rlog: RLog Channels

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# RLog Channels

An RLog Channel is a naming method for logging messages.

All logs are associated with a single channel, however there a variety of ways of subscribing to a log message.

## Channel Hierarchy

Channels are hierarchical. For example, if a log message is published on the "debug" channel:

        rDebug("hi");
        // same as
        static RLogChannel *myChannel = DEF_CHANNEL("debug", Log_Debug);
        rLog(myChannel, "hi");

In the example above, all subscribers to the "debug" channel receive the messages, but not subscribers to "debug/foo" or other sub-channels.

If a log is published under "debug/foo/bar":

        static RLogChannel *myChannel = DEF_CHANNEL("debug/foo/bar", Log_Debug);
        rLog(myChannel, "hi");

In that example, all subscribers to "debug/foo/bar", "debug/foo", and "debug" will receive the message.

All channels are considered to be derived from a root channel. It doesn't have a true name and is referenced as the empty string "". So, to capture all messages:

        // capture all messages and log them to stderr
        StdioNode stdLog( STDERR_FILENO );
        stdLog.subscribeTo( GetGlobalChannel("") ); // empty string is root channel

## Channel Components

Or in mathematical terms, the cross product of channels and components.

Channels are componentized. By default, all log messages using one of the rLog type macros is actually published on the component-specific version of the channel (the component being the value of RLOG_COMPONENT at compile time). So, instead of just saying a message was published on "debug" channel, we need to also say which component it was part of, which we could represent as a pair ( < COMPONENT, CHANNEL > ) -- eg <"rlog", "debug">.

This means that two separate components, both using rDebug() (for example) could be subscribed to separately, or together.

There is a way to subscribe to channels in the following ways:

- <COMPONENT, CHANNEL> : subscribe to a particular channel from a component
- <COMPONENT, *> : subscribe to all channels from a component
- <*, CHANNEL> : subscribe to a channel from all components
- <*, *> : subscribe to all channels from all components

    {
        // this is published on the channel "debug", and the component
        // [RLOG_COMPONENT]
        rDebug("hi");

        StdioNode stdLog( STDERR_FILENO );

        // subscribe to a particular channel, from the current component
        // ([RLOG_COMPONENT])
        stdLog.subscribeTo( RLOG_CHANNEL("debug/foo") );

        // subscribe to all channels from the current component ([RLOG_COMPONENT])
        // (the root channel is the empty string "")
        stdLog.subscribeTo( RLOG_CHANNEL("") );

        // subscribe to a particular channel from all components
        stdLog.subscribeTo( GetGlobalChannel("debug/foo") );

        // subscribe to all channels from all components
        stdLog.subscribeTo( GetGlobalChannel("") );
    }

As you can see from the pattern above, using the RLOG_CHANNEL() macro limits the selection to the current component. If you want to specify a component other then the current component, use GetComponentChannel() which takes the component name as the first argument.