**********************
Failover Syslog Server
**********************


There are often situations where syslog data from the local system should be 
sent to a central syslogd (for consolidation, archival and whatever other 
reasons). A common problem is that messages are lost when the central syslogd 
goes down.
Rsyslog has the capability to work with failover servers to prevent message 
loss. A prerequisite is that TCP based syslog or RELP forwarding is used to send
to the central server. The reason is that with UDP there is no reliable way to
detect the remote system has gone away.
Let's assume you have a primary and two secondary central servers. Then, you 
can use the following config file excerpt to send data to them:

.. code-block:: none

   if($msg contains "error") then {
        action(type="omfwd" target="primary-syslog.example.com" port="10514"
               protocol="tcp")
        action(type="omfwd" target="secondary-1-syslog.example.com" port="10514"
               action.execOnlyWhenPreviousIsSuspended="on")
        action(type="omfwd" target="secondary-2-syslog.example.com" port="10514"
               action.execOnlyWhenPreviousIsSuspended="on")
        action(type="omfile" tag="failover" file="/var/log/localbuffer"
               action.execOnlyWhenPreviousIsSuspended="on")
   }


The action processes all messages that contain "error". It tries to forward 
every message to primary-syslog.example.com (via tcp). If it can not reach that
server, it tries secondary-1-syslog.example.com, if that fails too, it tries 
secondary-2-syslog.example.com. If neither of these servers can be connected, 
the data is stored in /var/log/localbuffer. Please note that the secondaries 
and the local log buffer are only used if the one before them does not work. 
So ideally, /var/log/localbuffer will never receive a message. If one of the 
servers resumes operation, it automatically takes over processing again.

**Important:** Failover will **not** work when you define queues on the actions.
This is because a queue explicitely tells rsyslog that the action shall be
processed asynchronously. With asynchronous processing you do not have any
feedback capability. As such, the action will never fail.

**If you would like to use a queue on the forwarding process as whole, the solution
is** to put all actions into a ruleset and assing a queue to the ruleset. In
that configuration, the ruleset is process asynchronously, but inside the
rule set each action is processed synchronously and can provide feedback,
which permits to detect failed actions.

To do so, the above example can be rewritten as follows:

.. code-block:: none

   ruleset(name="forwarding" queue.type="linkedList" queue.filename="fwdq") {
        action(type="omfwd" target="primary-syslog.example.com" port="10514"
               protocol="tcp")
        action(type="omfwd" target="secondary-1-syslog.example.com" port="10514"
               action.execOnlyWhenPreviousIsSuspended="on")
        action(type="omfwd" target="secondary-2-syslog.example.com" port="10514"
               action.execOnlyWhenPreviousIsSuspended="on")
        action(type="omfile" tag="failover" file="/var/log/localbuffer"
               action.execOnlyWhenPreviousIsSuspended="on")
   }

   if($msg contains "error") then {
        call forwarding
   }


Please note that the example could also be rewritten in several other ways. To
keep things simple, we just provide one of those.
