Terminology

EPL terminology
Basic modules	Application	An Apama application consists of one or more collaborating monitors.
	Package	A mechanism for qualifying monitor and event names. Monitors and global events in the same package must each have a unique name within the package.
	Context	Contexts allow EPL applications to organize work into threads that the correlator can concurrently execute.
	Monitor	A monitor is the basic unit of program execution. Monitors have both data and logic. Monitors communicate by sending and receiving events.
	Event (data type)	An event is a data object, used primarily for communication between monitors and between the correlator and its environment. All events have an event type and an ordered set of event fields. An event type might also have zero or more defined event actions that operate on the event fields.
	Field	A data element of an event.
	Method	A method is a pre-defined action. A given EPL data type has a given set of methods that it supports.
Data types	Data type	EPL supports the following value data types: boolean, decimal, float, integer, and the following reference data types: action, chunk, context, dictionary, event, Exception, listener, location, sequence, StackTraceElement, stream, string.
	sequence	An EPL data type used to hold an ordered set of objects (referenced by position).
	dictionary	An EPL data type used to hold a keyed set of objects (referenced by key).
	location	An EPL data type that represents a rectangular area in a two-dimensional unitless Cartesian coordinate plane.
	chunk	An EPL data type that references an opaque data set, the data items of which are manipulated only in a correlator plug-in.
	listener	You can assign an event listener or a stream listener to a variable of this type and then subsequently call quit() on the listener to remove the listener from the correlator.
	action	An EPL data type that references an action. Actions in EPL are the equivalent of methods in object-oriented languages. Actions are user-defined methods that you can define in monitor definitions, event type definitions, and custom aggregate function definitions.
	context	An EPL data type that provides a reference to a context. A context lets the correlator concurrently process events.
	stream	An EPL data type that refers to a stream object. Each stream is a conduit or channel through which items flow. A stream transports items of only one type, which can be any Apama data type. Streams are internal to a monitor.
	Exception	Values of Exception type are objects that contain information about runtime errors.
	StackTrace Element	A StackTraceElement type value is an object that contains information about one entry in the stack trace.
Monitors	Monitor name	Each monitor has a name that can be used to delete the monitor from the correlator.
	Monitor definition	The set of source statements that define a monitor.
	Monitor instance	A monitor instance is created whenever a monitor is loaded into the correlator. Subsequent monitor instances are created whenever a monitor instance spawns.
	Sub-monitor	A monitor instance. When a monitor spawns it creates a sub-monitor, which is just another monitor instance.
Events	Event name	Every event must identify its event type. Event types are identified by a unique event name. The event name can also be used to remove the event definition from the correlator.
	Event definition	The set of source statements that define an event type.
	Event type	All events of a given event type have the same structure. An event type defines the event name, the ordered set of event fields and the set of event actions that can be called on the event fields.
	Event field	A data element of an event.
	Event action	An action defined within an event definition. The action can operate only on the fields of the event and any arguments passed into the action call.
Listeners	Event listener	A construct that monitors the events passed to, or routed within, a correlator context. When the event pattern matches the event pattern specified in an event listener, the correlator invokes the event listener’s code block.
	on statement	EPL statement that defines an event listener. An on statement specifies an event expression and a listener action.
	Stream listener	A construct that continuously watches for items from a stream and invokes the listener code block each time new items are available.
	from statement	EPL statement that defines a stream listener. A from statement specifies a source stream, a variable, and a code block. The from statement coassigns each stream output item to the specified variable and executes the statement or block once for each output item.
	Listener action	The action, statement or block part of a listener.
	Listener handle	It is possible to assign the handle (reference) to a listener to a listener variable. This variable can then be used to quit the listener.
	Event template	Specifies an event type and the set of (or set of ranges of) event field values to match.
	Event operator	Relational, logical, or temporal operator that applies to an event template and that you specify in an event expression.
	Event expression	An expression, constructed using event operators and event templates, that identifies an event or pattern of events to match.
Streams See also: stream, streamlistener, and from statement.	Stream query	A query that the correlator applies continuously to one or two streams. The output of a stream query is one continuous stream of derived items.
	Stream source template	An event template preceded by the all keyword. It uses no other event operators. A stream source template creates a stream that contains events that match the event template.
	Stream network	Network of stream source templates, streams, stream queries, and stream listeners. Upstream elements feed into downstream elements to generate derived, added-value items.
	Activation	When the passage of time or the arrival of an item causes a stream network or an element in a stream network to process items.