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Version 1.12.3

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9 External Configuration Data

9.1  General

To avoid hard-coding data values related to the test and/or System Under Test (SUT) in the test suites, the data can instead be specified through configuration files or strings that Common Test reads before the start of a test run. External configuration data makes it possible to change test properties without modifying the test suites using the data. Examples of configuration data follows:

  • Addresses to the test plant or other instruments
  • User login information
  • Names of files needed by the test
  • Names of programs to be executed during the test
  • Any other variable needed by the test

9.2  Syntax

A configuration file can contain any number of elements of the type:



 CfgVarName = atom()
 Value = term() | [{CfgVarName,Value}]

9.3  Requiring and Reading Configuration Data

In a test suite, one must require that a configuration variable (CfgVarName in the previous definition) exists before attempting to read the associated value in a test case or configuration function.

require is an assert statement, which can be part of the Test Suite Information Function or Test Case Information Function. If the required variable is unavailable, the test is skipped (unless a default value has been specified, see section Test Case Information Function for details). Also, function ct:require/1/2 can be called from a test case to check if a specific variable is available. The return value from this function must be checked explicitly and appropriate action be taken depending on the result (for example, to skip the test case if the variable in question does not exist).

A require statement in the test suite information case or test case information-list is to look like {require,CfgVarName} or {require,AliasName,CfgVarName}. The arguments AliasName and CfgVarName are the same as the arguments to ct:require/1,2. AliasName becomes an alias for the configuration variable, and can be used as reference to the configuration data value. The configuration variable can be associated with any number of alias names, but each name must be unique within the same test suite. The two main uses for alias names follows:

  • To identify connections (described later).
  • To help adapt configuration data to a test suite (or test case) and improve readability.

To read the value of a configuration variable, use function get_config/1,2,3.


 suite() -> 
     [{require, domain, 'CONN_SPEC_DNS_SUFFIX'}].


 testcase(Config) ->
     Domain = ct:get_config(domain),

9.4  Using Configuration Variables Defined in Multiple Files

If a configuration variable is defined in multiple files and you want to access all possible values, use function ct:get_config/3 and specify all in the options list. The values are then returned in a list and the order of the elements corresponds to the order that the configuration files were specified at startup.

9.5  Encrypted Configuration Files

Configuration files containing sensitive data can be encrypted if they must be stored in open and shared directories.

To have Common Test encrypt a specified file using function DES3 in application Crypto, call ct:encrypt_config_file/2,3 The encrypted file can then be used as a regular configuration file in combination with other encrypted files or normal text files. However, the key for decrypting the configuration file must be provided when running the test. This can be done with flag/option decrypt_key or decrypt_file, or a key file in a predefined location.

Common Test also provides decryption functions, ct:decrypt_config_file/2,3, for recreating the original text files.

9.6  Opening Connections Using Configuration Data

Two different methods for opening a connection using the support functions in, for example, ct_ssh, ct_ftp, and ct_telnet follows:

  • Using a configuration target name (an alias) as reference.
  • Using the configuration variable as reference.

When a target name is used for referencing the configuration data (that specifies the connection to be opened), the same name can be used as connection identity in all subsequent calls related to the connection (also for closing it). Only one open connection per target name is possible. If you attempt to open a new connection using a name already associated with an open connection, Common Test returns the already existing handle so the previously opened connection is used. This feature makes it possible to call the function for opening a particular connection whenever useful. An action like this does not necessarily open any new connections unless it is required (which could be the case if, for example, the previous connection has been closed unexpectedly by the server). Using named connections also removes the need to pass handle references around in the suite for these connections.

When a configuration variable name is used as reference to the data specifying the connection, the handle returned as a result of opening the connection must be used in all subsequent calls (also for closing the connection). Repeated calls to the open function with the same variable name as reference results in multiple connections being opened. This can be useful, for example, if a test case needs to open multiple connections to the same server on the target node (using the same configuration data for each connection).

9.7  User-Specific Configuration Data Formats

The user can specify configuration data on a different format than key-value tuples in a text file, as described so far. The data can, for example, be read from any files, fetched from the web over HTTP, or requested from a user-specific process. To support this, Common Test provides a callback module plugin mechanism to handle configuration data.

Default Callback Modules for Handling Configuration Data

Common Test includes default callback modules for handling configuration data specified in standard configuration files (described earlier) and in XML files as follows:

  • ct_config_plain - for reading configuration files with key-value tuples (standard format). This handler is used to parse configuration files if no user callback is specified.
  • ct_config_xml - for reading configuration data from XML files.

Using XML Configuration Files

An example of an XML configuration file follows:


Once read, this file produces the same configuration variables as the following text file:

 {ftp_host, [{ftp,"targethost"},

 {lm_directory, "/test/loadmodules"}.

Implement a User-Specific Handler

The user-specific handler can be written to handle special configuration file formats. The parameter can be either file names or configuration strings (the empty list is valid).

