docs/user-guide.rst

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============================================================================================ 
User's Guide 
============================================================================================ 
-------------------------------------------------------------------------------------------- 
RIC Message Router -- RMR 
-------------------------------------------------------------------------------------------- 


Overview
========

The RIC Message Router (RMR) is a library for peer-to-peer 
communication. Applications use the library to send and 
receive messages where the message routing and endpoint 
selection is based on the message type rather than DNS host 
name-IP port combinations. The library provides the following 
major features: 
 
 
* Routing and endpoint selection is based on *message type.* 
   
* Application is insulated from the underlying transport 
  mechanism and/or protocols. 
   
* Message distribution (round robin or fanout) is selectable 
  by message type. 
   
* Route management updates are received and processed 
  asynchronously and without overt application involvement. 
 
 


Purpose
-------

RMR's main purpose is to provide an application with the 
ability to send and receive messages to/from other peer 
applications with minimal effort on the application's part. 
To achieve this, RMR manages all endpoint information, 
connections, and routing information necessary to establish 
and maintain communication. From the application's point of 
view, all that is required to send a message is to allocate 
(via RMR) a message buffer, add the payload data, and set the 
message type. To receive a message, the application needs 
only to invoke the receive function; when a message arrives a 
message buffer will be returned as the function result. 


Message Routing
---------------

Applications are required to place a message type into a 
message before sending, and may optionally add a subscription 
ID when appropriate. The combination of message type, and 
subscription ID are refered to as the *message key,* and is 
used to match an entry in a routing table which provides the 
possible endpoints expecting to receive messages with the 
matching key. 


Round Robin Delivery
--------------------

An endpoint from RMR's perspective is an application to which 
RMR may establish a connection, and expect to send messages 
with one or more defined message keys. Each entry in the 
route table consists of one or more endpoint groups, called 
round robin groups. When a message matches a specific entry, 
the entry's groups are used to select the destination of the 
message. A message is sent once to each group, with messages 
being *balanced* across the endpoints of a group via round 
robin selection. Care should be taken when defining multiple 
groups for a message type as there is extra overhead required 
and thus the overall message latency is somewhat increased. 


Routing Table Updates
---------------------

Route table information is made available to RMR a static 
file (loaded once), or by updates sent from a separate route 
manager application. If a static table is provided, it is 
loaded during RMR initialization and will remain in use until 
an external process connects and delivers a route table 
update (often referred to as a dynamic update). Dynamic 
updates are listened for in a separate process thread and 
applied automatically; the application does not need to allow 
for, or trigger, updates. 


Latency And Throughput
----------------------

While providing insulation from the underlying message 
transport mechanics, RMR must also do so in such a manner 
that message latency and throughput are not impacted. In 
general, the RMR induced overhead, incurred due to the 
process of selecting an endpoint for each message, is minimal 
and should not impact the overall latency or throughput of 
the application. This impact has been measured with test 
applications running on the same physical host and the 
average latency through RMR for a message was on the order of 
0.02 milliseconds. 
 
As an application's throughput increases, it becomes easy for 
the application to overrun the underlying transport mechanism 
(e.g. NNG), consume all available TCP transmit buffers, or 
otherwise find itself in a situation where a send might not 
immediately complete. RMR offers different *modes* which 
allow the application to manage these states based on the 
overall needs of the application. These modes are discussed 
in the *Configuration* section of this document. 


General Use
===========

To use, the RMR based application simply needs to initialise 
the RMR environment, wait for RMR to have received a routing 
table (become ready), and then invoke either the send or 
receive functions. These steps, and some behind the scenes 
details, are described in the following paragraphs. 


Initialisation
--------------

The RMR function ``rmr_init()`` is used to set up the RMR 
environment and must be called before messages can be sent or 
received. One of the few parameters that the application must 
communicate to RMR is the port number that will be used as 
the listen port for new connections. The port number is 
passed on the initialisation function call and a TCP listen 
socket will be opened with this port. If the port is already 
in use RMR will report a failure; the application will need 
to reinitialise with a different port number, abort, or take 
some other action appropriate for the application. 
 
In addition to creating a TCP listen port, RMR will start a 
process thread which will be responsible for receiving 
dynamic updates to the route table. This thread also causes a 
TCP listen port to be opened as it is expected that the 
process which generates route table updates will connect and 
send new information when needed. The route table update port 
is **not** supplied by the application, but is supplied via 
an environment variable as this value is likely determined by 
the mechanism which is starting and configuring the 
application. 


The RMR Context
---------------

On successful initialisation, a void pointer, often called a 
*handle* by some programming languages, is returned to the 
application. This is a reference to the RMR control 
information and must be passed as the first parameter on most 
RMR function calls. RMR refers to this as the context, or 
ctx. 


Wait For Ready
--------------

An application which is only receiving messages does not need 
to wait for RMR to *become ready* after the call to the 
initialization function. However, before the application can 
successfully send a message, RMR must have loaded a route 
table, and the application must wait for RMR to report that 
it has done so. The RMR function ``rmr_ready()`` will return 
the value *true* (1) when a complete route table has been 
loaded and can be used to determine the endpoint for a send 
request. 


