Merge pull request #185 from lsst-sqre/tickets/DM-50042

DM-50042: Set Expires and Cache-Control on links replies

Author: rra

changelog.d/20250411_114846_rra_DM_50042.md

@@ -0,0 +1,3 @@
+### New features
+
+- Set `Expires` and `Cache-Control` headers on the links reply reflecting the expiration time of signed image URLs, informing clients that the response should not be cached beyond the expiration of those URLs. The lifetime of the links is specified as a new configuration option for now. That option will be removed once that lifetime is available from Butler.

src/datalinker/config.py

@@ -2,12 +2,14 @@
 
 from __future__ import annotations
 
+from datetime import timedelta
 from pathlib import Path
 from typing import Annotated
 
 from pydantic import Field, HttpUrl, SecretStr
 from pydantic_settings import BaseSettings, SettingsConfigDict
 from safir.logging import LogLevel, Profile
+from safir.pydantic import HumanTimedelta
 
 __all__ = [
     "Config",
@@ -42,6 +44,14 @@ class Config(BaseSettings):
         ),
     ] = "/api/hips"
 
+    links_lifetime: Annotated[
+        HumanTimedelta,
+        Field(
+            title="Lifetime of image links replies",
+            description="Should match the lifetime of signed URLs from Butler",
+        ),
+    ] = timedelta(hours=1)
+
     log_level: Annotated[
         LogLevel,
         Field(title="Log level of the application's logger"),

src/datalinker/handlers/external.py

@@ -1,5 +1,7 @@
 """Handlers for the app's external root, ``/datalinker/``."""
 
+from datetime import UTC, datetime
+from email.utils import format_datetime
 from typing import Annotated, Literal
 from urllib.parse import urlencode
 
@@ -276,6 +278,8 @@ def links(
             ],
         )
 
+    lifetime = int(config.links_lifetime.total_seconds())
+    expires = datetime.now(tz=UTC) + config.links_lifetime
     return _TEMPLATES.TemplateResponse(
         request,
         "links.xml",
@@ -286,5 +290,9 @@ def links(
             "image_size": image_uri.size(),
             "cutout_sync_url": str(config.cutout_sync_url),
         },
+        headers={
+            "Cache-Control": f"max-age={lifetime}",
+            "Expires": format_datetime(expires, usegmt=True),
+        },
         media_type="application/x-votable+xml",
     )

tests/handlers/external_test.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+from datetime import timedelta
+from email.utils import parsedate_to_datetime
 from unittest.mock import patch
 from urllib.parse import parse_qs, urlparse
 from uuid import uuid4
@@ -10,6 +12,7 @@
 from httpx import AsyncClient
 from jinja2 import Environment, PackageLoader, select_autoescape
 from lsst.daf.butler import LabeledButlerFactory
+from safir.datetime import current_datetime
 
 from datalinker.config import config
 
@@ -194,13 +197,19 @@ async def test_links(client: AsyncClient, mock_butler: MockButler) -> None:
         f"https://presigned-url.example.com/{mock_butler.uuid!s}"
         "?X-Amz-Signature=abcdef"
     )
+    expected_expires = current_datetime() + config.links_lifetime
 
     # Use iD to test the IVOA requirement of case insensitive parameters.
     r = await client.get(
         "/api/datalink/links",
         params={"iD": f"butler://label-http/{mock_butler.uuid!s}"},
     )
     assert r.status_code == 200
+    expires = parsedate_to_datetime(r.headers["Expires"])
+    assert expected_expires <= expires
+    assert expires <= expected_expires + timedelta(seconds=5)
+    lifetime = int(config.links_lifetime.total_seconds())
+    assert r.headers["Cache-Control"] == f"max-age={lifetime}"
 
     # The URL is already signed, so it should be passed through unchanged
     env = Environment(