Example of a Heka Lua Sandbox decoder that will take JSON log lines (and some extra decoder config) and stream that information into the `Fields` variables of the Heka message.

json_log_decode.lua

require "cjson"

-- Generic decoder for JSON logs. This will extract all JSON
-- keys and add them to the `Fields` variable of the created
-- Heka message.
--
-- Example use:
--
-- [NginxJsonLogDecoder]
-- type = "SandboxDecoder"
-- script_type = "lua"
-- filename = "/location/of/json_log_decode.lua"
--
-- [NginxJsonLogDecoder.config]
-- app = "some application"
--
-- [some-nginx-json-log-file]
-- type = "LogfileInput"
-- logfile = "/some/location/of/an/nginx/log/file.json.log"
-- decoder = "NginxJsonLogDecoder"

function process_message()
   local raw_message = read_message("Payload")
   local app = read_config("app")

   local json = cjson.decode(raw_message)
   if not json then
      -- When plain text is found, ship it in it's raw form.
      inject_message(raw_message)
      return 0
   end

   local message = {
      Type = "MyGenericNamespace.JsonLog"
   }

   if app then
      json["app"] = app
   end

   message.Payload = raw_message
   message.Fields = json

   inject_message(message)
   return 0
end