doc/src/library/failures.im

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	Copyright (c) 2019 Nokia
	Copyright (c) 2018-2019 AT&T Intellectual Property.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
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==================================================================================
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	Mnemonic:	failures.im
	Abstract:	This is the major section on how an application might handle failures
	Date:		2 August 2019
	Author:		E. Scott Daniels
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&h1(Handling Failures)
The vast majority of states reported by RMR are fatal; if encountered during setup or initialisation,
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 &ital(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.

&h2(Failure Notification)
When a soft failure is reported, the returned message buffer returned by the RMR function will be &cw(RMR_ERR_RETRY.)
These types of failures can occur for various reasons; one of two reasons is typically the underlying cause:

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&indent
&beg_list( &lic1 )
	&li The session to the targeted recipient (endpoint) is not connected.
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	&li The transport mechanism buffer pool is full and cannot accept another buffer.
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&end_list
&uindent
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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.

&h2(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.

&h2(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.
&space

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 &cw(RMR_ERR_RETRY,) but will &bold(not)
make any attempts to resend the message.
This zero-retry policy is enabled by invoking the &func(rmr_set_stimeout) with a value of 0; this can be done once
immediately after &func(rmr_init:) is invoked.

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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.


&h2(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. &func(rmr_init:).)
The following are the RMR state constants and a brief description of their meaning.

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&beg_dlist( 1.5i &ditext )
	&di(RMR_OK) 			state is good; operation finished successfully
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	&di(RMR_ERR_BADARG)		argument passed to function was unusable
	&half_space

	&di(RMR_ERR_NOENDPT)	send/call could not find an endpoint based on msg type
	&half_space

	&di(RMR_ERR_EMPTY)		msg received had no payload; attempt to send an empty message
	&half_space

	&di(RMR_ERR_NOHDR)		message didn't contain a valid header
	&half_space

	&di(RMR_ERR_SENDFAILED) send failed; errno may contain the transport provider reason
	&half_space

	&di(RMR_ERR_CALLFAILED) unable to send the message for a call function; errno may contain the transport provider reason
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	&di(RMR_ERR_NOWHOPEN)	no wormholes are open
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	&di(RMR_ERR_WHID)		the wormhole id provided was invalid
	&half_space

	&di(RMR_ERR_OVERFLOW)	operation would have busted through a buffer/field size
	&half_space

	&di(RMR_ERR_RETRY)		request (send/call/rts) failed, but caller should retry (EAGAIN for wrappers)
	&half_space

	&di(RMR_ERR_RCVFAILED)	receive failed (hard error)
	&half_space

	&di(RMR_ERR_TIMEOUT)	response message not received in a reasonable amount of time
	&half_space

	&di(RMR_ERR_UNSET)		the message hasn't been populated with a transport buffer
	&half_space

	&di(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)
	&half_space

	&di(RMR_ERR_INITFAILED) initialisation of something (probably message) failed
	&half_space

	&di(RMR_ERR_NOTSUPP)	the request is not supported, or RMr was not initialised for the request
&end_dlist
&uindent
.st &textsize
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Depending on the underlying transport mechanism, and the nature of the call that RMR attempted, the
system &cw(errno) value might reflect additional detail about the failure. 
Applications should &bold(not) rely on errno as some transport mechanisms do not set it with
any consistency.