Is there any way that I can either write a custom validator for this config variable or use an existing one to ensure that the array has 3 values and that each value matches a specific string ?
This is a custom plugin that you've written, right? There might be a way to supply a validation function, but otherwise you can make the check in the register method and raise a LogStash::ConfigurationError exception if @da_order doesn't comply with the protocol.
As @magnusbaeck correctly said, there is actually no way to pass your custom validation function for the config options, however this is a great extension point for our config validation, see https://github.com/elastic/logstash/issues/5395 for a followup issue in logstash.
For now the best you can do is using the register method.
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