Sometimes an ad network won’t fill a request for an ad. This could be for a variety of reasons: there are no ads available for the requesting region, a user has hit their frequency cap with that network for the day, etc.
For this reason, it is best to work with several ad networks and to implement “fail over” logic that tries to fill a request that has gone unfilled by one of the networks. For example, you might choose to show an AppLovin ad then fail over to another network if AppLovin doesn’t provide an ad.
This sort of setup is called a “waterfall”. It can help you increase your overall revenue, since you won’t have to rely on just one network for 100% of your requests. A simple pseudocode implementation of a waterfall would be as follows:
if AppLovin has an ad available to show show the ad else ask another network if they have an ad available to show