How to implement a Non Task queue based custom agent 

Answer

(1) getJobs function can be overridden in the custom agent class.
(2) This method will get the values set in the agent criteria from the parameters (input XML) coming into the method.
(3) The implementation is left to the user.
(4) Hence even though you get the values of Number of records to buffer, how to use it will depend on the implementation. If not done, then all the records satisfying the condition mentioned in the calling query will be fetched.
(5) Same holds true for the lastMessageCreated parameter. If the data is fetched in batches (5000 by default), then to track the last message processed in a batch this parameter can be set.
(6) When the agent is triggered for the first time, this parameter will be null.
(7) If the agent is running in a single threaded mode, the second getJobs call would not be fired until the execute job has finished processing the records.
(8) Even if the second trigger is waiting, till the processing of all records is comepleted by the first run, the getJobs query will not be fired for the 2nd trigger.

OOB Implemention:
Hence if there are 15K records in a table and the number to buffer is 5K, then the getJobs will fetch 5K records at a time(depending on the implementation) till all the 15k records are processed and only then the getJobs for the 2nd trigger will be fired