  Fog computing can provide computational resources and low-latency
communication at the network edge. But with it comes uncertainties that must be
managed in order to guarantee Service Level Agreements. Service observability
can help the environment better deal with uncertainties, delivering relevant
and up-to-date information in a timely manner to support decision making.
Observability is considered a superset of monitoring since it uses not only
performance metrics, but also other instrumentation domains such as logs and
traces. However, as Fog Computing is typically characterised by
resource-constrained nodes and network uncertainties, increasing observability
in fog can be risky due to the additional load injected into a restricted
environment. There is no work in the literature that evaluated fog
observability. In this paper, we first outline the challenges of achieving
observability in a Fog environment, based on which we present a formal
definition of fog observability. Subsequently, a real-world Fog Computing
testbed running a smart city use case is deployed, and an empirical evaluation
of fog observability using open-source tools is presented. The results show
that under certain conditions, it is viable to provide observability in a Fog
Computing environment using open-source tools, although it is necessary to
control the overhead modifying their default configuration according to the
application characteristics.
