<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd" [
  <!ENTITY message '<seeerl marker="#message">message()</seeerl>'>
  <!ENTITY dict
  '<seefile marker="diameter_dict#MESSAGE_RECORDS">diameter_dict(4)</seefile>'>
  <!ENTITY % also SYSTEM "seealso.ent" >
  <!ENTITY % here SYSTEM "seehere.ent" >
  %also;
  %here;
]>

<erlref>
<header>

<copyright>
<year>2011</year>
<year>2020</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
 
    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

</legalnotice>

<title>diameter_app(3)</title>
<prepared>Anders Svensson</prepared>
<responsible></responsible>
<docno></docno>
<approved></approved>
<checked></checked>
<date></date>
<rev></rev>
<file>diameter_app.xml</file>

</header>

<module since="OTP R14B03">diameter_app</module>
<modulesummary>
Callback module of a Diameter application.</modulesummary>

<description>

<p>
A diameter service as started by &mod_start_service;
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
&man_dict; 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>

<list>
<item>
<p>
&peer_up; and &peer_down; signal the
attainment or loss of connectivity with a Diameter peer.</p>
</item>

<item>
<p>
&pick_peer;,
&prepare_request;,
&prepare_retransmit;,
&handle_answer;
and &handle_error; are (or may be) called as a consequence of a call
to &mod_call; to send an outgoing
Diameter request message.</p>
</item>

<item>
<p>
&handle_request;
is called in response to an incoming Diameter request message.</p>
</item>

</list>

<p>
The arities 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 &mod_start_service;
and, for the call-specific callbacks, any extra arguments passed to
&mod_call;.</p>

</description>

<!-- ===================================================================== -->
<!-- ===================================================================== -->

<section>
<title>DATA TYPES</title>

<taglist>

<tag>
<marker id="capabilities"/><c>capabilities() = #diameter_caps{}</c></tag>
<item>
<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>
</item>

<tag>
<marker id="message"/><c>message() = &codec_message;</c></tag>
<item>
<p>
The representation of a Diameter message as passed to
&mod_call; or returned from a &handle_request; callback.</p>

</item>

<tag>
<marker id="packet"/><c>packet() = &codec_packet;</c></tag>
<item>
<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>
</item>

<tag>
<marker id="peer_ref"/><c>peer_ref() = term()</c></tag>
<item>
<p>
A term identifying a transport connection with a Diameter peer.</p>
</item>

<tag>
<marker id="peer"/><c>peer() = {&peer_ref;, &capabilities;}</c></tag>
<item>
<p>
A tuple representing a Diameter peer connection.</p>
</item>

<tag>
<marker id="state"/><c>state() = term()</c></tag>
<item>
<p>
The state maintained by the application callback functions
&peer_up;, &peer_down; and (optionally) &pick_peer;.
The initial state is configured in the call to
&mod_start_service; 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 since="OTP R14B03">Mod:peer_up(SvcName, Peer, State) -> NewState</name>
<fsummary>Invoked when a transport connection has been established</fsummary>
<type>
<v>SvcName = &mod_service_name;</v>
<v>Peer    = &peer;</v>
<v>State   = NewState = &state;</v>
</type>
<desc>
<p>
Invoked to signal the availability of a peer connection on the local
Erlang node.
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 <c>OKAY</c> and Diameter
messages can be both sent and received.</p>

<note>
<p>
A watchdog state machine can reach state <c>OKAY</c> from state
<c>SUSPECT</c> without a new capabilities exchange taking place.
A new transport connection (and capabilities exchange) results in a
new peer_ref().</p>
</note>

<note>
<p>
There is no requirement that a callback return before incoming
requests are received: &handle_request; callbacks must be
handled independently of &peer_up; and &peer_down;.</p>
</note>

</desc>
</func>

<func>
<name since="OTP R14B03">Mod:peer_down(SvcName, Peer, State) -> NewState</name>
<fsummary>Invoked when a transport connection has been lost.</fsummary>
<type>
<v>SvcName = &mod_service_name;</v>
<v>Peer    = &peer;</v>
<v>State   = NewState = &state;</v>
</type>
<desc>
<p>
Invoked to signal that a peer connection on the local Erlang node is
no longer available following a previous call to &peer_up;.
In particular, that the RFC 3539 watchdog state machine for the
connection has left state <c>OKAY</c> and the peer will no longer be a
candidate in &pick_peer; callbacks.</p>

</desc>
</func>

<func>
<name since="OTP R14B03">Mod:pick_peer(LocalCandidates, RemoteCandidates, SvcName, State)
      -> Selection | false</name>
<fsummary>Select a target peer for an outgoing request.</fsummary>
<type>
<v>LocalCandidates = RemoteCandidates = [&peer;]</v>
<v>SvcName = &mod_service_name;</v>
<v>State = NewState = &state;</v>
<v>Selection = {ok, Peer} | {Peer, NewState}</v>
<v>Peer = &peer; | false</v>
</type>
<desc>
<p>
Invoked as a consequence of a call to &mod_call; to select a destination
peer for an outgoing request.
The return value indicates the selected peer.</p>

