?_ttl= parameter and default_cache_ttl config

Refs #285, Closes #289

datasette/app.py

@@ -83,6 +83,9 @@ CONFIG_OPTIONS = (
     ConfigOption("allow_sql", True, """
         Allow arbitrary SQL queries via ?sql= parameter
     """.strip()),
+    ConfigOption("default_cache_ttl", 365 * 24 * 60 * 60, """
+        Default HTTP cache TTL (used in Cache-Control: max-age= header)
+    """.strip()),
 )
 DEFAULT_CONFIG = {
     option.name: option.default

datasette/views/base.py

@@ -257,7 +257,17 @@ class BaseView(RenderMixin):
             r.status = status_code
         # Set far-future cache expiry
         if self.ds.cache_headers:
-            r.headers["Cache-Control"] = "max-age={}".format(365 * 24 * 60 * 60)
+            ttl = request.args.get("_ttl", None)
+            if ttl is None or not ttl.isdigit():
+                ttl = self.ds.config["default_cache_ttl"]
+            else:
+                ttl = int(ttl)
+            if ttl == 0:
+                ttl_header = 'no-cache'
+            else:
+                ttl_header = 'max-age={}'.format(ttl)
+            r.headers["Cache-Control"] = ttl_header
+        r.headers["Referrer-Policy"] = "no-referrer"
         return r
 
     async def custom_sql(

docs/config.rst

@@ -92,3 +92,12 @@ allow_sql
 Enable/disable the ability for users to run custom SQL directly against a database. To disable this feature, run::
 
     datasette mydatabase.db --config allow_sql:off
+
+default_cache_ttl
+-----------------
+
+Default HTTP caching max-age header in seconds, used for ``Cache-Control: max-age=X``. Can be over-ridden on a per-request basis using the ``?_ttl=`` querystring parameter. Set this to ``0`` to disable HTTP caching entirely. Defaults to 365 days (31536000 seconds).
+
+::
+
+    datasette mydatabase.db --config default_cache_ttl:10

docs/json_api.rst

@@ -164,6 +164,10 @@ The Datasette table view takes a number of special querystring arguments:
     long, for example if you want to implement autocomplete search but only if
     it can be executed in less than 10ms.
 
+``?_ttl=SECONDS``
+    For how many seconds should this response be cached by HTTP proxies? Use
+    ``?_ttl=0`` to disable HTTP caching entirely for this request.
+
 ``?_next=TOKEN``
     Pagination by continuation token - pass the token that was returned in the
     ``"next"`` property by the previous page.

tests/test_api.py

@@ -928,6 +928,7 @@ def test_config_json(app_client):
         "allow_facet": True,
         "suggest_facets": True,
         "allow_sql": True,
+        "default_cache_ttl": 365 * 24 * 60 * 60,
     } == response.json
 
 
@@ -1127,3 +1128,14 @@ def test_suggest_facets_off():
             "/test_tables/facetable.json",
             gather_request=False
         ).json["suggested_facets"]
+
+
+@pytest.mark.parametrize('path,expected_cache_control', [
+    ("/test_tables/facetable.json", "max-age=31536000"),
+    ("/test_tables/facetable.json?_ttl=invalid", "max-age=31536000"),
+    ("/test_tables/facetable.json?_ttl=10", "max-age=10"),
+    ("/test_tables/facetable.json?_ttl=0", "no-cache"),
+])
+def test_ttl_parameter(app_client, path, expected_cache_control):
+    response = app_client.get(path, gather_request=False)
+    assert expected_cache_control == response.headers['Cache-Control']