Receiving Messages
------------------

The process of receiving is fairly straight forward. The 
application invokes the RMR ``rmr_rcv_msg()`` function which 
will block until a message is received. The function returns 
a pointer to a message block which provides all of the 
details about the message. Specifically, the application has 
access to the following information either directly or 
indirectly: 
 
 
* The payload (actual data) 
   
* The total payload length in bytes 
   
* The number of bytes of the payload which contain valid data 
   
* The message type and subscription ID values 
   
* The hostname and IP address of the source of the message 
  (the sender) 
   
* The transaction ID 
   
* Tracing data (if provided) 
 
 


The Message Payload
-------------------

The message payload contains the *raw* data that was sent by 
the peer application. The format will likely depend on the 
message type, and is expected to be known by the application. 
A direct pointer to the payload is available from the message 
buffer (see appendix B for specific message buffer details). 
 
Two payload-related length values are also directly 
available: the total payload length, and the number of bytes 
actually filled with data. The used length is set by the 
caller, and may or not be an accurate value. The total 
payload length is determined when the buffer is created for 
sending, and is the maximum number of bytes that the 
application may modify should the buffer be used to return a 
response. 


Message Type and Subscription ID
--------------------------------

The message type and subscription ID are both directly 
available from the message buffer, and are the values which 
were used to by RMR in the sending application to select the 
endpoint. If the application resends the message, as opposed 
to returning the message buffer as a response, the message 
number and/or the subscription ID might need to be changed to 
avoid potential issues[1]. 


Sender Information
------------------

The source, or sender information, is indirectly available to 
the application via the ``rmr_get_src()`` and 
``rmr_get_ip()`` functions. The former returns a string 
containing ``hostname:port,`` while the string 
``ip:port`` is returned by the latter. 


Transaction ID
--------------

The message buffer contains a fixed length set of bytes which 
applications can set to track related messages across the 
application concept of a transaction. RMR will use the 
transaction ID for matching a response message when the 
``rmr_call()`` function is used to send a message. 


Trace Information
-----------------

RMR supports the addition of an optional trace information to 
any message. The presence and size is controlled by the 
application, and can vary from message to message if desired. 
The actual contents of the trace information is determined by 
the application; RMR provides only the means to set, extract, 
and obtain a direct reference to the trace bytes. The trace 
data field in a message buffer is discussed in greater detail 
in the *Trace Data* section. 


Sending Messages
----------------

Sending requires only slightly more work on the part of the 
application than receiving a message. The application must 
allocate an RMR message buffer, populate the message payload 
with data, set the message type and length, and optionally 
set the subscription ID. Information such as the source IP 
address, hostname, and port are automatically added to the 
message buffer by RMR, so there is no need for the 
application to worry about these. 


Message Buffer Allocation
-------------------------

The function ``rmr_msg_alloc()`` allocates a *zero copy* 
buffer and returns a pointer to the RMR ``rmr_mbuf_t`` 
structure. The message buffer provides direct access to the 
payload, length, message type and subscription ID fields. The 
buffer must be preallocated in order to allow the underlying 
transport mechanism to allocate the payload space from its 
internal memory pool; this eliminates multiple copies as the 
message is sent, and thus is more efficient. 
 
If a message buffer has been received, and the application 
wishes to use the buffer to send a response, or to forward 
the buffer to another application, a new buffer does **not** 
need to be allocated. The application may set the necessary 
information (message type, etc.), and adjust the payload, as 
is necessary and then pass the message buffer to 
``rmr_send_msg()`` or ``rmr_rts_msg()`` to be sent or 
returned to the sender. 


Populating the Message Buffer
-----------------------------

The application has direct access to several of the message 
buffer fields, and should set them appropriately. 
 
 
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      * - **len** 
        - 
          This is the number of bytes that the application placed into 
          the payload. Setting length to 0 is allowed, and length may 
          be less than the allocated payload size. 
       
      * - **mtype** 
        - 
          The message type that RMR will use to determine the endpoint 
          used as the target of the send. 
       
      * - **sub_id** 
        - 
          The subscription ID if the message is to be routed based on 
          the combination of message type and subscription ID. If no 
          subscription ID is valid for the message, the application 
          should set the field with the RMR constant 
          ``RMR_VOID_SUBID.`` 
       
      * - **payload** 
        - 
          The application should obtain the reference (pointer) to the 
          payload from the message buffer and place any data into the 
          payload. The application is responsible for ensuring that the 
          maximum payload size is not exceeded. The application may 
          obtain the maximum size via the ``rmr_payload_size()`` 
          function. 
       
      * - **trace data** 
        - 
          Optionally, the application may add trace information to the 
          message buffer. 
           
 
 


Sending a Message Buffer
------------------------

Once the application has populated the necessary bits of a 
message, it may be sent by passing the buffer to the 
``rmr_send_msg()`` function. This function will select an 
endpoint to receive the message, based on message type and 
subscription ID, and will pass the message to the underlying 
transport mechanism for actual transmission on the 
connection. (Depending on the underlying transport mechanism, 
the actual connection to the endpoint may happen at the time 
of the first message sent to the endpoint, and thus the 
latency of the first send might be longer than expected.) 
 
On success, the send function will return a reference to a 
message buffer; the status within that message buffer will 
indicate what the message buffer contains. When the status is 
``RMR_OK`` the reference is to a **new** message buffer for 
the application to use for the next send; the payload size is 
the same as the payload size allocated for the message that 
was just sent. This is a convenience as it eliminates the 
need for the application to call the message allocation 
function at some point in the future, and assumes the 
application will send many messages which will require the 
same payload dimensions. 
 