<p>
The candidate lists contain only those peers that have advertised
support for the Diameter application in question during capabilities
exchange, that have not be excluded by a <c>filter</c> option in
the call to &mod_call;
and whose watchdog state machine is in the <c>OKAY</c> 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 <c>{filter, {all, [host, realm]}}</c>
option to &mod_call;)
will be placed at the head of the list.
<c>LocalCandidates</c> contains peers whose transport process resides
on the local Erlang node while
<c>RemoteCandidates</c> contains peers that have been communicated
from other nodes by services of the same name.</p>

<p>
A callback that returns a peer() will be followed by a
&prepare_request;
callback and, if the latter indicates that the request should be sent,
by either &handle_answer;
or &handle_error; depending
on whether or not an answer message is received from the peer.
If the transport becomes unavailable after &prepare_request; then a
new &pick_peer; callback may take place to
failover to an alternate peer, after which &prepare_retransmit; takes the
place of &prepare_request; in resending the
request.
There is no guarantee that a &pick_peer; 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>
The return values <c>false</c> and <c>{false, State}</c> (that is,
<c>NewState = State</c>) are equivalent, as are <c>{ok, Peer}</c> and
<c>{Peer, State}</c>.</p>

<note>
<p>
The &mod_service_opt; <c>use_shared_peers</c> determines whether or
not a service uses peers shared from other nodes.
If not then <c>RemoteCandidates</c> is the empty list.</p>
</note>

<warning>
<p>
The return value <c>{Peer, NewState}</c> is only allowed if
the Diameter application in question was configured with the
&mod_application_opt; <c>{call_mutates_state, true}</c>.
Otherwise, the <c>State</c> argument is always
the initial value as configured on the application, not any subsequent
value returned by a &peer_up;
or &peer_down; callback.</p>
</warning>

</desc>

</func>

<func>
<name since="OTP R14B03">Mod:prepare_request(Packet, SvcName, Peer) -> Action</name>
<fsummary>Return a request for encoding and transport.</fsummary>
<type>
<v>Packet = &packet;</v>
<v>SvcName = &mod_service_name;</v>
<v>Peer = &peer;</v>
<v>Action = Send | Discard | {eval_packet, Action, PostF}</v>
<v>Send = {send, &packet; | &message;}</v>
<v>Discard = {discard, Reason} | discard</v>
<v>PostF = &mod_eval;}</v>
</type>
<desc>
<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 <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 process.
Extra arguments passed to &mod_call; can be used to
communicate transport (or any other) data to the callback.</p>

<p>
A returned &packet; can set
the <c>header</c> field to a
<c>#diameter_header{}</c> to specify values that should
be preserved in the outgoing request, values otherwise being those in
the header record contained in <c>Packet</c>.
A returned <c>length</c>, <c>cmd_code</c> or <c>application_id</c> is
ignored.</p>

<p>
A returned <c>PostF</c> will be evaluated on any encoded
<c>#diameter_packet{}</c> prior to transmission, the <c>bin</c> field
containing the encoded binary.
The return value is ignored.</p>

<p>
Returning <c>{discard, Reason}</c> causes the request to be aborted
and the &mod_call; 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>

</desc>
</func>

<func>
<name since="OTP R14B03">Mod:prepare_retransmit(Packet, SvcName, Peer) -> Action</name>
<fsummary>Return a request for encoding and retransmission.</fsummary>
<type>
<v>Packet  = &packet;</v>
<v>SvcName = &mod_service_name;</v>
<v>Peer = &peer;</v>
<v>Action = Send | Discard | {eval_packet, Action, PostF}</v>
<v>Send = {send, &packet; | &message;}</v>
<v>Discard = {discard, Reason} | discard</v>
<v>PostF = &mod_eval;}</v>
</type>
<desc>
<p>
Invoked to return a request for encoding and retransmission.
Has the same role as &prepare_request; in the case that
a peer connection is lost an an alternate peer selected but the
argument &packet; is as returned
by the initial &prepare_request;.</p>

<p>
Returning <c>{discard, Reason}</c> causes the request to be aborted
and a &handle_error; callback to
take place with <c>Reason</c> as initial argument.
Returning <c>discard</c> is equivalent to returning <c>{discard,
discarded}</c>.</p>

</desc>
</func>

<func>
<name since="OTP R14B03">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 = &mod_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 &mod_call; unless the
<c>detach</c> option was specified.</p>

<p>
The decoded answer record and undecoded binary are in the <c>msg</c>
and <c>bin</c> fields of the argument
&packet; respectively.
<c>Request</c> is the outgoing request message as was returned from
&prepare_request; or &prepare_retransmit;.</p>

<p>
For any given call to &mod_call; there is at most one
&handle_answer; callback: any
duplicate answer (due to retransmission or otherwise) is discarded.
Similarly, only one of &handle_answer; or
&handle_error; is called.</p>

<p>
By default, an incoming answer message that cannot be successfully
decoded causes the request process to fail, causing
&mod_call;
to return <c>{error, failure}</c> unless the <c>detach</c> option was
specified.
In particular, there is no &handle_error; callback in this
case.
The &mod_application_opt;
<c>answer_errors</c> can be set to change this behaviour.</p>

