How do you save the Slack modal view ID for future updates?

I'm using Slack's modals + their new BlockKit interactive actions. I have a modal dialog with several section's that include select menu accessories. When one of those accessories changes (ex: user preference goes from Yes -> No), I want to update the modal's view to reflect the change in some contextual text.

The docs outline two ways to update a model. Because I'm not using the traditional modal inputs, I have to update the modal via the API and not via response_action. Proof: I don't even receive a view_submission payload at all... I receive a blockkit_action payload.

The docs conflict a bit. In the above link they say that to update a view you must pass in the returned ID of the view when it was opened:

> Remember the view.id that was included in the success response when
> you used views.open earlier? We hope you kept it, because you can now
> use it to update that view.

But when you look at the docs for the views.open and views.update, it appears there is a second option: external_id. The views.update docs say this:

> A unique identifier of the view set by the developer. Must be unique
> for all views on a team. Max length of 255 characters. Either view_id
> or external_id is required.

Initially I got everything working really nicely by choosing an external ID of myapp-mymodal-[userID]. But as soon as I tried opening the modal simultaneously on my Desktop + Mobile client, I started getting internal_error responses and have been unable to open the modal since!

So I started looking into saving the view ID, per the first quote. My problem is: that ID changes every time the modal opens. I don't understand how I am expected to possibly keep track of the 0-2 "active" view IDs that might be taking place across a user's Desktop + Mobile clients (or more than 2 if they have an iPad, a second laptop, etc).

I tried to look for some sort of unique client ID that I could either use to include in the external_id or in my view_id persistence logic, but given that Slack's API appears to be sesssion-less I couldn't find any such thing.

How do others solve this problem?

Try using the private_metadata field in the View

I initially did the same thing you did, which worked, but I wanted to avoid that edge case if possible. What I figured out, is that I could pass the external_id that I created when opening the view into the view's metadata, and then retrieve it when I receive an interaction.

When I set the external_id I set it to the userID + the current time (so it's always unique, even if the user opens the modal from different devices)

externalID := UserID + time.Now().String()

and pass that both as the external_id for the view, and as a string into the private_metadata.

PrivateMetadata string `json:"private_metadata"`

When the user interacts with one of my blocks and Slack sends me the interactive message, I retrieve the metadata like this:

Request.Payload.View.PrivateMetadata

I wrote my Slack app in Go, but I tried to make it clear for any language. Let me know if anything is unclear or needs expanding on - this is my first time giving back to the Stackoverflow community.