How to store ‘last seen’ for users in Phoenix
This week, I worked on some under-the-hood improvements to Plausible to give me better insights into my userbase. One of these was to store a
last_seen timestamp for all users. This is a private piece of data that I use to determine:
- How many users are actively logging on and checking their analytics
- What is the average usage frequency of Plausible?
- When should I consider an account as ‘rotting’? Meaning I’m about to lose them as a user/customer
Let’s see how this can be achieved in Phoenix. First, we’ll start with the schema.
We’ll need to generate a migration for the
$ mix ecto.gen.migration add_last_seen_to_users
and the migration itself:
defmodule Plausible.Repo.Migrations.AddLastSeenToUsers do use Ecto.Migration def change do alter table(:users) do add :last_seen, :naive_datetime, default: fragment("now()") end end end
At this point, you’ll want to add the field to your
User Ecto schema as well.
Now I want to update this field every time use uses the website. A natural solution here is to add a Plug that intercepts every request. However, updating the database on every request seems fairly wasteful. Besides, for my purposes, the timestamp doesn’t need to be accurate to the millisecond. This is a perfect case for throttling, which can be achieved by using the browser session.
defmodule PlausibleWeb.LastSeenPlug do import Plug.Conn use Plausible.Repo @one_hour 60 * 60 def init(opts) do opts end def call(conn, _opts) do last_seen = get_session(conn, :last_seen) user = conn.assigns[:current_user] cond do user && last_seen && last_seen < (unix_now() - @one_hour) -> persist_last_seen(user) put_session(conn, :last_seen, unix_now()) user && !last_seen -> put_session(conn, :last_seen, unix_now()) true -> conn end end defp persist_last_seen(user) do q = from(u in Plausible.Auth.User, where: u.id == ^user.id) Repo.update_all(q, [set: [last_seen: DateTime.utc_now()]]) end defp unix_now do DateTime.utc_now() |> DateTime.to_unix end end
I added this plug to my
browser pipeline in my router. Note that I have another plug running before this one which looks up the current user and makes
This approach satisfies every requirement I had for this feature:
- Persist the
last_seenwith an hour precision
- Runs a database update at most once per hour per user
- I specifically use
This blog documents the journey of building Plausible
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