Discovery Chain

1.6.0+: This feature is available in Consul versions 1.6.0 and newer.

This topic is part of a low-level API primarily targeted at developers building external Connect proxy integrations.

The service discovery process can be modeled as a “discovery chain” which passes through three distinct stages: routing, splitting, and resolution. Each of these stages is controlled by a set of configuration entries. By configuring different phases of the discovery chain a user can control how proxy upstreams are ultimately resolved to specific instances for load balancing.

Note: The discovery chain is currently only used to discover Connect proxy upstreams.

Configuration

The configuration entries used in the discovery chain are designed to be simple to read and modify for narrowly tailored changes, but at discovery-time the various configuration entries interact in more complex ways. For example:

  • If a service-resolver is created with a service redirect defined, then all references made to the original service in any other configuration entry is replaced with the redirect destination.

  • If a service-resolver is created with a default subset defined then all references made to the original service in any other configuration entry that did not specify a subset will be replaced with the default.

  • If a service-splitter is created with a service split, and the target service has its own service-splitter then the overall effect is flattened and only a single aggregate traffic split is ultimately configured in the proxy.

  • service-resolver redirect loops must be rejected as invalid.

  • service-router and service-splitter configuration entries require an L7 compatible protocol be set for the service via either a service-defaults or proxy-defaults config entry. Violations must be rejected as invalid.

  • If an upstream configuration datacenter parameter is defined then any configuration entry that does not explicitly refer to a desired datacenter should use that value from the upstream.

Compilation

To correctly interpret a collection of configuration entries as a valid discovery chain, we first compile them into a form more directly usable by the layers responsible for configuring Connect sidecar proxies.

You can interact with the compiler directly using the discovery-chain API.

Compilation Parameters

  • Service Name - The service being discovered by name.
  • Datacenter - The datacenter to use as the basis of compilation.
  • Overrides - Discovery-time tweaks to apply when compiling. These should be derived from either the proxy or upstream configurations if either are set.

Compilation Results

The response is a single wrapped CompiledDiscoveryChain field:

  1. {
  2.     "Chain": {...<CompiledDiscoveryChain>...}
  3. }

CompiledDiscoveryChain

The chain encodes a digraph of nodes and targets. Nodes are the compiled representation of various discovery chain stages and targets are instructions on how to use the health API to retrieve relevant service instance lists.

You should traverse the nodes starting with StartNode. The nodes can be resolved by name using the Nodes field. Targets can be resolved by name using the Targets field.

  • ServiceName (string) - The requested service.

  • Namespace (string) - The requested namespace.

  • Datacenter (string) - The requested datacenter.

  • CustomizationHash (string: <optional>) - A unique hash of any overrides that affected the compilation of the discovery chain.

    If set, this value should be used to prefix/suffix any generated load balancer data plane objects to avoid sharing customized and non-customized versions.

  • Protocol (string) - The overall protocol shared by everything in the chain.

  • StartNode (string) - The first key into the Nodes map that should be followed when traversing the discovery chain.

  • Nodes (map<string|DiscoveryGraphNode>) - All nodes available for traversal in the chain keyed by a unique name. You can walk this by starting with StartNode.

    The names should be treated as opaque values and are only guaranteed to be consistent within a single compilation.

  • Targets (map<string|DiscoveryTarget>) - A list of all targets used in this chain.

    The names should be treated as opaque values and are only guaranteed to be consistent within a single compilation.

DiscoveryGraphNode

A single node in the compiled discovery chain.

  • Type (string) - The type of the node. Valid values are: router, splitter, and resolver.

  • Name (string) - The unique name of the node.

  • Routes (array<DiscoveryRoute>) - Only set for Type:router. List of routes to render.

    • Definition (ServiceRoute) - Relevant portion of underlying service-router route.

    • NextNode (string) - The name of the next node in the chain in Nodes.

  • Splits (array<DiscoverySplit>) - Only set for Type:splitter. List of traffic splits.

    • Weight (float32) - Copy of underlying service-splitter weight field.

    • NextNode (string) - The name of the next node in the chain in Nodes.

  • Resolver (DiscoveryResolver: <optional>) - Only set for Type:resolver. How to resolve the service instances.

    • Default (bool) - Set to true if no service-resolver config entry is defined for this node and the default was synthesized.

    • ConnectTimeout (duration) - Copy of the underlying service-resolver ConnectTimeout field. If one is not defined the default of 5s is returned.

    • Target (string) - The name of the target to use found in Targets.

    • Failover (DiscoveryFailover: <optional>) - Compiled form of the underlying service-resolver Failover definition to use for this request.

      • Targets (array<string>) - List of targets found in Targets to failover to in order of preference.

DiscoveryTarget

  • ID (string) - The unique name of this target.

  • Service (string) - The service to query when resolving a list of service instances.

  • ServiceSubset (string: <optional>) - The subset of the service to resolve.

  • Namespace (string) - The namespace to use when resolving a list of service instances.

  • Datacenter (string) - The datacenter to use when resolving a list of service instances.

  • Subset (ServiceResolverSubset) - Copy of the underlying service-resolver Subsets definition for this target.

    • Filter (string: "") - The filter expression to be used for selecting instances of the requested service. If empty all healthy instances are returned.

    • 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.

  • MeshGateway (MeshGatewayConfig) - The mesh gateway configuration to use when connecting to this target’s service instances.

    • Mode (string: "") - One of none, local, or remote.

  • External (bool: false) - True if this target is outside of this consul cluster.

  • SNI (string) - This value should be used as the SNI value when connecting to this set of endpoints over TLS.

  • Name (string) - The unique name for this target for use when generating load balancer objects. This has a structure similar to SNI, but will not be affected by SNI customizations such as ExternalSNI.