»Service Resolver

v1.8.4+: On Kubernetes, the ServiceResolver custom resource is supported in Consul versions 1.8.4+.
v1.6.0+: On other platforms, this config entry is supported in Consul versions 1.6.0+.

The service-resolver config entry kind (ServiceResolver on Kubernetes) controls which service instances should satisfy Connect upstream discovery requests for a given service name.

If no resolver config is defined the chain assumes 100% of traffic goes to the healthy instances of the default service in the current datacenter+namespace and discovery terminates.

»Interaction with other Config Entries

Service resolver config entries are a component of L7 Traffic Management.

»Sample Config Entries

»Filter on service version

Create service subsets based on a version metadata and override the defaults:

Kind          = "service-resolver"
Name          = "web"
DefaultSubset = "v1"
Subsets = {
  "v1" = {
    Filter = "Service.Meta.version == v1"
  }
  "v2" = {
    Filter = "Service.Meta.version == v2"
  }
}

»Other datacenters

Expose a set of services in another datacenter as a virtual service:

Kind = "service-resolver"
Name = "web-dc2"
Redirect {
  Service    = "web"
  Datacenter = "dc2"
}

»Datacenter failover

Enable failover for all subsets:

Kind           = "service-resolver"
Name           = "web"
ConnectTimeout = "15s"
Failover = {
  "*" = {
    Datacenters = ["dc3", "dc4"]
  }
}

»Consistent load balancing

Apply consistent load balancing for requests based on x-user-id header:

Kind = "service-resolver"
Name = "web"

LoadBalancer = {
  Policy = "maglev"
  HashPolicies = [
    {
      Field = "header"
      FieldValue = "x-user-id"
    }
  ]
}

»Available Fields

Kind - Must be set to service-resolver
Name (string: <required>) - Set to the name of the service being configured.
Namespace (string: "default")
Enterprise
- Specifies the namespace the config entry will apply to.
Meta (map<string|string>: nil) - Specifies arbitrary KV metadata pairs. Added in Consul 1.8.4.
ConnectTimeout (duration: 0s) - The timeout for establishing new network connections to this service.
DefaultSubset (string: "") - The subset to use when no explicit subset is requested. If empty the unnamed subset is used.
Subsets (map[string]ServiceResolverSubset) - A map of subset name to subset definition for all usable named subsets of this service. The map key is the name of the subset and all names must be valid DNS subdomain elements.

This may be empty, in which case only the unnamed default subset will be usable.
- Filter (string: "") - The filter expression to be used for selecting instances of the requested service. If empty all healthy instances are returned. This expression can filter on the same selectors as the Health API endpoint.
- OnlyPassing (bool: false) - Specifies the behavior of the resolver's health check interpretation. If this is set to false, instances with checks in the passing as well as the warning states will be considered healthy. If this is set to true, only instances with checks in the passing state will be considered healthy.
Redirect (ServiceResolverRedirect: <optional>) - When configured, all attempts to resolve the service this resolver defines will be substituted for the supplied redirect EXCEPT when the redirect has already been applied.

When Redirect is set, all other fields besides Kind, Name, Namespace and Redirect will be ignored.
- Service (string: "") - A service to resolve instead of the current service.
- ServiceSubset (string: "") - A named subset of the given service to resolve instead of one defined as that service's DefaultSubset. If empty, the default subset is used.
  
  If this is specified at least one of Service, Datacenter, or Namespace should be configured.
- Namespace (string: "")
  Enterprise
  - The namespace to resolve the service from instead of the current one.
- Datacenter (string: "") - The datacenter to resolve the service from instead of the current one.
Failover (map[string]ServiceResolverFailover) - Controls when and how to reroute traffic to an alternate pool of service instances.

The map is keyed by the service subset it applies to and the special string "*" is a wildcard that applies to any subset not otherwise specified here.

Service, ServiceSubset, Namespace, and Datacenters cannot all be empty at once.
- Service (string: "") - The service to resolve instead of the default as the failover group of instances during failover.
- ServiceSubset (string: "") - The named subset of the requested service to resolve as the failover group of instances. If empty the default subset for the requested service is used.
- Namespace (string: "")
  Enterprise
  - The namespace to resolve the requested service from to form the failover group of instances. If empty the current namespace is used.
- Datacenters (array<string>) - A fixed list of datacenters to try during failover.
LoadBalancer (LoadBalancer) - Determines the load balancing policy and configuration for services issuing requests to this upstream. This option is available in Consul versions 1.9.0 and newer.
- Policy (string: "") - The load balancing policy used to select a host. One of: random, round_robin, least_request, ring_hash, maglev.
- RingHashConfig (RingHashConfig) - Configuration for the ring_hash policy type.
  - MinimumRingRize (int: 1024) - Determines the minimum number of entries in the hash ring.
  - MaximumRingRize (int: 8192) - Determines the maximum number of entries in the hash ring.
- LeastRequestConfig (LeastRequestConfig) - Configuration for the least_request policy type.
  - ChoiceCount (int: 2) - Determines the number of random healthy hosts from which to select the one with the least requests.
- HashPolicies (array<HashPolicies>) - List of hash policies to use for hashing load balancing algorithms. Hash policies are evaluated individually and combined such that identical lists result in the same hash. If no hash policies are present, or none are successfully evaluated, then a random backend host will be selected.
  - Field (string: "") - The attribute type to hash on. Must be one of header, cookie, or query_parameter. Cannot be specified along with SourceAddress.
  - FieldValue (string: "") - The value to hash. ie. header name, cookie name, URL query parameter name. Cannot be specified along with SourceAddress.
  - CookieConfig (CookieConfig) - Additional configuration for the "cookie" hash policy type. This is specified to have Envoy generate a cookie for a client on its first request.
    - Session (bool: false) - Generates a session cookie with no expiration.
    - TTL (duration: 0s) - TTL for generated cookies. Cannot be specified for session cookies.
    - Path (string: "") - The path to set for the cookie.
  - SourceIP (bool: false) - Determines whether the hash should be of the source IP address rather than of a field and field value. Cannot be specified along with Field or FieldValue.
  - Terminal (bool: false) - Will short circuit the computation of the hash when multiple hash policies are present. If a hash is computed when a Terminal policy is evaluated, then that hash will be used and subsequent hash policies will be ignored.

»Service Subsets

A service subset assigns a concrete name to a specific subset of discoverable service instances within a datacenter, such as "version2" or "canary".

A service subset name is useful only when composed with an actual service name, a specific datacenter, and namespace.

All services have an unnamed default subset that will return all healthy instances unfiltered.

Subsets are defined in service-resolver configuration entries, but are referenced by their names throughout the other configuration entry kinds.

»ACLs

Configuration entries may be protected by ACLs.

Reading a service-resolver config entry requires service:read on the resource.

Creating, updating, or deleting a service-resolver config entry requires service:write on the resource and service:read on any other service referenced by name in these fields:

Redirect.Service
Failover[].Service