out_forward.txt

# 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](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.

## Example Configuration

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

    :::text
    <match pattern>
      @type forward
      send_timeout 60s
      recover_wait 10s
      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 LINK:[Config File](config-file) article for the basic structure and syntax of the configuration file.

## Supported modes

* Synchronous
* Asynchronous

See [Output Plugin Overview](output-plugin-overview) for more details.

## Plugin helpers

* [socket](api-plugin-helper-socket)
* [server](api-plugin-helper-server)
* [timer](api-plugin-helper-timer)
* [thread](api-plugin-helper-thread)
* [compat_parameters](api-plugin-helper-compat_parameters)

## Parameters

[Common Parameters](plugin-common-parameters)

### @type

The value must be `forward`.

### &lt;server&gt; (at least one is required)

| required | multi | version |
|:--------:|:-----:|:-------:|
| true     | true  | 0.14.5  |

The destination servers. Each server has following parameters.

- host
- name
- port
- shared_key
- username
- password
- standby
- weight

#### host

| type   | default            | version |
|:------:|:------------------:|:-------:|
| string | required parameter | 0.14.5  |

The IP address or host name of the server.

#### name

| type   | default | version |
|:------:|:-------:|:-------:|
| string | nil     | 0.14.5  |

The name of the server. Used for logging and certificate verification in TLS transport (when host is address).

#### port

| type    | default | version |
|:-------:|:-------:|:-------:|
| integer | 24224   | 0.14.5  |

The port number of the host. Note that both TCP packets (event stream) and UDP packets (heartbeat message) are sent to this port.

#### shared_key

| type   | default | version |
|:------:|:-------:|:-------:|
| string | nil     | 0.14.5  |

The shared key per server.

#### username

| type   | default           | version |
|:------:|:-----------------:|:-------:|
| string | "" (empty string) | 0.14.5  |

The username for authentication.

#### password

| type   | default           | version |
|:------:|:-----------------:|:-------:|
| string | "" (empty string) | 0.14.5  |

The password for authentication.

#### standby

| type | default | version |
|:----:|:-------:|:-------:|
| bool | false   | 0.14.5  |

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.

    :::text
    <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>

#### weight

| type    | default | version |
|:-------:|:-------:|:-------:|
| integer | 60      | 0.14.5  |

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 ratio. The default weight is 60.

### require_ack_response

| type | default | version |
|:----:|:-------:|:-------:|
| bool | false   | 0.14.0  |

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

### ack_response_timeout

| type | default | version |
|:----:|:-------:|:-------:|
| time | 190     | 0.14.0  |

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

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

### send_timeout

| type | default | version |
|:----:|:-------:|:-------:|
| time | 60      | 0.14.0  |

The timeout time when sending event logs.

### recover_wait

| type | default | version |
|:----:|:-------:|:-------:|
| time | 10      | 0.14.0  |

The wait time before accepting a server fault recovery.

### heartbeat_type

| type | default   | available                 | version |
|:----:|:---------:|:-------------------------:|:-------:|
| enum | transport | transport, tcp, udp, none | 0.14.12 |

The transport protocol to use for heartbeats. Set "none" to disable heartbeat. 

### heartbeat_interval

| type | default | version |
|:----:|:-------:|:-------:|
| time | 1       | 0.14.0  |

The interval of the heartbeat packer.

### phi_failure_detector

| type | default | version |
|:----:|:-------:|:-------:|
| bool | true    | 0.14.0  |

Use the "Phi accrual failure detector" to detect server failure.

### phi_threshold

| type    | default | version |
|:-------:|:-------:|:-------:|
| integer | 16      | 0.14.0  |

The threshold parameter used to detect server faults.

NOTE: `phi_threshold` is deeply related to `heartbeat_interval`. If you are using longer `heartbeat_interval`, please use the larger `phi_threshold`. Otherwise you will see frequent detachments of destination servers. The default value 16 is tuned for `heartbeat_interval` 1s.

### hard_timeout

| type | default | version |
|:----:|:-------:|:-------:|
| time | 60      | 0.14.0  |

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

### expire_dns_cache

| type | default                | version |
|:----:|:----------------------:|:-------:|
| time | nil (persistent cache) | 0.14.0  |

Set TTL to expire DNS cache in seconds. Set 0 not to use DNS Cache.

### dns_round_robin

| type | default | version |
|:----:|:-------:|:-------:|
| bool | false   | 0.14.0  |

Enable client-side DNS round robin. Uniform randomly pick an IP address to send data when a hostname has several IP addresses.

NOTE: `heartbeat_type udp` is not available with `dns_round_robin true`. Use `heartbeat_type tcp` or `heartbeat_type none`. 

### ignore_network_errors_at_startup

| type | default | version |
|:----:|:-------:|:-------:|
| bool | false   | 0.14.12 |

Ignore DNS resolution and errors at startup time.

### tls_version

| type | default | available       | version |
|:----:|:-------:|:---------------:|:-------:|
| enum | TLSv1_2 | TLSv1_1, TLSv1_2| 0.14.12 |

The default version of TLS transport.

### tls_ciphers

| type   | default                                            | version |
|:------:|:--------------------------------------------------:|:-------:|
| string | ALL:!aNULL:!eNULL:!SSLv2 (OpenSSL > 1.0.0 default) | 0.14.12 |

The cipher configuration of TLS transport.

### tls_insecure_mode

| type | default | version |
|:----:|:-------:|:-------:|
| bool | false   | 0.14.12 |

Skip all verification of certificates or not.

