Service B initiates the connection to Service A in order to receive notifications, and Service B initiates the connection to Service A to query for changed data.
Service A never initiates a connection with Service B. If Service B went offline, Service A would never notice.
Service A: publish a notification indicating that some new data is available.
Service B: consume these notifications and call back to service A with queries for the changed data and perhaps surrounding context.
What would you recommend when something like this is desired?