Load balancing

When a filter needs to acquire a connection to a host in an upstream cluster, the cluster manager uses a load balancing policy to determine which host is selected. The load balancing policies are pluggable and are specified on a per upstream cluster basis in the configuration. Note that if no active health checking policy is configured for a cluster, all upstream cluster members are considered healthy.

Supported load balancers

Round robin

This is a simple policy in which each healthy upstream host is selected in round robin order.

Weighted least request

The least request load balancer uses an O(1) algorithm which selects two random healthy hosts and picks the host which has fewer active requests. (Research has shown that this approach is nearly as good as an O(N) full scan). If any host in the cluster has a load balancing weight greater than 1, the load balancer shifts into a mode where it randomly picks a host and then uses that host <weight> times. This algorithm is simple and sufficient for load testing. It should not be used where true weighted least request behavior is desired (generally if request durations are variable and long in length). We may add a true full scan weighted least request variant in the future to cover this use case.

Ring hash

The ring/modulo hash load balancer implements consistent hashing to upstream hosts. The algorithm is based on mapping all hosts onto a circle such that the addition or removal of a host from the host set changes only affect 1/N requests. This technique is also commonly known as “ketama” hashing. The consistent hashing load balancer is only effective when protocol routing is used that specifies a value to hash on. Currently the only implemented mechanism is to hash via HTTP header values in the HTTP router filter. The default minimum ring size is specified in runtime. The minimum ring size governs the replication factor for each host in the ring. For example, if the minimum ring size is 1024 and there are 16 hosts, each host will be replicated 64 times. The ring hash load balancer does not currently support weighting.


The random load balancer selects a random healthy host. The random load balancer generally performs better than round robin if no health checking policy is configured. Random selection avoids bias towards the host in the set that comes after a failed host.

Original destination

This is a special purpose load balancer that can only be used with an original destination cluster. Upstream host is selected based on the downstream connection metadata, i.e., connections are opened to the same address as the destination address of the incoming connection was before the connection was redirected to Envoy. New destinations are added to the cluster by the load balancer on-demand, and the cluster periodically cleans out unused hosts from the cluster. No other load balancing type can be used with original destination clusters.

Panic threshold

During load balancing, Envoy will generally only consider healthy hosts in an upstream cluster. However, if the percentage of healthy hosts in the cluster becomes too low, Envoy will disregard health status and balance amongst all hosts. This is known as the panic threshold. The default panic threshold is 50%. This is configurable via runtime. The panic threshold is used to avoid a situation in which host failures cascade throughout the cluster as load increases.

Zone aware routing

We use the following terminology:

  • Originating/Upstream cluster: Envoy routes requests from an originating cluster to an upstream cluster.
  • Local zone: The same zone that contains a subset of hosts in both the originating and upstream clusters.
  • Zone aware routing: Best effort routing of requests to an upstream cluster host in the local zone.

In deployments where hosts in originating and upstream clusters belong to different zones Envoy performs zone aware routing. There are several preconditions before zone aware routing can be performed:

  • Both originating and upstream cluster are not in panic mode.
  • Zone aware routing is enabled.
  • The originating cluster has the same number of zones as the upstream cluster.
  • The upstream cluster has enough hosts. See here for more information.

The purpose of zone aware routing is to send as much traffic to the local zone in the upstream cluster as possible while roughly maintaining the same number of requests per second across all upstream hosts (depending on load balancing policy).

Envoy tries to push as much traffic as possible to the local upstream zone as long as roughly the same number of requests per host in the upstream cluster are maintained. The decision of whether Envoy routes to the local zone or performs cross zone routing depends on the percentage of healthy hosts in the originating cluster and upstream cluster in the local zone. There are two cases with regard to percentage relations in the local zone between originating and upstream clusters:

  • The originating cluster local zone percentage is greater than the one in the upstream cluster. In this case we cannot route all requests from the local zone of the originating cluster to the local zone of the upstream cluster because that will lead to request imbalance across all upstream hosts. Instead, Envoy calculates the percentage of requests that can be routed directly to the local zone of the upstream cluster. The rest of the requests are routed cross zone. The specific zone is selected based on the residual capacity of the zone (that zone will get some local zone traffic and may have additional capacity Envoy can use for cross zone traffic).
  • The originating cluster local zone percentage is smaller than the one in upstream cluster. In this case the local zone of the upstream cluster can get all of the requests from the local zone of the originating cluster and also have some space to allow traffic from other zones in the originating cluster (if needed).

