There is no out of the box process to remove old ecc_agent_log records like with other log tables. This can result in years of data within the table. Customers can remove this data via a script but this can lead to performance related issues as the data set is large.
A table cleaner rule [sys_auto_flush] is needed to prevent the growth of this table.
Any instance with MID Servers will see records regularly inserted into ecc_agent_log. Records specific to a MID Server can be seen in each MID Server's form's related lists.
The main entries seen will be for "Running system command...", which will be logged for each ecc_queue output for a SystemCommand.
Running system command: FileChange
Running system command: credentials_reload
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