If the message contains any status other than ``RMR_OK,`` 
then the message could **not** be sent, and the reference is 
to the unsent message buffer. The value of the status will 
indicate whether the nature of the failure was transient ( 
``RMR_ERR_RETRY``) or not. Transient failures are likely to 
be successful if the application attempts to send the message 
at a later time. Unfortunately, it is impossible for RMR to 
know the exact transient failure (e.g. connection being 
established, or TCP buffer shortage), and thus it is not 
possible to communicate how long the application should wait 
before attempting to resend, if the application wishes to 
resend the message. (More discussion with respect to message 
retries can be found in the *Handling Failures* section.) 


Advanced Usage
==============

Several forms of usage fall into a more advanced category and 
are described in the following sections. These include 
blocking call, return to sender and wormhole functions. 


The Call Function
-----------------

The RMR function ``rmr_call()`` sends a message in the exact 
same manner as the ``rmr_send_msg()()`` function, with the 
endpoint selection based on the message key. But unlike the 
send function, ``rmr_call()`` will block and wait for a 
response from the application that is selected to receive the 
message. The matching message is determined by the 
transaction ID which the application must place into the 
message buffer prior to invoking ``rmr_call()``. Similarly, 
the responding application must ensure that the same 
transaction ID is placed into the message buffer before 
returning its response. 
 
The return from the call is a message buffer with the 
response message; there is no difference between a message 
buffer returned by the receive function and one returned by 
the ``rmr_call()`` function. If a response is not received in 
a reasonable amount of time, a nil message buffer is returned 
to the calling application. 


Returning a Response
--------------------

Because of the nature of RMR's routing policies, it is 
generally not possible for an application to control exactly 
which endpoint is sent a message. There are cases, such as 
responding to a message delivered via ``rmr_call()`` that the 
application must send a message and guarantee that RMR routes 
it to an exact destination. To enable this, RMR provides the 
``rmr_rts_msg(),`` return to sender, function. Upon receipt 
of any message, an application may alter the payload, and if 
necessary the message type and subscription ID, and pass the 
altered message buffer to the ``rmr_rts_msg()`` function to 
return the altered message to the application which sent it. 
When this function is used, RMR will examine the message 
buffer for the source information and use that to select the 
connection on which to write the response. 


Multi-threaded Calls
--------------------

The basic call mechanism described above is **not** thread 
safe, as it is not possible to guarantee that a response 
message is delivered to the correct thread. The RMR function 
``rmr_mt_call()`` accepts an additional parameter which 
identifies the calling thread in order to ensure that the 
response is delivered properly. In addition, the application 
must specifically initialise the multi-threaded call 
environment by passing the ``RMRFL_MTCALL`` flag as an option 
to the ``rmr_init()`` function. 
 
One advantage of the multi-threaded call capability in RMR is 
the fact that only the calling thread is blocked. Messages 
received which are not responses to the call are continued to 
be delivered via normal ``rmr_rcv_msg()`` calls. 
 
While the process is blocked waiting for the response, it is 
entirely possible that asynchronous, non-matching, messages 
will arrive. When this happens, RMR will queues the messages 
and return them to the application over the next calls to 
``rmr_rcv_msg().`` 


Wormholes
---------

As was mentioned earlier, the design of RMR is to eliminate 
the need for an application to know a specific endpoint, even 
when a response message is being sent. In some rare cases it 
may be necessary for an application to establish a direct 
connection to an RMR-based application rather than relying on 
message type and subscription ID based routing. The 
*wormhole* functions provide an application with the ability 
to create a direct connection and then to send and receive 
messages across the connection. The following are the RMR 
functions which provide wormhole communications: 
 
 
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      * - **rmr_wh_open** 
        - 
          Open a connection to an endpoint. Name or IP address and port 
          of the endpoint is supplied. Returns a wormhole ID that the 
          application must use when sending a direct message. 
       
      * - **rmr_wh_send_msg** 
        - 
          Sends an RMR message buffer to the connected application. The 
          message type and subscription ID may be set in the message, 
          but RMR will ignore both. 
       
      * - **rmr_wh_close** 
        - 
          Closes the direct connection. 
           
 
 


Handling Failures
=================

The vast majority of states reported by RMR are fatal; if 
encountered during setup or initialization, then it is 
unlikely that any message oriented processing should 
continue, and when encountered on a message operation 
continued operation on that message should be abandoned. 
Specifically with regard to message sending, it is very 
likely that the underlying transport mechanism will report a 
*soft,* or transient, failure which might be successful if 
the operation is retried at a later point in time. The 
paragraphs below discuss the methods that an application 
might deal with these soft failures. 


Failure Notification
--------------------

When a soft failure is reported, the returned message buffer 
returned by the RMR function will be ``RMR_ERR_RETRY.`` These 
types of failures can occur for various reasons; one of two 
reasons is typically the underlying cause: 
 
 
* The session to the targeted recipient (endpoint) is not 
  connected. 
   
* The transport mechanism buffer pool is full and cannot 
  accept another buffer. 
   
 
 
Unfortunately, it is not possible for RMR to determine which 
of these two cases is occurring, and equally as unfortunate 
the time to resolve each is different. The first, no 
connection, may require up to a second before a message can 
be accepted, while a rejection because of buffer shortage is 
likely to resolve in less than a millisecond. 


