1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
|
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2003</year><year>2013</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>Included Applications</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>included_applications.xml</file>
</header>
<marker id="included appl"></marker>
<section>
<title>Introduction</title>
<p>An application can <em>include</em> other applications.
An <em>included application</em> has its own application directory
and <c>.app</c> file, but it is started as part of the supervisor
tree of another application.</p>
<p>An application can only be included by one other application.</p>
<p>An included application can include other applications.</p>
<p>An application that is not included by any other application is
called a <em>primary application</em>.</p>
<marker id="inclappls"></marker>
<image file="../design_principles/inclappls.gif">
<icaption>Primary Application and Included Applications</icaption>
</image>
<p>The application controller automatically loads any included
applications when loading a primary application, but does not start
them. Instead, the top supervisor of the included application
must be started by a supervisor in the including application.</p>
<p>This means that when running, an included application is in fact
part of the primary application, and a process in an included
application considers itself belonging to the primary
application.</p>
</section>
<section>
<title>Specifying Included Applications</title>
<p>Which applications to include is defined by
the <c>included_applications</c> key in the <c>.app</c> file:</p>
<pre>
{application, prim_app,
[{description, "Tree application"},
{vsn, "1"},
{modules, [prim_app_cb, prim_app_sup, prim_app_server]},
{registered, [prim_app_server]},
{included_applications, [incl_app]},
{applications, [kernel, stdlib, sasl]},
{mod, {prim_app_cb,[]}},
{env, [{file, "/usr/local/log"}]}
]}.</pre>
</section>
<section>
<title>Synchronizing Processes during Startup</title>
<p>The supervisor tree of an included application is started as
part of the supervisor tree of the including application.
If there is a need for synchronization between processes in
the including and included applications, this can be achieved
by using <em>start phases</em>.</p>
<p>Start phases are defined by the <c>start_phases</c> key in
the <c>.app</c> file as a list of tuples <c>{Phase,PhaseArgs}</c>,
where <c>Phase</c> is an atom and <c>PhaseArgs</c> is a term.</p>
<p>The value of the <c>mod</c> key of the including application
must be set to <c>{application_starter,[Module,StartArgs]}</c>,
where <c>Module</c> as usual is the application callback module.
<c>StartArgs</c> is a term provided as argument to the callback
function <c>Module:start/2</c>:</p>
<code type="none">
{application, prim_app,
[{description, "Tree application"},
{vsn, "1"},
{modules, [prim_app_cb, prim_app_sup, prim_app_server]},
{registered, [prim_app_server]},
{included_applications, [incl_app]},
{start_phases, [{init,[]}, {go,[]}]},
{applications, [kernel, stdlib, sasl]},
{mod, {application_starter,[prim_app_cb,[]]}},
{env, [{file, "/usr/local/log"}]}
]}.
{application, incl_app,
[{description, "Included application"},
{vsn, "1"},
{modules, [incl_app_cb, incl_app_sup, incl_app_server]},
{registered, []},
{start_phases, [{go,[]}]},
{applications, [kernel, stdlib, sasl]},
{mod, {incl_app_cb,[]}}
]}.</code>
<p>When starting a primary application with included applications,
the primary application is started the normal way, that is:</p>
<list type="bulleted">
<item>The application controller creates an application master for
the application</item>
<item>The application master calls
<c>Module:start(normal, StartArgs)</c> to start the top
supervisor.</item>
</list>
<p>Then, for the primary application and each included application
in top-down, left-to-right order, the application master calls
<c>Module:start_phase(Phase, Type, PhaseArgs)</c> for each phase
defined for the primary application, in that order. If a phase
is not defined for an included application,
the function is not called for this phase and application.</p>
<p>The following requirements apply to the <c>.app</c> file for
an included application:</p>
<list type="bulleted">
<item>The <c>{mod, {Module,StartArgs}}</c> option must be included.
This option is used to find the callback module <c>Module</c> of the
application. <c>StartArgs</c> is ignored, as <c>Module:start/2</c>
is called only for the primary application.</item>
<item>If the included application itself contains included
applications, instead the
<c>{mod, {application_starter, [Module,StartArgs]}}</c> option
must be included.</item>
<item>The <c>{start_phases, [{Phase,PhaseArgs}]}</c> option must
be included, and the set of specified phases must be a subset
of the set of phases specified for the primary application.</item>
</list>
<p>When starting <c>prim_app</c> as defined above, the application
controller calls the following callback functions before
<c>application:start(prim_app)</c> returns a value:</p>
<code type="none">
application:start(prim_app)
=> prim_app_cb:start(normal, [])
=> prim_app_cb:start_phase(init, normal, [])
=> prim_app_cb:start_phase(go, normal, [])
=> incl_app_cb:start_phase(go, normal, [])
ok</code>
</section>
</chapter>
|
