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consul/contributing/config/README.md

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Agent Configuration

The Agent Configuration is the primary mechanism for configuring Consul. Agent Configuration also allows for specifying Config Entries, Services, and Checks that will be loaded when the agent starts.

Most configuration comes from hcl or json files, but some configuration can also be specified using command line flags, and some can be loaded with Auto-Config.

See also the checklist for adding a new field to the configuration.

Code

The Agent Configuration is implemented in agent/config, and the primary entrypoint is Load. Config loading is performed in phases:

  1. Command line flags are used to create a config.LoadOpts and passed to Load.
  2. Load reads all the config files and builds an ordered list of config.Source.
  3. Each config.Source is read to produce a config.Config.
  4. Each config.Config is merged ontop the previous.
  5. A config.RuntimeConfig is produced from the merged config.Config
  6. The config.RuntimeConfig is validated.
  7. Finally a result is returned with the RuntimeConfig and any warnings, or an error.

If Auto-Config is enabled, when it receives the config from the server, the entire process is repeated a second time with the addition config provided as another config.Source.

Default values can be specified in one of the default sources or set when converting from Config to RuntimeConfig in builder.build. Hopefully in the future we should remove one of those ways of setting default values.

Auto-Config

Auto-Config is enabled by the auto_config field in an Agent Configuration file. It is implemented in a couple packages.