Application Response
--------------------

The action which an application takes when a soft failure is 
reported ultimately depends on the nature of the application 
with respect to factors such as tolerance to extended message 
latency, dropped messages, and over all message rate. 


RMR Retry Modes
---------------

In an effort to reduce the workload of an application 
developer, RMR has a default retry policy such that RMR will 
attempt to retransmit a message up to 1000 times when a soft 
failure is reported. These retries generally take less than 1 
millisecond (if all 1000 are attempted) and in most cases 
eliminates nearly all reported soft failures to the 
application. When using this mode, it might allow the 
application to simply treat all bad return values from a send 
attempt as permanent failures. 
 
If an application is so sensitive to any delay in RMR, or the 
underlying transport mechanism, it is possible to set RMR to 
return a failure immediately on any kind of error (permanent 
failures are always reported without retry). In this mode, 
RMR will still set the state in the message buffer to 
``RMR_ERR_RETRY,`` but will **not** make any attempts to 
resend the message. This zero-retry policy is enabled by 
invoking the ``rmr_set_stimeout()`` with a value of 0; this 
can be done once immediately after ``rmr_init()`` is invoked. 
 
Regardless of the retry mode which the application sets, it 
will ultimately be up to the application to handle failures 
by queuing the message internally for resend, retrying 
immediately, or dropping the send attempt all together. As 
stated before, only the application can determine how to best 
handle send failures. 


Other Failures
--------------

RMR will return the state of processing for message based 
operations (send/receive) as the status in the message 
buffer. For non-message operations, state is returned to the 
caller as the integer return value for all functions which 
are not expected to return a pointer (e.g. 
``rmr_init()``.) The following are the RMR state constants 
and a brief description of their meaning. 
 
 
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      * - **RMR_OK** 
        - 
          state is good; operation finished successfully 
       
      * - **RMR_ERR_BADARG** 
        - 
          argument passed to function was unusable 
       
      * - **RMR_ERR_NOENDPT** 
        - 
          send/call could not find an endpoint based on msg type 
       
      * - **RMR_ERR_EMPTY** 
        - 
          msg received had no payload; attempt to send an empty message 
       
      * - **RMR_ERR_NOHDR** 
        - 
          message didn't contain a valid header 
       
      * - **RMR_ERR_SENDFAILED** 
        - 
          send failed; errno may contain the transport provider reason 
       
      * - **RMR_ERR_CALLFAILED** 
        - 
          unable to send the message for a call function; errno may 
          contain the transport provider reason 
       
      * - **RMR_ERR_NOWHOPEN** 
        - 
          no wormholes are open 
       
      * - **RMR_ERR_WHID** 
        - 
          the wormhole id provided was invalid 
       
      * - **RMR_ERR_OVERFLOW** 
        - 
          operation would have busted through a buffer/field size 
       
      * - **RMR_ERR_RETRY** 
        - 
          request (send/call/rts) failed, but caller should retry 
          (EAGAIN for wrappers) 
       
      * - **RMR_ERR_RCVFAILED** 
        - 
          receive failed (hard error) 
       
      * - **RMR_ERR_TIMEOUT** 
        - 
          response message not received in a reasonable amount of time 
       
      * - **RMR_ERR_UNSET** 
        - 
          the message hasn't been populated with a transport buffer 
       
      * - **RMR_ERR_TRUNC** 
        - 
          length in the received buffer is longer than the size of the 
          allocated payload, received message likely truncated (length 
          set by sender could be wrong, but we can't know that) 
       
      * - **RMR_ERR_INITFAILED** 
        - 
          initialisation of something (probably message) failed 
       
      * - **RMR_ERR_NOTSUPP** 
        - 
          the request is not supported, or RMR was not initialised for 
          the request 
           
 
 
Depending on the underlying transport mechanism, and the 
nature of the call that RMR attempted, the system 
``errno`` value might reflect additional detail about the 
failure. Applications should **not** rely on errno as some 
transport mechanisms do not set it with any consistency. 


Configuration and Control
=========================

With the assumption that most RMR based applications will be 
executed in a containerised environment, there are some 
underlying mechanics which the developer may need to know in 
order to properly provide a configuration specification to 
the container management system. The following paragraphs 
briefly discuss these. 
 


TCP Ports
---------

RMR requires two (2) TCP listen ports: one for general 
application-to-application communications and one for 
route-table updates. The general communication port is 
specified by the application at the time RMR is initialised. 
The port used to listen for route table updates is likely to 
be a constant port shared by all applications provided they 
are running in separate containers. To that end, the port 
number defaults to 4561, but can be configured with an 
environment variable (see later paragraph in this section). 


Host Names
----------

RMR is typically host name agnostic. Route table entries may 
contain endpoints defined either by host name or IP address. 
In the container world the concept of a *service name* might 
exist, and likely is different than a host name. RMR's only 
requirement with respect to host names is that a name used on 
a route table entry must be resolvable via the 
``gethostbyname`` system call. 


Environment Variables
---------------------

Several environment variables are recognised by RMR which, in 
general, are used to define interfaces and listen ports (e.g. 
the route table update listen port), or debugging 
information. Generally this information is system controlled 
and thus RMR expects this information to be defined in the 
environment rather than provided by the application. The 
following is a list of the environment variables which RMR 
recognises: 
 
 
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      * - **RMR_BIND_IF** 
        - 
          The interface to bind to listen ports to. If not defined 
          0.0.0.0 (all interfaces) is assumed. 
       
