Adaptive Concurrency


The adaptive concurrency filter is experimental and is currently under active development.

This filter should be configured with the name envoy.filters.http.adaptive_concurrency.

See the v2 API reference for details on each configuration parameter.


The adaptive concurrency filter dynamically adjusts the allowed number of requests that can be outstanding (concurrency) to all hosts in a given cluster at any time. Concurrency values are calculated using latency sampling of completed requests and comparing the measured samples in a time window against the expected latency for hosts in the cluster.

Concurrency Controllers

Concurrency controllers implement the algorithm responsible for making forwarding decisions for each request and recording latency samples to use in the calculation of the concurrency limit.

Gradient Controller

The gradient controller makes forwarding decisions based on a periodically measured ideal round-trip time (minRTT) for an upstream.

v2 API reference

Calculating the minRTT

The minRTT is periodically measured by only allowing a very low outstanding request count to an upstream cluster and measuring the latency under these ideal conditions. The length of this minRTT calculation window is variable depending on the number of requests the filter is configured to aggregate to represent the expected latency of an upstream.

A configurable jitter value is used to randomly delay the start of the minRTT calculation window by some amount of time. This is not necessary and can be disabled; however, it is recommended to prevent all hosts in a cluster from being in a minRTT calculation window (and having a concurrency limit of 3 by default) at the same time. The jitter helps negate the effect of the minRTT calculation on the downstream success rate if retries are enabled.

It is possible that there is a noticeable increase in request 503s during the minRTT measurement window because of the potentially significant drop in the concurrency limit. This is expected and it is recommended to enable retries for resets/503s.


It is recommended to use the previous_hosts retry predicate. Due to the minRTT recalculation jitter, it’s unlikely that all hosts in the cluster will be in a minRTT calculation window, so retrying on a different host in the cluster will have a higher likelihood of success in this scenario.

Once calculated, the minRTT is then used in the calculation of a value referred to as the gradient.

The Gradient

The gradient is calculated using summarized sampled request latencies (sampleRTT):

\[gradient = \frac{minRTT + B}{sampleRTT}\]

This gradient value has a useful property, such that it decreases as the sampled latencies increase. Notice that B, the buffer value added to the minRTT, allows for normal variance in the sampled latencies by requiring the sampled latencies the exceed the minRTT by some configurable threshold before decreasing the gradient value.

The buffer will be a percentage of the measured minRTT value whose value is modified via the buffer field in the minRTT calculation parameters. The buffer is calculated as follows:

\[B = minRTT * buffer_{pct}\]

The gradient value is then used to update the concurrency limit via:

\[limit_{new} = gradient * limit_{old} + headroom\]

Concurrency Limit Headroom

The headroom value is necessary as a driving factor to increase the concurrency limit when the sampleRTT is in the same ballpark as the minRTT. This value must be present in the limit calculation, since it forces the concurrency limit to increase until there is a deviation from the minRTT latency. In the absence of a headroom value, the concurrency limit could potentially stagnate at an unnecessary small value if the sampleRTT and minRTT are close to each other.

Because the headroom value is so necessary to the proper function for the gradient controller, the headroom value is unconfigurable and pinned to the square-root of the concurrency limit.


The adaptive concurrency filter’s control loop relies on latency measurements and adjustments to the concurrency limit based on those measurements. Because of this, the filter must operate in conditions where it has full control over request concurrency. This means that:

  1. The filter works as intended in the filter chain for a local cluster.

  2. The filter must be able to limit the concurrency for a cluster. This means there must not be requests destined for a cluster that are not decoded by the adaptive concurrency filter.

Example Configuration

An example filter configuration can be found below. Not all fields are required and many of the fields can be overridden via runtime settings.

name: envoy.filters.http.adaptive_concurrency
      value: 90
      concurrency_update_interval: 0.1s
        value: 10
      interval: 60s
      request_count: 50
    default_value: true
    runtime_key: "adaptive_concurrency.enabled"

The above configuration can be understood as follows:

  • Gather latency samples for a time window of 100ms. When entering a new window, summarize the requests (sampleRTT) and and update the concurrency limit using this sampleRTT.

  • When calculating the sampleRTT, use the p90 of all sampled latencies for that window.

  • Recalculate the minRTT every 60s and add a jitter (random delay) of 0s-6s to the start of the minRTT recalculation. The delay is dictated by the jitter value.

  • Collect 50 request samples to calculate the minRTT and use the p90 to summarize them.

  • The filter is enabled by default.


It is recommended that the adaptive concurrency filter come after the healthcheck filter in the filter chain to prevent latency sampling of health checks. If health check traffic is sampled, it could potentially affect the accuracy of the minRTT measurements.


The adaptive concurrency filter supports the following runtime settings:


Overrides whether the adaptive concurrency filter will use the concurrency controller for forwarding decisions. If set to false, the filter will be a no-op. Defaults to what is specified for enabled in the filter configuration.


Overrides the interval in which the ideal round-trip time (minRTT) will be recalculated.


Overrides the number of requests sampled for calculation of the minRTT.


Overrides the random delay introduced to the minRTT calculation start time. A value of 10 indicates a random delay of 10% of the configured interval. The runtime value specified is clamped to the range [0,100].


Overrides the interval in which the concurrency limit is recalculated based on sampled latencies.


Overrides the maximum allowed concurrency limit.


Overrides the padding added to the minRTT when calculating the concurrency limit.


Overrides the percentile value used to represent the collection of latency samples in calculations. A value of 95 indicates the 95th percentile. The runtime value specified is clamped to the range [0,100].


Overrides the concurrency that is pinned while measuring the minRTT.


The adaptive concurrency filter outputs statistics in the http.<stat_prefix>.adaptive_concurrency. namespace. The stat prefix comes from the owning HTTP connection manager. Statistics are specific to the concurrency controllers.

Gradient Controller Statistics

The gradient controller uses the namespace http.<stat_prefix>.adaptive_concurrency.gradient_controller.






Total requests that were blocked by the filter.



Set to 1 if the controller is in the process of a minRTT calculation. 0 otherwise.



The current concurrency limit.



The current gradient value.



The current headroom value in the concurrency limit calculation.



The current measured minRTT value.



The current measured sampleRTT aggregate.