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kumo.on('should_enqueue_log_record', function(message, hook_name))

This event is triggered when kumo.configure_log_hook has been used to enable it.

Since: Version 2023.08.22-4d895015

The functionality described in this outlined box requires version 2023.08.22-4d895015 of KumoMTA, or a more recent version.

The hook_name parameter was added. It corresponds to the name field that was passed to kumo.configure_log_hook and is present to allow you to decide whether a given message should get queued for a given hook instance.

When enabled, each log record will generate a new Message object with the following attributes:

  • Sender will be set to the sender of the originating message
  • Recipient will be set to the recipient of the originating message
  • the log_record meta value will be set to the Log Record that is being logged
  • The message body/data will be set to the textual representation of the log record. By default this will be the JSON serialization of the log record, but if you have enabled templating using the per_record parameter to kumo.configure_log_hook then it will be whatever your template evaluated to for that record.
  • The reception_protocol meta value will be set to "LogRecord"

That log message is passed to the should_enqueue_log_record hook, which must return a boolean value that indicates whether the log message should be queued and acted upon (return true), or whether the log message should be discarded.

kumo.on('should_enqueue_log_record', function(msg, hook_name)
  local log_record = msg:get_meta 'log_record'
  -- avoid an infinite loop caused by logging that we logged that we logged...
  -- Check the log record: if the record was destined for the webhook queue
  -- then it was a record of the webhook delivery attempt and we must not
  -- log its outcome via the webhook.
  if log_record.queue ~= 'webhook' then
    -- was some other event that we want to log via the webhook
    msg:set_meta('queue', 'webhook')
    return true
  return false

-- This is a user-defined event that matches up to the custom_lua
-- constructor used in `get_queue_config` below.
-- It returns a lua connection object that can be used to "send"
-- messages to their destination.
kumo.on('make.webhook', function(domain, tenant, campaign)
  local connection = {}
  local client = kumo.http.build_client {}
  function connection:send(message)
    local response = client
      :post(string.format('', WEBHOOK_PORT))
      :header('Content-Type', 'application/json')

    local disposition = string.format(
      '%d %s: %s',

    if response:status_is_success() then
      return disposition

    -- Signal that the webhook request failed.
    -- In this case the 500 status prevents us from retrying
    -- the webhook call again, but you could be more sophisticated
    -- and analyze the disposition to determine if retrying it
    -- would be useful and generate a 400 status instead.
    -- In that case, the message we be retryed later, until
    -- it reached it expiration.
    kumo.reject(500, disposition)
  return connection

kumo.on('get_queue_config', function(domain, tenant, campaign, routing_domain)
  if domain == 'webhook' then
    -- Use the `make.webhook` event to handle delivery
    -- of webhook log records
    return kumo.make_queue_config {
      protocol = {
        custom_lua = {
          constructor = 'make.webhook',
  return kumo.make_queue_config {}

Since: Version 2023.11.28-b5252a41

The functionality described in this outlined box requires version 2023.11.28-b5252a41 of KumoMTA, or a more recent version.

It is now possible to use kumo.on to register multiple handlers for this event. The handlers will be called in the order that they were registered. If a handler returns nil then the next handler will be called. Conversely, if a handler returns either true or false, its return value is taken as the definitive outcome and no further handlers will be called.

This behavior is intended to make it easier to compose multiple helpers or lua modules together.