Improve handling of download concurrency

Don't save download concurrency defaults in the database. Make it easier
for individual plugins to override.

closes: #8897
https://pulp.plan.io/issues/8897

CHANGES/8897.feature

@@ -0,0 +1 @@
+The default ``download_concurrency`` of 10 was found to be too high for many client types, it has been reduced to 8. Additionally, where before ``download_concurrency`` would be set to a default value upon creation, it will now be set NULL (but a default value will still be used).

CHANGES/plugin_api/8897.feature

@@ -0,0 +1 @@
+Added a field ``DEFAULT_DOWNLOAD_CONCURRENCY`` to the Remote base class - plugin writers can override the number of concurrent downloads for each type of remote. The default value is 8.

pulpcore/app/migrations/0066_download_concurrency_changes.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.24 on 2021-06-13 03:14
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0065_merge_20210615_1211'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='remote',
+            name='download_concurrency',
+            field=models.PositiveIntegerField(null=True, validators=[django.core.validators.MinValueValidator(1)]),
+        ),
+    ]

pulpcore/app/models/repository.py

@@ -256,6 +256,8 @@ class Remote(MasterModel):
     ON_DEMAND = "on_demand"
     STREAMED = "streamed"
 
+    DEFAULT_DOWNLOAD_CONCURRENCY = 8
+
     POLICY_CHOICES = (
         (IMMEDIATE, "When syncing, download all metadata and content now."),
         (
@@ -288,7 +290,7 @@ class Remote(MasterModel):
     proxy_username = models.TextField(null=True)
     proxy_password = models.TextField(null=True)
 
-    download_concurrency = models.PositiveIntegerField(default=10)
+    download_concurrency = models.PositiveIntegerField(null=True, validators=[MinValueValidator(1)])
     policy = models.TextField(choices=POLICY_CHOICES, default=IMMEDIATE)
 
     total_timeout = models.FloatField(
@@ -323,7 +325,10 @@ def download_factory(self):
         try:
             return self._download_factory
         except AttributeError:
-            self._download_factory = DownloaderFactory(self)
+            # if a download concurrency value was not provided (0), then use the default
+            # concurrency, which can be overridden on a per-remote-type basis.
+            concurrency = self.download_concurrency or self.DEFAULT_DOWNLOAD_CONCURRENCY
+            self._download_factory = DownloaderFactory(self, download_concurrency=concurrency)
             return self._download_factory
 
     @property

pulpcore/app/serializers/repository.py

@@ -136,7 +136,12 @@ class RemoteSerializer(ModelSerializer):
         help_text="Timestamp of the most recent update of the remote.", read_only=True
     )
     download_concurrency = serializers.IntegerField(
-        help_text="Total number of simultaneous connections.", required=False, min_value=1
+        help_text=(
+            "Total number of simultaneous connections. If not provided or set to zero (0), "
+            "then the default value will be used."
+        ),
+        required=False,
+        min_value=1,
     )
     policy = serializers.ChoiceField(
         help_text="The policy to use when downloading content.",

pulpcore/download/factory.py

@@ -65,15 +65,17 @@ class DownloaderFactory:
     to session continuation implementation in various servers.
     """
 
-    def __init__(self, remote, downloader_overrides=None):
+    def __init__(self, remote, downloader_overrides=None, download_concurrency=None):
         """
         Args:
             remote (:class:`~pulpcore.plugin.models.Remote`): The remote used to populate
                 downloader settings.
             downloader_overrides (dict): Keyed on a scheme name, e.g. 'https' or 'ftp' and the value
                 is the downloader class to be used for that scheme, e.g.
                 {'https': MyCustomDownloader}. These override the default values.
+            download_concurrency (int): How many files may be downloaded concurrently.
         """
+        download_concurrency = download_concurrency or remote.DEFAULT_DOWNLOAD_CONCURRENCY
         self._remote = remote
         self._download_class_map = copy.copy(PROTOCOL_MAP)
         if downloader_overrides:
@@ -85,7 +87,7 @@ def __init__(self, remote, downloader_overrides=None):
             "file": self._generic,
         }
         self._session = self._make_aiohttp_session_from_remote()
-        self._semaphore = asyncio.Semaphore(value=remote.download_concurrency)
+        self._semaphore = asyncio.Semaphore(value=download_concurrency)
         atexit.register(self._session_cleanup)
 
     def _session_cleanup(self):

pulpcore/download/http.py

@@ -201,9 +201,9 @@ async def _run(self, extra_data=None):
         """
         Download, validate, and compute digests on the `url`. This is a coroutine.
 
-        This method is decorated with a backoff-and-retry behavior to retry HTTP 429 and
-        some 5XX errors. It retries with exponential backoff 10 times before allowing
-        a final exception to be raised.
+        This method is decorated with a backoff-and-retry behavior to retry some errors.
+        It retries with exponential backoff 10 times before allowing a final exception to
+        be raised.
 
         This method provides the same return object type and documented in
         :meth:`~pulpcore.plugin.download.BaseDownloader._run`.

pulpcore/tests/functional/api/using_plugin/test_crud_repos.py

@@ -254,7 +254,7 @@ def test_read(self):
         self._compare_results(self.remote_attrs, self.remote)
 
     def test_update(self):
-        data = {"download_concurrency": 66, "policy": "immediate"}
+        data = {"download_concurrency": 23, "policy": "immediate"}
         self.remotes_api.partial_update(self.remote.pulp_href, data)
         time.sleep(1)  # without this, the read returns the pre-patch values
         new_remote = self.remotes_api.read(self.remote.pulp_href)