Plugin Development - Plugin Configuration

Most of the time, it makes sense for your plugin to be configurable to answer all of your users’ needs. Your plugin’s configuration is stored in the datastore for Kong to retrieve it and pass it to your handler.lua methods when the plugin is being executed.

The configuration consists of a Lua table in Kong that we call a schema. It contains key/value properties that the user will set when enabling the plugin through the Admin API. Kong provides you with a way of validating the user’s configuration for your plugin.

Your plugin’s configuration is being verified against your schema when a user issues a request to the Admin API to enable or update a plugin on a given Service, Route, or Consumer.

For example, a user performs the following request:

$ curl -X POST http://kong:8001/services/<service-name-or-id>/plugins \
    -d "name=my-custom-plugin" \
    -d "config.foo=bar"

If all properties of the config object are valid according to your schema, then the API would return 201 Created and the plugin would be stored in the database along with its configuration:

{
  foo = "bar"
}

If the configuration is not valid, the Admin API would return 400 Bad Request and the appropriate error messages.

Module

kong.plugins.<plugin_name>.schema

schema.lua specifications

This module is to return a Lua table with properties that will define how your plugins can later be configured by users. Available properties are:

All the plugins inherit some default fields which are:

In most of the cases you can ignore most of those and use the defaults. Or let the user specify value when enabling a plugin.

Here is an example of a potential schema.lua file (with some overrides applied):

local typedefs = require "kong.db.schema.typedefs"
return {
  name = "<plugin-name>",
  fields = {
    {
      -- this plugin will only be applied to Services or Routes
      consumer = typedefs.no_consumer
    },
    {
      -- this plugin will only run within Nginx HTTP module
      protocols = typedefs.protocols_http
    },
    {
      config = {
        type = "record",
        fields = {
          -- Describe your plugin's configuration's schema here.        
        },
      },
    },
  },
  entity_checks = {
    -- Describe your plugin's entity validation rules
  },
}

Describing your configuration schema

The config.fields property of your schema.lua file describes the schema of your plugin’s configuration. It is a flexible array of field definitions where each field is a valid configuration property for your plugin, describing the rules for that property. For example:

{
  name = "<plugin-name>",
  fields = {
    config = {
      type = "record",
      fields = {
        {
          some_string = {
            type = "string",
            required = false,
          },
        },
        {
          some_boolean = {
            type = "boolean",
            default = false,
          },
        },
        {
          some_array = {
            type = "array",
            elements = {
              type = "string",
              one_of = {
                "GET",
                "POST",
                "PUT",
                "DELETE",
              },
            },
          },
        },
      },
    },
  },
}

Here is the list of some common (not all) accepted rules for a property (see the fields table above for examples):

There are many more, but the above are commonly used.

You can also add field validators, to mention a few:

There are some additional validators, but you get a good idea how you can specify validation rules on fields from the above table.

Examples

This schema.lua file is for the key-auth plugin:

-- schema.lua
local typedefs = require "kong.db.schema.typedefs"
return {
  name = "key-auth",
  fields = {
    {
      consumer = typedefs.no_consumer
    },
    {
      protocols = typedefs.protocols_http
    },
    {
      config = {
        type = "record",
        fields = {
          {
            key_names = {
              type = "array",
              required = true,
              elements = typedefs.header_name,
              default = {
                "apikey",
              },
            },
          },
          {
            hide_credentials = {
              type = "boolean",
              default = false,
            },
          },
          {
            anonymous = {
              type = "string",
              uuid = true,
              legacy = true,
            },
          },
          {
            key_in_body = {
              type = "boolean",
              default = false,
            },
          },
          {
            run_on_preflight = {
              type = "boolean",
              default = true,
            },
          },
        },
      },
    },
  },
}

Hence, when implementing the access() function of your plugin in handler.lua and given that the user enabled the plugin with the default values, you’d have access to:

-- handler.lua
local BasePlugin = require "kong.plugins.base_plugin"
local kong = kong
local CustomHandler = BasePlugin:extend()
CustomHandler.VERSION  = "1.0.0"
CustomHandler.PRIORITY = 10
function CustomHandler:new()
  CustomHandler.super.new(self, "my-custom-plugin")
end
function CustomHandler:access(config)
  CustomHandler.super.access(self)
  kong.log.inspect(config.key_names)        -- { "apikey" }
  kong.log.inspect(config.hide_credentials) -- false
end
return CustomHandler

Note that the above example uses the kong.log.inspect function of the Plugin Development Kit to print out those values to the Kong logs.

A more complex example, which could be used for an eventual logging plugin:

-- schema.lua
local typedefs = require "kong.db.schema.typedefs"
return {
  name = "my-custom-plugin",
  fields = {
    {
      config = {
        type = "record",
        fields = {
          {
            environment = {
              type = "string",
              required = true,
              one_of = {
                "production",
                "development",
              },
            },
          },
          {
            server = {
              type = "record",
              fields = {
                {
                  host = typedefs.host {
                    default = "example.com",
                  },
                },
                {
                  port = {
                    type = "number",
                    default = 80,
                    between = {
                      0,
                      65534
                    },
                  },
                },  
              },
            },
          },
        },
      },
    },
  },
}

Such a configuration will allow a user to post the configuration to your plugin as follows:

$ curl -X POST http://kong:8001/services/<service-name-or-id>/plugins \
    -d "name=my-custom-plugin" \
    -d "config.environment=development" \
    -d "config.server.host=http://localhost"

And the following will be available in handler.lua:

-- handler.lua
local BasePlugin = require "kong.plugins.base_plugin"
local kong = kong
local CustomHandler = BasePlugin:extend()
CustomHandler.VERSION  = "1.0.0"
CustomHandler.PRIORITY = 10
function CustomHandler:new()
  CustomHandler.super.new(self, "my-custom-plugin")
end
function CustomHandler:access(config)
  CustomHandler.super.access(self)
  kong.log.inspect(config.environment) -- "development"
  kong.log.inspect(config.server.host) -- "http://localhost"
  kong.log.inspect(config.server.port) -- 80
end
return CustomHandler

You can also see a real-world example of schema in the Key-Auth plugin source code.

Next: Accessing the Datastore ›