Subset Load Balancing Policy (proto)

This extension has the qualified name envoy.load_balancing_policies.subset

Note

This extension is intended to be robust against both untrusted downstream and upstream traffic.

Tip

This extension extends and can be used with the following extension category:

envoy.load_balancing_policies

This extension must be configured with one of the following type URLs:

type.googleapis.com/envoy.extensions.load_balancing_policies.subset.v3.Subset

extensions.load_balancing_policies.subset.v3.Subset

[extensions.load_balancing_policies.subset.v3.Subset proto]

Optionally divide the endpoints in this cluster into subsets defined by endpoint metadata and selected by route and weighted cluster metadata.

{
  "fallback_policy": ...,
  "default_subset": {...},
  "subset_selectors": [],
  "locality_weight_aware": ...,
  "scale_locality_weight": ...,
  "panic_mode_any": ...,
  "list_as_any": ...,
  "metadata_fallback_policy": ...,
  "subset_lb_policy": {...}
}

fallback_policy: (extensions.load_balancing_policies.subset.v3.Subset.LbSubsetFallbackPolicy) The behavior used when no endpoint subset matches the selected route’s metadata. The value defaults to NO_FALLBACK.

default_subset: (Struct) Specifies the default subset of endpoints used during fallback if fallback_policy is DEFAULT_SUBSET. Each field in default_subset is compared to the matching LbEndpoint.Metadata under the envoy.lb namespace. It is valid for no hosts to match, in which case the behavior is the same as a fallback_policy of NO_FALLBACK.

subset_selectors

(repeated extensions.load_balancing_policies.subset.v3.Subset.LbSubsetSelector) For each entry, LbEndpoint.Metadata’s envoy.lb namespace is traversed and a subset is created for each unique combination of key and value. For example:

{ "subset_selectors": [
    { "keys": [ "version" ] },
    { "keys": [ "stage", "hardware_type" ] }
]}

A subset is matched when the metadata from the selected route and weighted cluster contains the same keys and values as the subset’s metadata. The same host may appear in multiple subsets.

locality_weight_aware

(bool) If true, routing to subsets will take into account the localities and locality weights of the endpoints when making the routing decision.

There are some potential pitfalls associated with enabling this feature, as the resulting traffic split after applying both a subset match and locality weights might be undesirable.

Consider for example a situation in which you have 50/50 split across two localities X/Y which have 100 hosts each without subsetting. If the subset LB results in X having only 1 host selected but Y having 100, then a lot more load is being dumped on the single host in X than originally anticipated in the load balancing assignment delivered via EDS.

scale_locality_weight: (bool) When used with locality_weight_aware, scales the weight of each locality by the ratio of hosts in the subset vs hosts in the original subset. This aims to even out the load going to an individual locality if said locality is disproportionately affected by the subset predicate.

panic_mode_any

(bool) If true, when a fallback policy is configured and its corresponding subset fails to find a host this will cause any host to be selected instead.

This is useful when using the default subset as the fallback policy, given the default subset might become empty. With this option enabled, if that happens the LB will attempt to select a host from the entire cluster.

list_as_any: (bool) If true, metadata specified for a metadata key will be matched against the corresponding endpoint metadata if the endpoint metadata matches the value exactly OR it is a list value and any of the elements in the list matches the criteria.

metadata_fallback_policy

(extensions.load_balancing_policies.subset.v3.Subset.LbSubsetMetadataFallbackPolicy) Fallback mechanism that allows to try different route metadata until a host is found. If load balancing process, including all its mechanisms (like fallback_policy) fails to select a host, this policy decides if and how the process is repeated using another metadata.

The value defaults to METADATA_NO_FALLBACK.

subset_lb_policy: (config.cluster.v3.LoadBalancingPolicy, REQUIRED) The child LB policy to create for endpoint-picking within the chosen subset.

extensions.load_balancing_policies.subset.v3.Subset.LbSubsetSelector

[extensions.load_balancing_policies.subset.v3.Subset.LbSubsetSelector proto]

Specifications for subsets.

