How to set Hash Key value in envoy proxy for RING_HASH load balancing - load-balancing

Question

I am trying to set up RING_HASH load balancing on the envoy proxy based on some request header. From documentation it looks like I have to set hash key in filter_metadata
filter_metadata:
envoy.lb:
hash_key: "YOUR HASH KEY"
But, I am not able to find out what are the possible values/expressions of hash_key. I tried to configure a fixed value for the hash key, but still requests does not go to the same upstream server.
My envoy.yaml
admin:
address:
socket_address: { address: 0.0.0.0, port_value: 9901 }
static_resources:
listeners:
- name: listener_0
address:
socket_address: { address: 0.0.0.0, port_value: 10000 }
filter_chains:
- filters:
- name: envoy.filters.network.http_connection_manager
typed_config:
"#type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
stat_prefix: ingress_http
codec_type: AUTO
route_config:
name: local_route
virtual_hosts:
- name: local_service
domains: ["*"]
routes:
- match: { prefix: "/" }
route: { cluster: some_service }
http_filters:
- name: envoy.filters.http.router
clusters:
- name: some_service
connect_timeout: 0.25s
type: STATIC
lb_policy: RING_HASH
metadata:
filter_metadata:
envoy.lb:
hash_key: "fixed_value"
load_assignment:
cluster_name: some_service
endpoints:
- lb_endpoints:
- endpoint:
address:
socket_address:
address: 172.19.0.2
port_value: 8080
- endpoint:
address:
socket_address:
address: 172.19.0.3
port_value: 8080
- endpoint:
address:
socket_address:
address: 172.19.0.4
port_value: 8080

Answer 1

I guess, envoy.lb metadata is used to find hash of upstream host not to configure key for request.
There is another configuration, hash_policy which should be used.
My final working envoy configuration looks like this. The sticky session is based on header sticky-key
admin:
address:
socket_address: { address: 0.0.0.0, port_value: 9901 }
static_resources:
listeners:
- name: listener_0
address:
socket_address: { address: 0.0.0.0, port_value: 10000 }
filter_chains:
- filters:
- name: envoy.filters.network.http_connection_manager
typed_config:
"#type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
stat_prefix: ingress_http
codec_type: AUTO
route_config:
name: local_route
virtual_hosts:
- name: local_service
domains: ["*"]
routes:
- match: { prefix: "/" }
route:
cluster: some_service
hash_policy:
- header:
header_name: sticky-key
http_filters:
- name: envoy.filters.http.router
clusters:
- name: some_service
connect_timeout: 0.25s
type: STATIC
lb_policy: RING_HASH
load_assignment:
cluster_name: some_service
endpoints:
- lb_endpoints:
- endpoint:
address:
socket_address:
address: 172.19.0.2
port_value: 8080
- endpoint:
address:
socket_address:
address: 172.19.0.3
port_value: 8080
- endpoint:
address:
socket_address:
address: 172.19.0.4
port_value: 8080