The code for this page was making assumptions about properties which
aren’t present on rejected broadcasts.
This commit accounts for those properties and presents the relevant
elements on the page.
There are basically two kinds of 4G masts:
Frequency | Range | Bandwidth
----------|-------------|----------------------------------
800MHz | Long (500m) | Low (can handle a bit of traffic)
1800Mhz | Short (5km) | High (can handle lots of traffic)
The 1800Mhz masts are better in terms of how much traffic they can
handle and how fast a connection they provide. But because they have
quite short range, it’s only economical to install them in very built up
areas†.
In more rural areas the 800MHz masts are better because they cover a
wider area, and have enough bandwidth for the lower population density.
The net effect of this is that cell broadcasts in rural areas are likely
to bleed further, because the masts they are being broadcast from are
less precise.
We can use population density as a proxy for how likely it is to be
covered by 1800Mhz masts, and therefore how much bleed we should expect.
So this commit varies the amount of bleed shown based on the population
density.
I came up with the formula based on 3 fixed points:
- The most remote areas (for example the Scottish Highlands) should have
the highest average bleed, estimated at 5km
- An town, like Crewe, should have about the same bleed as we were
estimating before (1.5km) – Pete D thinks this is about right based on
his knowledge of the area around his office in Crewe
- The most built up areas, like London boroughs, could have as little as
500m of bleed
Based on these three figures I came up with the following formula, which
roughly gives the right bleed distance (`b`) for each of their population
densities (`d`):
```
b = 5900 - (log10(d) × 1_250)
```
Plotted on a curve it looks like this:
This is based on averages – remember that the UI shows where is _likely_
to receive the alert, based on bleed, not where it’s _possible_ to
receive the alert.
Here’s what it looks like on the map:
---
†There are some additional subtleties which make this not strictly true:
- The 800Mhz masts are also used in built up areas to fill in the gaps
between the areas covered by the 1800Mhz masts
- Switching between masts is inefficient, so if you’re moving fast
through a built up area (for example on a train) your phone will only
use the 800MHz masts so that you have to handoff from one mast to
another less often
This adds to the `__le__` method on the `BroadcastMessage` class to
allow two BroadcastMessages with no `updated_at` to be compared.
Previously, the method expected at least one message to have
`updated_at` set, but this is not necessarily the case.
Broadcasts created by the API are different in that:
- they aren’t created by any user, so don’t have a `created_by_id`
- they are created instantly, not in steps, so don’t have an
`updated_at` time
This commit alters the views to account for when these pieces of
information aren’t present.
At the moment the admin app expects all broadcasts to have a template,
and expects the content of the alert to come from the template.
This commit makes it so those pages can still get a `Template` instance,
but populated with content straight from the `content` field in the
database.
We think that in some cases alerts will be composed in the moment, and
therefore making people first create a template is:
- not a good use of their time
- adding some conceptual complexity which they don’t need
This commit makes it possible to type some words and have them go
straight into the `content` field in the database.
In the future we might want to progressively enhance the radio buttons
so they show on the same page (like we do with the grey buttons on the
templates page).
We have lots of functions for converting various types of data into
strings to be displayed to the user somewhere.
This commit collects all these functions into their own module, rather
than having them cluttering up `app/__init__.py` or buried amongst
various other things that have ended up in `app/utils.py`.
For emails and text messages we sort by the time the user (or API) sent
them.
This makes sense for broadcasts too, since most users will receive the
alert within seconds of it being broadcast.
For alerts that haven’t started yet we can sort by `updated_at`, which
is when the user preparing the broadcast submitted it for approval.
Our style for areas is pale blue background with black keylines or bold
black text.
This commit makes the display of area names on the dashboard consistent
with that visual style.
This also means that we’re not truncating the list of areas, which is
appropriate because no one area is more important than any of the
others.
We’ve had some feedback from user research that difference between
‘will get alert’ and ‘likely to get alert’ is not clear, and it’s hard
to tell if the latter is inclusive of the former. This leads people to
question the validity of these numbers, which is important, because an
the estimate should give you some idea of the impact of what you’re
about to do.
This commit reformats the number as a range, for example 1,000 to 2,000
phones.
If the range is small, eg 40,000,000 to 40,800,000 then this suggests
a false level of accuracy. So instead we just give one number and say
it’s an estimate, eg ‘40,000,000 phones estimated’
If you’re adding another area to your broadcast it’s likely to be close
to one of the areas you’ve already added.
But we make you start by choosing a library, then you have to find the
local authority again from the long list. This is clunky, and it
interrupts the task the user is trying to complete.
