Subsystems

Subsystems

Subsystems provide a logical grouping mechanism for log messages, independent of channels and backends.

They allow fine-grained control over which messages are included in logs without restructuring channel topology.

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Concept

A subsystem is a logical tag associated with a log message.

It represents a functional area or feature of the application, such as:

• networking
• storage
• defragmentation
• cache
• database

Subsystems are orthogonal to channels.

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Why Subsystems Exist

Channels are well suited for:

• routing messages
• selecting backends
• splitting output into different files

Subsystems solve a different problem:

• selective inclusion or exclusion of message groups
• early filtering within a single log stream

This is especially useful when:

• a single large log file is used
• some functional areas are only occasionally relevant

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Usage

Subsystem identifiers are specified as part of logging macros.

Subsystems are checked early in the routing pipeline. If a message specifies a subsystem that is currently disabled, the message is dropped immediately and does not reach any channel or backend.

Subsystem identifiers may be included in the output as the #subsystem field when OutputFlags.Subsystem is enabled.

Subsystems can be enabled or disabled dynamically, allowing runtime control over log verbosity.

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Subsystems vs Channels

Aspect	Channels	Subsystems
Purpose	Routing and output	Logical grouping and filtering
Controls routing	Yes	No
Controls file selection	Yes	No
Filters messages	Yes (level / enabled state)	Yes (by subsystem state)
Typical use	Separate logs	Filter within one log

While subsystems do not control routing or backend selection, disabling a subsystem prevents messages from reaching any backend, including file backends.

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Typical Example

An application writes all logs to a single file, but uses a separate subsystem for storage defragmentation.

When defragmentation details are not needed, the subsystem can be disabled, preventing those messages from being written to the log file without affecting other output.