      * - **RMR_RTG_SVC** 
        - 
          The port RMR will listen on for route manager connections. If 
          not defined 4561 is used. 
       
      * - **RMR_SEED_RT** 
        - 
          Where RMR expects to find the name of the seed (static) route 
          table. If not defined no static table is read. 
       
      * - **RMR_RTG_ISRAW** 
        - 
          If the value set to 0, RMR expects the route table manager 
          messages to be messages with and RMR header. If this is not 
          defined messages are assumed to be "raw" (without an RMR 
          header. 
       
      * - **RMR_VCTL_FILE** 
        - 
          Provides a file which is used to set the verbose level of the 
          route table collection thread. The first line of the file is 
          read and expected to contain an integer value to set the 
          verbose level. The value may be changed at any time and the 
          route table thread will adjust accordingly. 
       
      * - **RMR_SRC_NAMEONLY** 
        - 
          If the value of this variable is greater than 0, RMR will not 
          permit the IP address to be sent as the message source. Only 
          the host name will be sent as the source in the message 
          header. 
           
 
 


Logging
-------

RMR does **not** use any logging libraries; any error or 
warning messages are written to standard error. RMR messages 
are written with one of three prefix strings: 
 
 
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      * - **[CRI]** 
        - 
          The event is of a critical nature and it is unlikely that RMR 
          will continue to operate correctly if at all. It is almost 
          certain that immediate action will be needed to resolve the 
          issue. 
       
      * - **[ERR]** 
        - 
          The event is not expected and RMR is not able to handle it. 
          There is a small chance that continued operation will be 
          negatively impacted. Eventual action to diagnose and correct 
          the issue will be necessary. 
       
      * - **[WRN]** 
        - 
          The event was not expected by RMR, but can be worked round. 
          Normal operation will continue, but it is recommended that 
          the cause of the problem be investigated. 
           
 
 


Notes
=====

 
 [1] It is entirely possible to design a routing table, and 
 application group, such that the same message type is is 
 left unchanged and the message is forwarded by an 
 application after updating the payload. This type of 
 behaviour is often referred to as service chaining, and can 
 be done without any "knowledge" by an application with 
 respect to where the message goes next. Service chaining is 
 supported by RMR in as much as it allows the message to be 
 resent, but the actual complexities of designing and 
 implementing service chaining lie with the route table 
 generator process. 
 
 
 
 


Appendix A -- Quick Reference
=============================

Please  refer  to  the RMR manual pages on the Read the Docs 
site 
 
https://docs.o-ran-sc.org/projects/o-ran-sc-ric-plt-lib-rmr/en/latest/index.html 
 


Appendix B -- Message Buffer Details
====================================

The RMR message buffer is a C structure which is exposed  in 
the  ``rmr.h``  header  file. It is used to manage a message 
received from a peer endpoint, or a message  that  is  being 
sent  to  a  peer.  Fields include payload length, amount of 
payload actually  used,  status,  and  a  reference  to  the 
payload.  There are also fields which the application should 
ignore, and could be hidden in the header file, but we chose 
not  to.  These fields include a reference to the RMR header 
information,  and  to  the  underlying  transport  mechanism 
message  struct  which may or may not be the same as the RMR 
header reference. 


The Structure
-------------

The following is the C structure. Readers are  cautioned  to 
examine  the ``rmr.h`` header file directly; the information 
here may be out of date (old document in  some  cache),  and 
thus it may be incorrect. 
 
 
:: 
 
   
  typedef struct {
      int    state;            // state of processing
      int    mtype;            // message type
      int    len;              // length of data in the payload (send or received)
      unsigned char* payload;  // transported data
      unsigned char* xaction;  // pointer to fixed length transaction id bytes
      int    sub_id;           // subscription id
      int    tp_state;         // transport state (errno)
   
                               // these things are off limits to the user application
      void*    tp_buf;         // underlying transport allocated pointer (e.g. nng message)
      void*    header;         // internal message header (whole buffer: header+payload)
      unsigned char* id;       // if we need an ID in the message separate from the xaction id
      int      flags;          // various MFL_ (private) flags as needed
      int      alloc_len;      // the length of the allocated space (hdr+payload)
      void*    ring;           // ring this buffer should be queued back to
      int      rts_fd;         // SI fd for return to sender
      int      cookie;         // cookie to detect user misuse of free'd msg
  } rmr_mbuf_t;
 
 


State vs Transport State
------------------------

The  state  field reflects the state at the time the message 
buffer is returned to the calling application.  For  a  send 
operation,  if  the state is not ``RMR_OK`` then the message 
buffer references the payload that could not  be  sent,  and 
when the state is ``RMR_OK`` the buffer references a *fresh* 
payload that the application may fill in. 
 
