Troubleshooting

Multiple sources of configuration information can be difficult and confusing to track down why a particular configured value is the value that it is. Several diagnostic capabilities are provided to help you trace how a value was configured.

Source tracing is supported for up to 8 configuration providers. The limit of 8 is a design choice to keep the cost of tracing low. It’s unlikely an application would have more than 8 configuration providers at a time.

Tracing

The configuration system provides limited support for the tracing crate.

Configuration Reload

Whenever a configuration reloads, it will log a Level::TRACE message indicating a reload has occurred. If the reload fails, a Level::ERROR message will be recorded with the error details so that background failures can be observed and triaged.

Whenever an existing configuration value is overridden by another configuration provider, a Level::TRACE message will log the key, old value, new value, who provided the old value, and who override the key with a new value. This can help identify unexpected configuration resolution in an application.

Formatting

A configuration can also be inspected by simply printing it out. The implementation of the Display trait will output the configuration hierarchy section by section. The implementation also supports an alternate output form that includes the corresponding providers associated with a configured value. The default alternate behavior expands all providers that set the configuration value. The number of providers that are output can be controlled by the formatting width. The configuration providers that are output are ranked most recent to oldest. A value of 0, indicates the last effective provider for a configuration value.

Default Output

The default output using format!("{config}") has the hierarchical format:

Section1:
  Key1 = Value1
Section2:
  Key2 = Value2

Show Providers

The alternate output using format!("{config:#}") has the format:

Section1:
  Key1 = Value1 (Memory)
Section2:
  Key2 = Value2 (Memory → overrides.json)

Show Effective Provider Only

The alternate and width output using format!("{config:#1}") has the format:

Section1:
  Key1 = Value1 (Memory)
Section2:
  Key2 = Value2 (overrides.json)

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