Remove kibana.defaultAppId

[flash1293]

Currently there are two settings around which page to load when the user navigates to a Kibana instance without a path - the defaultRoute advanced setting that allows to load another app than the default kibana app (e.g. app/canvas) and the yml file setting kibana.defaultAppId that's used to route to a sub app within the kibana app.

On the one hand this is inherently confusing because two different things are described as "app" (without much details in the documentation) and on the other hand the kibana app will be broken into multiple apps in the next major version, making the extra setting unnecessary.

While maintaining BWC is possible IMHO it is too complicated in this case to justify the effort, so I propose a breaking change in 8.0.

kibana.defaultAppId should be removed completely and all places currently referencing defaultAppId should be changed to the defaultRoute ui setting. This change has to be documented in the breaking change log.

Which release will ship the breaking change?

8.0

Describe the change. How will it manifest to users?

If the user has set a kibana.defaultAppId key in their kibana.yml, Kibana will refuse to start up.

How many users will be affected?

We don't have telemetry on this, but this setting has been superseded a long time, so impact should be minimal. There's already a warning in the logs in recent Kibana versions if it is used.

What can users do to address the change manually?

Users should remove the kibana.defaultAppId setting and replace it using an advanced setting "defaultRoute"

How could we make migration easier with the Upgrade Assistant?

Theoretically we could show a warning in the assistant if the setting is present by exposing it to the browser and pass it to the assistant plugin, but I'm not sure whether it's worth it. The Elasticsearch UI team might now better.

Are there any edge cases?

Test Data

Set kibana.defaultAppId to discover. This works in current version of Kibana, but will break in 8.0

Related #46331

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[flash1293]

@elastic/kibana-platform is the plan proposed here sound? I'm not sure whether I'm missing an aspect.

[joshdover]

Sounds reasonable to me, but I also don't have a lot of context on this. If I remember correctly, I think @legrego was actually the last one to touch this feature.

[legrego]

Yes, let's please drop defaultAppId for 8.0. Folks should be using the defaultRoute UI setting now, as this is space aware and more flexible. We should deprecate this as soon as possible so that we're at least warning users in the logs when we first start up if they have this customized.

[cjcenizal]

@flash1293 #6902 looks very similar to this issue. It contains some historical context which might not be relevant any more. Should we close it in favor of this one?

[flash1293]

Closed the other issue, thanks @cjcenizal