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A46-xds-nack-semantics-improvement.md

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A46: xDS NACK Semantics Improvement

Abstract

This proposal clarifies xDS NACK semantics used in gRPC.

Background

The xDS spec says that, at the wire protocol level, a NACK is sent on a per-response basis, not a per-resource basis. Although not explicitly stated in the spec, this implies that the client must reject (i.e., not actually use) even the valid resources from that response, since that would allow the server to know the client's actual state (although see Rationale below). That is the behavior that gRPC currently implements.

However, this approach causes problems in a case where a single resource is invalid and it causes the client to ignore all of the resources in the update, especially for LDS and CDS, where the server must send all resources in every response. For example, if there is a single invalid Cluster resource in a CDS response, all of the Cluster resources will be rejected. If this happens on the first CDS response after a client starts up (i.e., when the client has not already accepted previous versions of the CDS resources that it can continue to use), that will cause the client to have no valid CDS resources at all, which means that the problem will prevent all clusters from functioning instead of affecting only the invalid resource. This is particularly problematic now that gRPC shares its XdsClient between channels, because a single invalid Cluster resource can basically cause all of the client's channels to stop working all at once.

This behavior makes it challenging for xDS servers to safely deploy changes in environments in which the clients are not centrally controlled. For example, older gRPC clients support only the ROUND_ROBIN LB policy, as per gRFC A27, but newer clients now support the RING_HASH policy, as per gRFC A42. If a Cluster resource specifies an unsupported LB policy, clients will consider the resource invalid, which will cause them to NACK the response. So if an xDS server cannot be sure that all of its clients have been upgraded to a version that supports the RING_HASH policy, then it cannot safely send a Cluster resource configuring that policy, because that change would cause older clients to stop functioning.

Note that this document is not attempting to solve the general problem that an individual resource that sets a supported field to an unsupported value will be considered invalid; that behavior is intentional, and there is nothing we can do to prevent the need for clients to be upgraded to use a configuration resource that requires features that they do not yet support. However, this document is intending to solve the problem of that one invalid resource causing other valid resources to be ignored.

This document proposes a behavior change to address this problem.

Related Proposals:

Proposal

We will change gRPC's behavior such that when a response is NACKed, gRPC will still use all valid resources from the response; it will ignore only the invalid resources.

Note that the xDS wire protocol behavior is not changing at all; the protocol currently still requires NACKs to be done on a per-response basis instead of a per-resource basis (although the latter is something that is expected to be added to the protocol in the future). The only change is that the client will actually use the valid resources in the response.

Temporary environment variable protection

N/A

Rationale

Note that gRPC's original behavior was intended to make sure that the control plane had a clear picture of what configuration the client is using. However, it turns out that Envoy currently has cases where it will apply some valid resources from a response that it NACKs, which means that the control plane already did not have that kind of clear picture. To ensure that the control plane will have that kind of clear picture, there will be subsequent work to extend the xDS protocol to allow per-resource NACKing instead of per-response NACKing, although that will be a broader project. Note that the CSDS service in the gRPC xDS client (see gRFC A40) can be used to get the current state of the resources in the client, which is likely to be more accurate than the view from the xDS server.

We considered the following alternatives:

  • We could have just waited for the xDS protocol changes that will allow per-resource NACKing instead of per-response NACKing. However, that is likely to take more work to design and implement in both clients and control planes, and we wanted to do something quickly to alleviate the problem described above.

  • For the case of unknown LB policies specifically, we could fall back to ROUND_ROBIN instead of considering the resource invalid. However, this is not semantically correct behavior; it could cause unexpected load on servers and a large number of unnecessary connections to backends. Also, while it would address the specific case that triggered this design, it would not address the general case of seeing an unsupported value in a supported field.

Implementation

C-core implementation: grpc/grpc#27276

TODO: Add info for other languages.

Open issues (if applicable)

N/A