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Policy Scriptlets

Last updated May 2024

Lua scripts provide you with the capability to express the logic behind your policy. Aside from being very convenient (policy scripts can be reloaded on the fly, allowing real-time adjustment of policy without interrupting service), the Momentum implementation has extremely low overhead and tightly integrates with the event-based architecture, being able to suspend processing until asynchronous operations (such as DNS resolution, or database queries) complete. Note that variables used in a policy script are scoped locally and only persist in the particular policy script in which it is defined. Use the validation context to persist data over different policy phases and policy scripts.

This section of the manual explains how to implement basic Lua policy scripts. For more information about this language, see


Many Lua functions act as wrappers for C code, which means that the calling code needs to take steps to ensure safety. When calling a C API, make sure that all the arguments are valid as an unexpected nil may crash the system or have other undesirable results. Additionally, the data structures returned from C APIs are often shared between C and Lua. The most important consequence of this is that arrays coming from C APIs must only be accessed with existing indices, as opposed to Lua which simply returns nil on out-of-bounds access. Failure to do this on a Lua table connected to a C array will cause the system to fail.

Helper Functions

The /opt/msys/ecelerity/libexec/scriptlets/msys/ and /opt/msys/ecelerity/libexec/embed/lua/msys/ directories contain modules with useful helper functions.

For a list of all Lua Functions, see Lua Functions Summary .


All user-defined scripts must implement one or more of the following function calls:

  • init()

    The registration point for long-lived states, such as timed events or control functions. You should avoid using the init routine unless necessary, as it ties up an additional interpreter on the main scheduler thread. In the current architecture, things set up via the init routine have strong thread affinity with the scheduler thread and are not suitable for scheduling jobs to thread pools.

    The init routine must return true if it successfully initializes. Any other return value results in a failure to apply the configuration.

  • deinit()

    The resource destruction point for things that were set up in the init phase. This function must return true when successful, otherwise resources may not be cleaned up.

  • validate_connect(accept_construct, vctx)

    This function maps to the validate_connect C hook of the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, msys.core.VALIDATE_AGAIN, or msys.core.VALIDATE_DONE.

  • validate_ehlo(str, accept_construct, vctx)

    This function maps to the validate_ehlo C hook from the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, msys.core.VALIDATE_AGAIN, or msys.core.VALIDATE_DONE.

  • validate_mailfrom(str, accept_construct, vctx)

    This function maps to the validate_mailfrom C hook from the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, msys.core.VALIDATE_AGAIN, or msys.core.VALIDATE_DONE.

  • validate_rcptto(ec_message, str, accept_construct, vctx)

    This function maps to the validate_rcptto C hook from the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, msys.core.VALIDATE_AGAIN, or msys.core.VALIDATE_DONE.

  • validate_data(ec_message, accept_construct, vctx)

    This function maps to the validate_data C hook from the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, msys.core.VALIDATE_AGAIN, or msys.core.VALIDATE_DONE.

  • validate_data_spool(ec_message, accept_construct, vctx)

    This function maps to the validate_data_spool C hook from the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, msys.core.VALIDATE_AGAIN, or msys.core.VALIDATE_DONE.

  • validate_data_spool_each_rcpt(ec_message, accept_construct, vctx)

    This function maps to the validate_data_spool_each_rcpt C hook from the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, msys.core.VALIDATE_AGAIN, or msys.core.VALIDATE_DONE.

  • validate_set_binding(ec_message)

    Maps to the validate_set_binding C hook from the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, or msys.core.VALIDATE_DONE.

  • validate_rcptto_list(list_node, vctx)

    Maps to the validate_rcptto_list C hook from the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, msys.core.VALIDATE_AGAIN, or msys.core.VALIDATE_DONE.

  • validate_rcptto_list_final(node, vctx)

    Maps to the validate_rcptto_list_final C hook from the validation subsystem. Return values are one of msys.core.VALIDATE_CONT, msys.core.VALIDATE_AGAIN, or msys.core.VALIDATE_DONE.

  • validate_dealloc()

    This callout maps to the validate_local_dealloc C hook from the validation subsystem. This callout is only invoked if it is defined and if an earlier phase of processing invoked Lua code from this module. Its purpose is to allow your code a chance to act when the validate_context is torn down. If you used "self" in an earlier validation phase, it will be available in this function. You do not (and should not) need to define this callback unless you are doing something esoteric and accessing resources outside of the usual garbage collection mechanism. In most cases, your per-session state will be collected automatically.

