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<img alt="Erlang logo" src="../../../../doc/erlang-logo.png"><br><small><a href="users_guide.html">User's Guide</a><br><a href="index.html">Reference Manual</a><br><a href="release_notes.html">Release Notes</a><br><a href="../pdf/diameter-1.3.pdf">PDF</a><br><a href="../../../../doc/index.html">Top</a></small><p><strong>Diameter</strong><br><strong>Reference Manual</strong><br><small>Version 1.3</small></p>
<br><a href="javascript:openAllFlips()">Expand All</a><br><a href="javascript:closeAllFlips()">Contract All</a><p><small><strong>Table of Contents</strong></small></p>
<ul class="flipMenu">
<li id="no" title="diameter " expanded="false">diameter<ul>
<li><a href="diameter.html">
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</a></li>
<li title="add_transport-2"><a href="diameter.html#add_transport-2">add_transport/2</a></li>
<li title="call-4"><a href="diameter.html#call-4">call/4</a></li>
<li title="origin_state_id-0"><a href="diameter.html#origin_state_id-0">origin_state_id/0</a></li>
<li title="remove_transport-2"><a href="diameter.html#remove_transport-2">remove_transport/2</a></li>
<li title="service_info-2"><a href="diameter.html#service_info-2">service_info/2</a></li>
<li title="services-0"><a href="diameter.html#services-0">services/0</a></li>
<li title="session_id-1"><a href="diameter.html#session_id-1">session_id/1</a></li>
<li title="start-0"><a href="diameter.html#start-0">start/0</a></li>
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<li title="diameterc"><a href="diameterc.html">diameterc</a></li>
<li id="loadscrollpos" title="diameter_app " expanded="true">diameter_app<ul>
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</a></li>
<li title="Mod:peer_up-3"><a href="diameter_app.html#Mod:peer_up-3">Mod:peer_up/3</a></li>
<li title="Mod:peer_down-3"><a href="diameter_app.html#Mod:peer_down-3">Mod:peer_down/3</a></li>
<li title="Mod:pick_peer-4"><a href="diameter_app.html#Mod:pick_peer-4">Mod:pick_peer/4</a></li>
<li title="Mod:prepare_request-3"><a href="diameter_app.html#Mod:prepare_request-3">Mod:prepare_request/3</a></li>
<li title="Mod:prepare_retransmit-3"><a href="diameter_app.html#Mod:prepare_retransmit-3">Mod:prepare_retransmit/3</a></li>
<li title="Mod:handle_answer-4"><a href="diameter_app.html#Mod:handle_answer-4">Mod:handle_answer/4</a></li>
<li title="Mod:handle_error-4"><a href="diameter_app.html#Mod:handle_error-4">Mod:handle_error/4</a></li>
<li title="Mod:handle_request-3"><a href="diameter_app.html#Mod:handle_request-3">Mod:handle_request/3</a></li>
</ul>
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</a></li>
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<li title="encode-2"><a href="diameter_codec.html#encode-2">encode/2</a></li>
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</a></li>
<li title="Mod:start-3"><a href="diameter_transport.html#Mod:start-3">Mod:start/3</a></li>
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<div id="content">
<div class="innertube">
<!-- refpage --><center><h1>diameter_app</h1></center>
<h3>MODULE</h3>
<div class="REFBODY">diameter_app</div>
<h3>MODULE SUMMARY</h3>
<div class="REFBODY">
Callback module of a Diameter application.</div>
<h3>DESCRIPTION</h3>
<div class="REFBODY"><p>
<p>
A diameter service as started by <span class="bold_code"><a href="diameter.html#start_service-2">diameter:start_service/2</a></span>
configures one of more Diameter applications, each of whose
configuration specifies a callback that handles messages specific to
the application.
The messages and AVPs of the application are defined in a
dictionary file whose format is documented in
<span class="bold_code"><a href="diameter_dict.html">diameter_dict(4)</a></span> while the callback module is documented here.