Load Balancer Subsets

Envoy may be configured to divide hosts within an upstream cluster into subsets based on metadata attached to the hosts. Routes may then specify the metadata that a host must match in order to be selected by the load balancer, with the option of falling back to a predefined set of hosts, including any host.

Subsets use the load balancer policy specified by the cluster. The original destination policy may not be used with subsets because the upstream hosts are not known in advance. Subsets are compatible with zone aware routing, but be aware that the use of subsets may easily violate the minimum hosts condition described above.

If subsets are configured and a route specifies no metadata or no subset matching the metadata exists, the subset load balancer initiates its fallback policy. The default policy is NO_ENDPOINT, in which case the request fails as if the cluster had no hosts. Conversely, the ANY_ENDPOINT fallback policy load balances across all hosts in the cluster, without regard to host metadata. Finally, the DEFAULT_SUBSET causes fallback to load balance among hosts that match a specific set of metadata.

Subsets must be predefined to allow the subset load balancer to efficiently select the correct subset of hosts. Each definition is a set of keys, which translates to zero or more subsets. Conceptually, each host that has a metadata value for all of the keys in a definition is added to a subset specific to its key-value pairs. If no host has all the keys, no subsets result from the definition. Multiple definitions may be provided, and a single host may appear in multiple subsets if it matches multiple definitions.

During routing, the route’s metadata match configuration is used to find a specific subset. If there is a subset with the exact keys and values specified by the route, the subset is used for load balancing. Otherwise, the fallback policy is used. The cluster’s subset configuration must, therefore, contain a definition that has the same keys as a given route in order for subset load balancing to occur.

This feature can only be enabled using the V2 configuration API. Furthermore, host metadata is only supported when using the EDS discovery type for clusters. Host metadata for subset load balancing must be placed under the filter name "envoy.lb". Similarly, route metadata match criteria use the "envoy.lb" filter name. Host metadata may be hierarchical (e.g., the value for a top-level key may be a structured value or list), but the subset load balancer only compares top-level keys and values. Therefore when using structured values, a route’s match criteria will only match if an identical structured value appears in the host’s metadata.


We’ll use simple metadata where all values are strings. Assume the following hosts are defined and associated with a cluster:

Host Metadata
host1 v: 1.0, stage: prod
host2 v: 1.0, stage: prod
host3 v: 1.1, stage: canary
host4 v: 1.2-pre, stage: dev

The cluster may enable subset load balancing like this:

name: cluster-name
type: EDS
    path: '.../eds.conf'
  seconds: 10
lb_policy: LEAST_REQUEST
  fallback_policy: DEFAULT_SUBSET
    stage: prod
  - keys:
    - v
    - stage
  - keys:
    - stage

The following table describes some routes and the result of their application to the cluster. Typically the match criteria would be used with routes matching specific aspects of the request, such as the path or header information.

Match Criteria Balances Over Reason
stage: canary host3 Subset of hosts selected
v: 1.2-pre, stage: dev host4 Subset of hosts selected
v: 1.0 host1, host2 Fallback: No subset selector for “v” alone
other: x host1, host2 Fallback: No subset selector for “other”
(none) host1, host2 Fallback: No subset requested

Metadata match criteria may also be specified on a route’s weighted clusters. Metadata match criteria from the selected weighted cluster are merged with and override the criteria from the route:

Route Match Criteria Weighted Cluster Match Criteria Final Match Criteria
stage: canary stage: prod stage: prod
v: 1.0 stage: prod v: 1.0, stage: prod
v: 1.0, stage: prod stage: canary v: 1.0, stage: canary
v: 1.0, stage: prod v: 1.1, stage: canary v: 1.1, stage: canary
(none) v: 1.0 v: 1.0
v: 1.0 (none) v: 1.0

Example Host With Metadata

An EDS LbEndpoint with host metadata:

      protocol: TCP
      port_value: 8888
      version: '1.0'
      stage: 'prod'

Example Route With Metadata Criteria

An RDS Route with metadata match criteria:

  prefix: /
  cluster: cluster-name
        version: '1.0'
        stage: 'prod'