  Yield farming has been an immensely popular activity for cryptocurrency
holders since the explosion of Decentralized Finance (DeFi) in the summer of
2020. In this Systematization of Knowledge (SoK), we study a general framework
for yield farming strategies with empirical analysis. First, we summarize the
fundamentals of yield farming by focusing on the protocols and tokens used by
aggregators. We then examine the sources of yield and translate those into
three example yield farming strategies, followed by the simulations of yield
farming performance, based on these strategies. We further compare four major
yield aggregrators -- Idle, Pickle, Harvest and Yearn -- in the ecosystem,
along with brief introductions of others. We systematize their strategies and
revenue models, and conduct an empirical analysis with on-chain data from
example vaults, to find a plausible connection between data anomalies and
historical events. Finally, we discuss the benefits and risks of yield
aggregators.