When the state is not ``RMR_OK,`` C programmes  may  examine 
the  global ``errno`` value which RMR will have left set, if 
it was set, by the underlying transport mechanism.  In  some 
cases,  wrapper  modules are not able to directly access the 
C-library ``errno``  value,  and  to  assist  with  possible 
transport  error  details,  the  send and receive operations 
populate ``tp_state`` with the value of ``errno.`` 
 
Regardless of whether  the  application  makes  use  of  the 
``tp_state,`` or the ``errno`` value, it should be noted that 
the underlying transport mechanism may not  actually  update 
the errno value; in other words: it might not be accurate. In 
addition, RMR populates the ``tp_state`` value in the message 
buffer **only** when the state is not ``RMR_OK.`` 


Field References
----------------

The  transaction  field  was exposed in the first version of 
RMR, and in hindsight this shouldn't have been done.  Rather 
than  break  any  existing  code the reference was left, but 
additional fields such as  trace  data,  were  not  directly 
exposed  to  the  application.  The application developer is 
strongly encouraged to use the functions which get  and  set 
the  transaction  ID rather than using the pointer directly; 
any data overruns will not be detected if the  reference  is 
used directly. 
 
In contrast, the payload reference should be used directly by 
the application  in  the  interest  of  speed  and  ease  of 
programming.  The same care to prevent writing more bytes to 
the payload buffer than it can hold must  be  taken  by  the 
application.  By the nature of the allocation of the payload 
in transport space, RMR is unable to add guard bytes  and/or 
test for data overrun. 


Actual Transmission
-------------------

When RMR sends the application's message, the message buffer 
is **not** transmitted. The transport buffer (tp_buf)  which 
contains  the RMR header and application payload is the only 
set of bytes which are transmitted. While it may seem to the 
caller  like  the function ``rmr_send_msg()`` is returning a 
new message buffer, the same struct is reused and only a new 
transport  buffer  is  allocated.  The intent is to keep the 
alloc/free cycles to a minimum. 
 


Appendix C -- Glossary
======================

Many terms in networking can be  interpreted  with  multiple 
meanings, and several terms used in various RMR documentation 
are RMR specific. The following definitions are the meanings 
of  terms  used within RMR documentation and should help the 
reader to understand the intent of meaning. 
 
    .. list-table:: 
      :widths: 25,70 
      :header-rows: 0 
      :class: borderless 
       
      * - **application** 
        - 
          A programme which uses RMR to send and/or  receive  messages 
          to/from another RMR based application. 
       
      * - **Critical error** 
        - 
          An error that RMR has encountered which will prevent further 
          successful  processing  by  RMR.  Critical  errors  usually  
          indicate that the application should abort. 
       
      * - **Endpoint** 
        - 
          An RMR based application that is defined as being capable of 
          receiving one or more types of messages  (as  defined  by  a 
          *message key.*) 
       
      * - **Environment variable** 
        - 
          A key/value pair which is set externally to the application, 
          but which is available to the  application  (and  referenced 
          libraries)  through  the ``getenv`` system call. Environment 
          variables are the main method of  communicating  information 
          such as port numbers to RMR. 
       
      * - **Error** 
        - 
          An abnormal condition that RMR has encountered, but will not 
          affect the overall processing by RMR, but may impact certain 
          aspects  such  as the ability to communicate with a specific 
          endpoint. Errors generally indicate that something,  usually 
          external to RMR, must be addressed. 
       
      * - **Host name** 
        - 
          The  name  of  the host as returned by the ``gethostbyname`` 
          system call. In a containerised environment this might be the 
          container  or service name depending on how the container is 
          started. From RMR's point of view, a host name can be used to 
          resolve an *endpoint* definition in a *route* table.) 
       
      * - **IP** 
        - 
          Internet  protocol.  A low level transmission protocol which 
          governs   the  transmission  of  datagrams  across  network  
          boundaries. 
       
      * - **Listen socket** 
        - 
          A  *TCP*  socket used to await incoming connection requests. 
          Listen sockets are defined by an interface and  port  number 
          combination  where  the  port  number  is  unique  for  the  
          interface. 
       
      * - **Message** 
        - 
          A series of bytes transmitted from the application to another 
          RMR based application. A message is comprised of RMR specific 
          data (a header), and application data (a payload). 
       
      * - **Message buffer** 
        - 
          A data structure used to describe a message which is  to  be 
          sent  or  has been received. The message buffer includes the 
          payload length, message  type,  message  source,  and  other 
          information. 
       
      * - **Messgae type** 
        - 
          A  signed  integer  (0-32000)  which  identifies the type of 
          message being transmitted, and is one of the two  components 
          of a *routing key.* See *Subscription ID.* 
       
      * - **Payload** 
        - 
          The  portion  of  a  message which holds the user data to be 
          transmitted to the remote *endpoint.* The  payload  contents 
          are completely application defined. 
       
      * - **RMR context** 
        - 
          A  set of information which defines the current state of the 
          underlying transport connections that RMR is  managing.  The 
          application  will be give a context reference (pointer) that 
          is supplied to most RMR functions as the first parameter. 
       
      * - **Round robin** 
        - 
          The method of selecting an *endpoint* from a list such  that 
          all  *endpoints* are selected before starting at the head of 
          the list. 
       
      * - **Route table** 
        - 
          A series of "rules" which define the possible *endpoints* for 
          each *message key.* 
       
      * - **Route table manager** 
        - 
          An  application responsible for building a *route table* and 
          then   distributing   it   to   all  applicable  RMR  based  
          applications. 
       
      * - **Routing** 
        - 
          The  process  of  selecting  an *endpoint* which will be the 
          recipient of a message. 
       