The callback module implements the Diameter application-specific
functionality of a service.</p>
<p>
A callback module must export all of the functions documented below.
The functions themselves are of three distinct flavours:</p>
<ul>
<li>
<p>
<span class="bold_code"><a href="#Mod:peer_up-3">peer_up/3</a></span> and <span class="bold_code"><a href="#Mod:peer_down-3">peer_up/3</a></span> signal the
attainment or loss of connectivity with a Diameter peer.</p>
</li>
<li>
<p>
<span class="bold_code"><a href="#Mod:pick_peer-4">pick_peer/4</a></span>,
<span class="bold_code"><a href="#Mod:prepare_request-3">prepare_request/3</a></span>,
<span class="bold_code"><a href="#Mod:prepare_retransmit-3">prepare_retransmit/3</a></span>,
<span class="bold_code"><a href="#Mod:handle_answer-4">handle_answer/4</a></span>
and <span class="bold_code"><a href="#Mod:handle_error-4">handle_error/4</a></span> are (or may be) called as a consequence of a call
to <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span> to send an outgoing
Diameter request message.</p>
</li>
<li>
<p>
<span class="bold_code"><a href="#Mod:handle_request-3">handle_request/3</a></span>
is called in response to an incoming Diameter request message.</p>
</li>
</ul>
</p></div>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>
The arities given for the the callback functions here assume no extra
arguments.
All functions will also be passed any extra arguments configured with
the callback module itself when calling <span class="bold_code"><a href="diameter.html#start_service-2">diameter:start_service/2</a></span>
and, for the call-specific callbacks, any extra arguments passed to
<span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span>.</p>
</p></div>
</div>
<h3><a name="id74116">DATA TYPES</a></h3>
<div class="REFBODY">
<dl>
<a name="capabilities"></a>
<dt><strong><span class="code">capabilities() = #diameter_caps{}</span></strong></dt>
<dd>
<p>
A record containing the identities of
the local Diameter node and the remote Diameter peer having an
established transport
connection, as well as the capabilities as
determined by capabilities exchange.
Each field of the record is a 2-tuple consisting of
values for the (local) host and (remote) peer.
Optional or possibly multiple values are encoded as lists of values,
mandatory values as the bare value.</p>
</dd>
<a name="message"></a>
<dt><strong><span class="code">message() = <span class="bold_code"><a href="diameter_codec.html#message">diameter_codec:message()</a></span></span></strong></dt>
<dd>
<p>
The representation of a Diameter message as passed to
<span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span> or returned from a <span class="bold_code"><a href="#Mod:handle_request-3">handle_request/3</a></span> callback.</p>
</dd>
<a name="packet"></a>
<dt><strong><span class="code">packet() = <span class="bold_code"><a href="diameter_codec.html#packet">diameter_codec:packet()</a></span></span></strong></dt>
<dd>
<p>
A container for incoming and outgoing Diameter messages that's passed
through encode/decode and transport.
Fields should not be set in return values except as documented.</p>
</dd>
<a name="peer_ref"></a>
<dt><strong><span class="code">peer_ref() = term()</span></strong></dt>
<dd>
<p>
A term identifying a transport connection with a Diameter peer.</p>
</dd>
<a name="peer"></a>
<dt><strong><span class="code">peer() = {<span class="bold_code"><a href="#peer_ref">peer_ref()</a></span>, <span class="bold_code"><a href="#capabilities">capabilities()</a></span>}</span></strong></dt>
<dd>
<p>
A tuple representing a Diameter peer connection.</p>
</dd>
<a name="state"></a>
<dt><strong><span class="code">state() = term()</span></strong></dt>
<dd>
<p>
The state maintained by the application callback functions
<span class="bold_code"><a href="#Mod:peer_up-3">peer_up/3</a></span>, <span class="bold_code"><a href="#Mod:peer_down-3">peer_up/3</a></span> and (optionally) <span class="bold_code"><a href="#Mod:pick_peer-4">pick_peer/4</a></span>.