### tls_allow_self_signed_cert

| type | default | version |
|:----:|:-------:|:-------:|
| bool | false   | 0.14.12 |

Allow self signed certificates or not.

### tls_verify_hostname

| type | default | version |
|:----:|:-------:|:-------:|
| bool | true    | 0.14.12 |

Verify hostname of servers and certificates or not in TLS transport.

### tls_cert_path

| type            | default | version |
|:---------------:|:-------:|:-------:|
| array of string | nil     | 0.14.12 |

The additional CA certificate path for TLS.

### tls_client_cert_pat

| type   | default | version |
|:------:|:-------:|:-------:|
| string | nil     | 1.3.2   |

The client certificate path for TLS

### tls_client_private_key_path

| type   | default | version |
|:------:|:-------:|:-------:|
| string | nil     | 1.3.2   |

The client private key path for TLS.

### tls_client_private_key_passphrase

| type   | default | version |
|:------:|:-------:|:-------:|
| string | nil     | 1.3.2   |

The client private key passphrase for TLS.

### keepalive

| type | default | version |
|:----:|:-------:|:-------:|
| bool | fales    | 1.4.3  |

Enable keepalive connection.

### keepalive_timeout

| type | default | version |
|:----:|:-------:|:-------:|
| time | nil     | 1.4.3  |

Expired time of keepalive. Default value is nil, which means to keep connection as long as possible.

### &lt;security&gt; section

| required | multi | version |
|:--------:|:-----:|:-------:|
| false    | false | 0.14.5  |

This section contains parameters related to authentication.

- self_hostname
- shared_key

#### self_hostname

| type   | default            | version |
|:------:|:------------------:|:-------:|
| string | required parameter | 0.14.5  |

The hostname.

#### shared_key

| type   | default            | version |
|:------:|:------------------:|:-------:|
| string | required parameter | 0.14.5  |

Shared key for authentication. If you want to specify `shared_key` for specific server, use `<server>` section.

### &lt;secondary&gt;

| required | multi | version |
|:--------:|:-----:|:-------:|
| false    | false | 0.14.0  |

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

For more details, see [Secondary Output](output-plugin-overview#secondary-output).

### verify_connection_at_startup

| type | default | version |
|:----:|:-------:|:-------:|
| bool | false   | 1.3.1   |

Verify that a connection can be made with one of out_forward nodes at the time of startup.

## Tips & Tricks

### How to connect to a TLS/SSL enabled server

If you've [set up TLS/SSL encryption in the receiving server](in_forward#how-to-enable-tls/ssl-encryption), you need to tell the output forwarder to use encryption by setting the `transport` parameter:

```
<match debug.**>
  @type forward
  transport tls
  <server>
    host 192.168.1.2
    port 24224
  </server>
</match>
```

If you're using a self-singed certificate, copy the certificate file to the forwarding server, then add the following settings:

```
<match debug.**>
  @type forward
  transport tls
  tls_cert_path /path/to/fluentd.crt # Set the path to the certificate file.
  tls_verify_hostname true           # Set false to ignore cert hostname.
  <server>
    host 192.168.1.2
    port 24224
  </server>
</match>
```

After updating the settings, please confirm that the forwarded data is being received by the destination node properly.

### How to Enable Password Authentication

If you want to connect to [a server that requires password authentication](in_forward#how-to-enable-password-authentication), you need to set your credentials in the configuration file.

    <match debug.**>
      @type forward
      <server>
        host 192.168.1.2
        port 24224
      </server>
      <security>
        self_hostname HOSTNAME
        shared_key secret
      </security>
    </match>

Note that, as to the option `self_hostname`, you need to set the name of the server on which your `out_forward` instance is running. In the current implementation, it is considered invalid if your `in_forward` and `out_forward` shares the same hostname.

### How to enable gzip compression

Since v0.14.7, Fluentd supports transparent data compression. You can use this feature to reduce the transferred payload size.

To enable this feature, set the `compress` option as follows:

```
<match debug.**>
  @type forward
  compress gzip
  <server>
    host 192.168.1.2
    port 24224
  </server>
</match>
```

You don't need any configuration in the receiving server. Data compression is auto-detected and handled transparently by the destination node.

### What is a Phi accrual failure detector?

Fluentd implements an adaptive failure detection mechanism called "Phi accrual failure detector". Here is how it works:

 1. Each `in_forward` node sends heartbeat packets to its `out_foward` server at a regular interval.
 2. The `out_forward` server records the arrival time of heartbeat packets sent by each node.
 3. If the server does not receive a heartbeat from one of its nodes for "a long time", it assumes the node is down.

But how long should the server wait before detaching a node? The phi accrual failure detector answers this question by computing the probability of a node being down based on the assumption that heartbeat intervals follow normal distribution. Internally it represent the confidence of a node being down by a continuous function *&phi;(t)* which grows as the time from the last packet increases.

For example, suppose that the historical average interval is 1 seconds and the standard deviation is 1, it's not likely that the node is still being active when its heartbeat does not arrive for the last 10 seconds.

For details, please read the original paper: [Hayashibara, Naohiro, et al. "The &phi; accrual failure detector." IEEE, 2004.](https://scholar.google.com/scholar?cluster=12946656837229314866)

## 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.

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

Please note that there is one [known issue](http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2019944) where VMware will occasionally lose small UDP packets used for heartbeat.