An object-oriented event-driven architecture for the VLT
Telescope Control Software
G.Chiozzi
European Southern Observatory, Karl-Schwarshild-Str. 2D-85748 Garching bei Muenchen, Germany
Email: gchiozzi@eso.org
Abstract
The control software for the Very Large Telescope follows the “Standard Architecture” and is distributed over several workstations, that provide high-level and coordination services, and VME basedsystems, for real-time control purposes.
The adoption of object-oriented design techniques and the support of a C++ application framework
for the implementation of Event-Driven systems reduces considerably the complexity of the applications. The framework provides a general application skeleton and services to automatically receive andanalyse events. These are then dispatched to specialized objects, designed to handle specific events.
Since all the run-time and configuration data of the whole VLT is stored in a distributed real-time
database, there is a strict coupling between the structure of the database and the applications. As a consequence, the real-time database, although based on the hierarchical model, has been structured to provide support for object-oriented design and implementation.
This paper describes the architecture of the Telescope Control Software (TCS) for the VLT and theobject-oriented infrastructure on which it is based.

1INTRODUCTION

The control software for the Very Large Telescope (described in more details elsewhere in these proceedings[5]) follows the “Standard Architecture”[7] and is distributed over several workstations, that provide highlevel and coordination services, and VME based systems, for real-time control purposes. The communication
between all the processes running on these machines is based on a message system and a distributed hierarchicaldatabase.
This architecture implies that all the applications, but in particular the coordination processes running at workstation level, must be ready at any moment to accept and handle a lot of different kinds of messages, such as newcommands, alarms, and notifications from the controlled sub-components.
As a consequence, the design and the implementation of the coordination applications has an high degree ofcomplexity, with an obvious impact on development and debugging time.
The adoption of object-oriented design techniques and the support of an application framework for the implementation of Event-Driven systems reduces considerably this complexity. The framework, based over a set ofC++ classes, provides a general application skeleton and services to automatically receive and analyse events.
These are then dispatched to specialized objects, designed to handle a specific set of events without having to take
into account other parallel but independent conditions.
Given these services, the design and implementation of an application consists of the design of a set of smaller
and independent objects specialized in the handling of specific events, such as a command, a change in some database values, the tic of a periodic timer, etc.
Since all the run-time and configuration data of the whole VLT is stored in a distributed real-time database,
there is a strict coupling between the structure of the database and the applications. As a consequence, the realtime database, although based on the hierarchical model, has been structured to provide support for object-oriented design and implementation.
This architectural approach has a number of advantages, both in the design/development and maintenancephases; in particular it enforces the respect of standards and provides a mean of sharing and reusing code in theVLT software team.

2VLT CONTROL SOFTWARE
The control software for the VLT is responsible for the control of the 4 main telescopes and of the auxiliary

ones, the interferometer and the instruments attached to the light beams. The main telescopes can be operated
individually or in combination; on an individual telescope multiple instruments can be used simultaneously (onefor observation, the others for calibration, preparation or maintenance). This correspond to a distributed environment of 120-150 coordinating workstations and microprocessor based VME systems (Local Control Units,LCUs)[6].
The basic software for the VLT control system, called the Central Control Software (CCS)[3][4], is a large setof modules designed to provide services common to many applications. Most of these common services are available both on workstation and LCU platforms and provide on both the same application programming interface.
The main group of functions provided in the CCS are:• message handling• on-line real-time database• error and alarm handling• logging• process I/O• event monitoring
The real-time database definition language and a framework of C++ classes for the development of event
driven applications provide support for object oriented programming. These are described in more detail in thefollowing paragraphs.

3TELESCOPE CONTROL SOFTWAREThe Telescope Control Software (TCS)[8] controls the main telescopes and the attached equipment that iscommon to instruments. In particular, for every telescope it controls the main axes (altitude and azimuth), the mirrors, three instrument adapters, the CCDs for auto guiding and active optics, the enclosure and other auxiliary
equipment.
The complete VLT will have four equivalent copies of TCS, one per telescope.
3.1

Software architecture
The TCS software can be grouped into four categories:• user interface• coordinating software• subsystem application software• special interface software
The subsystem application software implements all the functions that can be performed locally in an LCU,
without any knowledge of other components of the system. It controls basic actions and motions of subsystem
devices such as the main structure of the telescope, mirror supports, adapters and the enclosure.

TCS
specialinterface
softwarecoordinatingsoftware

TCSTCS
UIF

interfaceexternalfunctionalmodules

softwareadapter’sinterface

subsystems

Fig. 1- categories of TCS software

The coordinating software is hierarchically above the subsystem software. It performs actions of combined,
coordinating nature, and it often uses one or more subsystems to execute its actions. It runs mainly on the telescope control workstation, although there are major parts running in LCUs.
The special interface software consist of libraries to access, via TCS, external systems such as star catalogues
and the Astronomical Site Monitor subsystem.