The initial state is configured in the call to
<span class="bold_code"><a href="diameter.html#start_service-2">diameter:start_service/2</a></span> that configures the application on a service.
Callback functions returning a state are evaluated in a common
service-specific process while
those not returning state are evaluated in a request-specific
process.</p>
</dd>
</dl>
<a name="peer_up"></a>
</div>
<h3>EXPORTS</h3>
<p><a name="Mod:peer_up-3"><span class="bold_code">Mod:peer_up(SvcName, Peer, State) -> NewState</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">SvcName = <span class="bold_code"><a href="diameter.html#service_name">diameter:service_name()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Peer = <span class="bold_code"><a href="#peer">peer()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">State = NewState = <span class="bold_code"><a href="#state">state()</a></span></span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>
Invoked to signal the availability of a peer connection.
In particular, capabilities exchange with the peer has indicated
support for the application in question, the RFC 3539 watchdog state
machine for the connection has reached state <span class="code">OKAY</span> and Diameter
messages can be both sent and received.</p>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>
A watchdog state machine can reach state <span class="code">OKAY</span> from state
<span class="code">SUSPECT</span> without a new capabilities exchange taking place.
A new transport connection (and capabilities exchange) results in a
new peer_ref().</p>
</p></div>
</div>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>
There is no requirement that a callback return before incoming
requests are received: <span class="bold_code"><a href="#Mod:handle_request-3">handle_request/3</a></span> callbacks must be
handled independently of <span class="bold_code"><a href="#Mod:peer_up-3">peer_up/3</a></span> and <span class="bold_code"><a href="#Mod:peer_down-3">peer_up/3</a></span>.</p>
</p></div>
</div>
</p></div>
<p><a name="Mod:peer_down-3"><span class="bold_code">Mod:peer_down(SvcName, Peer, State) -> NewState</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">SvcName = <span class="bold_code"><a href="diameter.html#service_name">diameter:service_name()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Peer = <span class="bold_code"><a href="#peer">peer()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">State = NewState = <span class="bold_code"><a href="#state">state()</a></span></span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>
Invoked to signal that a peer connection is no longer available
following a previous call to <span class="bold_code"><a href="#Mod:peer_up-3">peer_up/3</a></span>.
In particular, that the RFC 3539 watchdog state machine for the
connection has left state <span class="code">OKAY</span> and the peer will no longer be a
candidate in <span class="bold_code"><a href="#Mod:pick_peer-4">pick_peer/4</a></span> callbacks.</p>
</p></div>
<p><a name="Mod:pick_peer-4"><span class="bold_code">Mod:pick_peer(Candidates, _Reserved, SvcName, State)
-> Selection | false</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Candidates = [<span class="bold_code"><a href="#peer">peer()</a></span>]</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">SvcName = <span class="bold_code"><a href="diameter.html#service_name">diameter:service_name()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">State = NewState = <span class="bold_code"><a href="#state">state()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Selection = {ok, Peer} | {Peer, NewState}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Peer = <span class="bold_code"><a href="#peer">peer()</a></span> | false</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>
Invoked as a consequence of a call to <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span> to select a destination
peer for an outgoing request.
The return value indicates the selected peer.</p>
<p>
The candidate list contains only those peers that have advertised
support for the Diameter application in question during capabilities
exchange, that have not be excluded by a <span class="code">filter</span> option in
the call to <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span>
and whose watchdog state machine is in the <span class="code">OKAY</span> state.
The order of the elements is unspecified except that any
peers whose Origin-Host and Origin-Realm matches that of the
outgoing request (in the sense of a <span class="code">{filter, {all, [host, realm]}}</span>
option to <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span>)
will be placed at the head of the list.</p>
<p>
A callback that returns a peer() will be followed by a
<span class="bold_code"><a href="#Mod:prepare_request-3">prepare_request/3</a></span>
callback and, if the latter indicates that the request should be sent,
by either <span class="bold_code"><a href="#Mod:handle_answer-4">handle_answer/4</a></span>
or <span class="bold_code"><a href="#Mod:handle_error-4">handle_error/4</a></span> depending
on whether or not an answer message is received from the peer.