The callback module implementing the handler is responsible for checking the correctness of configuration strings.

To validate the configuration strings, the callback module is to have function Callback:check_parameter/1 exported.

The input argument is passed from Common Test, as defined in the test specification, or specified as an option to ct_run or ct:run_test.

The return value is to be any of the following values, indicating if the specified configuration parameter is valid:

  • {ok, {file, FileName}} - the parameter is a file name and the file exists.
  • {ok, {config, ConfigString}} - the parameter is a configuration string and it is correct.
  • {error, {nofile, FileName}} - there is no file with the specified name in the current directory.
  • {error, {wrong_config, ConfigString}} - the configuration string is wrong.

The function Callback:read_config/1 is to be exported from the callback module to read configuration data, initially before the tests start, or as a result of data being reloaded during test execution. The input argument is the same as for function check_parameter/1.

The return value is to be either of the following:

  • {ok, Config} - if the configuration variables are read successfully.
  • {error, {Error, ErrorDetails}} - if the callback module fails to proceed with the specified configuration parameters.

Config is the proper Erlang key-value list, with possible key-value sublists as values, like the earlier configuration file example:

 [{ftp_host, [{ftp, "targethost"}, {username, "tester"}, {password, "letmein"}]},
  {lm_directory, "/test/loadmodules"}]

9.8  Examples of Configuration Data Handling

A configuration file for using the FTP client to access files on a remote host can look as follows:

 {ftp_host, [{ftp,"targethost"},

 {lm_directory, "/test/loadmodules"}.

The XML version shown earlier can also be used, but it is to be explicitly specified that the ct_config_xml callback module is to be used by Common Test.

The following is an example of how to assert that the configuration data is available and can be used for an FTP session:

 init_per_testcase(ftptest, Config) ->
     {ok,_} = ct_ftp:open(ftp),

 end_per_testcase(ftptest, _Config) ->

 ftptest() ->

 ftptest(Config) ->
     Remote = filename:join(ct:get_config(lm_directory), "loadmodX"),
     Local = filename:join(?config(priv_dir,Config), "loadmodule"),
     ok = ct_ftp:recv(ftp, Remote, Local),

The following is an example of how the functions in the previous example can be rewritten if it is necessary to open multiple connections to the FTP server:

 init_per_testcase(ftptest, Config) ->
     {ok,Handle1} = ct_ftp:open(ftp_host),
     {ok,Handle2} = ct_ftp:open(ftp_host),
     [{ftp_handles,[Handle1,Handle2]} | Config].

 end_per_testcase(ftptest, Config) ->
     lists:foreach(fun(Handle) -> ct_ftp:close(Handle) end, 

 ftptest() ->

 ftptest(Config) ->
     Remote = filename:join(ct:get_config(lm_directory), "loadmodX"),
     Local = filename:join(?config(priv_dir,Config), "loadmodule"),
     [Handle | MoreHandles] = ?config(ftp_handles,Config),
     ok = ct_ftp:recv(Handle, Remote, Local),

9.9  Example of User-Specific Configuration Handler

A simple configuration handling driver, asking an external server for configuration data, can be implemented as follows:

 -export([read_config/1, check_parameter/1]).

     ServerModule = list_to_atom(ServerName),

     ServerModule = list_to_atom(ServerName),
     case code:is_loaded(ServerModule) of
         {file, _}->
             {ok, {config, ServerName}};
             case code:load_file(ServerModule) of
                 {module, ServerModule}->
                     {ok, {config, ServerName}};
                 {error, nofile}->
                     {error, {wrong_config, "File not found: " ++ ServerName ++ ".beam"}}

The configuration string for this driver can be config_server, if the config_server.erl module that follows is compiled and exists in the code path during test execution:

 -export([start/0, stop/0, init/1, get_config/0, loop/0]).

 -define(REGISTERED_NAME, ct_test_config_server).

     case whereis(?REGISTERED_NAME) of
             spawn(?MODULE, init, [?REGISTERED_NAME]),

     register(Name, self()),

     call(self(), get_config).

     call(self(), stop).

 call(Client, Request)->
     case whereis(?REGISTERED_NAME) of
             {error, {not_started, Request}};
             Pid ! {Client, Request},
                     {ok, Reply}
             after 4000->
                 {error, {timeout, Request}}

         {Pid, stop}->
             Pid ! ok;
         {Pid, get_config}->
             {D,T} = erlang:localtime(),
             Pid !
                 [{localtime, [{date, D}, {time, T}]},
                  {node, erlang:node()},
                  {now, erlang:now()},
                  {config_server_pid, self()},
                  {config_server_vsn, ?vsn}],

     case whereis(?REGISTERED_NAME) of

Here, the handler also provides for dynamically reloading of configuration variables. If ct:reload_config(localtime) is called from the test case function, all variables loaded with config_driver:read_config/1 are updated with their latest values, and the new value for variable localtime is returned.