      * - **Routing key** 
        - 
          A combination of *message type* and *subscription ID*  which 
          RMR uses to select the destination *endpoint* when sending a 
          message. 
       
      * - **Source** 
        - 
          The sender of a message. 
       
      * - **Subscription ID** 
        - 
          A  signed  integer  value  (0-32000)  which  identifies  the 
          subscription  characteristic  of  a  message.  It is used in 
          conjunction with the *message type* to determine the *routing 
          key.* 
       
      * - **Target** 
        - 
          The *endpoint* selected to receive a message. 
       
      * - **TCP** 
        - 
          Transmission  Control  Protocol. A connection based internet 
          protocol which provides for lossless packet  transportation, 
          usually over IP. 
       
      * - **Thread** 
        - 
          Also  called  a  *process  thread,  or  pthread.*  This is a 
          lightweight process which executes in concurrently with  the 
          application  and  shares  the  same  address space. RMR uses 
          threads to manage asynchronous functions such as route table 
          updates. &Term An optional portion of the message buffer that 
          the application may  populate  with  data  that  allows  for 
          tracing  the  progress  of  the  transaction  or application 
          activity across components. RMR makes no use of this data. 
       
      * - **Transaction ID** 
        - 
          A fixed number of bytes in the *message* buffer)  which  the 
          application  may  populate  with  information related to the 
          transaction. RMR makes use of the transaction ID for matching 
          response  messages  with  the  &c function is used to send a 
          message. 
       
      * - **Transient failure** 
        - 
          An error state that is believed to be short lived  and  that 
          the  operation,  if  retried  by  the  application, might be 
          successful.   C   programmers   will   recognise   this  as  
          ``EAGAIN.`` 
       
      * - **Warning** 
        - 
          A  warning occurs when RMR has encountered something that it 
          believes isn't correct, but has a defined work round. 
       
      * - **Wormhole** 
        - 
          A  direct  connection  managed  by  RMR  between  the  user  
          application and a remote, RMR based, application. 
           
 
 


Appendix D -- Code Examples
===========================

The  following  snippet of code illustrate some of the basic 
operation of the RMR library. Please refer to  the  examples 
and  test directories in the RMR repository for complete RMR 
based programmes. 


Sender Sample
-------------

The following code segment shows how a message buffer can be 
allocated, populated, and sent. The snippet also illustrates 
how the result from the ``rmr_send_msg()`` function is  used 
to send the next message. It does not illustrate error and/or 
retry handling. 
 
 
:: 
 
   
  #include <unistd.h>
  #include <errno.h>
  #include <string.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <sys/epoll.h>
  #include <time.h>
   
  #include <rmr/rmr.h>
   
  int main( int argc, char** argv ) {
      void* mrc;                            // msg router context
      struct epoll_event events[1];        // list of events to give to epoll
      struct epoll_event epe;                // event definition for event to listen to
      int     ep_fd = -1;                    // epoll's file des (given to epoll_wait)
      int rcv_fd;                            // file des for epoll checks
      int nready;                            // number of events ready for receive
      rmr_mbuf_t*        sbuf;                // send buffer
      rmr_mbuf_t*        rbuf;                // received buffer
      int    count = 0;
      int    rcvd_count = 0;
      char*    listen_port = "43086";
      int        delay = 1000000;            // mu-sec delay between messages
      int        mtype = 0;
      int        stats_freq = 100;
   
      if( argc > 1 ) {                    // simplistic arg picking
          listen_port = argv[1];
      }
      if( argc > 2 ) {
          delay = atoi( argv[2] );
      }
      if( argc > 3 ) {
          mtype = atoi( argv[3] );
      }
   
      fprintf( stderr, "<DEMO> listen port: %s; mtype: %d; delay: %d\\n",
          listen_port, mtype, delay );
   
      if( (mrc = rmr_init( listen_port, 1400, RMRFL_NONE )) == NULL ) {
          fprintf( stderr, "<DEMO> unable to initialise RMR\\n" );
          exit( 1 );
      }
   
      rcv_fd = rmr_get_rcvfd( mrc );  // set up epoll things, start by getting the FD from RMR
      if( rcv_fd < 0 ) {
          fprintf( stderr, "<DEMO> unable to set up polling fd\\n" );
          exit( 1 );
      }
      if( (ep_fd = epoll_create1( 0 )) < 0 ) {
          fprintf( stderr, "[FAIL] unable to create epoll fd: %d\\n", errno );
          exit( 1 );
      }
      epe.events = EPOLLIN;
      epe.data.fd = rcv_fd;
   
      if( epoll_ctl( ep_fd, EPOLL_CTL_ADD, rcv_fd, &epe ) != 0 )  {
          fprintf( stderr, "[FAIL] epoll_ctl status not 0 : %s\\n", strerror( errno ) );
          exit( 1 );
      }
   
      sbuf = rmr_alloc_msg( mrc, 256 );    // alloc 1st send buf; subsequent bufs alloc on send
      rbuf = NULL;                        // don't need to alloc receive buffer
   
      while( ! rmr_ready( mrc ) ) {        // must have route table
          sleep( 1 );                        // wait til we get one
      }
      fprintf( stderr, "<DEMO> rmr is ready\\n" );
   
   
      while( 1 ) {            // send messages until the cows come home
          snprintf( sbuf->payload, 200,
              "count=%d received= %d ts=%lld %d stand up and cheer!",    // create the payload
              count, rcvd_count, (long long) time( NULL ), rand() );
   
          sbuf->mtype = mtype;                            // fill in the message bits
          sbuf->len =  strlen( sbuf->payload ) + 1;        // send full ascii-z string
          sbuf->state = 0;
          sbuf = rmr_send_msg( mrc, sbuf );                // send & get next buf to fill in
          while( sbuf->state == RMR_ERR_RETRY ) {            // soft failure (device busy?) retry
              sbuf = rmr_send_msg( mrc, sbuf );            // w/ simple spin that doesn't give up
          }
          count++;
   
