  The High-Luminosity LHC (HL-LHC) will usher in a new era in high-energy
physics. The HL-LHC experimental conditions entail an instantaneous luminosity
of up to $7.5 \times 10^{34}$ cm$^{-2}$ s$^{-1}$ and up to 200 simultaneous
collisions per bunch crossing (pileup). To cope with those conditions, the CMS
detector will undergo a series of improvements, in what is known as the Phase-2
upgrade. In particular, the upgrade of the Data Acquisition and of the
High-Level Trigger (DAQ-HLT) will have to address a much higher event rate and
more complex events. In this paper, we will discuss the aspects of the HLT
upgrade, detailing the development of the online reconstruction, the
construction, characterisation and timing/rate measurement of a simplified HLT
menu, the role of heterogeneous architectures in the HLT and the plan of work
and milestones until the beginning of Phase-2.
