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Script Processor

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Script Processor

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The script processor executes Javascript code to process an event. The processor uses a pure Go implementation of ECMAScript 5.1 and has no external dependencies. This can be useful in situations where one of the other processors doesn’t provide the functionality you need to filter events.

The processor can be configured by embedding Javascript in your configuration file or by pointing the processor at external file(s).

processors:
  - script:
      lang: javascript
      id: my_filter
      source: >
        function process(event) {
            event.Tag("js");
        }

This loads filter.js from disk.

processors:
  - script:
      lang: javascript
      id: my_filter
      file: ${path.config}/filter.js

Parameters can be passed to the script by adding params to the config. This allows for a script to be made reusable. When using params the code must define a register(params) function to receive the parameters.

processors:
  - script:
      lang: javascript
      id: my_filter
      params:
        threshold: 15
      source: >
        var params = {threshold: 42};
        function register(scriptParams) {
            params = scriptParams;
        }
        function process(event) {
            if (event.Get("severity") < params.threshold) {
                event.Cancel();
            }
        }

If the script defines a test() function it will be invoked when the processor is loaded. Any exceptions thrown will cause the processor to fail to load. This can be used to make assertions about the behavior of the script.

function process(event) {
    if (event.Get("event.code") === 1102) {
        event.Put("event.action", "cleared");
    }
}

function test() {
    var event = process(new Event({event: {code: 1102}));
    if (event.Get("event.action") !== "cleared") {
        throw "expected event.action === cleared";
    }
}

Configuration options

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The script processor has the following configuration settings:

lang: This field is required and its value must be javascript.
tag: This is an optional identifier that is added to log messages. If defined it enables metrics logging for this instance of the processor. The metrics include the number of exceptions and a histogram of the execution times for the process function.
source: Inline Javascript source code.
file: Path to a script file to load. Relative paths are interpreted as relative to the path.config directory. Globs are expanded.
files: List of script files to load. The scripts are concatenated together. Relative paths are interpreted as relative to the path.config directory. And globs are expanded.
params: A dictionary of parameters that are passed to the register of the script.
tag_on_exception: Tag to add to events in case the Javascript code causes an exception while processing an event. Defaults to _js_exception.
timeout: This sets an execution timeout for the process function. When the process function takes longer than the timeout period the function is interrupted. You can set this option to prevent a script from running for too long (like preventing an infinite while loop). By default there is no timeout.
max_cached_sessions: This sets the maximum number of Javascript VM sessions that will be cached to avoid reallocation. The default is 4.

Event API

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The Event object passed to the process method has the following API.

Method	Description
`Get(string)`	Get a value from the event (either a scalar or an object). If the key does not exist `null` is returned. If no key is provided then an object containing all fields is returned. Example: `var value = event.Get(key);`
`Put(string, value)`	Put a value into the event. If the key was already set then the previous value is returned. It throws an exception if the key cannot be set because one of the intermediate values is not an object. Example: `var old = event.Put(key, value);`
`Rename(string, string)`	Rename a key in the event. The target key must not exist. It returns true if the source key was successfully renamed to the target key. Example: `var success = event.Rename("source", "target");`
`Delete(string)`	Delete a field from the event. It returns true on success. Example: `var deleted = event.Delete("user.email");`
`Cancel()`	Flag the event as cancelled which causes the processor to drop event. Example: `event.Cancel(); return;`
`Tag(string)`	Append a tag to the `tags` field if the tag does not already exist. Throws an exception if `tags` exists and is not a string or a list of strings. Example: `event.Tag("user_event");`
`AppendTo(string, string)`	`AppendTo` is a specialized `Put` method that converts the existing value to an array and appends the value if it does not already exist. If there is an existing value that’s not a string or array of strings then an exception is thrown. Example: `event.AppendTo("error.message", "invalid file hash");`

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