Junctions make it possible to send the messages to different channels, process the messages differently on each channel, and then join every channel together again. You can define any number of channels in a junction: every channel receives a copy of every message that reaches the junction. Every channel can process the messages differently, and at the end of the junction, the processed messages of every channel return to the junction again, where further processing is possible.
A junction includes one or more channels. A channel usually includes at least one filter, though that is not enforced. Otherwise, channels are identical to log statements, and can include any kind of objects, for example, parsers, rewrite rules, destinations, and so on. (For details on using channels, as well as on using channels outside junctions, see Using channels in configuration objects.)
NOTE: Certain parsers can also act as filters:
You can also use log-path flags in the channels of the junction. Within the junction, a message is processed by every channel, in the order the channels appear in the configuration file. Typically if your channels have filters, you also set the flags(final) option for the channel. However, note that the log-path flags of the channel apply only within the junction, for example, if you set the final flag for a channel, then the subsequent channels of the junction will not receive the message, but this does not affect any other log path or junction of the configuration. The only exception is the flow-control flag: if you enable flow-control in a junction, it affects the entire log path. For details on log-path flags, see Log path flags.
junction {
channel { <other-syslog-ng-objects> <log-path-flags>};
channel { <other-syslog-ng-objects> <log-path-flags>};
...
};
Example: Using junctions
For example, suppose that you have a single network source that receives log messages from different devices, and some devices send messages that are not RFC-compliant (some routers are notorious for that). To solve this problem in earlier versions of syslog-ng OSE, you had to create two different network sources using different IP addresses or ports: one that received the RFC-compliant messages, and one that received the improperly formatted messages (for example, using the flags(no-parse) option). Using junctions this becomes much more simple: you can use a single network source to receive every message, then use a junction and two channels. The first channel processes the RFC-compliant messages, the second everything else. At the end, every message is stored in a single file. The filters used in the example can be host() filters (if you have a list of the IP addresses of the devices sending non-compliant messages), but that depends on your environment.
log {
source {
syslog(
ip(10.1.2.3)
transport("tcp")
flags(no-parse)
);
};
junction {
channel {
filter(f_compliant_hosts);
parser {
syslog-parser();
};
};
channel {
filter(f_noncompliant_hosts);
};
};
destination {
file("/var/log/messages");
};
};
Since every channel receives every message that reaches the junction, use the flags(final) option in the channels to avoid the unnecessary processing the messages multiple times:
log {
source {
syslog(
ip(10.1.2.3)
transport("tcp")
flags(no-parse)
);
};
junction {
channel {
filter(f_compliant_hosts);
parser {
syslog-parser();
};
flags(final);
};
channel {
filter(f_noncompliant_hosts);
flags(final);
};
};
destination {
file("/var/log/messages");
};
};
NOTE: Junctions differ from embedded log statements, because embedded log statements are like branches: they split the flow of messages into separate paths, and the different paths do not meet again. Messages processed on different embedded log statements cannot be combined together for further processing. However, junctions split the messages to channels, then combine the channels together.
An alternative, more straightforward way to implement conditional evaluation is to configure conditional expressions using if {}, elif {}, and else {} blocks. For details, see if-else-elif: Conditional expressions.
Flags influence the behavior of syslog-ng, and the way it processes messages. The following flags may be used in the log paths, as described in Log paths.
Table 11: Log statement flags
| catchall |
This flag means that the source of the message is ignored, only the filters of the log path are taken into account when matching messages. A log statement using the catchall flag processes every message that arrives to any of the defined sources. |
| drop-unmatched |
This flag means that the message is dropped along a log path when it does not match a filter or is discarded by a parser. Without using the drop-unmatched flag, syslog-ng OSE would continue to process the message along alternative paths. |
| fallback |
This flag makes a log statement 'fallback'. Fallback log statements process messages that were not processed by other, 'non-fallback' log statements.
Processed means that every filter of a log path matched the message. Note that in the case of embedded log paths, the message is considered to be processed if it matches the filters of the outer log path, even if it does not match the filters of the embedded log path. For details, see Example: Using log path flags. |
| final |
This flag means that the processing of log messages processed by the log statement ends here, other log statements appearing later in the configuration file will not process the messages processed by the log statement labeled as 'final'. Note that this does not necessarily mean that matching messages will be stored only once, as there can be matching log statements processed before the current one (syslog-ng OSE evaluates log statements in the order they appear in the configuration file).
