  One of the most important issues in data stream processing systems is to use
operator migration to handle highly variable workloads in a cost-efficient
manner and adapt to the needs at any given time on demand. Operator migration
is a complex process that involves changes in the state and stream management
of a running query, typically without any loss of data, and with as little
disruption to the execution as possible. This survey provides an overview of
solutions for operator migration from a historical perspective as well as the
perspective of the goal of migration. It introduces a conceptual model of
operator migration to establish a unified terminology and classify existing
solutions. Existing work in the area is analyzed to separate the mechanism of
migration from the decision to migrate the data. In case of the latter, a
cost-benefit analysis is emphasized that is important for operator migration
but is often only implicitly addressed, or is neglected altogether. A
description of the available solutions provides the reader with a good
understanding of the design alternatives from an algorithmic viewpoint. We
complement this with an empirical study to provide quantitative insights on the
impact of different design alternatives on the mechanisms of migration.