If the transport becomes unavailable after <span class="bold_code"><a href="#Mod:prepare_request-3">prepare_request/3</a></span> then a
new <span class="bold_code"><a href="#Mod:pick_peer-4">pick_peer/4</a></span> callback may take place to
failover to an alternate peer, after which <span class="bold_code"><a href="#Mod:prepare_retransmit-3">prepare_retransmit/3</a></span> takes the
place of <span class="bold_code"><a href="#Mod:prepare_request-3">prepare_request/3</a></span> in resending the
request.
There is no guarantee that a <span class="bold_code"><a href="#Mod:pick_peer-4">pick_peer/4</a></span> callback to select
an alternate peer will be followed by any additional callbacks since a
retransmission to an alternate peer is abandoned if an answer is
received from a previously selected peer.</p>
<p>
Returning <span class="code">false</span> or <span class="code">{false, NewState}</span> causes <span class="code">{error,
no_connection}</span> to be returned from <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span>.</p>
<p>
The return values <span class="code">false</span> and <span class="code">{false, State}</span> (that is,
<span class="code">NewState = State</span>) are equivalent, as are <span class="code">{ok, Peer}</span> and
<span class="code">{Peer, State}</span>.</p>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>
The return value <span class="code">{Peer, NewState}</span> is only allowed if
the Diameter application in question was configured with the
<span class="bold_code"><a href="diameter.html#application_opt">diameter:application_opt()</a></span> <span class="code">{call_mutates_state, true}</span>.
Otherwise, the <span class="code">State</span> argument is always
the intial value as configured on the application, not any subsequent
value returned by a <span class="bold_code"><a href="#Mod:peer_up-3">peer_up/3</a></span>
or <span class="bold_code"><a href="#Mod:peer_down-3">peer_up/3</a></span> callback.</p>
</p></div>
</div>
</p></div>
<p><a name="Mod:prepare_request-3"><span class="bold_code">Mod:prepare_request(Packet, SvcName, Peer) -> Action</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Packet = <span class="bold_code"><a href="#packet">packet()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">SvcName = <span class="bold_code"><a href="diameter.html#service_name">diameter:service_name()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Peer = <span class="bold_code"><a href="#peer">peer()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Action = Send | Discard | {eval_packet, Action, PostF}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Send = {send, <span class="bold_code"><a href="#packet">packet()</a></span> | <span class="bold_code"><a href="#message">message()</a></span>}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Discard = {discard, Reason} | discard</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">PostF = <span class="bold_code"><a href="diameter.html#evaluable">diameter:evaluable()</a></span>}</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>
Invoked to return a request for encoding and transport.
Allows the sender to use the selected peer's capabilities
to modify the outgoing request.
Many implementations may simply want to return <span class="code">{send, Packet}</span></p>
<p>
A returned <span class="bold_code"><a href="#packet">packet()</a></span> should set the
request to be encoded in its
<span class="code">msg</span> field and can set the <span class="code">transport_data</span> field in order
to pass information to the transport process.
Extra arguments passed to <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span> can be used to
communicate transport (or any other) data to the callback.</p>
<p>
A returned <span class="bold_code"><a href="#packet">packet()</a></span> can set
the <span class="code">header</span> field to a
<span class="code">#diameter_header{}</span> to specify values that should
be preserved in the outgoing request, values otherwise being those in
the header record contained in <span class="code">Packet</span>.
A returned <span class="code">length</span>, <span class="code">cmd_code</span> or <span class="code">application_id</span> is
ignored.</p>
<p>
A returned <span class="code">PostF</span> will be evaluated on any encoded
<span class="code">#diameter_packet{}</span> prior to transmission, the <span class="code">bin</span> field
containing the encoded binary.
