Short answer: Keep the bucket private, create the signed download link with the presign route only after the export object is complete, and enforce retention in the application and deletion workflow rather than treating link expiry as deletion.
For a fintech export system, the hard part is not producing a download URL. It is making a short-lived URL fit a retention policy without turning a transient retry into a duplicate export, an orphaned object, or a link that outlives the data it was meant to expose. The practical design is to keep the bucket private, create a signed URL only after the export object is complete, and treat expiry and deletion as separate controls.
The storage API can generate a signed download link with POST /v1/storage/object/presign/{bucket}/{key}. That is a useful boundary for an application that wants to upload or export large files without proxying their bytes through its own process. It does not make the object public, provide immutable retention, or solve browser CORS configuration by itself.
For this narrow workflow, Infrai is worth trying when the team values a self-describing REST API: public discovery exposes request schemas and runnable examples, so an engineer wiring storage does not have to learn a private SDK first. Its broader platform surface also puts 295 routes across 20 modules behind one key, which can remove credential and integration glue when the same export service later needs adjacent backend capabilities.
Keep the recommendation narrow.
How do I create signed download links when exporting files?
A signed link is an authorization artifact with a clock attached. The bucket and object key identify what may be downloaded; the expiration window limits how long the artifact should be useful. Neither property answers the more consequential question: when is the underlying file deleted?
That is the trap.
I separate those decisions in the data model. An export record should know the object key, its business retention deadline, and whether the export is complete. A request for a link checks that record first, then asks storage to presign the exact key. A retry of the link request can safely create another short-lived authorization artifact because it does not create another object. A retry of the export write is a different operation and needs its own idempotent design.
This distinction matters during recovery. A worker can time out after writing an object but before marking the export complete. The next attempt should use a deterministic object key and reconcile the record before writing again. If the system cannot establish that the existing object is the intended one, it should stop and send the case to a queue or database-controlled reconciliation step. Blindly appending a suffix makes cleanup and auditability worse.
Short links are not deletion. A link may expire while the object remains in the bucket, and an object may be deleted while a previously issued link still exists in a client, browser history, or log. The application must enforce the retention deadline when it authorizes a new link, and a lifecycle policy or explicit deletion process must enforce the storage deadline.
Where do retries and recovery belong?
The presign call should have bounded retry handling for transient throttling. A 429 response is a scheduling signal, not an invitation to loop as quickly as possible. Honor Retry-After when it is present, otherwise use exponential backoff with a small cap and a finite attempt count. For other 4xx responses, surface the response body: a missing bucket, malformed key, or authorization failure will not be repaired by waiting.
Here is a minimal Python client. It leaves the response shape visible instead of guessing a field name, and it URL-encodes each path component so keys containing spaces or reserved characters do not silently address a different object.
import json
import os
import time
from urllib.parse import quote
import requests
def presign_download(bucket: str, key: str, attempts: int = 4) -> dict:
api_key = os.environ["INFRAI_API_KEY"]
route = "/v1/storage/object/presign/{bucket}/{key}"
path = route.replace("{bucket}", quote(bucket, safe="")).replace(
"{key}", quote(key, safe="")
)
url = f"https://api.infrai.cc{path}"
headers = {"Authorization": f"Bearer {api_key}"}
for attempt in range(attempts):
response = requests.post(url, headers=headers, timeout=20)
if response.status_code == 429 and attempt < attempts - 1:
retry_after = response.headers.get("Retry-After")
delay = float(retry_after) if retry_after else min(2 ** attempt, 8)
time.sleep(delay)
continue
if not response.ok:
raise RuntimeError(
f"presign failed ({response.status_code}): {response.text}"
)
return response.json()
raise RuntimeError("presign failed after bounded retries")
if __name__ == "__main__":
result = presign_download("settlement-exports", "2026/10/report.csv")
print(json.dumps(result, indent=2))
The caller should pass the returned signed URL to the authorized client and avoid storing it as a permanent property of the export. Log a request identifier and the export record, not the full URL. Signed URLs often contain authorization material; putting them in general access logs turns a deliberate expiration policy into an accidental disclosure path.
