  Real-Time Bidding (RTB) display advertising is a method for purchasing
display advertising inventory in auctions that occur within milliseconds. The
performance of RTB campaigns is generally measured with a series of Key
Performance Indicators (KPIs) - measurements used to ensure that the campaign
is cost-effective and that it is purchasing valuable inventory. While an RTB
campaign should ideally meet all KPIs, simultaneous improvement tends to be
very challenging, as an improvement to any one KPI risks a detrimental effect
toward the others. Here we present an approach to simultaneously controlling
multiple KPIs with a PID-based feedback-control system. This method generates a
control score for each KPI, based on both the output of a PID controller module
and a metric that quantifies the importance of each KPI for internal business
needs. On regular intervals, this algorithm - Sequential Control - will choose
the KPI with the greatest overall need for improvement. In this way, our
algorithm is able to continually seek the greatest marginal improvements to its
current state. Multiple methods of control can be associated with each KPI, and
can be triggered either simultaneously or chosen stochastically, in order to
avoid local optima. In both offline ad bidding simulations and testing on live
traffic, our methods proved to be effective in simultaneously controlling
multiple KPIs, and bringing them toward their respective goals.
