add new redis template usage per day key

We've run into issues with redis expiring keys while we try and write
to them - short lived redis TTLs aren't really sustainable for keys
where we mutate the state. Template usage is a hash contained in redis
where we increment a count keyed by template_id each time a message is
sent for that template. But if the key expires, hincrby (redis command
for incrementing a value in a hash) will re-create an empty hash.

This is no good, as we need the hash to be populated with the last
seven days worth of data, which we then increment further. We can't
tell whether the hincrby created the key, so a different approach
entirely was needed:

* New redis key: <service_id>-template-usage-<YYYY-MM-DD>. Note: This
  YYYY-MM-DD is BTC time so it lines up nicely with ft_billing table
* Incremented to from process_notification - if it doesn't exist yet,
  it'll be created then.
* Expiry set to 8 days every time it's incremented to.

Then, at read time, we'll just read the last eight days of keys from
Redis, and sum them up. This works because we're only ever incrementing
from that one place - never setting wholesale, never recreating the
data from scratch. So we know that if the data is in redis, then it is
good and accurate data.

One thing we *don't* know and *cannot* reason about is what no key in
redis means. It could be either of:

* This is the first message that the service has sent today.
* The key was deleted from redis for some reason.

Since we set the TTL to so long, we'll never be writing to a key that
previously expired. But if there is a redis (or operator) error and the
key is deleted, then we'll have bad data - after any data loss we'll
have to rebuild the data.

tests/app/notifications/test_process_notification.py

@@ -1,11 +1,13 @@
 import datetime
 import uuid
-import pytest
+from unittest.mock import call
 
+import pytest
 from boto3.exceptions import Boto3Error
 from sqlalchemy.exc import SQLAlchemyError
 from freezegun import freeze_time
 from collections import namedtuple
+from flask import current_app
 
 from app.models import (
     Notification,
@@ -205,9 +207,11 @@ def test_persist_notification_with_optionals(sample_job, sample_api_key, mocker)
 
 
 @freeze_time("2016-01-01 11:09:00.061258")
-def test_persist_notification_increments_cache_if_key_exists(sample_template, sample_api_key, mocker):
+def test_persist_notification_doesnt_touch_cache_for_old_keys_that_dont_exist(sample_template, sample_api_key, mocker):
     mock_incr = mocker.patch('app.notifications.process_notifications.redis_store.incr')
     mock_incr_hash_value = mocker.patch('app.notifications.process_notifications.redis_store.increment_hash_value')
+    mocker.patch('app.notifications.process_notifications.redis_store.get', return_value=None)
+    mocker.patch('app.notifications.process_notifications.redis_store.get_all_from_hash', return_value=None)
 
     persist_notification(
         template_id=sample_template.id,
@@ -221,11 +225,20 @@ def test_persist_notification_increments_cache_if_key_exists(sample_template, sa
         reference="ref"
     )
     mock_incr.assert_not_called()
-    mock_incr_hash_value.assert_not_called()
+    mock_incr_hash_value.assert_called_once_with(
+        str(sample_template.service_id) + "-template-usage-2016-01-01",
+        sample_template.id
+    )
 
+
+@freeze_time("2016-01-01 11:09:00.061258")
+def test_persist_notification_increments_cache_if_key_exists(sample_template, sample_api_key, mocker):
+    mock_incr = mocker.patch('app.notifications.process_notifications.redis_store.incr')
+    mock_incr_hash_value = mocker.patch('app.notifications.process_notifications.redis_store.increment_hash_value')
     mocker.patch('app.notifications.process_notifications.redis_store.get', return_value=1)
     mocker.patch('app.notifications.process_notifications.redis_store.get_all_from_hash',
                  return_value={sample_template.id, 1})
+
     persist_notification(
         template_id=sample_template.id,
         template_version=sample_template.version,
@@ -236,9 +249,12 @@ def test_persist_notification_increments_cache_if_key_exists(sample_template, sa
         api_key_id=sample_api_key.id,
         key_type=sample_api_key.key_type,
         reference="ref2")
+
     mock_incr.assert_called_once_with(str(sample_template.service_id) + "-2016-01-01-count", )
-    mock_incr_hash_value.assert_called_once_with(cache_key_for_service_template_counter(sample_template.service_id),
-                                                 sample_template.id)
+    assert mock_incr_hash_value.mock_calls == [
+        call(str(sample_template.service_id) + "-template-counter-limit-7-days", sample_template.id),
+        call(str(sample_template.service_id) + "-template-usage-2016-01-01", sample_template.id),
+    ]
 
 
 @pytest.mark.parametrize('research_mode, requested_queue, expected_queue, notification_type, key_type',
@@ -446,3 +462,41 @@ def test_persist_email_notification_stores_normalised_email(
 
     assert persisted_notification.to == recipient
     assert persisted_notification.normalised_to == expected_recipient_normalised
+
+
+@pytest.mark.parametrize('utc_time, day_in_key', [
+    ('2016-01-01 23:00:00', '2016-01-01'),
+    ('2016-06-01 22:59:00', '2016-06-01'),
+    ('2016-06-01 23:00:00', '2016-06-02'),
+])
+def test_persist_notification_increments_and_expires_redis_template_usage(
+    utc_time,
+    day_in_key,
+    sample_template,
+    sample_api_key,
+    mocker
+):
+    mock_incr_hash_value = mocker.patch('app.notifications.process_notifications.redis_store.increment_hash_value')
+    mock_expire = mocker.patch('app.notifications.process_notifications.redis_store.expire')
+    mocker.patch('app.notifications.process_notifications.redis_store.get', return_value=None)
+    mocker.patch('app.notifications.process_notifications.redis_store.get_all_from_hash', return_value=None)
+
+    with freeze_time(utc_time):
+        persist_notification(
+            template_id=sample_template.id,
+            template_version=sample_template.version,
+            recipient='+447111111122',
+            service=sample_template.service,
+            personalisation={},
+            notification_type='sms',
+            api_key_id=sample_api_key.id,
+            key_type=sample_api_key.key_type,
+        )
+    mock_incr_hash_value.assert_called_once_with(
+        '{}-template-usage-{}'.format(str(sample_template.service_id), day_in_key),
+        sample_template.id
+    )
+    mock_expire.assert_called_once_with(
+        '{}-template-usage-{}'.format(str(sample_template.service_id), day_in_key),
+        current_app.config['EXPIRE_CACHE_EIGHT_DAYS']
+    )