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<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>2011</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
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Erlang Public License along with this software. If not, it can be
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Software distributed under the License is distributed on an "AS IS"
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<title>diameter_app(3)</title>
<prepared>Anders Svensson</prepared>
<responsible></responsible>
<docno></docno>
<approved></approved>
<checked></checked>
<date></date>
<rev>%REV%</rev>
<file>diameter_app.xml</file>
</header>
<module>diameter_app</module>
<modulesummary>
Callback module of a Diameter application.</modulesummary>
<description>
<p>
A diameter service as started by <seealso
marker="diameter#start_service">diameter:start_service/2</seealso>
configures one of more Diameter applications, each of whose
configuration specifies a callback that handles messages specific to
its application.
The messages and AVPs of the Diameter application are defined in a
specification file whose format is documented in
<seealso marker="diameter_dict">diameter_dict(4)</seealso>
while the callback module is documented here.
The callback module implements the Diameter application-specific
functionality of a service.</p>
<note>
<p>
The arities of the callback functions below assume no extra arguments.
All functions will also be passed any extra arguments configured with
the callback module itself when calling <seealso
marker="diameter#start_service">diameter:start_service/2</seealso>
and, except for peer_up, peer_down and handle_request, any extra
arguments passed to <seealso
marker="diameter#call">diameter:call/4</seealso>.</p>
</note>
<p>
A callback module must export all of the functions documented below.
The functions themselves are of three distinct flavours:</p>
<list>
<item>
<p>
<seealso marker="#peer_up">peer_up/3</seealso> and
<seealso marker="#peer_down">peer_down/3</seealso> signal the attainment
or loss of communicativity with a Diameter peer.</p>
</item>
<item>
<p>
<seealso marker="#pick_peer">pick_peer/4</seealso>,
<seealso marker="#prepare_request">prepare_request/3</seealso>,
<seealso marker="#prepare_retransmit">prepare_retransmit/3</seealso>,
<seealso marker="#handle_answer">handle_answer/4</seealso>
and <seealso marker="#handle_error">handle_error/4</seealso> are (or may
be) called as a consequence of a call to <seealso
marker="diameter#call">diameter:call/4</seealso> to send an outgoing
Diameter request message.</p>
</item>
<item>
<p>
<seealso marker="#handle_request">handle_request/3</seealso>
is called in response to an incoming Diameter request message.</p>
</item>
</list>
</description>
<!-- ===================================================================== -->
<!-- ===================================================================== -->
<section>
<title>DATA TYPES</title>
<taglist>
<tag><c>capabilities() = #diameter_caps{}</c></tag>
<item>
<p>
A record containing the identities of
the local and remote Diameter peers 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>
</item>
<tag><c>message() = record() | list()</c></tag>
<item>
<p>
The representation of a Diameter message as passed to
<seealso marker="diameter#call">diameter:call/4</seealso>.
The record representation is as outlined in
<seealso
marker="diameter_dict#MESSAGE_RECORDS">diameter_dict(4)</seealso>:
a message as defined in a dictionary file is encoded as a record with
one field for each component AVP.
Equivalently, a message can also be encoded as a list whose head is
the atom-valued message name (the record name minus any
prefix in the dictionary file) and whose tail is a list of
<c>{FieldName, FieldValue}</c> pairs.</p>
<p>
A third representation allows a message to be specified as a list
whose head is a <c>diameter_header</c> record and whose tail is a list
of <c>diameter_avp</c> records.
This representation is used by diameter itself when relaying requests
as directed by the return value of a
<seealso marker="#handle_request">handle_request/3</seealso>
callback.
It differs from the other other two in that it bypasses the checks for
messages that do not agree with their definitions in the dictionary in
question: messages are sent exactly as specified.</p>
</item>
<tag><c>packet() = #diameter_packet{}</c></tag>
<item>
<p>
A container for incoming and outgoing Diameters message that's passed
through encode/decode and transport.
Defined in diameter.hrl.
Fields should not be altered except as documented.</p>
</item>
<tag><c>peer_ref() = term()</c></tag>
<item>
<p>
A term identifying a transport connection with a Diameter peer.
Should be treated opaquely.</p>
</item>
<tag><c>peer() = {peer_ref(), capabilities()}</c></tag>
<item>
<p>
A tuple representing a Diameter peer connection.</p>
</item>
<tag><c>service_name() = term()</c></tag>
<item>
<p>
The service supporting the Diameter application.
