Add support rule evaluation via query frontend docs

Signed-off-by: SungJin1212 <[email protected]>

docs/guides/rule-evaluations-via-query-frontend.md

@@ -0,0 +1,40 @@
+---
+title: "Rule evaluations via query frontend"
+linkTitle: "Rule evaluations via query frontend"
+weight: 10
+slug: rule-evalutions-via-query-frontend
+---
+
+This guide explains how to configure the Ruler to evaluate rules via Query Frontends instead of the Ingesters and the pros and cons of rule evaluation via Query Frontend.
+
+## How to enable
+
+By default, the Ruler queries both Ingesters and Store Gateway depending on the Rule time range for evaluating rules (alerting rules or recording rules). If you have set `-ruler.frontend-address` then the Ruler queries the Query Frontend for evaluation rules.
+The address should be the gRPC listen address in host:port format.
+
+You can configure via args:
+```
+-ruler.frontend-address=query-frontend.svc.cluster.local:9095
+```
+And via yaml:
+```yaml
+ruler:
+  frontend_address: query-frontend.svc.cluster.local:9095
+```
+
+In addition, you can configure gRPC client (Ruler -> Query Frontend) config, please refer to [ruler config](../configuration/config-file-reference.md#ruler_config) in frontend_client section.
+
+## Pros and Cons
+If this feature is enabled, the query execute path is as follows:
+
+Ruler -> Query Frontend -> Query Scheduler -> Querier -> Ingester/Store Gateway
+
+There are more hops than before (Ruler -> Ingester/Store Gateway).
+### Pros
+The rule evaluation performance is improved in such a situation where the number of Queriers pulling queries from the Query Scheduler is enough as we can use Query Frontend features like the vertical query sharding.
+Also, the Ruler can use fewer resources as it doesn't need to run query engine to evaluate rules.
+
+### Cons
+If there are not enough Queriers, add rule queries to Query Scheduler could cause query starvation problem and rules cannot get evaluated.
+Another is, currently, the only support format of the query response of the Query Frontend is the JSON format. The JSON format only contains partial information of the native histogram.
+So, if your rules use the native histogram, it is not allowed yet.