Use cache for tasks that save notifications

These tasks need to repeatedly get the same template and service from
the database. We should be able to improve their performance by getting
the template and service from the cache instead, like we do in the REST
endpoint code.

tests/app/celery/test_tasks.py

@@ -49,6 +49,7 @@ from app.models import (
     SMS_TYPE,
     ReturnedLetter,
     NOTIFICATION_CREATED)
+from app.serialised_models import SerialisedService, SerialisedTemplate
 from app.utils import DATETIME_FORMAT
 
 from tests.app import load_example_csv
@@ -1888,3 +1889,119 @@ def test_save_api_email_dont_retry_if_notification_already_exists(sample_service
     assert notifications[0].created_at == datetime(2020, 3, 25, 14, 30)
     # should only have sent the notification once.
     mock_provider_task.assert_called_once_with([data['id']], queue=expected_queue)
+
+
+@pytest.mark.parametrize('task_function, delivery_mock, recipient, template_args', (
+    (
+        save_email,
+        'app.celery.provider_tasks.deliver_email.apply_async',
+        'test@example.com',
+        {'template_type': 'email', 'subject': 'Hello'},
+    ), (
+        save_sms,
+        'app.celery.provider_tasks.deliver_sms.apply_async',
+        '07700 900890',
+        {'template_type': 'sms'}
+    ), (
+        save_letter,
+        'app.celery.letters_pdf_tasks.get_pdf_for_templated_letter.apply_async',
+        '123 Example Street\nCity of Town\nXM4 5HQ',
+        {'template_type': 'letter', 'subject': 'Hello'}
+    ),
+))
+def test_save_tasks_use_cached_service_and_template(
+    notify_db_session,
+    mocker,
+    task_function,
+    delivery_mock,
+    recipient,
+    template_args,
+):
+    service = create_service()
+    template = create_template(service=service, **template_args)
+
+    notification = _notification_json(template, to=recipient)
+    delivery_mock = mocker.patch(delivery_mock)
+    service_dict_mock = mocker.patch(
+        'app.serialised_models.SerialisedService.get_dict',
+        wraps=SerialisedService.get_dict,
+    )
+    template_dict_mock = mocker.patch(
+        'app.serialised_models.SerialisedTemplate.get_dict',
+        wraps=SerialisedTemplate.get_dict,
+    )
+
+    for _ in range(3):
+        task_function(
+            service.id,
+            uuid.uuid4(),
+            encryption.encrypt(notification),
+        )
+
+    # We talk to the database once for the service and once for the
+    # template; subsequent calls are caught by the in memory cache
+    assert service_dict_mock.call_args_list == [
+        call(service.id),
+    ]
+    assert template_dict_mock.call_args_list == [
+        call(str(template.id), str(service.id), 1),
+    ]
+
+    # But we save 3 notifications and enqueue 3 tasks
+    assert len(Notification.query.all()) == 3
+    assert len(delivery_mock.call_args_list) == 3
+
+
+@freeze_time('2020-03-25 14:30')
+@pytest.mark.parametrize('notification_type, task_function, expected_queue, recipient', (
+    ('sms', save_api_sms, QueueNames.SEND_SMS, '+447700900855'),
+    ('email', save_api_email, QueueNames.SEND_EMAIL, 'jane.citizen@example.com'),
+))
+def test_save_api_tasks_use_cache(
+    sample_service,
+    mocker,
+    notification_type,
+    task_function,
+    expected_queue,
+    recipient,
+):
+    mock_provider_task = mocker.patch(
+        f'app.celery.provider_tasks.deliver_{notification_type}.apply_async'
+    )
+    service_dict_mock = mocker.patch(
+        'app.serialised_models.SerialisedService.get_dict',
+        wraps=SerialisedService.get_dict,
+    )
+
+    template = create_template(sample_service, template_type=notification_type)
+    api_key = create_api_key(service=template.service)
+
+    def create_encrypted_notification():
+        return encryption.encrypt({
+            "to": recipient,
+            "id": str(uuid.uuid4()),
+            "template_id": str(template.id),
+            "template_version": template.version,
+            "service_id": str(template.service_id),
+            "personalisation": None,
+            "notification_type": template.template_type,
+            "api_key_id": str(api_key.id),
+            "key_type": api_key.key_type,
+            "client_reference": 'our email',
+            "reply_to_text": "our.email@gov.uk",
+            "document_download_count": 0,
+            "status": NOTIFICATION_CREATED,
+            "created_at": datetime.utcnow().strftime(DATETIME_FORMAT),
+        })
+
+    assert len(Notification.query.all()) == 0
+
+    for _ in range(3):
+        task_function(encrypted_notification=create_encrypted_notification())
+
+    assert service_dict_mock.call_args_list == [
+        call(str(template.service_id))
+    ]
+
+    assert len(Notification.query.all()) == 3
+    assert len(mock_provider_task.call_args_list) == 3