Specified to <seealso
marker="diameter#start_service">diameter:start_service/2</seealso>
when starting the service.</p>
</item>
<tag><c>state() = term()</c></tag>
<item>
<p>
The state maintained by the application callback functions
<seealso marker="#peer_up">peer_up/3</seealso>,
<seealso marker="#peer_down">peer_down/3</seealso> and (optionally)
<seealso marker="#pick_peer">pick_peer/4</seealso>.
The initial state is configured in the call to
<seealso
marker="diameter#start_service">diameter:start_service/2</seealso>
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>
</item>
</taglist>
<marker id="peer_up"/>
</section>
<!-- ===================================================================== -->
<!-- ===================================================================== -->
<funcs>
<func>
<name>Mod:peer_up(SvcName, Peer, State) -> NewState</name>
<fsummary>Invoked when a transport connection has been established</fsummary>
<type>
<v>SvcName = service_name()</v>
<v>Peer = peer()</v>
<v>State = NewState = state()</v>
</type>
<desc>
<p>
Invoked when a transport connection has been established
and a successful capabilities exchange has indicated that the peer
supports the Diameter application of the application on which
the callback module in question has been configured.</p>
<marker id="peer_down"/>
</desc>
</func>
<func>
<name>Mod:peer_down(SvcName, Peer, State) -> NewState</name>
<fsummary>Invoked when a transport connection has been lost.</fsummary>
<type>
<v>SvcName = service_name()</v>
<v>Peer = peer()</v>
<v>State = NewState = state()</v>
</type>
<desc>
<p>
Invoked when a transport connection has been lost following a previous
call to <seealso marker="peer_up">peer_up/3</seealso>.</p>
<marker id="pick_peer"/>
</desc>
</func>
<func>
<name>Mod:pick_peer(Cands, Reserved, SvcName, State)
-> {ok, Peer} | {Peer, NewState} | false</name>
<fsummary>Select a target peer for an outgoing request.</fsummary>
<type>
<v>Cands = [Peer]</v>
<v>Peer = peer() | false</v>
<v>SvcName = service_name()</v>
<v>State = NewState = state()</v>
</type>
<desc>
<p>
Invoked as a consequence of a call to <seealso
marker="diameter#call">diameter:call/4</seealso> to select a destination
peer for an outgoing request, the return value indicating the selected peer.
A new application state can also be returned but only if the Diameter
application in question was
configured with the option <c>call_mutates_state</c> set to
<c>true</c>, as documented for <seealso
marker="diameter#start_service">diameter:start_service/2</seealso>.</p>
<p>
The candidate peers list will only include those
which are selected by any <c>filter</c> option specified in the call to
<seealso marker="diameter#call">diameter:call/4</seealso>.</p>
<!--
The local candidates are those whose transport process is executing on
the local Erlang node, the remote list those that are available on
other nodes.</p> -->
<p>
The return values <c>false</c> and <c>{false, State}</c> are
equivalent when callback state is mutable, as are
<c>{ok, Peer}</c> and <c>{Peer, State}</c>.
Returning a peer as <c>false</c> causes <c>{error, no_connection}</c>
to be returned from <seealso marker="diameter#call">diameter:call/4</seealso>.
Returning a peer() from an initial pick_peer/4 callback will result in a
<seealso marker="#prepare_request">prepare_request/3</seealso> callback
followed by either <seealso
marker="#handle_answer">handle_answer/4</seealso>
or <seealso marker="#handle_error">handle_error/4</seealso> depending
on whether or not an answer message is received from the peer.
If transport with the peer is lost before this then a new <seealso
marker="#pick_peer">pick_peer/4</seealso> callback takes place to
select an alternate peer.</p>
<p>
Note that there is no guarantee that a <seealso
marker="#pick_peer">pick_peer/4</seealso> callback to select
an alternate peer will be followed by any additional callbacks, only
that the initial <seealso
marker="#pick_peer">pick_peer/4</seealso> will be, since a
retransmission to an alternate peer is abandoned if an answer is
received from a previously selected peer.</p>
<marker id="prepare_request"/>
</desc>
</func>
<func>
<name>Mod:prepare_request(Packet, SvcName, Peer) -> Action</name>
<fsummary>Return a request for encoding and transport.</fsummary>
<type>
<v>Packet = packet()</v>
<v>SvcName = service_name()</v>
<v>Peer = peer()</v>
<v>Action = {send, packet() | message()} | {discard, Reason} | discard</v>
</type>
<desc>
<p>
Invoked to return a request for encoding and transport.
