Embedded log statements

1: Using embedded log statements

Starting from version 3.0, AxoSyslog can handle embedded log statements (also called log pipes). Embedded log statements are useful for creating complex, multi-level log paths with several destinations and use filters, parsers, and rewrite rules.

For example, if you want to filter your incoming messages based on the facility parameter, and then use further filters to send messages arriving from different hosts to different destinations, you would use embedded log statements.

How embedded log statements work

Embedded log statements include sources — and usually filters, parsers, rewrite rules, or destinations — and other log statements that can include filters, parsers, rewrite rules, and destinations. The following rules apply to embedded log statements:

Only the beginning (also called top-level) log statement can include sources.
Embedded log statements can include multiple log statements on the same level (that is, a top-level log statement can include two or more log statements).
Embedded log statements can include several levels of log statements (that is, a top-level log statement can include a log statement that includes another log statement, and so on).
After an embedded log statement, you can write either another log statement, or the flags() option of the original log statement. You cannot use filters or other configuration objects. This also means that flags (except for the flow-control flag) apply to the entire log statement, you cannot use them only for the embedded log statement.
Embedded log statements that are on the same level receive the same messages from the higher-level log statement. For example, if the top-level log statement includes a filter, the lower-level log statements receive only the messages that pass the filter.

Embedded log statement configuration

Embedded log filters can be used to optimize the processing of log messages, for example, to re-use the results of filtering and rewriting operations.

1 - Using embedded log statements

Embedded log statements (for details, see Embedded log statements) re-use the results of processing messages (for example, the results of filtering or rewriting) to create complex log paths. Embedded log statements use the same syntax as regular log statements, but they cannot contain additional sources. To define embedded log statements, use the following syntax:

   log {
        source(s1); source(s2); ...
    
        optional_element(filter1|parser1|rewrite1);
        optional_element(filter2|parser2|rewrite2);
        ...
        destination(d1); destination(d2); ...
    
        #embedded log statement
        log {
            optional_element(filter1|parser1|rewrite1);
            optional_element(filter2|parser2|rewrite2);
            ...
            destination(d1); destination(d2); ...
    
            #another embedded log statement
            log {
                optional_element(filter1|parser1|rewrite1);
                optional_element(filter2|parser2|rewrite2);
                ...
                destination(d1); destination(d2); ...
            };
        };
        #set flags after the embedded log statements
        flags(flag1[, flag2...]);
    };

Example: Using embedded log paths

The following log path sends every message to the configured destinations: both the d_file1 and the d_file2 destinations receive every message of the source.

   log {
        source(s_localhost);
        destination(d_file1);
        destination(d_file2);
    };

The next example is equivalent to the one above, but uses an embedded log statement.

   log {
        source(s_localhost);
        destination(d_file1);
        log {
            destination(d_file2);
        };
    };

The following example uses two filters:

messages coming from the host 192.168.1.1 are sent to the d_file1 destination, and
messages coming from the host 192.168.1.1 and containing the string example are sent to the d_file2 destination.

   log {
        source(s_localhost);
        filter {
            host(192.168.1.1);
        };
        destination(d_file1);
        log {
            message("example");
            destination(d_file2);
        };
    };

The following example collects logs from multiple source groups and uses the source() filter in the embedded log statement to select messages of the s_network source group.

   log {
        source(s_localhost);
        source(s_network);
        destination(d_file1);
        log {
            filter {
                source(s_network);
            };
        destination(d_file2);
        };
    };