larex-action-sdk 0.6.1

Framework-neutral Python SDK for building LAREX Action processors.

LAREX Action SDK

This SDK is work in progress. The public API can still change before LAREX Actions and the SDK are considered stable.

Framework-neutral Python SDK for building LAREX Action processors with signed dispatch verification, typed payloads, and cooperative run cancellation.

The core package verifies LAREX dispatch requests, parses typed run/input payloads, sends heartbeats, downloads selected files, uploads result manifests, and helps processors acknowledge cancellation cleanly. FastAPI support is available as an optional convenience extra.

Installation

uv add "larex-action-sdk[fastapi]"

For framework-neutral usage only:

uv add larex-action-sdk

FastAPI Processor

import os

from larex_actions import ActionContext
from larex_actions.fastapi import create_larex_action_app


async def process(ctx: ActionContext) -> None:
    action_input = await ctx.pull_input()
    results = ctx.result_builder()

    for page in action_input.pages:
        async with ctx.step(f"Processing {page.name}", progress_percent=25):
            await ctx.check_cancelled()
            if page.xml:
                xml_bytes = await ctx.download_bytes(page.xml[0])
                results.add_xml_bytes(
                    page_id=page.id,
                    content=xml_bytes,
                    file_name=f"{page.name}-processed.xml",
                )

    await ctx.complete(results, "Done")


app = create_larex_action_app(
    processor_id="my-processor",
    dispatch_secret=os.environ["LAREX_DISPATCH_HMAC_SECRET"],
    handler=process,
)

The FastAPI adapter always exposes /dispatch and /health. Set LAREX_ACTION_ROUTE_PREFIXES to also expose prefixed routes when a reverse proxy keeps an external path prefix:

LAREX_ACTION_ROUTE_PREFIXES=/kraken,/ocr

With that setting, the same processor also accepts /kraken/dispatch, /kraken/health, /ocr/dispatch, and /ocr/health. LAREX must sign and call the same path the processor receives; do not strip the prefix in the reverse proxy before the request reaches the processor.

Target-Aware Runs

LAREX can dispatch page, region, and textline targeted runs. The SDK exposes the requested target on both dispatch and pulled input payloads:

payload_target = ctx.payload.target
action_input = await ctx.pull_input()
input_target = action_input.target

Processors still receive full page files according to the Action YAML inputs. Target metadata contains selected region/textline ids only. LAREX sends full page images/XML and lets processors resolve geometry from PAGE XML, including whether to crop, mask, pad, deskew, or process the full image.

Processors return normal PAGE XML via ResultBuilder.add_xml_bytes(...) or add_xml_path(...). For region or textline targeted runs, LAREX imports only the selected target scope from the returned PAGE XML.

Framework-Neutral Dispatch Verification

from larex_actions import DispatchVerifier

payload = DispatchVerifier(
    processor_id="my-processor",
    dispatch_secret=secret,
).verify(
    method=request_method,
    path_and_query=request_path_and_query,
    headers=request_headers,
    body=request_body,
)

You can then pass payload.model_dump(mode="json", by_alias=True) to your own queue/worker system and use ActionClient.from_dispatch(payload) in async workers.

Cooperative Cancellation

LAREX cancellation is cooperative. The processor keeps polling the heartbeat endpoint and LAREX responds with cancelRequested: true when the run should stop.

• Use await ctx.check_cancelled() at safe interruption points.
• ctx.check_cancelled() performs a heartbeat request, so avoid calling it in a hot inner loop without pacing.
• await ctx.heartbeat(..., raise_on_cancel=True) also raises ActionCancelled when a cancellation is pending.
• await ctx.run_subprocess(...) polls for cancellation while a child process is running, sends a final status="cancelled" heartbeat, and terminates the child process gracefully before escalating to kill.
• Once cancellation has been requested, the SDK refuses result uploads and acknowledges cancellation instead.

Security

• Dispatch requests are verified with the X-LAREX-Action-* HMAC headers.
• Timestamps and nonces are checked to reduce replay risk.
• The FastAPI adapter rejects dispatch bodies larger than max_dispatch_body_bytes.
• Per-run bearer secrets and dispatch HMAC secrets are never included in model reprs.
• Processor YAML must still declare the inputs and outputs LAREX should expose or accept.

Development

uv sync --all-extras
uv run ruff format .
uv run ruff check .
uv run pyright
uv run pytest
uv build

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