We thought about redirecting you somewhere deep into the hierarchy,
perhaps by sending you to either:
- the parent of the last area you’d chosen
- the common ancestor of all the areas you’d chosen
This approach would however mean you’d need a way to navigate back up
the hierarchy if we’d dropped you in the wrong place. And we don’t have
a pattern for that at the moment.
So instead this commit adds some ‘shortcuts’ to the chose library page,
giving you a choice of all the parents of the areas you’ve currently
selected. In most cases this will be one (unitary authority) or two
(county and district) choices, but it will scale to adding areas from
multiple different authorities.
It does mean an extra click compared to the redirect approach, but this
is still fewer, easier clicks compared to now.
This meant a couple of under-the-hood changes:
- making `BroadcastArea`s hashable so it’s possible to do
`set([BroadcastArea(…), BroadcastArea(…), BroadcastArea(…)])`
- making `BroadcastArea`s aware of which library they live in, so we can
link to the correct _Choose area_ page
At the moment there are some areas which have:
- a `count_of_phones` value of `None`
- no sub-areas
This is wrong, but until we fix the data the phone counting code needs
to handle this.
This commit:
- adds the `or 0` in the right place (where it will catch these areas
with missing data)
- adds a test which checks these areas, and compares them to other kinds
of areas
We need to give people a better feel for the consequences of
broadcasting an alert. We’ve seen in research that some users will
assume it is subscription based, or opt-in, rather than going to every
phone in the area.
I reckon that the most effective way to communicate this is to put some
numbers next to the areas, to give people an idea of how many people
will get alerted.
We can estimate how many phones are in an area by:
- taking the population of all electoral wards in that area
- multiplying it by the percentage of people who own an internet
connected phone[1]
The Office for National Statistics publish both these datasets.
The number of people who own an intenet connected phone varies a lot by
age. Since the population data for each ward is broken down by age we
can factor this in. Simplified, the calculation looks like this:
- take the _Abbey_ ward of _Barking and Dagenham_
- in this ward there are 26 people aged 80
- 40% of people over 65 have an internet-connected phone
- therefore 10 of these 80-year-olds would be likely to receive a
broadcast
- (repeat for all other ages)
These numbers won’t be exact, but should be enough to give people a feel
for the severity of what they’re about to do. We can see if they acheive
this aim in user research.
1. This is a proxy for the number of people who are likely to have a 4G
capable phone, because only 4G capable phones will be receiving
broadcasts to begin with
When creating broadcast message, or updating it.
We want to send simple polygons to API so we can
later relay them to the broadcast provider.
Test that update broadcast message sends polygons correctly
We have a bunch of stuff for doing lat/long transformation in the
`BroadcastMessage` class. This is not a good separation of concerns, now
that we have a separate class for dealing with polygons and coordinates.
This commit does two things:
- uses our new polygon-simplifying library to process the polygons
before storing them, rather than processing them in real time
- stores only the polygons in the database, rather than the whole
GeoJSON feature, because we don’t need any of the other information
about the feature
Simplifying polygons means reducing the number of points used to render
them. This commit implements simplification such that, for any given
input polygons, the combined point count of the simplified polygons is
less than 100.
When simplifying the polygons we are trying to get the smallest number
of points while meeting these two rules:
1. No part of the area the user has chosen can be cut off
2. The area of the simplified polygon should be as small as possible
This commit introduces two techniques we weren’t using before:
1. Dilating and eroding the area to fill in concave details of the
shape, like inlets and harbours[1]
2. Making the simplification threshold proportionate to the perimeter of
all polygons, so bigger and crinklier polygons get more
simplification applied
It also shows the estimated bleed as a separate polygon. This lets us
make it bigger (so it’s more closer the the approximate bleed) without
having to send a bigger area to the CBC and compounding the amount of
actual bleed.
1. Inspired by this blog post about ‘removing the crinkley bits’ from
Vancouver Island:
http://blog.cleverelephant.ca/2010/11/removing-complexities.html
Since we added the end time picker:
- we have discovered that broadcasts can’t be longer than 24h
- we have observed that most users confuse picking the end time for
scheduling the message, or don’t understand exactly what it means for
the broadcast to ‘end’
- we’ve developed the concept of ‘training mode’, which you should be
going through before sending a real broadcast
We also think that, for most scenarios, you won’t necessarily know when
a broadcast should end at the time of starting it because the cause of
the danger is not within your control. So giving you control of the
end time before the broadcast has even been approved is a confusing
distraction.
Having to pick a time at all also makes the whole process feel more
planned and less immediate. Whereas in reality all the phones in the
area will be getting the message in seconds. It’s only people coming
into the area later to whom the ‘ongoing’ aspect of the broadcast
applies.