Processed means that every filter of a log path matched the message. Note that in the case of embedded log paths, the message is considered to be processed if it matches the filters of the outer log path, even if it does not match the filters of the embedded log path. For details, see Example: Using log path flags. |
| flow-control |
Enables flow-control to the log path, meaning that syslog-ng will stop reading messages from the sources of this log statement if the destinations are not able to process the messages at the required speed. If disabled, syslog-ng will drop messages if the destination queues are full. If enabled, syslog-ng will only drop messages if the destination queues/window sizes are improperly sized. For details, see Managing incoming and outgoing messages with flow-control. |
|
|
Caution:
The final, fallback, and catchall flags apply only for the top-level log paths, they have no effect on embedded log paths. |
Example: Using log path flags
Let's suppose that you have two hosts (myhost_A and myhost_B) that run two applications each (application_A and application_B), and you collect the log messages to a central syslog-ng server. On the server, you create two log paths:
-
one that processes only the messages sent by myhost_A, and
-
one that processes only the messages sent by application_A.
This means that messages sent by application_A running on myhost_A will be processed by both log paths, and the messages of application_B running on myhost_B will not be processed at all.
-
If you add the final flag to the first log path, then only this log path will process the messages of myhost_A, so the second log path will receive only the messages of application_A running on myhost_B.
-
If you create a third log path that includes the fallback flag, it will process the messages not processed by the first two log paths, in this case, the messages of application_B running on myhost_B.
-
Adding a fourth log path with the catchall flag would process every message received by the syslog-ng server.
log { source(s_localhost); destination(d_file); flags(catchall); };
The following example shows a scenario that can result in message loss. Do NOT use such a configuration, unless you know exactly what you are doing. The problem is if a message matches the filters in the first part of the first log path, syslog-ng OSE treats the message as 'processed'. Since the first log path includes the final flag, syslog-ng OSE will not pass the message to the second log path (the one with the fallback flag). As a result, syslog-ng OSE drops messages that do not match the filter of the embedded log path.
# Do not use such a configuration, unless you know exactly what you are doing.
log {
source(s_network);
# Filters in the external log path.
# If a message matches this filter, it is treated as 'processed'
filter(f_program);
filter(f_message);
log {
# Filter in the embedded log path.
# If a message does not match this filter, it is lost, it will not be processed by the 'fallback' log path
filter(f_host);
destination(d_file1);
};
flags(final);
};
log {
source(s_network);
destination(d_file2);
flags(fallback);
};
Example: Using the drop-unmatched flag
In the following example, if a log message arrives whose $MSG part does not contain the string foo, then syslog-ng OSE will discard the message and will not check compliance with the second if condition.
...
if {
filter { message('foo') };
flags(drop-unmatched)
};
if {
filter { message('bar') };
};
...
(Without the drop-unmatched flag, syslog-ng OSE would check if the message complies with the second if condition, that is, whether or not the message contains the string bar .)
This section describes the internal message-processing model of syslog-ng, as well as the flow-control feature that can prevent message losses.
The syslog-ng 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, syslog-ng polls every source and reads the available messages. These messages are processed and put into the output buffer of syslog-ng (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 syslog-ng 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.
Figure 14: Managing log messages in syslog-ng
TCP and unix-stream sources can receive the logs from several incoming connections (for example, many different clients or applications). For such sources, syslog-ng reads messages from every connection, thus the log-fetch-limit() parameter applies individually to every connection of the source.
Figure 15: Managing log messages of TCP sources in syslog-ng
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 syslog-ng application uses flow-control in the following cases:
NOTE: The way flow-control works has changed significantly in version syslog-ng OSE3.22. If you are using an older version of syslog-ng OSE, consult the documentation of the version you are using for details about flow-control.
The flow-control of syslog-ng introduces a control window to the source that tracks how many messages can syslog-ng accept from the source. Every message that syslog-ng reads from the source lowers the window size by one, every message that syslog-ng successfully sends from the output buffer increases the window size by one. If the window is full (that is, its size decreases to zero), syslog-ng 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, syslog-ng 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 syslog-ng OSE3.22, 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 syslog-ng 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 syslog-ng 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 syslog-ng 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 syslog-ng 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);
};
|
|
Caution:
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 syslog-ng application handles outgoing messages the following way:
Figure 16: Handling outgoing messages in syslog-ng OSE
-
Output queue: Messages from the output queue are sent to the target syslog-ng server. The syslog-ng 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, syslog-ng 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, syslog-ng 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, syslog-ng sets the size of the overflow queue automatically. For details on sizing the log-fifo-size() parameter, see Configuring flow-control.
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 syslog-ng 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 (disk-buf-size() minus mem-buf-size()).