{
  "keys": [],
  "single_host_per_subset": ...,
  "fallback_policy": ...,
  "fallback_keys_subset": []
}

keys: (repeated string) List of keys to match with the weighted cluster metadata.

single_host_per_subset

(bool) Selects a mode of operation in which each subset has only one host. This mode uses the same rules for choosing a host, but updating hosts is faster, especially for large numbers of hosts.

If a match is found to a host, that host will be used regardless of priority levels.

When this mode is enabled, configurations that contain more than one host with the same metadata value for the single key in keys will use only one of the hosts with the given key; no requests will be routed to the others. The cluster gauge lb_subsets_single_host_per_subset_duplicate indicates how many duplicates are present in the current configuration.

fallback_policy: (extensions.load_balancing_policies.subset.v3.Subset.LbSubsetSelector.LbSubsetSelectorFallbackPolicy) The behavior used when no endpoint subset matches the selected route’s metadata.

fallback_keys_subset: (repeated string) Subset of keys used by KEYS_SUBSET fallback policy. It has to be a non empty list if KEYS_SUBSET fallback policy is selected. For any other fallback policy the parameter is not used and should not be set. Only values also present in keys are allowed, but fallback_keys_subset cannot be equal to keys.

Enum extensions.load_balancing_policies.subset.v3.Subset.LbSubsetSelector.LbSubsetSelectorFallbackPolicy

[extensions.load_balancing_policies.subset.v3.Subset.LbSubsetSelector.LbSubsetSelectorFallbackPolicy proto]

Allows to override top level fallback policy per selector.

NOT_DEFINED: (DEFAULT) ⁣If NOT_DEFINED top level config fallback policy is used instead.

NO_FALLBACK: ⁣If NO_FALLBACK is selected, a result equivalent to no healthy hosts is reported.

ANY_ENDPOINT: ⁣If ANY_ENDPOINT is selected, any cluster endpoint may be returned (subject to policy, health checks, etc).

DEFAULT_SUBSET: ⁣If DEFAULT_SUBSET is selected, load balancing is performed over the endpoints matching the values from the default_subset field.

KEYS_SUBSET: ⁣If KEYS_SUBSET is selected, subset selector matching is performed again with metadata keys reduced to fallback_keys_subset. It allows for a fallback to a different, less specific selector if some of the keys of the selector are considered optional.

Enum extensions.load_balancing_policies.subset.v3.Subset.LbSubsetFallbackPolicy

[extensions.load_balancing_policies.subset.v3.Subset.LbSubsetFallbackPolicy proto]

If NO_FALLBACK is selected, a result equivalent to no healthy hosts is reported. If ANY_ENDPOINT is selected, any cluster endpoint may be returned (subject to policy, health checks, etc). If DEFAULT_SUBSET is selected, load balancing is performed over the endpoints matching the values from the default_subset field.

NO_FALLBACK: (DEFAULT) ⁣

ANY_ENDPOINT: ⁣

DEFAULT_SUBSET: ⁣

Enum extensions.load_balancing_policies.subset.v3.Subset.LbSubsetMetadataFallbackPolicy

[extensions.load_balancing_policies.subset.v3.Subset.LbSubsetMetadataFallbackPolicy proto]

METADATA_NO_FALLBACK: (DEFAULT) ⁣No fallback. Route metadata will be used as-is.

FALLBACK_LIST

⁣A special metadata key fallback_list will be used to provide variants of metadata to try. Value of fallback_list key has to be a list. Every list element has to be a struct - it will be merged with route metadata, overriding keys that appear in both places. fallback_list entries will be used in order until a host is found.

fallback_list key itself is removed from metadata before subset load balancing is performed.

Example:

for metadata:

version: 1.0
fallback_list:
  - version: 2.0
    hardware: c64
  - hardware: c32
  - version: 3.0

at first, metadata:

{"version": "2.0", "hardware": "c64"}

will be used for load balancing. If no host is found, metadata:

{"version": "1.0", "hardware": "c32"}

is next to try. If it still results in no host, finally metadata:

{"version": "3.0"}

is used.