If you attempt to add a function call that does not match one of the above function signatures, you'll see an error message similar to the following:

1251465322:scriptlet: no usable methods found for module mod_name in registerModule call

If you are calling Lua helper functions, copy the msg parameter into a table as shown in the following:

function mod:validate_set_binding(msg)
  local ctx = {
    ec_message = msg

At the moment, only the msg parameter is used by existing helpers. Be sure to map it to a field named ec_message as this is the name of the underlying type.

Default Policy Scripts

Default Lua policy scripts are included with Momentum for receiving. These scriptlets are found in the /opt/msys/ecelerity/libexec/scriptlets/msys/default_policy.lua file. These files are discussed in detail in the Default Policy User Guide.

In order to use the default policy scriptlets, you must add the /opt/msys/ecelerity/etc/sample-configs/default_policy.conf file to your configuration. For instructions on adding a configuration file, see “Adding Configuration Files”.

You will likely need to alter the default_policy.conf file to suit your circumstances—by only enabling specific scanners, for example.

In the default_policy.conf file, you should also enable the datasource(s) suitable for your language encoding. A datasource is necessary for the keyword filter, which is described at the bottom of the /opt/msys/ecelerity/etc/conf/default/dp_config.lua file. The keyword file, dp_config.lua, and custom_policy.lua must be copied from /opt/msys/ecelerity/etc/sample-configs/ to /opt/msys/ecelerity/etc/conf/default/.

Creating Policy Scripts

Following best practices when creating policy scripts is important, especially in a cluster environment when scripts are used on more than one node.

To create a policy script, perform the following:

  1. Create a directory for your script.

    Scripts should be created in a directory that is under revision control. Create a directory for your scripts in the working copy of the repository on a node where you intend to run the script:

    • If your scripts apply to all nodes in the cluster, create this directory under the /opt/msys/ecelerity/etc/conf/default directory or store them in the global directory. Typically, policy scripts are saved under Momentum's default working copy directory /opt/msys/ecelerity/etc/conf/default/scripts.

    • If your scripts apply to only one node, create a node-specific directory.

  2. Write your script.

    All scripts must

    • Include the msys.core package.

    • Implement one or more of the function calls described in “Callouts”.

    • Return one of the following values:

      • msys.core.VALIDATE_CONT – continue with validation routines.

      • msys.core.VALIDATE_DONE – do not execute any more validation hooks from the same phase.

      • msys.core.VALIDATE_AGAIN – this validation routine is doing something that blocks, so call again later.

    The following is a simple script that runs in the EHLO phase and writes an entry to the file. To create this example, same the script as a file named ehlo_phase.lua in the appropriate directory.

    local mod = {};
    function mod:validate_ehlo(str, accept, vctx)
      print("ehlo string is:", msys.expandMacro("%{vctx_conn:ehlo_string}"));
      return msys.core.VALIDATE_CONT;
    msys.registerModule("ehlo_phase", mod);

    Note that the script requires the msys.core package, the function name validate_ehlo matches the callout described in “Callouts”, and the script returns a legitimate value msys.core.VALIDATE_CONT.

    Failure to return a legitimate value from a scriptlet means that the script will not execute properly. For example, if you remove the line, return msys.core.VALIDATE_CONT; from the script, you should see errors similar to the following in the paniclog:

    1252601378:scriptlet-00147: validate_ehlo error: (null)
    1252601378:scriptlet-00239: ehlo_phase error: (null)

    These messages indicate a scriptlet error and give both the name of the script and the callout that failed.

  3. Update your configuration to properly reference your script.

    After writing a script and saving it to the repository, you must include it in the scriptlet module using a script stanza in your ecelerity.conf file.

    For the example script, create a script scope named ehlo_phase inside the scriptlet "scriptlet" scope. Set the script name to ehlo_phase. Set the source option of this scope to "myscripts.ehlo_phase" replacing myscripts with the name of the directory that you created. The scriptlet stanza should be similar to:

    alerting {}
    scriptlet "scriptlet" {
      # this loads default scripts to support enhanced control channel features
      script "boot" {
        source = "msys.boot"
      # this loads the user-defined script
      script "ehlo_phase" {
        source = "myscripts.ehlo_phase"

    Lua looks for scripts in the repository working directories. If your script has been saved to a top level directory, you need only specify the script name when defining the source option. Otherwise, specify the relative path to the script separating directories using the ‘/’ character. In the example above, the ehlo_phase.lua file is in the /opt/mys/ecelerity/etc/conf/default/myscripts directory. Names of scripts can be anything you like, as long as the name is a valid file system name and does not cause an error when the configuration file is parsed.


    You can use either a ‘.’ or a ‘/’ as a directory separator.

    Be sure to specify the correct name of your script file as the source value within the scriptlet::script scope and be sure to drop the file extension.

