Lookup tables are a powerful construct to obtain class information based on message content. It works on top of a data-file which maps key (to be looked up) to value (the result of lookup).
The idea is to use a message properties (or derivatives of it) as an index into a table which then returns another value. For example, $fromhost-ip could be used as an index, with the table value representing the type of server or the department or remote office it is located in.
Read more about it here Lookup Tables
In the following example the hostname is looked up in the given table and the corresponding value is returned.
lookup_table(name="host_bu" file="/var/lib/host_billing_unit_mapping.json") set $.bu = lookup("host_bu", $hostname);
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