PRODUCT NAME:  MIRA High Availability Management        SPD 31.86.00
               Software for VMS, Version 1.0 

DESCRIPTION

System Overview

MIRA Applications Switch (MIRA AS) are High Availability systems based on a 
Dual-Host architecture plus Watchdog modules installed in each computer and 
communicating via a dedicated link.

MIRA High Availability Management Software (MIRA HAMS) is a software 
architecture for controlling MIRA Applications Switch systems, handling the 
following main tasks:

oo Error detection at system level as well as at device level provided 
  that device errors are logged/recorded by their associated driver.

oo Application failover and recovery initiation, upon error detected and 
  in accordance with user-configured action templates.

The complete application recovery (regain the lost process context) must 
be accomplished by the application itself. Process/context shadowing is 
not a feature of MIRA HAMS.

Before running an application, the user must configure system environment 
and error recovery strategy. This should be done on both computers, and 
consists of filling in templates associating the devices dedicated to the 
application and the actions to be taken in the event of device failure.

The two computers of a MIRA Applications Switch system operate 
independently. MIRA HAMS is installed on each computer and handles 
connected applications in such a way that a complete failover is decided 
only if absolutely necessary, thus increasing the system availability. 

Upon detecting a device failure which does not impact the whole 
functionality of one computer, and provided that the user has filled-in 
the action tables accordingly, MIRA HAMS is able to decide the failover 
of only those applications that are associated with this failed device.  
The unaffected applications are kept running on the current computer.

MIRA Applications Switch systems provide a hardware and software 
environment required to create high availability applications which can 
recover from failure without operator intervention.

MIRA Applications Switch systems are particularly suited to dedicated 
control applications allowing very short breaks of system availability 
which do not affect its overall integrity during the 
service/maintenance/replacement of the failed component. 

Terminology

LOCAL System - System currently operating or executing commands.

REMOTE System - System at the other end of the watchdog link.

MASTER Application - Application running on any of the two computers with 
a backup application declared in STANDBY mode on the other computer.

MASTER BACKUP Application - Application in STANDBY mode which temporarily 
becomes MASTER following the failure of its partner on the remote system.

SYSTEM failure - Failure affecting the integrity of the whole system 
which is not recoverable by the application resulting in a SYSTEM 
FAILOVER.

NON-SYSTEM failure - Failure affecting a specific device and/or its 
associated application(s). Recovery depends on customer needs and may 
result in APPLICATION FAILOVER (all or part).

Failure Detection Process

MIRA HAMS is installed on each computer and exchanges status messages via 
the dedicated watchdog link. 

oo System Failure

  If the software on either computer is unable to exchange messages 
  within a user-specified period or detects a system failure, a system 
  failover occurs on the computer which is still in operation.

oo Non-System Failure

  Once a device error threshold has been reached or whenever a MIRA 
  Applications Switch application fails, a non-system error is generated. 
  If the software on either computer sends a message with application or 
  application device failure state, application failover or system 
  failover occurs on the remote computer.

Failover Operation

In the event of system failover, the following automatically occurs on 
the computer still operating:

oo All the applications in STANDBY mode acquire the MASTER BACKUP state, 
  thus initiating application(s) recovery.

oo All the applications in MASTER mode keep the MASTER state.

The following automatically occurs on the failed computer: 

oo If the feature known as "DCLOW" is enabled, a power fail is simulated 
  thus speeding up the release of the dual-port disks that were mounted 
  so that they can be mounted on the other system (eg: RA type disks).

  In the event of application failover, the local application is aborted.  
  The partner application on the remote computer, in STANDBY mode, 
  acquires the MASTER BACKUP state, thus initiating the application 
  recovery.

application failover => recovery initiation => recovery completion

     --------------->	     |	<-------------
       HAMS driven                     Application driven

MIRA HAMS can perform failover, plus application recovery initialization, 
in less than one second on normally loaded systems.

On-Line Testing

On-line Tests can be invoked by the operator to test the specific MIRA 
Applications Swtich hardware on a system. Tests can be used without 
disturbing the applications running on either computer.