          // check to see if anything was received and pull all messages in
          while( (nready = epoll_wait( ep_fd, events, 1, 0 )) > 0 ) { // 0 is non-blocking
              if( events[0].data.fd == rcv_fd ) {     // waiting on 1 thing, so [0] is ok
                  errno = 0;
                  rbuf = rmr_rcv_msg( mrc, rbuf );    // receive and ignore; just count
                  if( rbuf ) {
                      rcvd_count++;
                  }
              }
          }
   
          if( (count % stats_freq) == 0 ) {            // occasional stats out to tty
              fprintf( stderr, "<DEMO> sent %d   received %d\\n", count, rcvd_count );
          }
   
          usleep( delay );
      }
  }
   
 


Receiver Sample
---------------

The receiver code is even simpler than the sender code as it 
does  not  need  to  wait  for a route table to arrive (only 
senders need to do that), nor does it need  to  allocate  an 
initial  buffer.  The  example  assumes  that  the sender is 
transmitting a zero terminated string as the payload. 
 
 
:: 
 
   
  #include <unistd.h>
  #include <errno.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <time.h>
   
  #include <rmr/rmr.h>
   
   
  int main( int argc, char** argv ) {
      void* mrc;                     // msg router context
      long long total = 0;
      rmr_mbuf_t* msg = NULL;        // message received
      int stat_freq = 10;            // write stats after reciving this many messages
      int i;
      char*    listen_port = "4560"; // default to what has become the standard RMR port
      long long count = 0;
      long long bad = 0;
      long long empty = 0;
   
      if( argc > 1 ) {
          listen_port = argv[1];
      }
      if( argc > 2 ) {
          stat_freq = atoi( argv[2] );
      }
      fprintf( stderr, "<DEMO> listening on port: %s\\n", listen_port );
      fprintf( stderr, "<DEMO> stats will be reported every %d messages\\n", stat_freq );
   
      mrc = rmr_init( listen_port, RMR_MAX_RCV_BYTES, RMRFL_NONE );
      if( mrc == NULL ) {
          fprintf( stderr, "<DEMO> ABORT:  unable to initialise RMr\\n" );
          exit( 1 );
      }
   
      while( ! rmr_ready( mrc ) ) {    // wait for RMR to get a route table
          fprintf( stderr, "<DEMO> waiting for ready\\n" );
          sleep( 3 );
      }
      fprintf( stderr, "<DEMO> rmr now shows ready\\n" );
   
      while( 1 ) {                              // receive until killed
          msg = rmr_rcv_msg( mrc, msg );        // block until one arrives
   
          if( msg ) {
              if( msg->state == RMR_OK ) {
                  count++;                      // nothing fancy, just count
              } else {
                  bad++;
              }
          } else {
              empty++;
          }
   
          if( (count % stat_freq) == 0  ) {
              fprintf( stderr, "<DEMO> total received: %lld; errors: %lld; empty: %lld\\n",
                  count, bad, empty );
          }
      }
  }
   
 


Receive and Send Sample
-----------------------

The following code snippet receives messages and responds to 
the  sender if the message type is odd. The code illustrates 
how the received message may be used to return a message  to 
the source. Variable type definitions are omitted for clarity 
and should be obvious. 
 
It should also be noted that things like  the  message  type 
which  id returned to the sender (99) is a random value that 
these applications would have agreed on in  advance  and  is 
**not** an RMR definition. 
 
 
:: 
 
  mrc = rmr_init( listen_port, MAX_BUF_SZ, RMRFL_NOFLAGS );
  rmr_set_stimeout( mrc, 1 );        // allow RMR to retry failed sends for ~1ms
   
  while( ! rmr_ready( mrc ) ) {        // we send, therefore we need a route table
      sleep( 1 );
  }
   
  mbuf = NULL;                        // ensure our buffer pointer is nil for 1st call
   
  while( TRUE ) {
      mbuf = rmr_rcv_msg( mrc, mbuf );        // wait for message
   
      if( mbuf == NULL || mbuf->state != RMR_OK ) {
          break;
      }
   
      if( mbuf->mtype % 2 ) {                // respond to odd message types
          plen = rmr_payload_size( mbuf );        // max size
   
                                                  // reset necessary fields in msg
          mbuf->mtype = 99;                       // response type
          mbuf->sub_id = RMR_VOID_SUBID;          // we turn subid off
          mbuf->len = snprintf( mbuf->payload, plen, "pong: %s", get_info() );
   
          mbuf = rmr_rts_msg( mrc, mbuf );        // return to sender
          if( mbuf == NULL || mbuf->state != RMR_OK ) {
              fprintf( stderr, "return to sender failed\\n" );
          }
      }
  }
   
  fprintf( stderr, "abort: receive failure\\n" );
  rmr_close( mrc );