Wrapping a Slow Generative-Music API — Submit, Poll, Callback, Download, Trim

Echo Echo is a songwriting-inspiration tool built by a small team: you pick a mood, theme, style, instruments, tempo and energy, and it returns a short instrumental sketch. The interesting engineering is not the UI; it's wrapping a slow, third-party, asynchronous generation API so that the rest of the app can treat it like a function call. This post covers that integration layer.

Structured Input, Then a Prompt

The frontend sends structured fields, not free text:

@dataclass
class GenerationInput:
    mood: str; theme: str; style: str
    instruments: list[str]; tempo: int; energy: int

build_prompt turns these into a single instruction ("a 30 second instrumental songwriting inspiration sketch…") plus a style string, with a fallback phrase when no instruments are chosen. For the alternative ACE-Step backend, the same inputs become a comma-separated tag prompt, with the 1–10 energy slider mapped to words (1 → "very calm" … 10 → "extreme") and tempo expressed as "90 BPM". Keeping one structured input and several prompt builders means a different model needs a new function, not a new UI.

The Job Lifecycle

Generation takes much longer than a normal HTTP request, so the provider's API is job-based:

1. Submit a task and receive a task_id.
2. Wait for completion, either by polling or by the provider calling you back.
3. Download the resulting audio URL.
4. Post-process it.

The code is a small async pipeline using httpx.AsyncClient: submit_to_suno → poll_suno → download_audio, each timed and logged.

A Polling Loop With a Deadline and a Failure Set

poll_suno checks the task's record-info endpoint every five seconds until a deadline (ten minutes). Three rules make it robust:

• A status of SUCCESS returns the record.
• Any status in a known failure set — task creation failed, audio generation failed, callback exception, sensitive-word rejection — raises immediately with the provider's error message. Waiting out a timeout for a job you already know has failed is wasted time for the user.
• Falling out of the loop raises a TimeoutError.

asyncio.sleep yields to the event loop between polls, so one slow generation doesn't block other requests. response.raise_for_status() and a per-request timeout keep a failing provider from hanging the loop.

Why the Callback URL Is Validated at Startup

The API can POST to a callback URL when it finishes. That only works if the URL is publicly reachable, and a very common local-development mistake is configuring localhost. Instead of letting that fail mysteriously minutes later, build_callback_url() raises a configuration error immediately if PUBLIC_BASE_URL is missing or contains localhost or 127.0.0.1, and the error message tells you to use a tunnel such as ngrok or cloudflared. validate_kieai_config() runs it at startup.

This is the general principle: turn a late, confusing runtime failure into an early, specific configuration error. The callback path itself is configurable and normalised to start with /.

Post-Processing

The provider's audio may be longer than the product's 30-second target. After downloading, trim_audio uses pydub (backed by ffmpegffmpegA command-line toolkit for recording, converting and streaming audio and video in almost any format.) to cut to a configurable duration and export an MP3. The code first locates an audio converter and raises a clear error if none is installed, rather than failing deep inside pydub. Original and trimmed filenames are stored separately so the original is never destroyed.

Song ids are short generated codes checked against the set of existing ids to avoid collisions; the result record stores the task id, the prompt, the source URL and the creation time so any track can be traced back to the request that produced it.

Error Handling Details

• The provider's error body is sanitised before logging, so secrets in a response aren't written to logs.
• Missing API keys produce a direct message ("add it to .env") instead of an unhelpful 401.
• Vocal generation reuses the same polling code, and attaches a callback URL only if one is configured.

What I'd Improve

• Polling plus callbacks is redundant if one is reliable; choose callback with polling as a fallback and persist task state so a server restart doesn't lose an in-flight job.
• Exponential backoffbackoffWaiting progressively longer between retries after failures — often exponentially, with random jitter — so a struggling service isn't hammered. on the poll interval.
• Cache identical requests so repeated parameter combinations don't spend credits.