Metrics API (like Health API)

[adria0]

Openethereum needs to add prometheus metrics https://github.com/openethereum/openethereum/issues/11725, and I am looking for the best option to do it.

I realized about the health API, but we cannot use it for now:

1. There is no way to define more that one endpoint for it
2. We cannot use or modify the current /api/health path because it could break current user's monitorization tools.
3. We need to output text instead json (string without " and no carriage return escapes)

I would like to purpose extending the functionality of health_api in the following way, without introducing any breaking change in the current api:

• add a method .raw_api(path,rpc_method) in the builder, which allows to dump the result as Vec without JSONify.
• allow multiple raw api handlers

I am aware that this breaks a little the spirit of a json-based library but opens the possibility to handle this case, but also in a very flexible way that could help other future use cases.

If something like this is ok, I can write the feature and do a PR for it.

[tomusdrw]

Sounds good. I supposed the RPC method that is used as raw_api will be required to output something that is AsRef[u8]? Or require Display? Or simply a String/Vec<u8>? (I think I prefer Display requirement).

How would an error be handled? Just format!("{}", error) with a non-200 error code?

[adria0]

Sounds good. I supposed the RPC method that is used as raw_api will be required to output something that is AsRef[u8]? Or require Display? Or simply a String/Vec<u8>? (I think I prefer Display requirement).

I like the idea of having a Display, also. 👍

How would an error be handled? Just format!("{}", error) with a non-200 error code?

Yep, I think that just a generic 500 Internal Server Error with format error will work. Do have in mind any use case that will need other reponse codes like 4xx ones?

[tomusdrw]

Yep, I think that just a generic 500 Internal Server Error with format error will work. Do have in mind any use case that will need other reponse codes like 4xx ones?

4xx should be used to indicate client errors, I just checked that health API returns 503 Service Unavailable which I feel would suit here as well:

The server cannot handle the request (because it is overloaded or down for maintenance). Generally, this is a temporary state.[66]