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config
config
Configuration file.
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