Allows the sender to access the selected peer's capabilities
in order to set (for example) <c>Destination-Host</c> and/or
<c>Destination-Realm</c> in the outgoing request, although the
callback need not be limited to this usage.
Many implementations may simply want to return <c>{send, Packet}</c></p>
<p>
A returned packet() should set the request to be encoded in its
<c>msg</c> field and can set the <c>transport_data</c> field in order
to pass information to the transport module.
Extra arguments passed to <seealso
marker="diameter#call">diameter:call/4</seealso> can be used to
communicate transport data to the callback.</p>
<p>
Any returned packet() can set the <c>header</c> field to a
<c>diameter_header</c> record in order to specify values that should
be preserved in the outgoing request.
A specified <c>message_length</c> is ignored.</p>
<p>
Returning <c>{discard, Reason}</c> causes the request to be aborted
and the <seealso
marker="diameter#call">diameter:call/4</seealso> for which the
callback has taken place to return <c>{error, Reason}</c>.
Returning <c>discard</c> is equivalent to returning <c>{discard,
discarded}</c>.</p>
<marker id="prepare_retransmit"/>
</desc>
</func>
<func>
<name>Mod:prepare_retransmit(Packet, SvcName, Peer) -> Result</name>
<fsummary>Return a request for encoding and retransmission.</fsummary>
<type>
<v>Packet = packet()</v>
<v>SvcName = service_name()</v>
<v>Peer = peer()</v>
<v>Result = {send, packet() | message()} | {discard, Reason} | discard</v>
</type>
<desc>
<p>
Invoked to return a request for encoding and retransmission.
Has the same role as <seealso
marker="#prepare_request">prepare_request/3</seealso> in the case that
a peer connection is lost an an alternate peer selected but the
Packet passed to <c>prepare_retransmit/3</c> is as returned by
<c>prepare_request/3</c>.</p>
<p>
Returning <c>{discard, Reason}</c> causes the request to be aborted
and a <seealso
marker="#handle_error">handle_error/4</seealso> callback to
take place with <c>Reason</c> as initial argument.
Returning <c>discard</c> is equivalent to returning <c>{discard,
discarded}</c>.</p>
<marker id="handle_answer"/>
</desc>
</func>
<func>
<name>Mod:handle_answer(Packet, Request, SvcName, Peer) -> Result</name>
<fsummary>Receive an answer message from a peer.</fsummary>
<type>
<v>Packet = packet()</v>
<v>Request = message()</v>
<v>SvcName = service_name()</v>
<v>Peer = peer()</v>
<v>Result = term()</v>
</type>
<desc>
<p>
Invoked when an answer message is received from a peer.
The return value is returned from the call to <seealso
marker="diameter#call">diameter:call/4</seealso> for which the
callback takes place.</p>
<p>
The decoded answer record is in the <c>msg</c> field of <c>Packet</c>,
the undecoded binary in the <c>packet</c> field.
<c>Request</c> is the outgoing request message as was returned from
<seealso marker="#prepare_request">prepare_request/3</seealso> or
<seealso marker="#prepare_retransmit">prepare_retransmit/3</seealso>
before the request was passed to the transport.</p>
<p>
For any given call to <seealso
marker="diameter#call">diameter:call/4</seealso> there is at most one
call to the handle_answer callback of the application in question: any
duplicate answer (due to retransmission or otherwise) is discarded.
Similarly, only one of <c>handle_answer/4</c> or <c>handle_error/4</c> is
called for any given request.</p>
<p>
By default, an incoming answer message that cannot be successfully
decoded causes the request process in question to fail, causing the
relevant call to <seealso
marker="diameter#call">diameter:call/4</seealso>
to return <c>{error, failure}</c>.
There is no <c>handle_error/4</c> callback in this case.
Application configuration may change this behaviour as described for
<seealso
marker="diameter#start_service">diameter:start_service/2</seealso>.</p>
<marker id="handle_error"/>
</desc>
</func>
<func>
<name>Mod:handle_error(Reason, Request, SvcName, Peer) -> Result</name>
<fsummary>Return an error from a outgoing request.</fsummary>
<type>
<v>Reason = timeout | failover | term()</v>
<v>Request = message()</v>
<v>SvcName = service_name()</v>
<v>Peer = peer()</v>
<v>Result = term()</v>
</type>
<desc>
<p>
Invoked when an error occurs before an answer message is received from
a peer in response to an outgoing request.
