Indicators are pluggable modules that communicate the results of event stream processing to the external world. Indicators can act as visualizers that present a graphical view of their event inputs. They can also be warning agents (monitors) that send alerts, warnings or other control events to the outside world.
In their implementation indicators can be classes that implement the UpdateListener interface and that can thus be attached directly to one or more statements. Indicactors can also be attached to one or more EPStatement instances. This makes is possible for indicators to merge data as well as pull data from trigger and statement views.
Indicators may be integration components that plug together with other software, an some indicators will be supplied by Esper. Esper currently only has one indicator module as described below.