forward Output Plugin

The out_forward Buffered Output plugin forwards events to other fluentd nodes. This plugin supports load-balancing and automatic fail-over (a.k.a. active-active backup). For replication, please use the out_copy plugin.

The out_forward plugin detects server faults using a “φ accrual failure detector” algorithm. You can customize the parameters of the algorithm. When a server fault recovers, the plugin makes the server available automatically after a few seconds.

The out_forward plugin supports at-most-once and at-least-once semantics. The default is at-most-once.

NOTE: Do NOT use this plugin for inter-DC or public internet data transfer without secure connections. All the data is not encrypted, and this plugin is not designed for high-latency network environment. If you require a secure connection between nodes, please consider using in_secure_forward.

Example Configuration

out_forward is included in Fluentd's core. No additional installation process is required.

<match pattern>
  type forward
  send_timeout 60s
  recover_wait 10s
  heartbeat_interval 1s
  phi_threshold 16
  hard_timeout 60s

  <server>
    name myserver1
    host 192.168.1.3
    port 24224
    weight 60
  </server>
  <server>
    name myserver2
    host 192.168.1.4
    port 24224
    weight 60
  </server>
  ...

  <secondary>
    type file
    path /var/log/fluent/forward-failed
  </secondary>
</match>

NOTE: Please see the Config File article for the basic structure and syntax of the configuration file.

Parameters

type (required)

The value must be forward.

<server> (at least one is required)

The destination servers. Each server must have following information.

• name: The name of the server. This parameter is used in error messages.
• host (required): The IP address or host name of the server.
• port: The port number of the host. The default is 24224. Note that both TCP packets (event stream) and UDP packets (heartbeat message) are sent to this port.
• weight: The load balancing weight. If the weight of one server is 20 and the weight of the other server is 30, events are sent in a 2:3 raito. The default weight is 60.

require_ack_response

Change the protocol to at-least-once. The plugin waits the ack from destination's in_forward plugin.

ack_response_timeout

This option is used when require_ack_response is true. The default is 190. This default value is based on popular tcp_syn_retries.

If set 0, this plugin doesn't wait the ack response.

<secondary> (optional)

The backup destination that is used when all servers are unavailable.

send_timeout

The timeout time when sending event logs. The default is 60 seconds.

recover_wait

The wait time before accepting a server fault recovery. The default is 10 seconds.

heartbeat_type

The transport protocol to use for heartbeats. The default is "udp", but you can select "tcp" as well.

heartbeat_interval

The interval of the heartbeat packer. The default is 1 second.

phi_failure_detector

Use the "Phi accrual failure detector" to detect server failure. The default value is true.

phi_threshold

The threshold parameter used to detect server faults. The default value is 16.

hard_timeout

The hard timeout used to detect server failure. The default value is equal to the send_timeout parameter.

standby

Marks a node as the standby node for an Active-Standby model between Fluentd nodes. When an active node goes down, the standby node is promoted to an active node. The standby node is not used by the out_forward plugin until then.

<match pattern>
  type forward
  ...

  <server>
    name myserver1
    host 192.168.1.3
    weight 60
  </server>
  <server>  # forward doesn't use myserver2 until myserver1 goes down
    name myserver2
    host 192.168.1.4
    weight 60
    standby
  </server>
  ...
</match>

INCLUDE: _buffer_parameters

INCLUDE: _log_level_params

Troubleshooting

"no nodes are available"

Please make sure that you can communicate with port 24224 using not only TCP, but also UDP. These commands will be useful for checking the network configuration.

$ telnet host 24224
$ nmap -p 24224 -sU host

Please note that there is one known issue where VMware will occasionally lose small UDP packets used for heartbeat.