Managing incoming and outgoing messages with flow-control

1: Flow-control and multiple destinations

2: Configuring 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.

Reading messages

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.

Reading messages from a stream

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.

Note

The way flow-control works has changed significantly in version AxoSyslog 3.22. If you are using an older version of AxoSyslog, consult the documentation of the version you are using for details about flow-control.

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.

Note

If the source can handle multiple connections (for example, network() and syslog()), the size of the control window is divided by the value of the max-connections() parameter and this smaller control window is applied to each connection of the source.

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);
    };

Warning

Hazard of data loss! For destinations other than file, soft flow-control is not available. Thus, it is possible to lose log messages on those destinations. To avoid data loss on those destinations, use hard flow-control.

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:

Disk buffering

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.

1 - Flow-control and multiple destinations

Using flow-control on a source has an important side-effect if the messages of the source are sent to multiple destinations. If flow-control is in use and one of the destinations cannot accept the messages, the other destinations do not receive any messages either, because AxoSyslog stops reading the source. For example, if messages from a source are sent to a remote server and also stored locally in a file, and the network connection to the server becomes unavailable, neither the remote server nor the local file will receive any messages.

Note

Creating separate log paths for the destinations that use the same flow-controlled source does not avoid the problem.

If you use flow-control and reliable disk-based buffering together with multiple destinations, the flow-control starts slowing down the source only when:

one destination is down, and
the number of messages stored in the disk buffer of the destination reaches (capacity-bytes() minus flow-control-window-bytes()).

2 - Configuring flow-control

For details on how flow-control works, see Managing incoming and outgoing messages with flow-control. The summary of the main points is as follows:

The AxoSyslog application normally reads a maximum of log-fetch-limit() number of messages from a source.
From TCP and unix-stream sources, AxoSyslog reads a maximum of log-fetch-limit() from every connection of the source. The number of connections to the source is set using the max-connections() parameter.
Every destination has an output buffer. The size of this buffer is set automatically for log paths that use flow-control, and can be set using the log-fifo-size() option for other log paths.
Flow-control uses a control window to determine if there is free space in the output buffer for new messages. Every source has its own control window, the log-iw-size() option sets the size of the static control window. Optionally, you can enable a dynamic control window for the source using the dynamic-window-size() option.
When a source accepts multiple connections, the size of the control window is divided by the value of the max-connections() parameter and this smaller control window is applied to each connection of the source.
The dynamic control window is automatically distributed among the active connections of the source.
If the control window is full, AxoSyslog stops reading messages from the source until some messages are successfully sent to the destination.
If the output buffer becomes full, and neither disk-buffering nor flow-control is used, messages may be lost.

Warning

If you modify the max-connections() or the log-fetch-limit() parameter, do not forget to adjust the log-iw-size() and dynamic-window-size() parameters accordingly.

Example: Sizing parameters for flow-control

Suppose that AxoSyslog has a source that must accept up to 300 parallel connections. Such situation can arise when a network source receives connections from many clients, or if many applications log to the same socket.

Set the max-connections() parameter of the source to 300. However, the log-fetch-limit() (default value: 10) parameter applies to every connection of the source individually, while the log-iw-size() (default value: 1000) parameter applies to the source. In a worst-case scenario, the destination does not accept any messages, while all 300 connections send at least log-fetch-limit() number of messages to the source during every poll loop. Therefore, the control window must accommodate at least max-connections()*log-fetch-limit() messages to be able to read every incoming message of a poll loop. In the current example this means that log-iw-size() should be greater than 300\*10=3000. If the control window is smaller than this value, the control window might fill up with messages from the first connections — causing AxoSyslog to read only one message of the last connections in every poll loop.

The output buffer of the destination must accommodate at least log-iw-size() messages, but use a greater value: in the current example 3000\*10=30000 messages. That way all incoming messages of ten poll loops fit in the output buffer. If the output buffer is full, AxoSyslog does not read any messages from the source until some messages are successfully sent to the destination.

   source s_localhost {
        network(
            ip(127.0.0.1)
            port(1999)
            max-connections(300)
        );
    };
    destination d_tcp {
        network("10.1.2.3"
            port(1999)
            localport(999)
            log-fifo-size(30000)
        );
    };
    log {
        source(s_localhost);
        destination(d_tcp);
        flags(flow-control);
    };

If other sources send messages to this destination, then the output buffer must be further increased. For example, if a network host with maximum 100 connections also logs into the destination, then increase the log-fifo-size() by 10000.

   source s_localhost {
        network(
            ip(127.0.0.1)
            port(1999)
            max-connections(300)
        );
    };
    source s_tcp {
        network(
            ip(192.168.1.5)
            port(1999)
            max-connections(100)
        );
    };
    destination d_tcp {
        network("10.1.2.3"
            port(1999)
            localport(999)
            log-fifo-size(40000)
        );
    };
    log {
        source(s_localhost);
        destination(d_tcp);
        flags(flow-control);
    };