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A24-lb-policy-config.md

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Load Balancing Policy Configuration

Abstract

This document describes a new mechanism for specifying load balancing policy configuration via the gRPC service config. It also moves away from the special case currently used for selecting the grpclb LB policy and toward a more general-purpose mechanism for selecting LB policies.

Background

There are cases where a service owner may need to provide configuration in the gRPC service config to be used by the load balancing policy on the client. Currently, the service config provides a single loadBalancingPolicy field that indicates the name of the LB policy to use but does not provide a way to encode any configuration for that policy. This proposal replaces the existing field with a new field that specifies both the LB policy name and some associated configuration.

In addition, we are currently in the process of moving away from our custom grpclb load balancing protocol and to a new design using the Envoy xDS API (a separate gRFC for that will be published in the future). As part of this, we want to move away from the special case that we currently use to select the grpclb LB policy, which is that it gets triggered whenever the resolver returns at least one balancer address. Instead, we would like to use data in the service config to tell the new xds policy which balancers to contact.

Related Proposals:

There will be a subsequent gRFC describing the new xds LB policy.

Proposal

We propose to deprecate the existing loadBalancingPolicy service config field. In its place, we will add a new field called loadBalancingConfig that will encode both the policy name and a corresponding configuration for the policy.

Here is how the service config definition will now look. First, in proto form:

// Configuration for round_robin LB policy.
message RoundRobinConfig {
}

// Configuration for xds LB policy.
message XdsConfig {
  // Required.  Name of balancer to connect to.
  string balancer_name = 1;
  // Optional.  What LB policy to use for intra-locality routing.
  // If unset, will use whatever algorithm is specified by the balancer.
  // Multiple LB policies can be specified; clients will iterate through
  // the list in order and stop at the first policy that they support. If none
  // are supported, the service config is considered invalid.
  repeated LoadBalancingConfig child_policy = 2;
  // Optional.  What LB policy to use in fallback mode.  If not
  // specified, defaults to round_robin.
  // Multiple LB policies can be specified; clients will iterate through
  // the list in order and stop at the first policy that they support. If none
  // are supported, the service config is considered invalid.
  repeated LoadBalancingConfig fallback_policy = 3;
}

// Configuration for grpclb LB policy.
message GrpcLbConfig {
  // Optional.  What LB policy to use for routing between the backend
  // addresses.  If unset, defaults to round_robin.
  // Currently, the only supported values are round_robin and pick_first.
  // Note that this will be used both in balancer mode and in fallback mode.
  // Multiple LB policies can be specified; clients will iterate through
  // the list in order and stop at the first policy that they support.
  repeated LoadBalancingConfig child_policy = 1;
}

// Selects LB policy and provides corresponding configuration.
//
// In general, all instances of this field should be repeated.
// Clients will iterate through the list in order and stop at the first
// policy that they support.  This allows the service config to specify
// custom policies that may not be known to all clients.
message LoadBalancingConfig {
  // Exactly one LB policy may be configured.
  // If no LB policy is configured, then the default behavior is to pick
  // the first available backend.
  oneof policy {
    // For each new LB policy supported by gRPC, a new field must be added
    // here.  The field's name must be the LB policy name and its type is a
    // message that provides whatever configuration parameters are needed
    // by the LB policy.  The configuration message will be passed to the
    // LB policy when it is instantiated on the client.
    //
    // If the LB policy does not require any configuration parameters, the
    // message for that LB policy may be empty.
    //
    // Note that if an LB policy contains another nested LB policy
    // (e.g., a top-level policy picks the locality and then delegates to
    // another policy to pick the endpoint within that locality), its
    // configuration message may include a nested instance of the
    // LoadBalancingConfig message to configure the nested LB policy.
    RoundRobinConfig round_robin = 1 [json_name = "round_robin"];
    XdsConfig xds = 2;
    GrpcLbConfig grpclb = 3;
  }
}

message ServiceConfig {
  // Load balancing policy.
  //
  // Note that load_balancing_policy is deprecated in favor of
  // load_balancing_config; the former will be used only if the latter
  // is unset.
  //
  // If no LB policy is configured here, then the default behavior is to
  // pick the first available backend.
  //
  // If the deprecated load_balancing_policy field is used, note that if the
  // resolver returns at least one balancer address (as opposed to backend
  // addresses), gRPC will use grpclb (see
  // https://github.com/grpc/grpc/blob/master/doc/load-balancing.md),
  // regardless of what policy is configured here.  However, if the resolver
  // returns at least one backend address in addition to the balancer
  // address(es), the client may fall back to the requested policy if it
  // is unable to reach any of the grpclb load balancers.
  enum LoadBalancingPolicy {
    UNSPECIFIED = 0;
    ROUND_ROBIN = 1;
  }
  LoadBalancingPolicy load_balancing_policy = 1 [deprecated=true];
  // Multiple LB policies can be specified; clients will iterate through
  // the list in order and stop at the first policy that they support. If none
  // are supported, the service config is considered invalid.
  repeated LoadBalancingConfig load_balancing_config = 4;

  // ...other existing fields...
}

For example, to use the xds policy with a child policy of round_robin, something like this could be used:

load_balacing_config: {
  xds: {
    balancer_name: "dns:///balancer.example.com:8080"
    child_policy: {
      round_robin: {}
    }
  }
}

In JSON form (which is what is actually used in gRPC core), it would look like this:

{
   "loadBalancingConfig":[
      {
         "xds":{
            "balancerName":"dns:///balancer.example.com:8080",
            "childPolicy":[
               {
                  "round_robin":{}
               }
            ]
         }
      }
   ]
}

Rationale

When the new xds LB policy is ready, we will need a migration path to migrate clients from grpclb to xds. The new service config field described in this document has been explicitly designed to aid that migration.

There are several types of clients that we'll need to support for this migration:

  • Old clients that do not understand the new loadBalancingConfig service config field.
  • New clients that understand the new loadBalancingConfig service config field and support the xds LB policy.
  • In-between clients that understand the new loadBalancingConfig service config field but do not yet support the xds LB policy.

To ease migration from grpclb to xds, a resolver can return both the balancer addresses used by grpclb and a service config with the new loadBalancingConfig field (a repeated field with semantics of choosing the first LB policy supported by the client) set indicating to use xds if available and if not to fall back to grpclb. The new loadBalancingConfig service config field will override the special case that selects the grpclb policy whenever the resolver returns at least one balancer address. (The old loadBalancingPolicy field will continue to be overridden by the presence of balancer addresses.)

This gives us what we want in all cases:

  • Old clients will ignore the loadBalancingConfig field and will use grpclb based on the presence of the balancer addresses.
  • New clients will use xds based on the loadBalancingConfig field.
  • In-between clients will see the loadBalancingConfig field, but they will not support the xds policy, so they will use the second entry (for grpclb).

Note: This assumes that the old clients do not yet implement gRPC Service Config Error Handling. This means that we need to add support for the new loadBalancingConfig service config field (already done in C-core) before we implement the new error handling. Otherwise, a client that does support Service Config Error Handling but does not yet support the new loadBalancingConfig field would reject the entire service config.

Note: This also assumes that there are no clients that understand the loadBalancingConfig field and have an incomplete implementation of the xds policy. To avoid this, we will temporarily use the name xds_experimental while the policy is in development.

Implementation

Already implemneted in C-core.

Java and Go TBD.

Open issues (if applicable)

N/A