Subflows

A Subflow is a flow invoked by another flow instead of by its own external trigger — a reusable function at flow granularity. The @activepieces/piece-subflows core piece supplies both halves: the Callable Flow trigger that makes a flow callable, and the actions a parent uses to reach it. A parent calls a subflow once (Call Flow, optionally waiting for a response through a waitpoint) or fans out many calls from one streaming step (Stream CSV to Subflows). The Respond action sends data back to a waiting parent. There is no dedicated transport: every call is a webhook POST to /v1/webhooks/:flowId.

Entities & services

No server entity of its own — subflows are ordinary flows plus two conventions on the webhook path.

• Callable Flow trigger — marks a flow as callable; receives the parent's data payload and an optional callbackUrl.
• Call Flow action — one invocation, optional wait-for-response via a waitpoint, can fail the parent run on subflow error.
• Stream CSV to Subflows action — streaming fan-out, one fire-and-forget call per Batch of CSV rows.
• Respond action — posts { status, data } to the parent's callbackUrl.
• common.ts — the Callable Flow dropdown (listFlowsWithSubflowTrigger), findFlowByExternalIdOrThrow, request/response types, callback key.
• Parent linkage rides two headers the webhook request converter reads: x-parent-run-id and x-fail-parent-on-failure.

How it works

• Call Flow: resolves the target by externalId → POSTs { data, callbackUrl? } to the subflow's production webhook. With wait-for-response it creates a WEBHOOK waitpoint, passes its resume URL as callbackUrl, and pauses until Respond calls back; the RESUME branch rethrows when the subflow answered status: 'error'. Without it the step returns as soon as the webhook is acknowledged.
• Stream CSV to Subflows: input is a streaming Property.File (streaming: true, so it resolves to an ApStreamingFile and accepts a URL, an upload, or a previous step's file — a plain Property.File would materialize an ApFile Buffer before run() starts and OOM), a Callable Flow dropdown target, batchSize (default 100, capped at 10,000), delimiter (comma/tab) and optional extraData merged into every call.
  • The action pipes file.body straight into a streaming csv-parse parser — the engine owns the fetch, so the piece carries no HTTP client.
  • Parser construction lives in subflows/csv.ts (createCsvParser), pinned by test/csv.test.ts. Its bom: true, relax_column_count: true and group_columns_by_name: true are all load-bearing, not defensive — see the CSV gotchas in building-pieces. Because of group_columns_by_name, a row value is string | string[]: duplicate header names arrive as an array rather than silently dropping a column.
  • Payload per call: data = { batchIndex, headers, rows, extraData }. Dispatch is fire-and-forget — no callbackUrl, x-fail-parent-on-failure: false. extraData is re-serialized into every batch, so a large {{step.output}} reference counts against the webhook bodyLimit independently of batchSize.
  • fanOutBatches bounds in-flight dispatches (5) and awaits Promise.race when the window is full, so parsing back-pressures instead of buffering the file.
  • Returns { headers, firstRow, rowsProcessed, batchesDispatched }.
• Failure: a batch POST is retried by httpClient (retries: 2, so 3 attempts, 1s/2s backoff, 5xx and network errors only — every 4xx aborts on the first try). That is deliberate for a 413 (an oversized batch will never succeed) and for a 404 from a subflow deleted mid-run; the one it costs you is a 429, unreachable self-hosted (rate-limit.ts registers global: false and webhook routes never opt in) but reachable behind a Cloud edge WAF at MAX_IN_FLIGHT=5. After that the fan-out stops reading the stream and throws with the failed batchIndex and the cause. Already-dispatched subflows keep running — at-least-once, no fan-in, no rollback. A parse error on the read side takes the same exit: in-flight dispatches are drained before throwing, so rowsProcessed / batchesDispatched still tell the user how far it got.

Gotchas

• text/csv is a binary content type on the webhook path, on purpose. It was a 415 before — webhook routes get Fastify's built-ins (json, text/plain) plus webhook-module.ts's explicit list, and the only catch-all parser is scoped to the /ingest PostHog proxy. Adding it was therefore additive, not a behaviour change. It buys compatibility, not capacity: application/octet-stream and multipart/form-data already stream to the File service under the same AP_MAX_FILE_SIZE_MB ceiling (the converted { fileUrl } payload is far below MAX_WEBHOOK_PAYLOAD_SIZE_MB), so the win is senders whose header you cannot change — a partner's nightly export, an S3 event notification, a SaaS "POST my report" hook. Don't send a large CSV as text/plain: Fastify's string parser and the preParsing rawBody hook each buffer the whole body, and it dies at 25MB. Consequence to keep in mind: rawBody is not captured for binary types, so HMAC-over-raw-body sees undefined — fine here because the payload is { fileUrl } anyway. text/tab-separated-values has the same gap and is not yet covered, though the action offers a Tab delimiter.
• Time, not memory, is the ceiling. Streaming bounds memory; the step is still capped by FLOW_TIMEOUT_SECONDS (default 600s, fixed on Cloud). A file whose fan-out cannot finish inside that window is out of scope for v1 — it fails loud, never a silent partial fan-out. See 000015.
• Fan-out is not Call Flow. Wait-for-response is incompatible with many batches inside one step; only Call Flow can wait.
• "Retry on failure" is hidden on the fan-out action, on purpose. The framework default (action.ts) is defaultValue: false but visible, and step retry re-runs run() from the top — for a fire-and-forget fan-out that means re-reading the file from row 0 and re-dispatching every batch that already landed. A failure at batch 900 would duplicate 900 batches. So the action declares retryOnFailure: { defaultValue: false, hide: true }; continueOnFailure stays visible. Call Flow leaves both visible because one invocation is safe to retry.
• batchSize is capped at 10,000, because "streaming bounds memory" only holds for sane batch sizes. Peak memory is roughly (MAX_IN_FLIGHT + 1) × batchSize × rowSize, so an unbounded batchSize defeats the whole design. The cap that bites first in practice is not memory but the webhook bodyLimit (max(AP_MAX_FILE_SIZE_MB + 4, AP_MAX_FLOW_RUN_LOG_SIZE_MB + 4, 25) MB, 54MB by default) — a wide-row file will 413 well before 10,000 rows.
• The dropdown lists published flows carrying a Callable Flow trigger and labels disabled ones (inactive); streaming to a disabled flow throws before the first request.

Editions

Community, Enterprise, Cloud — core piece, no plan flag.

Key files

Entry point: streamCsvToSubflows.run, which drives fanOutBatches over a streaming CSV parser.

• packages/pieces/core/subflows/src/index.ts — piece definition
• packages/pieces/core/subflows/src/lib/actions/call-flow.ts — Call Flow (one call, optional wait-for-response)
• packages/pieces/core/subflows/src/lib/actions/stream-csv-to-flow.ts — Stream CSV to Subflows (streaming fan-out)
• packages/pieces/core/subflows/src/lib/actions/respond.ts — Respond (subflow → parent)
• packages/pieces/core/subflows/src/lib/triggers/callable-flow.ts — Callable Flow trigger
• packages/pieces/core/subflows/src/lib/fan-out.ts — batching + bounded-concurrency dispatch loop, transport-agnostic
• packages/pieces/core/subflows/src/lib/common.ts — flow dropdown, lookup helpers, request/response types
• packages/server/api/src/app/webhooks/webhook-request-converter.ts — text/csv in BINARY_CONTENT_TYPE_PATTERNS, parent-run headers