APEs can prevent database outages, handle bad or slow connections, and work over a WAN. APEs use Microsoft Message Queuing (MSMQ) to enable the local storage for up to 1 GB of polled data per polling engine to prevent data loss when the connection between the polling engine and the database is temporarily lost.
In my experience when doing upgrades. The system doesn't work while the main polling engine is down. Now, that just seems to be the website doesn't work, even though we have an additional web server. The polling on the APE does seem to continue to work, but it seems that you are asking about the system as a whole. APE is not HA. It sounds like you actually need the HA license. The additional polling engine is for load distribution of polling only.
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