🚦 kong-plugin-qos-classifier

What is QoS?

Quality of service (QoS) is the use of mechanisms or technologies to control traffic and ensure the performance of critical applications. The term QoS originated in the field of telecommunication and is widely used today in computer networking for traffic prioritization.

What is kong-plugin-qos-classifier?

This is a custom kong plugin which can classify requests into configured QoS classes based on thresholds defined in terms of requests/sec and inject the value in the configured HTTP header.

Use-cases of the plugin

This plugin can help in progressive and pro-active load shedding, at flow level, whenever unexpected spikes in traffic are seen. This is discussed in detail in part 1 and part 2 series of blogs.

Features of the plugin

• Categorizes the requests into various classes based on the req/s threshold defined.
• Sends custom value of a predefined header to the upstream service based on the class identified, based on which the upstream service can gracefully handle degradations by load shedding desired flows.
• Supports terminating the requests after breaching the highest threshold with custom status code.
• Also supports sending custom header to clients when a request is terminated. Eg, sending a Location header with a static CDN URL and 302 status code.
• The classification of requests happen locally in a kong node without any dependency on any central datastore, which makes it performant.
• An endpoint, like that from kong-plugin-cluster-stats, could be configured to dynamically update the calculation based on the current number of kong nodes in the cluster.

Prometheus metrics

The plugin also emits the following prometheus metrics with kong version >= 2.6.0

# HELP kong_qos_request_threshold Threshold for QoS class differentiation
# TYPE kong_qos_request_threshold gauge
kong_qos_request_threshold{class="<class_name>",route="<route>",service="<service>"} 1
# HELP kong_qos_total_requests Total requests received so far
# TYPE kong_qos_total_requests counter
kong_qos_total_requests{class="<class_name>",route="<route>",service="<service>"} 10

Installation and Loading of the plugin

Follow standard procedure to install and load the plugin.

This plugin also requires the following shared_dicts to be defined in nginx configuration. The following is the kong configuration for the same:

nginx_http_lua_shared_dict = qos_shared 12m; lua_shared_dict qos_lock 100k

Enabling the plugin

The plugin can be enabled at service, route or global levels. In case of multiple instances of plugins enabled for a request, evaluation only happens for the highest level. The order of precedence is (starting from the highest level):

• route
• service
• global

Configuring the plugin

The plugin has the following configuration object:

{
  "config": {
    "termination": {
      "status_code": 302,
      "header_name": "Location",
      "header_value": "https://cred.club"
    },
    "upstream_header_name": "X-QOS-CLASS",
    "node_count": {
      "http_timeout_in_ms": 15,
      "update_initial_delay_in_sec": 5,
      "update_url": "http://localhost:8001/cluster-stats",
      "initial": 2,
      "update_frequency_in_sec": 1
    },
    "classes": {
      "class_1": {
        "threshold": 4,
        "header_value": "green"
      },
      "class_2": {
        "threshold": 6,
        "header_value": "red"
      },
      "class_3": {
        "threshold": null,
        "header_value": null
      },
      "class_4": {
        "threshold": null,
        "header_value": null
      }
    }
  }
}

Configuration parameters

Parameter	Description
upstream_header_name	HTTP header to be sent to upstream services which carries the class of the request (default: X-QOS-CLASS)
classes.class_1.threshold	Threshold in req/s for 1st class of requests
classes.class_1.header_value	Value of the header identified by upstream_header_name when the requests is within classes.class_1.threshold
node_count.initial	Initial number of kong nodes
termination.status_code	HTTP status code to send when the threshold of the highest class is crossed (default: 429)
termination.header_name	HTTP header to send when the request is terminated
termination.header_value	Value of termination.header_name header to be sent
node_count.update_url	HTTP endpoint to hit to get updated number of nodes. The response should have num_nodes json field. If this is not set, node counts are not updated dynamically and node_count.initial is used
node_count.http_timeout_in_ms	HTTP timeout in ms for node_count.update_url endpoint (default: 10)
node_count.update_frequency_in_sec	Interval at which to update the node count (default: 1)
node_count.update_initial_delay_in_sec	Number of seconds to wait before initiating node update calls. This is to make sure that the cluster stabalises before making these calls, eg in case of a rolling deployment of the cluster. Till this time, the configured node_count.initial is used for the calculation (default: 15)