A way to scrub telemetry_ecto events of sensitive data

[lukad]

We recently noticed that ecto telemetry events emitted for postgres errors contains the full executed query including the actual values passed to queries or the actual values conflicting with constraints for example.

Here is a screenshot of an example:

This is a problem because we don't want to leak sensitive data such as personally identifying information to whatever system consumes these events.

Describe the solution you'd like
I think a builtin way to transform events, which allows library users to implement data scrubbing, would be a good addition to telemetry_ecto.

Our current workaround looks like this, we have our own module that attaches an event handler for repo events. The module has an event handler that forwards all events to OpentelemetryEcto.handle_event but scrubs errors of sensitive data.

defmodule MyApp.OpentelemetryEcto do
  def setup(event_prefix, config \\ []) do
    event = event_prefix ++ [:query]
    :telemetry.attach({__MODULE__, event}, event, &__MODULE__.handle_event/4, config)
  end

  def handle_event(event, measurements, %{result: {:error, error}} = data, config) do
    error = scrub_error(error)
    OpentelemetryEcto.handle_event(event, measurements, %{data | result: {:error, error}}, config)
  end

  def handle_event(event, measurements, data, config) do
    OpentelemetryEcto.handle_event(event, measurements, data, config)
  end

  defp scrub_error(%Postgrex.Error{} = error) do
    ...
  end
end

Ideally OpentelemetryEcto provides a way to specify a module or function that will be called for all handled events to transform them.

The most basic way to achieve this could be OpentelemetryEcto.setup([:my_app, :repo], transform: fn _even -> ... end).

Describe alternatives you've considered
The workaround above works fine, but I believe it's worth having a builtin and documented way of doing this.

What do you think? I'd be happy to contribute a MR.

[bryannaegele]

What happens if you set redact: true on the schema field?

[lukad]

We already have redact: true on all ecto schema fields we don't want to log. It has no effect here because the errors we're scrubbing are coming from postgrex which doesn't know anything about ecto schemas.

[danschultzer]

You would normally want to scrub it with a processor: https://opentelemetry.io/docs/security/config-best-practices/#scrub-sensitive-data

[lukad]

I see. I think there is value in scrubbing the data earlier like how I'm doing it because it allows me to scrub the %Postgrex.Error{} structs instead of having to do it in a much more unreliable string based way.
If you think this should not be a direct concern of telemetry_ecto then please feel free to close the issue :)

[tsloughter]

And since it gets turned into a string you can't even do it with that processor.

[tsloughter]

Postgrex.Error would have to be flattened to attributes like postgrex.error.<...>.