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When a Main object is used to organize WCT components its "app" objects have well defined life cycle.

1. If the "app" implements the IConfigurable interface its configure() method will be called soon after any user configuration is parsed. After all configurable components are called, execution is returned to the user code.
2. The user code may then call Main (which is a callable object). It is typically called once per overall process execution but the user code may elect to do differently. When called, the Main object will call execute() on every component which implements IApplication and which has been given to Main as an "app" object". The order of calling is that of the order of the "apps" configuration attribute.
3. If an app object (or any component) implements ITerminal then its finalize() method is called. The order of calling is the same order as the configuration sequence.

When using the wire-cell command line you may specify "app" objects with:

$ wire-cell -a app1 -a app2 [...] -c config.jsonnet

The names used must be associated with some component in the configuration.

Typically we "bake" in which applications to use by listing them in the configuration using the special component type "wire-cell". This only works for the wire-cell CLI. Other environments such as for interfacing with art provide similar special configuration means.

Here's the tail end of a configuration file that shows the Pgrapher app being configured and told to the wire-cell CLI:

local app = {
    type: "Pgrapher",
    data: {
	edges: g.edges(graph),
    },
};

local cmdline = {
    type: "wire-cell",
    data: {
	plugins: ["WireCellGen", "WireCellPgraph", "WireCellSio", "WireCellSigProc"],
	apps: ["Pgrapher"]
    }
};

// Finally, the configuration sequence which is emitted.
[cmdline] + g.uses(graph) + [app]

Typically, only one "app" object runs in a WCT job. In the rare case that multiple "app" objects run, there may be some care about the ordering. The order is as:

1. IConfigurable are constructed in their order of the configuration sequence
2. IApplication are constructed in order of the "apps" list and if they are not also IConfigurable.
3. IConfigurable have configure() called in their order of the configuration sequence
4. User code calls Main typically once which calls IApplication::execute() on each "app" object in order of the "apps" list.
5. User code destroys Main which calls ITerminal::finalize() in the configuration sequence order.q

An "app" object may be used to enact a global initialization and finalization, eg to manage some global resource or code. Currently WCT can not guarantee truly first/last semantics. But, one can get close by adding an "app" to an existing job early in the "apps" list and late in the configuration sequence. Eg:

local graph = ...;
local global = { name: "Global", ... };
local cmdline = {
    type: "wire-cell",
    data: {
	apps: ["Global", "Pgrapher"]
	...
    }
};
[cmdline] + g.uses(graph) + [app, global]

The "Global" app object may do relatively early operations in its constructor. As long as no other components try to access the "global" resource in their own constructors then "Global" will have time to do what it needs. Since "Global" app is first in the "apps" sequence it may "execute" just prior to the primary "app" object (here, "Pgrapher"). Then it is placed last in the configuration sequence so that it may get the final finalize() call.