Diagnostics

MDM Diagnostics must be run on the faulty computer. In case of partial 
system failure, the user has to provoke a complete system failover by 
using an operator command.  Diagnostics and repair can then be done 
normally on the faulty system without affecting the applications running 
on the computer still in operation.

Re-Start

Once a failed computer is repaired and restarted: 

oo Local applications having a remote partner application in master backup 
  state acquire the master state. 

oo The remote partner application in master backup mode acquires the 
  standby state. 

oo The local application having a remote partner application in master 
  state acquires the standby state. 

oo The remote application in master state keeps the master state. 

Thus, the user-configured load sharing mode will be re-established.

Swap Operation

A Customer application or the operator can command SYSTEM or APPLICATION 
SWAP. This operation can be disabled by an application. It is only valid 
from a local application in master state and with a remote partner 
application in standby state. 

In the event of SYSTEM SWAP, on both computers:

oo All the applications in standby state acquire the master state.

oo All the applications in master state acquire the standby state.

oo All the applications in master backup state keep this state.

In the event of APPLICATION SWAP:

oo The local application in master/standby state acquires the 
  standby/master state.

oo The remote partner application in standby/master state acquires the 
  master/standby state. 

APPLICATION SWAP gives load sharing possibilities and allows use of 100% 
of the available CPU resource of the MIRA II system.

Failover and Swap Summary

           Application state BEFORE

                      On Computer 1          On Computer 2               

Failover of	      Master		     Standby
Application on	      Master		     Unknown
Computer 1	      Master Backup	     Unknown
		      Standby		     Master 

                      On Computer 1          On Computer 2

Failover of 	      Master		     Standby
Application on	      Standby		     Master
Computer 2	      Unknown		     Master
		      Unknown		     Master Backup

	    Application state AFTER

                      On Computer 1          On Computer 2

Failover of 	      Unknown		     Master Backup
Application on	      Unknown		     Unknown
Computer 1	      Unknown		     Unknown
		      Unknown		     Master

                      On Computer 1          On Computer 2

Failover of 	      Master		     Unknown
Application on	      Master Backup	     Unknown
Computer 2	      Unknown		     Unknown
		      Unknown		     Unknown

	    Application state BEFORE

                      On Computer 1          On Computer 2               

Swap of		      Master		     Standby
Application on	      Standby		     Master
Computer 1	      Master 		     Unknown
		      Master Backup	     Unknown

                      On Computer 1          On Computer 2

Swap of 	      Master		     Standby
Application on	      Standby		     Master
Computer 2	      Unknown		     Master
		      Unknown		     Master Backup

	     Application state AFTER

                      On Computer 1          On Computer 2

Swap of 	      Standby		     Master 
Application on	      Standby		     Master
Computer 1	      Master		     Unknown
		      Master Backup	     Unknown

                      On Computer 1          On Computer 2

Swap of 	      Master		     Standby
Application on	      Master 		     Standby
Computer 2	      Unknown		     Master
		      Unknown		     Master Backup

Operator Commands

The following set of operator commands enable the operator to control the 
system:

HELP		     Displays information on commands.

EXIT		     Exits to DCL.

START		     Starts MIRA II operation.

STOP		     Stops MIRA II operation.

SHOW SYSTEM	     Displays local/remote system states.

TEST		     Tests local/remote watchdog link.

SET LOG_FILE	     Creates new log files.

SET SYSTEM	     Associates system or system device failure with an 
                     action.        

CLEAR SYSTEM	     Cancels effect of set system.

Application Management Commands

SWAP			 Forces swap of application states.

FAILOVER		 Forces failover of application/system.

RUN			 Starts local and remote application.

SHOW APPLICATION	 Displays information about particular 
                         application.

SET APPLICATION		 Adds entry in application action table.

CLEAR APPLICATION	 Removes entry from application action table.

SHOW CONNECTIONS	 Displays informations about the running 
                         applications.

