hard-coded instead of delegated through a slot

For the conceptual explanation of slots and slotflows, see Subflow Block Advanced Usage.