Add component healthcheck api design

0010-R-components-healthcheck.md

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+# Components Healthcheck In Dapr
+
+* Author(s): Deepanshu Agarwal (@DeepanshuA)
+* State: Draft
+* Updated: 07/20/2023
+
+## Overview
+As a user, I should be able to enquire the health of dapr system, which is possible currently - but this health check should also include health of connected dapr components as well.
+
+## Tenets
+1. It should be extensible i.e. tomorrow if any feature to check health of Actors etc. is required, it should not require a new endpoint.
+
+## Current Scenario
+There are many components in Dapr which don't yet implement Ping. 
+Ping is not mandatory to be implemented by Components, which is the correct behavior, as it could lead to false positives.
+For this components health-check, components not implementing Ping will be omitted out.
+
+## API Design
+### Endpoint:
+Instead of an additional endpoint, the Approach underneath works with a query parameter, in addition to `healthz` endpoint.
+
+Following are the possible responses for `healthz` API:
+
+| HTTP | Response Codes | 
+| -------- | -------- | 
+| 204     | dapr is healthy     | 
+| 500     | dapr is not healthy     | 
+
+Hence, if dapr is healthy, then the response code changes, as per the enlisted cases below.
+If dapr is NOT healthy, then the compoenents Health check should anyways NOT be checked.
+
+http://localhost:3500/v1.0/healthz?include_components=true
+
+### Approach: 
+- Maintain a cache with status of all components loaded successfully and keep updating this cache in a background go routine at a configurable `pingUpdateFrequency`. By default, `pingUpdateFrequency` to be 5 minutes.
+
+- This cache will not start to be built, right at the boot of daprd sidecar. There will be flag (let's say `collectPings`), which will be `false` at the beginning of the daprd sidecar and which will be turned `true`, once all the components are ready.
+Once, `collectPings` is `true`, the cache will start to be populated.
+
+- But, what happens if a component fails to initialize? 
+If a mandatory component fails to initialize, then daprd will not come up healthy and thus, anyways health check will report unhealthy.
+If an optional component fails to initialize OR if it is not healthy afterwards as well, then it is governed by a query parameter `ignoreOptionalComponent`, which is `true` by default. So, as the name suggests, if this query param is not set to `false`, then optional components failure to initialize OR failure to report healthy will not result in a un-healthy status. Rather, the http status code 207 will be reported back.
+
+- For components which don't yet implement Ping, they will be ignored for their health check.
+
+- Response of healthz endpoint will be always only a http status code.
+The result i.e. `healthCheckStatus` and `errorCode`/`message`, if required will be provided as part of `metadata` API. This is done, as dapr healthz endpoint is publicly accessible, so for security constraints, it doesn't deem good to provide component names etc. as a response of `healthz` endpoint.
+
+- To implement only http endpoint, at least for the first version of this API.
+
+- This endpoint is not supposed to support query for a particular component Health check.