Connection Limit

The connection limit features allows users to limit the number of concurrently active TCP connections on a Gateway or a Listener. When the connection limit is reached, new connections are closed immediately by Envoy proxy. It’s possible to configure a delay for connection rejection.

Users may want to limit the number of connections for several reasons:

  • Protect resources like CPU and Memory.
  • Ensure that different listeners can receive a fair share of global resources.
  • Protect from malicious activity like DoS attacks.

Envoy Gateway introduces a new CRD called Client Traffic Policy that allows the user to describe their desired connection limit settings. This instantiated resource can be linked to a Gateway.

The Envoy connection limit implementation is distributed: counters are not synchronized between different envoy proxies.

When a Client Traffic Policy is attached to a gateway, the connection limit will apply differently based on the Listener protocol in use:

  • HTTP: all HTTP listeners in a Gateway will share a common connection counter, and a limit defined by the policy.
  • HTTPS/TLS: each HTTPS/TLS listener will have a dedicated connection counter, and a limit defined by the policy.

Prerequisites

Install Envoy Gateway

  • Follow the steps from the Quickstart to install Envoy Gateway and the HTTPRoute example manifest. Before proceeding, you should be able to query the example backend using HTTP.

Install the hey load testing tool

  • The hey CLI will be used to generate load and measure response times. Follow the installation instruction from the Hey project docs.

Test and customize connection limit settings

This example we use hey to open 10 connections and execute 1 RPS per connection for 10 seconds.

hey -c 10 -q 1 -z 10s  -host "www.example.com" http://${GATEWAY_HOST}/get
Summary:
  Total:	10.0058 secs
  Slowest:	0.0275 secs
  Fastest:	0.0029 secs
  Average:	0.0111 secs
  Requests/sec:	9.9942

[...]

Status code distribution:
  [200]	100 responses

There are no connection limits, and so all 100 requests succeed.

Next, we apply a limit of 5 connections.

cat <<EOF | kubectl apply -f -
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: ClientTrafficPolicy
metadata:
  name: connection-limit-ctp
  namespace: default
spec:
  targetRef:
    group: gateway.networking.k8s.io
    kind: Gateway
    name: eg
    namespace: default
  connection:
    connectionLimit:
      value: 5    
EOF

Save and apply the following resource to your cluster:

---
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: ClientTrafficPolicy
metadata:
  name: connection-limit-ctp
  namespace: default
spec:
  targetRef:
    group: gateway.networking.k8s.io
    kind: Gateway
    name: eg
    namespace: default
  connection:
    connectionLimit:
      value: 5    

Execute the load simulation again.

hey -c 10 -q 1 -z 10s  -host "www.example.com" http://${GATEWAY_HOST}/get
Summary:
  Total:	11.0327 secs
  Slowest:	0.0361 secs
  Fastest:	0.0013 secs
  Average:	0.0088 secs
  Requests/sec:	9.0640

[...] 

Status code distribution:
  [200]	50 responses

Error distribution:
  [50]	Get "http://localhost:8888/get": EOF

With the new connection limit, only 5 of 10 connections are established, and so only 50 requests succeed.