The return value is ignored.</p>
<p>
Returning <span class="code">{discard, Reason}</span> causes the request to be aborted
and the <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span> for which the
callback has taken place to return <span class="code">{error, Reason}</span>.
Returning <span class="code">discard</span> is equivalent to returning <span class="code">{discard,
discarded}</span>.</p>
</p></div>
<p><a name="Mod:prepare_retransmit-3"><span class="bold_code">Mod:prepare_retransmit(Packet, SvcName, Peer) -> Action</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Packet = <span class="bold_code"><a href="#packet">packet()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">SvcName = <span class="bold_code"><a href="diameter.html#service_name">diameter:service_name()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Peer = <span class="bold_code"><a href="#peer">peer()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Action = Send | Discard | {eval_packet, Action, PostF}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Send = {send, <span class="bold_code"><a href="#packet">packet()</a></span> | <span class="bold_code"><a href="#message">message()</a></span>}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Discard = {discard, Reason} | discard</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">PostF = <span class="bold_code"><a href="diameter.html#evaluable">diameter:evaluable()</a></span>}</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>
Invoked to return a request for encoding and retransmission.
Has the same role as <span class="bold_code"><a href="#Mod:prepare_request-3">prepare_request/3</a></span> in the case that
a peer connection is lost an an alternate peer selected but the
argument <span class="bold_code"><a href="#packet">packet()</a></span> is as returned
by the initial <span class="bold_code"><a href="#Mod:prepare_request-3">prepare_request/3</a></span>.</p>
<p>
Returning <span class="code">{discard, Reason}</span> causes the request to be aborted
and a <span class="bold_code"><a href="#Mod:handle_error-4">handle_error/4</a></span> callback to
take place with <span class="code">Reason</span> as initial argument.
Returning <span class="code">discard</span> is equivalent to returning <span class="code">{discard,
discarded}</span>.</p>
</p></div>
<p><a name="Mod:handle_answer-4"><span class="bold_code">Mod:handle_answer(Packet, Request, SvcName, Peer) -> Result</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Packet = <span class="bold_code"><a href="#packet">packet()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Request = <span class="bold_code"><a href="#message">message()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">SvcName = <span class="bold_code"><a href="diameter.html#service_name">diameter:service_name()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Peer = <span class="bold_code"><a href="#peer">peer()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Result = term()</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>
Invoked when an answer message is received from a peer.
The return value is returned from <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span> unless the
<span class="code">detach</span> option was specified.</p>
<p>
The decoded answer record and undecoded binary are in the <span class="code">msg</span>
and <span class="code">bin</span> fields of the argument
<span class="bold_code"><a href="#packet">packet()</a></span> respectively.
<span class="code">Request</span> is the outgoing request message as was returned from
<span class="bold_code"><a href="#Mod:prepare_request-3">prepare_request/3</a></span> or <span class="bold_code"><a href="#Mod:prepare_retransmit-3">prepare_retransmit/3</a></span>.</p>
<p>
For any given call to <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span> there is at most one
<span class="bold_code"><a href="#Mod:handle_answer-4">handle_answer/4</a></span> callback: any
duplicate answer (due to retransmission or otherwise) is discarded.
Similarly, only one of <span class="bold_code"><a href="#Mod:handle_answer-4">handle_answer/4</a></span> or
<span class="bold_code"><a href="#Mod:handle_error-4">handle_error/4</a></span> is called.</p>
<p>
By default, an incoming answer message that cannot be successfully
decoded causes the request process to fail, causing
<span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span>
to return <span class="code">{error, failure}</span> unless the <span class="code">detach</span> option was
specified.
In particular, there is no <span class="bold_code"><a href="#Mod:handle_error-4">handle_error/4</a></span> callback in this
case.
