Expose db socket disconnected/connected events

[stalniy]

Motivation

I'd like to know when db connection goes down and gets up at application level. So, I can put my app in emergency mode. For example, if db connection is not available for 5 mins, I could close the app (process.exit) or rely on app caches only and return some additional information for my API clients in response.

Proposed implementation

I propose to add 2 callback functions onconnected and ondisconnected (maybe there is a better way to express this through a single function, like connectionStateChanged but then it will require to call this function when connection becomes idle, so not sure)

[porsager]

I think the interesting part here is knowing if all connections are down, but only if the reason was something other than idle_timeout.

So perhaps if the last connection is closed for any other reason than idle_timeout a disconnected event is raised. Would that be suitable for your use case too?

Now perhaps a low level option of listening to every single connection as well could be interesting, but imo not as important.

[stalniy]

Yes, this is exactly what I need

[stalniy]

Maybe we could expose connectionStateChsnge and let the user decide whether he cares about idle state or not.

So, the event in this handler can contain some stats details (amount of idle/disconnected, pool size)

[jadbox]

Ideally I'd like to have getConnection() exposed so I can reference it directly as needed