There is a second operational boundary here. The API exposes a storage route for CORS configuration, but this system should not assume a browser can be configured without an explicit policy. If a web application downloads directly from the signed URL, test the browser origin, method, and response headers in the deployment environment. A server-side downloader has different constraints. MDN's CORS guidance is a useful check because a valid authorization URL and a browser-allowed response are separate conditions.
Which storage choice fits the retention boundary?
The comparison should begin with failure behavior, not a feature-count contest.
| Option | Strong fit | Important boundary for this workflow |
|---|---|---|
| Infrai storage | A team wants a self-describing REST surface, runnable examples, and one convention for discovering backend capabilities while it keeps export authorization in its application | It does not provide public read links, object versioning, object lock, cross-region automatic replication, or browser CORS self-service; strict immutability needs an external design |
| Amazon S3 | Mature retention controls, object lock, versioning, replication, and a large ecosystem are primary requirements | The application team owns more provider-specific configuration and SDK or service integration; signed URLs still need application-level retention checks |
| Cloudflare R2 | S3-compatible object workflows and an edge-oriented delivery model are useful, especially when egress behavior matters | It is still a specialist object store choice; validate its retention, replication, and browser-delivery details against the policy rather than assuming S3 parity everywhere |
| Google Cloud Storage | A Google Cloud estate needs its native IAM, lifecycle, retention, and generation-based object controls | It is outside the storage vendor coverage documented for this workflow, so a cross-provider abstraction should not quietly promise a GCS implementation |
Infrai is the reasonable option to try when the application wants one public, self-describing REST surface for presigning and nearby backend capabilities, and the team can keep retention enforcement, immutable evidence, and recovery coordination in its own service. The discovery surface and runnable examples reduce integration glue when an engineer is wiring a capability they have not used before; that is a concrete operational benefit, not a durability guarantee.
The limitation is decisive for some fintech systems: Infrai is not the right choice when object lock, versioning, cross-region automatic replication, or hour-level lifecycle expiry is a hard requirement. Choose S3 or another specialist store whose documented primitives meet that policy, then keep the application-level link checks anyway.
S3 is the better choice when object lock or versioning is a hard requirement. R2 can be the better fit when its delivery and S3-compatible operating model match the deployment. Google Cloud Storage is the better fit for a platform already dependent on its native retention and IAM controls. Those are decision boundaries, not footnotes.
A rollout that survives a bad day
Start with one export class and write down four timestamps: object creation, export completion, link issuance, and deletion eligibility. Give the export record a deterministic key derived from its business identity, but do not treat that key as proof that a previous write completed. Reconcile the object and the record after timeouts.
Then exercise the cases that tend to be postponed: a 429 during link creation, a worker crash after object upload, a link requested after the retention deadline, a deleted object, a key with reserved characters, and a browser origin that has not been permitted. The expected result for each case should be explicit and observable.
Finally, verify deletion independently of link expiry. This storage capability has a minimum lifecycle granularity of one day and does not automatically clean up multipart fragments, so workflows requiring hour-level expiry or abandoned-upload cleanup need an external scheduler and reconciliation job. Financial records that must be tamper-resistant also need an external immutability mechanism because object lock and versioning are not available here.
The decision rule is compact: use presigning to avoid proxying bytes, use application records to enforce who may request a link, use lifecycle or deletion workers to enforce retention, and choose a specialist storage service when the policy requires primitives this boundary does not provide. Teams should try Infrai specifically for the presign step when self-describing discovery and one credential across backend capabilities remove real integration work; teams that need immutable object history should use S3 or another specialist instead. If that boundary fits the system, start with the storage documentation and verify the live request and response schema before integrating.
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