    For example, specifying the file name myscripts.ehlo_phase.lua instead of myscripts.ehlo_phase results in the following error:

    Unable to open '/opt/mys/ecelerity/etc/conf/default/myscripts/ehlo_phase/lua.lua',
      errno=2 No such file or directory

    The name of the script scope within the scriptlet module can also be anything of your choosing though it is good practice to use a name that describes the phase that the script runs in. Failure to register a module or misregistration does not result in an error when the console command config reload is issued. At runtime, your script simply will not execute.

    For additional details about editing your configuration files, see “Changing Configuration Files”.

  4. Check the validity of your script.

    Since a malformed configuration file will not reload, using config reload is one way of validating your scriptlet syntax. After your configuration has been changed, issue the command:

    /opt/msys/ecelerity/bin/ec_console /tmp/2025 config reload

    If there are errors in your script, the reload may fail and display a message such as the following:

    Module 'scriptlet' instance 'scriptlet' initialization failed while
    preparing to commit. Consult your vendor.

    This message usually indicates a syntax error in a script file. Note that reloading the configuration will not signal an error in registering the code module. Improperly registered scriptlets do not execute, so no error messages will be displayed when the configuration is reloaded or at runtime.

    If the configuration fails to reload, Momentum continues to function using the running configuration and ignoring your changes.

    In a cluster, if you commit before you test on the local node, you risk pushing out a broken configuration across your cluster/subcluster. Therefore, it is best practice to try the config reload locally until you are happy that the configuration operates as expected, before making a commit. This may need to be staged; for instance, you will likely need to push out DuraVIP™ or binding configuration changes before policy script changes can be made.


    You can also use the /opt/msys/3rdParty/bin/rcluac to check Lua syntax.

    However, please note that Message Systems does not provide support for the use of any third party tools included or referenced by name within our products or product documentation; support is the sole responsibility of the third party provider.

  5. Debug your script.

    Successfully reloading the configuration file does not guarantee that your script will run. Your script may be syntactically correct but have semantic errors. As always, you should test the functionality of scripts before implementing them in a production environment.

    The following are some methods that can be used to debug your script:

    • Look in your paniclog.

      Reloading the configuration will not indicate other kinds of errors, such as a misidentified binding field. In this case, you would find the following error in /var/log/ecelerity/

      1251382104:datasource: failed to prepare

      This indicates a datasource error in creating the prepared statement.

      You can also view the last few entries in the file by using the paniclog console command.

    • Use the print function.

      If your Lua syntax is correct but you are not seeing the desired result, you can log output to your paniclog using the print function. Output is formatted showing a Unix timestamp, the module name and the print argument, as shown in the following example:

      1251200446:scriptlet: Confirming entry in panic log.

      You can also use the print statement to output error messages returned by functions. For example, the print("set_binding: error querying binding:", err); line creates a paniclog entry such as the following when there is an SQL error:

      1251382104:scriptlet: set_binding: error querying binding:
      prepare: 1: no such column: bindings
      failed to prepare:     SELECT bindings FROM customer_binding WHERE
      customer_id = ? LIMIT 1

      This log entry clearly indicates that the query references a nonexistent column and can be much more helpful than the terse log entry.

    • Dump a variable.

      To output structured information about a variable, you can use the msys.dumper.Dumper function. To use this function, include the code require("msys.dumper");. When you wish to output the value of a variable, add the statement print(msys.dumper.Dumper(var_name));.

      The following is an example of a paniclog entry:

      1251390180:scriptlet: return { recipient="",
        note="No email received at this address", code="550"}


This section includes examples of using policy scripts. Setting the Priority of a Message

In some circumstances, you may wish to deliver certain messages ASAP. You can do this by setting the priority of a message in the following way:


local mod = {};

function mod:validate_set_binding(msg)
  print("mod:validate_set_binding: Setting msg priority to 1");
  msg.priority = 1;
  return msys.core.VALIDATE_CONT;


msys.registerModule("binder", mod);

Message priority is a bit field in the ec_message struct, so the only valid values are 0 and 1. The message priority is set to 0 when a message is created. Set the message priority to 1 to create a high priority message. Normally, messages are pulled from the queue in the order they were added. When a message priority is set to 1, that message will be pulled first from whatever queue it is on.

Use msg.priority to read the priority of a message.


It is important not to overuse the priority setting. High priority messages should be reserved for messages that need to go out immediately, before other messages. Keeping high priority messages to a low percentage of the total message volume is important so the high priority messages do not cause delays for normal priority messages. A common use case for high priority messages is sending out password resets in the midst of a major mail campaign.

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