The <span class="bold_code"><a href="diameter.html#application_opt">diameter:application_opt()</a></span>
<span class="code">answer_errors</span> can be set to change this behaviour.</p>
</p></div>
<p><a name="Mod:handle_error-4"><span class="bold_code">Mod:handle_error(Reason, Request, SvcName, Peer) -> Result</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Reason = timeout | failover | term()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Request = <span class="bold_code"><a href="#message">message()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">SvcName = <span class="bold_code"><a href="diameter.html#service_name">diameter:service_name()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Peer = <span class="bold_code"><a href="#peer">peer()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Result = term()</span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>
Invoked when an error occurs before an answer message is received
in response to an outgoing request.
The return value is returned from <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span> unless the
<span class="code">detach</span> option was specified.</p>
<p>
Reason <span class="code">timeout</span> indicates that an answer message has not been
received within the time specified with the corresponding <span class="bold_code"><a href="diameter.html#call_opt">diameter:call_opt()</a></span>.
Reason <span class="code">failover</span> indicates
that the transport connection to the peer to which the request has
been sent has become unavailable and that not alternate peer was
not selected.</p>
</p></div>
<p><a name="Mod:handle_request-3"><span class="bold_code">Mod:handle_request(Packet, SvcName, Peer) -> Action</span></a><br></p>
<div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES">
<span class="bold_code">Packet = <span class="bold_code"><a href="#packet">packet()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">SvcName = term()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Peer = <span class="bold_code"><a href="#peer">peer()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Action = Reply
| {relay, [Opt]}
| discard
| {eval|eval_packet, Action, PostF}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Reply = {reply, <span class="bold_code"><a href="#packet">packet()</a></span> | <span class="bold_code"><a href="#message">message()</a></span>}
| {protocol_error, 3000..3999}</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">Opt = <span class="bold_code"><a href="diameter.html#call_opt">diameter:call_opt()</a></span></span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">PostF = <span class="bold_code"><a href="diameter.html#evaluable">diameter:evaluable()</a></span></span><br>
</div>
</div>
<div class="REFBODY"><p>
<p>
Invoked when a request message is received from a peer.
The application in which the callback takes place (that is, the
callback module as configured with <span class="bold_code"><a href="diameter.html#start_service-2">diameter:start_service/2</a></span>)
is determined by the Application Identifier in the header of the
incoming request message, the selected module being the one
whose corresponding dictionary declares
itself as defining either the application in question or the Relay
application.</p>
<p>
The argument <span class="bold_code"><a href="#packet">packet()</a></span> has the following signature.</p>
<div class="example"><pre>
#diameter_packet{header = #diameter_header{},
avps = [#diameter_avp{}],
msg = record() | undefined,
errors = [<span class="bold_code"><a href="diameter_dict.html#DATA_TYPES">Unsigned32()</a></span> | {<span class="bold_code"><a href="diameter_dict.html#DATA_TYPES">Unsigned32()</a></span>, #diameter_avp{}}],
bin = binary(),
transport_data = term()}
</pre></div>
<p>
The <span class="code">msg</span> field will be <span class="code">undefined</span> in case the request has
been received in the relay application.
Otherwise it contains the record representing the request as outlined
in <span class="bold_code"><a href="diameter_dict.html#MESSAGE_RECORDS">diameter_dict(4)</a></span>.</p>
<p>
The <span class="code">errors</span> field specifies any Result-Code's identifying errors
that were encountered in decoding the request.
In this case diameter will set both Result-Code and
Failed-AVP AVP's in a returned
answer <span class="bold_code"><a href="#message">message()</a></span> before sending it to the peer:
the returned <span class="bold_code"><a href="#message">message()</a></span> need only set any other required AVP's.
Note that the errors detected by diameter are all of the 5xxx series
(Permanent Failures).
The <span class="code">errors</span> list is empty if the request has been received in
the relay application.</p>
<p>
The <span class="code">transport_data</span> field contains an arbitrary term passed into
diameter from the transport module in question, or the atom
<span class="code">undefined</span> if the transport specified no data.