</desc>
</func>

<func>
<name since="OTP R14B03">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 = &mod_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
in response to an outgoing request.
The return value is returned from &mod_call; unless the
<c>detach</c> option was specified.</p>

<p>
Reason <c>timeout</c> indicates that an answer message has not been
received within the time specified with the corresponding &mod_call_opt;.
Reason <c>failover</c> 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>

</desc>
</func>

<func>
<name since="OTP R14B03">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
           | {relay, [Opt]}
           | discard
           | {eval|eval_packet, Action, PostF}</v>
<v>Reply   = {reply, &packet; | &message;}
           | {answer_message, 3000..3999|5000..5999}
           | {protocol_error, 3000..3999}</v>
<v>Opt     = &mod_call_opt;</v>
<v>PostF   = &mod_eval;</v>
</type>
<desc>
<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 &mod_start_service;)
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 &packet; has the following signature.</p>

<pre>
#diameter_packet{header = #diameter_header{},
                 avps   = [#diameter_avp{}],
                 msg    = record() | undefined,
                 errors = [&dict_Unsigned32; | {&dict_Unsigned32;, #diameter_avp{}}],
                 bin    = binary(),
                 transport_data = term()}
</pre>

<p>
The <c>msg</c> field will be <c>undefined</c> in case the request has
been received in the relay application.
Otherwise it contains the record representing the request as outlined
in &dict;.</p>

<p>
The <c>errors</c> field specifies any results codes identifying errors
found while decoding the request.
This is used to set Result-Code and/or Failed-AVP in a returned
answer unless the callback returns a <c>#diameter_packet{}</c>
whose <c>errors</c> field is set to either a non-empty list of its
own, in which case this list is used instead, or the atom <c>false</c>
to disable any setting of Result-Code and Failed-AVP.
Note that the errors detected by diameter are of the 3xxx
and 5xxx series, Protocol Errors and Permanent Failures respectively.
The <c>errors</c> 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 if a &message; is returned but must be set
explicitly in a returned &packet;.</p>

<p>
The semantics of each of the possible return values are as follows.</p>

<taglist>

<tag><c>{reply, &packet; | &message;}</c></tag>
<item>
<p>
Send the specified answer message to the peer.
In the case of a &packet;, the
message to be sent must be set in the
<c>msg</c> field and the <c>header</c> field can be set to a
<c>#diameter_header{}</c> to specify values that should be
preserved in the outgoing answer, appropriate values otherwise
being set by diameter.</p>
</item>

<tag><c>{answer_message, 3000..3999|5000..5999}</c></tag>
<item>
<p>
Send an answer message to the peer containing the specified
Result-Code.
Equivalent to</p>
<pre>
{reply, ['answer-message' | Avps]
</pre>
<p>
where <c>Avps</c> sets the Origin-Host, Origin-Realm, the specified
Result-Code and (if the request contained one) Session-Id AVPs, and
possibly Failed-AVP as described below.</p>

<p>
Returning a value other than 3xxx or 5xxx will cause the request
process in question to fail, as will returning a 5xxx value if the
peer connection in question has been configured with the RFC 3588
common dictionary <c>diameter_gen_base_rfc3588</c>.
(Since RFC 3588 only allows 3xxx values in an answer-message.)</p>

<p>
When returning 5xxx, Failed-AVP will be populated with the AVP of the
first matching Result-Code/AVP pair in the <c>errors</c> field of the
argument &packet;, if found.
If this is not appropriate then an answer-message should be
constructed explicitly and returned in a <c>reply</c> tuple
instead.</p>
</item>

<tag><c>{relay, Opts}</c></tag>
<item>
<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 &pick_peer;
and subsequent callbacks take place just as if &mod_call; 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 <c>Opts</c> should not specify <c>detach</c>.
A subsequent &handle_answer;
callback for the relayed request must return its first
argument, the &packet; containing the answer message.
Note that the <c>extra</c> 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
&handle_error; callback)
causes the request to be answered with 3002 (DIAMETER_UNABLE_TO_DELIVER).</p>
</item>

<tag><c>discard</c></tag>
<item>
<p>
Discard the request.
No answer message is sent to the peer.</p>
</item>

<tag><c>{eval, Action, PostF}</c></tag>
<item>
<p>
Handle the request as if <c>Action</c> has been returned and then
evaluate <c>PostF</c> in the request process.
The return value is ignored.</p>
</item>

<tag><c>{eval_packet, Action, PostF}</c></tag>
<item>
<p>
Like <c>eval</c> but evaluate <c>PostF</c> on any encoded
<c>#diameter_packet{}</c> prior to transmission, the <c>bin</c> field
containing the encoded binary.
The return value is ignored.</p>
</item>

<tag><c>{protocol_error, 3000..3999}</c></tag>
<item>
<p>
Equivalent to <c>{answer_message, 3000..3999}</c>.</p>
</item>

</taglist>

<note>
<p>
Requests containing errors may be answered by diameter, without a
callback taking place, depending on the value of the
&mod_application_opt; <c>request_errors</c>.</p>
</note>

</desc>
</func>

</funcs>

</erlref>
