[syslog-ng] Process stored logs

I'm building an infra across the geo's to collect logs at a central repository. I have set up syslog-ng  in 3 geo's (say for e.g X, Y, Z) to collect logs form servers in respective Geo. A forurth server (say for eg. A)  where the logs are forwarded from the 3 log servers to aggregate all the logs from all GEO's.   The problem where I fail to understand is, if my aggregator server (A) goes down, how do i process my stored logs on (X,) (Y), (Z), so that i do not loose any logs during my downtime.   Is it possible to process backlog of logs on the server or do i have to change my infra so that i do not loose these logs? Thanks Noel (hsxtrt)