The term is preserved if a <span class="bold_code"><a href="#message">message()</a></span> is returned but must be set
explicitly in a returned <span class="bold_code"><a href="#packet">packet()</a></span>.</p>
<p>
The semantics of each of the possible return values are as follows.</p>
<dl>
<dt><strong><span class="code">{reply, <span class="bold_code"><a href="#packet">packet()</a></span> | <span class="bold_code"><a href="#message">message()</a></span>}</span></strong></dt>
<dd>
<p>
Send the specified answer message to the peer.
In the case of a <span class="bold_code"><a href="#packet">packet()</a></span>, the
message to be sent must be set in the
<span class="code">msg</span> field and the <span class="code">header</span> field can be set to a
<span class="code">#diameter_header{}</span> to specify values that should be
preserved in the outgoing answer, appropriate values otherwise
being set by diameter.</p>
</dd>
<dt><strong><span class="code">{protocol_error, 3000..3999}</span></strong></dt>
<dd>
<p>
Send an answer message to the peer containing the specified
protocol error.
Equivalent to</p>
<div class="example"><pre>
{reply, ['answer-message' | Avps]
</pre></div>
<p>
where <span class="code">Avps</span> sets the Origin-Host, Origin-Realm, the specified
Result-Code and (if the request sent one) Session-Id AVP's.</p>
<p>
Note that RFC 6733 mandates that only answers with a 3xxx series
Result-Code (protocol errors) may set the E bit.
Returning a non-3xxx value in a <span class="code">protocol_error</span> tuple
will cause the request process in question to fail.</p>
</dd>
<dt><strong><span class="code">{relay, Opts}</span></strong></dt>
<dd>
<p>
Relay a request to another peer in the role of a Diameter relay agent.
If a routing loop is detected then the request is answered with
3005 (DIAMETER_LOOP_DETECTED).
Otherwise a Route-Record AVP (containing the sending peer's Origin-Host) is
added to the request and <span class="bold_code"><a href="#Mod:pick_peer-4">pick_peer/4</a></span>
and subsequent callbacks take place just as if <span class="bold_code"><a href="diameter.html#call-4">diameter:call/4</a></span> had been called
explicitly.
The End-to-End Identifier of the incoming request is preserved in the
header of the relayed request.</p>
<p>
The returned <span class="code">Opts</span> should not specify <span class="code">detach</span>.
A subsequent <span class="bold_code"><a href="#Mod:handle_answer-4">handle_answer/4</a></span>
callback for the relayed request must return its first
argument, the <span class="code">#diameter_packet{}</span> record containing the answer
message.
Note that the <span class="code">extra</span> option can be specified to supply arguments
that can distinguish the relay case from others if so desired.
Any other return value (for example, from a
<span class="bold_code"><a href="#Mod:handle_error-4">handle_error/4</a></span> callback)
causes the request to be answered with 3002 (DIAMETER_UNABLE_TO_DELIVER).</p>
</dd>
<dt><strong><span class="code">discard</span></strong></dt>
<dd>
<p>
Discard the request.
No answer message is sent to the peer.</p>
</dd>
<dt><strong><span class="code">{eval, Action, PostF}</span></strong></dt>
<dd>
<p>
Handle the request as if <span class="code">Action</span> has been returned and then
evaluate <span class="code">PostF</span> in the request process.
The return value is ignored.</p>
</dd>
<dt><strong><span class="code">{eval_packet, Action, PostF}</span></strong></dt>
<dd>
<p>
Like <span class="code">eval</span> but evaluate <span class="code">PostF</span> on any encoded
<span class="code">#diameter_packet{}</span> prior to transmission, the <span class="code">bin</span> field
containing the encoded binary.
The return value is ignored.</p>
</dd>
</dl>
<p>
Note that protocol errors detected by diameter will result in an
answer message without <span class="code">handle_request/3</span> being invoked.</p>
</p></div>
</div>
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