The return value is returned from the call to <seealso
marker="diameter#call">diameter:call/4</seealso> for which the
callback takes place.</p>
<p>
Reason <c>timeout</c> indicates that an answer message has not been
received within the required time.
Reason <c>failover</c> indicates
that the transport connection to the peer to which the request has
been sent has been lost but that not alternate node was available,
possibly because a <seealso marker="#pick_peer">pick_peer/4</seealso>
callback returned false.
</p>
<marker id="handle_request"/>
</desc>
</func>
<func>
<name>Mod:handle_request(Packet, SvcName, Peer) -> Action</name>
<fsummary>Receive an incoming request.</fsummary>
<type>
<v>Packet = packet()</v>
<v>SvcName = term()</v>
<v>Peer = peer()</v>
<v>Action = Reply | NoReply | Relay | {eval, Action, ContF}</v>
<v>Reply = {reply, message()}
| {protocol_error, ResultCode}</v>
<v>NoReply = discard</v>
<v>Relay = {relay, Opts}</v>
<v>Opts = list()</v>
<v>ContF = diameter:evaluable()</v>
<v>ResultCode = 3000..3999</v>
</type>
<desc>
<p>
Invoked when a request message is received from a peer.</p>
<p>
The application in which the callback takes place (that is, the
callback module as configured with <seealso
marker="diameter#start_service">diameter:start_service/2</seealso>)
is determined by the Application Identifier in the header of the
incoming Diameter request message, the selected module being the one
whose corresponding <seealso
marker="diameter_dict#MESSAGE_RECORDS">dictionary</seealso> declares
itself as defining the application in question, or the RFC 3588 relay
application if the specific application is unsupported but the relay
application has been advertised.</p>
<p>
The packet() in which the incoming request is communicated has the
following signature.</p>
<code>
#diameter_packet{header = #diameter_header{},
avps = [#diameter_avp{}],
msg = record() | undefined,
errors = [integer() | {integer(), #diameter_avp{}}],
bin = binary(),
transport_data = term()}
</code>
<p>
The <c>msg</c> field will be <c>undefined</c> only in case the request has
been received in the relay application.
Otherwise it contains the record representing the request as outlined
in <seealso
marker="diameter_dict#MESSAGE_RECORDS">diameter_dict(4)</seealso>.</p>
<p>
The <c>errors</c> field specifies any non-protocol errors that were
encountered in decoding the request and can be returned in a
<c>reply</c> tuple to have diameter set the Result-Code and Failed-AVP
AVP's appropriately.
The list is empty if the request has been received in the relay
application.</p>
<p>
The <c>transport_data</c> field contains an arbitrary term passed into
diameter from the transport module in question, or the atom
<c>undefined</c> if the transport specified no data.
The term is preserved in the packet() containing any answer message
sent back to the transport process unless another value is explicitly
specified.</p>
<p>
The semantics of each of the possible return values are as follows.
(TODO: more.)</p>
<taglist>
<tag><c>{reply, Answer}</c></tag>
<item>
<p>
Send the specified answer message to the peer.</p>
</item>
<tag><c>{relay, Opts}</c></tag>
<item>
<p>
Relay a request to another peer.</p>
</item>
<tag><c>{protocol_error, ResultCode}</c></tag>
<item>
<p>
Send an answer message to the peer containing the specified 3xxx
protocol error.</p>
<p>
RFC 3588 mandates that only answers with a 3xxx series
Result-Code (protocol errors) may set the E bit.
Returning a non-3xxx value in a <c>{protocol_error, ResultCode}</c>
tuple will cause the request process in question to fail.</p>
</item>
<tag><c>discard</c></tag>
<item>
<p>
Discard the request.</p>
</item>
<tag><c>{eval, Action, ContF}</c></tag>
<item>
<p>
Handle the request as if <c>Action</c> has been returned and then
evaluate the evaluable() <c>ContF</c> in the request process.</p>
</item>
</taglist>
<p>
Note that diameter will respond to protocol errors in an incoming
request without invoking the a <c>handle_request/3</c> callback.</p>
</desc>
</func>
</funcs>
</erlref>
