Currently, App Broker does not directly support High Availability in Configuration Manager. If you promote the passive server to the active server, then App Broker will need to be reconfigured to point to the newly promoted server. The following steps will need to be performed:
1. Be sure that the ESD Web Service is installed on the passive server (The server to be promoted).
2. Go to Settings->Deployment->ConfigMgr and change the deployment settings to point to the newly promoted server.
3. Run the following query against the App Broker Database, to update the Server value to reflect the newly promoted server
update WD_SiteToAdvert set Server = '<newlyPromotedServer>' where Server = '<oldActiveServer>'
4. Restart the ESD Service.
There is an idea requesting more robust support for High Availability, without manual configuration. Please vote for this idea, if you feel that the idea should be implemented.