The best place to explain what’s happening with the phones is at the
approval stage and once you’ve sent your first (training mode)
broadcast. It’s easier to explain what’s happened if it’s in direct
response to something you’ve just done.
Later on we should add some kind of email reminder after 12 hours to
make sure you still want the broadcast live, again after 18 hours, etc.
We could let you schedule an end time once the broadcast is live, but
don’t think there’s a strong need. Knowing enough that you want to
cancel is one thing, but knowing enough to want to cancel but wanting to
wait a bit… nah.
Broadcasting is not a precise technology, because:
- cell towers are directional
- their range varies depending on whether they are 2, 3, 4, or 5G
(the higher the bandwidth the shorter the range)
- in urban areas the towers are more densely packed, so a phone is
likely to have a greater choice of tower to connect to, and will
favour a closer one (which has a stronger signal)
- topography and even weather can affect the range of a tower
So it’s good for us to visually indicate that the broadcast is not as
precise as the boundaries of the area, because it gives the person
sending the message an indication of how the technology works.
At the same time we have a restriction on the number of polygons we
think and area can have, so we’ve done some work to make versions of
polygons which are simplified and buffered (see
https://github.com/alphagov/notifications-utils/pull/769 for context).
Serendipitously, the simplified and buffered polygons are larger and
smoother than the detailed polygons we’ve got from the GeoJSON files. So
they naturally give the impression of covering an area which is wider
and less precise.
So this commit takes those simple polygons and uses them to render the
blue fill. This makes the blue fill extend outside the black stroke,
which is still using the detailed polygons direct from the GeoJSON.
At the moment they will get a ‘technical difficulties’ error if they
try.
We probably want to do something around letting people self-approve
broadcasts in trial mode, but for now just telling them they can’t is a
better experience than ‘technical difficulties’ (and will probably be
close to what they should see on a live service as well).
Different emergencies will need broadcasts to last for a variable amount
of time. We give users some control over this by letting them stop a
broadcast early. But we should also let them set a maximum broadcast
time, for:
- when the duration of the danger is known
- when the broadcast has been live long enough to alert everyone who
needs to know about it
This code re-uses the pattern for scheduling jobs, which has some
constraints that are probably OK for now:
- end time is limited to an hour
- longest duration is 3 whole days (eg if you start broadcasting Friday
you have the choice of Saturday, Sunday and all of Monday, up to
midnight)
If a broadcast definitely shouldn’t go out (for example because it has a
spelling mistake or is going to the wrong areas) then we should have a
way of removing it. Once it’s removed no-one else can approve it, and it
isn’t cluttering up the dashboard.
This is a link (because it’s a secondary action) and red (because it’s
destructive, in that it’s throwing away someone’s work).
Since new broadcasts will go into `pending-approval`, we now need a way
of approving them.
This commit adds a button to this page to start (or approve) the
broadcast. This button is wrapped in a bordered box, to emphasise that
it’s something consequential.
We don’t want one person going full yolo and start broadcasting without
any oversight. This commit changes the flow so that the button on the
‘preview’ page puts the broadcast into `pending-approval`, rather than
directly into `broadcasting`.
There was a lot of duplicate code here for updating the status and other
properties of a broadcast.
This commit abstracts them into reusable methods. They’re named with an
underscore to suggest that they shouldn’t be used externally.
This commit adds a page to view a single broadcast. This is important
for two reasons:
- users need an audit of what happened when, and who else was involved
in approving or cancelling a broadcast
- we need a place to put actions (approving, cancelling) on a broadcast
so that you can confirm details of the message and the areas before
performing the action
Shows the broadcasts with the longest time still to live at the top. At
the moment this will be the same as the newest broadcasts, so we may
want to revisit this sort order when we have broadcasts of variable
duration.
For no-longer-live broadcasts we show the most-recently-finished at the
top, whether it finished naturally or was cancelled.
Currently this is a `get` request from the dashboard. Once we have a page
for viewing an individual broadcast it should probably show there
instead and be a `get`/confirm/`post` loop like for deleting a template.
Shows current and previous broadcasts. Does not add a page for viewing
an individual broadcast.
Broadcasts are split into live and previous.
Draft broadcasts are excluded from the dashboard.
This commit removes the code the puts areas into the session and instead
creates and then updates a draft broadcast in the database.
This is so we can avoid session-related bugs, and potentially having a
large session when we start adding personalisation etc.
Once a broadcast is ready to go it is set to `broadcasting` straight
away with no approval. We’ll revisit this as we learn more about how
users might want to manage who can create and approve broadcasts.
The tests are a bit light in terms of checking what’s on the page, but
clicking through the pages is probably good enough for now.
