Using disk-based and memory buffering

The AxoSyslog application can store messages on the local hard disk if the destination (for example, the central log server) or the network connection to the destination becomes unavailable. The AxoSyslog application automatically sends the stored messages to the destination when the connection is reestablished. The disk buffer is used as a queue: when the connection to the destination is reestablished, AxoSyslog sends the messages to the destination in the order they were received.

Every destination driver supports the disk-buffer() option. The network(), syslog(), tcp(), and tcp6() destination drivers can also use the disk-buffer option, except when using the udp transport method.

Every such destination uses a separate disk buffer (similarly to the output buffers controlled by log-fifo-size()). The hard disk space is not pre-allocated, so ensure that there is always enough free space to store the disk buffers even when the disk buffers are full.

If AxoSyslog is restarted (using the /etc/init.d/syslog-ng restart command, or another appropriate command on your platform), it automatically saves any unsent messages from the disk buffer and in-memory queues. After the restart, AxoSyslog sends the saved messages to the destination. In other words, the disk buffer is persistent. The disk buffer is also resistant to AxoSyslog crashes.

The AxoSyslog application supports two types of disk buffering: reliable and normal. For details, see Enabling reliable disk-based buffering and Enabling normal disk-based buffering, respectively.

Message handling and normal disk-based buffering

When you use disk-based buffering, and the reliable() option is set to no, AxoSyslog handles outgoing messages the following way:

• Output queue: In-memory queue. If there is space left in it, AxoSyslog puts the message into this queue first . Messages stored here are processed faster, because AxoSyslog can skip writing to, and reading from the disk, as well as serializing or deserializing the message, saving I/O and processor time as a result. The contents of the in-memory output queue are persisted to the disk-buffer file during AxoSyslog reload, restart or stop, but they cannot be persisted if in the event of power failures, or if AxoSyslog crashes. By default, the output queue can hold 1000 messages (you can adjust this number using the quot-size() option).

• Disk-buffer file: Disk queue. If there is no space left in the output queue, the message is stored on the disk-buffer file. Messages stored here are persisted on the disk, even in case of power failures or if AxoSyslog crashes. Using the disk-buffer file takes considerable amount of disk I/O and processor time. The size of this queue can be set with the capacity-bytes() option.

• Overflow queue: In-memory queue. This queue is used to trigger flow-control if it is set. The contents of the in-memory overflow queue are persisted to the disk-buffer file in case of AxoSyslog reload, restart or stop, but they are not persisted in case of power failures or if AxoSyslog crashes. Setting the size of the overflow queue can be done with the flow-control-window-size() option.

Message handling and reliable disk-based buffering

When you use disk-based buffering, and the reliable() option is set to yes, AxoSyslog handles outgoing messages the following way.

The flow-control-window-bytes() option determines when flow-control is triggered. After the size of the disk-buffer file reaches (capacity-bytes() minus flow-control-window-bytes()), messages are written into both the disk-buffer file and the overflow queue, indicating that flow-control needs to slow down the message source. These messages are not taken out from the control window (governed by log-iw-size()), causing the control window to fill up.

If the control window is full, the flow-control completely stops reading incoming messages from the source. (As a result, flow-control-window-bytes() must be at least as large as log-iw-size() times the average message size.)

• Output queue: In-memory and disk queue. If there is space left in it, AxoSyslog puts the message into this queue first. In case of reliable disk-buffer, in addition to storing the message in memory, it is stored directly in the disk-buffer file as well for safety reasons (see the next point). Messages stored here are processed faster, because AxoSyslog can skip reading from the disk, and deserializing the message, saving I/O and processor time. By default, the output queue can hold 1000 messages (you can adjust it using the quot-size() option).

• Disk-buffer file: Disk queue. If there is no space left in the output queue, the message is stored on the disk-buffer file. Messages stored here are persisted on the disk, and survive AxoSyslog crash or power failure. Using the disk-buffer file takes considerable amount of disk I/O and processor time. The size of this queue can be set with the capacity-bytes() option.

• Overflow queue: In-memory and disk queue. This queue is used to trigger flow-control if it is set. Similarly to the output queue, in case of reliable disk-buffer in addition to storing the message in memory, it is stored directly in the disk-buffer file as well for safety reasons. Setting the size of the overflow queue can be done with the flow-control-window-bytes() option.