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Configuration file.

Description

A configuration file contains values for configuration parameters for the applications in the system. The erl command-line argument -config Name tells the system to use data in the system configuration file Name.config.

The erl command-line argument -configfd works the same way as the -config option but specifies a file descriptor to read configuration data from instead of a file.

The configuration data from configuration files and file descriptors are read in the same order as they are given on the command line. For example, erl -config a -configfd 3 -config b -configfd 4 would cause the system to read configuration data in the following order a.config, file descriptor 3, b.config, and file descriptor 4. If a configuration parameter is specified more than once in the given files and file descriptors, the last one overrides the previous ones.

Configuration parameter values in a configuration file or file descriptor override the values in the application resource files (see app(4)). The values in the configuration file are always overridden by command-line flags (see erts:erl(1)).

The value of a configuration parameter is retrieved by calling application:get_env/1,2.

File Syntax

The configuration file is to be called Name.config, where Name is any name.

File .config contains a single Erlang term and has the following syntax:

[{Application1, [{Par11, Val11}, ...]},
 ...
 {ApplicationN, [{ParN1, ValN1}, ...]}].

  • Application = atom() - Application name.

  • Par = atom() - Name of a configuration parameter.

  • Val = term() - Value of a configuration parameter.

sys.config

When starting Erlang in embedded mode, it is assumed that exactly one system configuration file is used, named sys.config. This file is to be located in $ROOT/releases/Vsn, where $ROOT is the Erlang/OTP root installation directory and Vsn is the release version.

Release handling relies on this assumption. When installing a new release version, the new sys.config is read and used to update the application's configurations.

This means that specifying another .config file, or more .config files, leads to an inconsistent update of application configurations. There is, however, a way to point out other config files from a sys.config. How to do this is described in the next section.

Including Files from sys.config and -configfd Configurations

There is a way to include other configuration files from a sys.config file and from a configuration that comes from a file descriptor that has been pointed out with the -configfd command-line argument.

The syntax for including files can be described by the Erlang type language like this:

[{Application, [{Par, Val}]} | IncludeFile].
  • IncludeFile = string() - Name of a .config file. The extension .config can be omitted. It is recommended to use absolute paths. If a relative path is used in a sys.config, IncludeFile is searched, first, relative to the sys.config directory, then relative to the current working directory of the emulator. If a relative path is used in a -configfd configuration, IncludeFile is searched, first, relative to the dictionary containing the boot script (see also the -boot command-line argument) for the emulator, then relative to the current working directory of the emulator. This makes it possible to use sys.config for pointing out other .config files in a release or in a node started manually using -config or -configfd with the same result whatever the current working directory is.

When traversing the contents of a sys.config or a -configfd configuration and a filename is encountered, its contents are read and merged with the result so far. When an application configuration tuple {Application, Env} is found, it is merged with the result so far. Merging means that new parameters are added and existing parameter values are overwritten.

Example:

sys.config:

["/home/user/myconfig1"
 {myapp,[{par1,val1},{par2,val2}]},
 "/home/user/myconfig2"].

myconfig1.config:

[{myapp,[{par0,val0},{par1,val0},{par2,val0}]}].

myconfig2.config:

[{myapp,[{par2,val3},{par3,val4}]}].

This yields the following environment for myapp:

[{par0,val0},{par1,val1},{par2,val3},{par3,val4}]

The run-time system will abort before staring up if an include file specified in sys.config or a -configfd configuration does not exist, or is erroneous. However, installing a new release version will not fail if there is an error while loading an include file, but an error message is returned and the erroneous file is ignored.

See Also

app(4), erts:erl(1), OTP Design Principles