Managing incoming and outgoing messages with flow-control

This section describes the internal message-processing model of syslog-ng, as well as the flow-control feature that can prevent message losses.

The AxoSyslog application monitors (polls) the sources defined in its configuration file, periodically checking each source for messages. When a log message is found in one of the sources, AxoSyslog polls every source and reads the available messages. These messages are processed and put into the output buffer of AxoSyslog (also called fifo). From the output buffer, the operating system sends the messages to the appropriate destinations.

In large-traffic environments many messages can arrive during a single poll loop, therefore AxoSyslog reads only a fixed number of messages from each source. The log-fetch-limit() option specifies the number of messages read during a poll loop from a single source.

TCP and unix-stream sources can receive the logs from several incoming connections (for example, many different clients or applications). For such sources, AxoSyslog reads messages from every connection, thus the log-fetch-limit() parameter applies individually to every connection of the source.

Log paths without flow-control

Every destination has its own output buffer. The output buffer is needed because the destination might not be able to accept all messages immediately. The log-fifo-size() parameter sets the size of the output buffer. The output buffer must be larger than the log-fetch-limit() of the sources, to ensure that every message read during the poll loop fits into the output buffer. If the log path sends messages to a destination from multiple sources, the output buffer must be large enough to store the incoming messages of every source.

Log paths with flow-control

The AxoSyslog application uses flow-control in the following cases:

• Hard flow-control: the flow-control flag is enabled for the particular log path.

• Soft flow-control: the log path includes a file destination.

The flow-control of AxoSyslog introduces a control window to the source that tracks how many messages can AxoSyslog accept from the source. Every message that AxoSyslog reads from the source lowers the window size by one, every message that AxoSyslog successfully sends from the output buffer increases the window size by one. If the window is full (that is, its size decreases to zero), AxoSyslog stops reading messages from the source. The initial size of the control window is by default 100. If a source accepts messages from multiple connections, all messages use the same control window.

When using flow-control, AxoSyslog automatically sets the size of the output buffer so that it matches the size of the control window of the sources. Note that starting with AxoSyslog 3.22, log-fifo-size() only affects log paths that are not flow-controlled.

Dynamic flow-control

In addition to the static control window set using the log-iw-size() option, you can also allocate a dynamic window to the source. The AxoSyslog application uses this window to dynamically increase the static window of the active connections. The dynamic window is distributed evenly among the active connections of the source. The AxoSyslog application periodically checks which connections of the source are active, and redistributes the dynamic window. If only one of the connections is active, it receives the entire dynamic window, while other connections receive only their share of the static window.

Using dynamic flow-control on your AxoSyslog server is useful when the source has lots of connections, but only a small subset of the active clients send messages at high rate, and the memory of the AxoSyslog server is limited. In other cases, it is currently not recommended, because it can result in higher memory usage and fluctuating performance compared to using only the static window.

When flow-control is used, every source has its own control window. As a worst-case situation, memory of the host must be greater than the total size of the messages of every control window, plus the size of the dynamic window, that is, the log-iw-size()+dynamic-window-size(). This applies to every source that sends logs to the particular destination. Thus if two sources having several connections and heavy traffic send logs to the same destination, the control window of both sources must fit into the memory of the host. Otherwise, some messages might not fit in the memory, and messages may be lost.

If dynamic flow-control is disabled (which is the default behavior), the value of the log-iw-size() option cannot be lower than 100. If dynamic flow-control is enabled, you can decrease the value of the log-iw-size() option (to the minimum of 1).

In case of soft flow-control there is no message lost if the destination can accept messages. It is possible to lose messages if it cannot accept messages (for example, the file destination is not writable, or the disk becomes full), and all buffers are full. Soft flow-control cannot be configured, it is automatically available for file destinations.

Hard flow-control: In case of hard flow-control there is no message lost. To use hard flow-control, enable the flow-control flag in the log path. Hard flow-control is available for all destinations.

Example: Soft flow-control

   source s_file {
        file("/tmp/input_file.log");
    };
    destination d_file {
        file("/tmp/output_file.log");
    };
    destination d_tcp {
        network("127.0.0.1"
            port(2222)
        );
    };
    log {
        source(s_file);
        destination(d_file);
        destination(d_tcp);
    };

Example: Hard flow-control

   source s_file {
        file("/tmp/input_file.log");
    };
    destination d_file {
        file("/tmp/output_file.log");
    };
    destination d_tcp {
        network("127.0.0.1"
            port(2222)
        );
    };
    log {
        source(s_file);
        destination(d_file);
        destination(d_tcp);
        flags(flow-control);
    };

Handling outgoing messages

The AxoSyslog application handles outgoing messages the following way:

• Output queue: Messages from the output queue are sent to the target AxoSyslog server. The AxoSyslog application puts the outgoing messages directly into the output queue, unless the output queue is full. The output queue can hold 64 messages, this is a fixed value and cannot be modified.

• Disk buffer: If the output queue is full and disk-buffering is enabled, AxoSyslog puts the outgoing messages into the disk buffer of the destination.

• Overflow queue: If the output queue is full and the disk buffer is disabled or full, AxoSyslog puts the outgoing messages into the overflow queue of the destination. (The overflow queue is identical to the output buffer used by other destinations.) The log-fifo-size() parameter specifies the number of messages stored in the overflow queue, unless flow-control is enabled. When dynamic flow-control is enabled, AxoSyslog sets the size of the overflow queue automatically. For details on sizing the log-fifo-size() parameter, see Configuring flow-control.