Error Management Commands

SHOW DEVICE		  Displays device/application action tables.

SET DEVICE		  Adds entry in device action table.

CLEAR DEVICE		  Removes entry from device action table.

User Application Interface

The User Application Interface provides a set of user-callable 
subroutines allowing exhanges of information via MIRA HAMS.  MIRA HAMS 
must be active on each computer.

These subroutines comply with the VMS Standard for Procedure Calling and 
Condition Handling and are supported for VAX PASCAL, VAX MACRO, VAX Ada, 
VAX BASIC, VAX COBOL, VAX BLISS-32, VAX PL/I, VAX C and VAX FORTRAN and 
include the following functions:

HAMS$CONNECT			  Connects the user application (maximum 
                                  16 simultaneous connections supported)

HAMS$DISCONNECT			  Disconnects the user application 

HAMS$SWAP			  Swaps MASTER and STANDBY applications

HAMS$FAILOVER			  Forces application failover 

HAMS$DISABLE_SWAP		  Disables Application Swap Operations

HAMS$ENABLE_SWAP		  Enables Application Swap Operations

HAMS$GET_EVENT			  Gets MIRA HAMS event status.

The user applications can also be notified, if requested, of changes in 
application status and invoke the necessary recovery procedures.

Clock Synchronization

The two computer clocks can be synchronized within a few milliseconds. 
This optional feature may be managed automatically by MIRA HAMS.

SOURCE CODE

The following source code modules are provided with Single-Use License, 
binary options on TK50 distribution media:

oo Ada Application Interface routines

INSTALLATION

Digital recommends that a customer's first purchase of this software 
product include Digital Installation Services.  These services provide 
for installation of the software product by an experienced Digital 
Software Specialist. 

For subsequent purchases of this product only experienced customers 
should attempt installation.  Digital recommends that all other customers 
purchase Digital's Installation Services. 

Customer Responsibilities

Before installation of the software, the customer must:

oo Previously have installed all requisite software and hardware including 
  terminals.

oo Make available for a reasonable period of time, as mutually agreed by 
  Digital and the customer, all hardware, communication facilities and 
  terminals that are to be used during installation.

Delays caused by any failure to meet the responsibilities will be charged 
at the then prevailing rate for time and materials. 

HARDWARE REQUIREMENTS

MIRA Applications Switch Dual-Host MicroVAX configurations as specified 
in the System Support Addendum (SSA 31.86.00-x).

SOFTWARE REQUIREMENTS *

VMS Operating System

VAX Ada 

* Refer to the Systems Support Addendum (SSA 31.86.00-x) for availability 
  and versions of required software.

ORDERING INFORMATION

Software Licenses: QL-YHMA*-**
Software Media: QA-YHMAA-**
Software Documentation: QA-YHMAA-GZ
Software Product Services: QT-YHMA*-** 
  
* Denotes variant fields.  For additional information on available 
  licenses, services and media refer to the appropriate price book.

SOFTWARE LICENSING

This software is furnished under the licensing provisions of Digital 
Equipment Corporation's Standard Terms and Conditions. 

For more information about Digital's licensing terms and policies, 
contact your local Digital office.

LICENSE MANAGEMENT FACILITY SUPPORT

This layered product supports the VMS License Management Facility.

License units for this product are allocated on a CPU-capacity basis.

For more information on the License Management Facility, refer to the VMS 
Operating System Software Product Description (25.01.xx) or the VMS 
Operating System documentation set. 

For more information about Digital's licensing terms and policies, 
contact your local Digital office.

SOFTWARE PRODUCT SERVICES

A variety of service options are available from Digital.  For more 
information contact your local Digital office.

SOFTWARE WARRANTY

Warranty for this product is provided by Digital with the purchase of a 
license for the product as defined in the Software Warranty Addendum of 
this SPD.

The DIGITAL Logo is registered trademark of Digital Equipment Corporation.
    
VAX, VMS, and MicroVAX are trademarks of Digital Equipment Corporation.

June 1990
AE-PBFYA-TE