While the software running on the LCUs has been written in C using traditional techniques, the workstation
software is designed using an object oriented methodology and coded in C++.
3.2

Coordinating software
The coordination software provides a public interface for external applications requiring services to TCS and aset of “functional” modules. These are the core modules of TCS software and perform all the coordination work,
interacting with each other and with the subsystems.
Some of these modules are for:• Presetting (setting the telescope to a new target)• Tracking (maintaining object position following its movement in the sky, without guide star feedback)• Autoguiding (corrections to telescope position based on guide star feedback)• Enclosure control (dome rotation, doors, windscreen, louvers)• Active optics (main mirror axial support and mirror two-position corrections)
All inter-module communication and all communication with subsystems, make use of the CCS message system. Each module has a command interface which is the one that is normally used for all inter-module communication and which is also used by the TCS user interface panels.
3.3

Coordination module’s architecture
All coordination modules have the same architecture and their implementation is founded on an object oriented framework based on C++ classes and real-time database classes. Thanks to this approach, maintenance of
already existing modules and development of new modules is much easier and, at the same time, VLT conventions
and rules are automatically enforced because they are implemented in the base classes used in the implementation.
All the modules are implemented through one or more independent processes.
If a module is made up of more than one process, only one of them is responsible for providing the public
interface to the external world, i.e. to receive commands from external modules and to send back the corresponding replies.
The other processes are just slave processes used to perform specific sub-tasks and to improve parallelism.
All coordination modules receive commands from external applications, analyse and elaborate the incoming
data and initiate all the necessary actions sending requests to other coordinating modules and to subsystems. They
must always be ready to process a new command within a typical “command response time” of 100 milliseconds
and many actions must be initiated and handled in parallel as far as possible.

4THE EVENT HANDLER

In order to meet this requirement, every process is designed in an event-driven way and the implementation isbased around the event-handler provided by the evhHANDLER class, which is a core component of the C++
library provided by CCS[1].
Every process contains an instance of this class.

evh based applicationcommands and messages

evhTASK
subclass
callback management

External
application

evhHANDLER

automatic replies

commands andmessages dispatched

replies/commands

Fig.2 - Event Handler ArchitectureThis object receives and parses all the incoming events, such as:• CCS commands• CCS replies and error replies to sent commands• CCS time out notifications• CCS periodic timer notifications• CCS Data Base Event messages• UNIX signals• UNIX file inputs
It then searches an internal database for a list of functions or object methods to be called in order to handle thatspecific event. Several callbacks, in principle unlimited, can be registered for a single event. For synchronization
purposes, it is also possible to define callbacks to be invoked after a combination of events. The callback list isdynamic and the evhHANDLER class provides methods to add and delete elements from the list.
Every list of callbacks must return to the event handler within the “command response time” of 100 milliseconds and the event handler itself takes care of checking this condition, issuing error messages if it is not met(coordinating modules have only soft and not hard real-time requirements).
The design of every process consists mainly in the design of the independent objects providing the methods tobe attached as callbacks to the event-handler.
Usually there is one object for every task (a TASK object) that can be executed in parallel inside a process andall the objects have their own independent “life”. Whenever an object is waiting for commands or for messages
and other events from subsystems, the event-handler is ready to receive events and dispatch them to other objects.

5THE TASK CLASSES

In order to help to implement the TASK objects and enforce at the same time that the VLT standard rules andprotocols are followed, a wide set of base classes are provided.
For example, it is required that every VLT module must be able to handle a certain number of “standard commands”. As a consequence an evhSTD_COMMANDS class exist that implements all the standard commands inconsistent and reasonable way. A fully compliant VLT application can be built with a few lines of C++ code justallocating an instance of evhHANDLER and one of evhSTD_COMMANDS classes. If a command must be
implemented in a different way, the related callbacks can be overloaded or a new object can be allocated for its
handling.
evhTASK

evhDB_TASK

evhDB_CMD_SEND

evhDB_CMD_SERIAL
mswMOD_LIST_MANAGER
trkwsTRANSFER

evhSTD_COMMANDS

mswSTD_COMMANDS
trkwsSETUP_STD_COMMANDS

mswCONTROL_MAIN
mswSWITCH_FOCUS
prsAH_MAIN_TASK

prsAH_MAIN_TASK_VLT
prsAH_MAIN_TASK_NTT

Fig.3 - Some of the classes in the hierarchy of evhTASKOther classes provide a standard architecture to send commands to one or more external modules and collect
the replies. The developer must just derive a sub-class where one or more callbacks are overloaded to specify theobject’s behaviour when successful replies, error conditions or time-out events are received. This scheme can also
handle complex synchronization and queuing protocols, saving a lot of development time and, most importantly,
warranting that the same concept is implemented consistently and in the same way everywhere. This is of great
benefit for maintenance and debugging.

6THE REAL-TIME DATABASE

All data that can be of interest for external modules, to get a picture of the status of the system, or for systemtuning and configuration, are stored in the real-time database.
This is a hierarchical database distributed on the different workstations and LCUs, mapping the physical andlogical objects that constitute the VLT control system and describing how they are logically contained, one inside
the other (how one component is “part-of” another one).

Each local database has a partial image only of the units of the system that the particular workstation or LCU
has to use. The whole database is the merge of all the local sections.
There will be a lot of different places in the database describing objects with the same or a very similar internal structure and behaviour. For example the VLT system will have a lot of motors, encoders or moving axes thathave the same general characteristics.
This description of the VLT real-time database fits very well in an object-oriented model, but the implementation of the database itself is based on a commercial hierarchical database (RTAP, from Hewlett-Packard) that does
not support object oriented concepts.
For this reason we have developed a preprocessor and a class browser that extend the semantic and syntax of
the RTAP database definition language, introducing the concept of class, with inheritance and overloading[2].

Fig.4 - User Interface for the DBL class Browser

RTAP syntax allows only to define “points” as data structures containing a set of “attributes” of predefinedbasic data types. The hierarchical structure is built defining a new point as a sub-point of an already existing one.
If the same data structure (for example describing a motor) is used in more places, the whole definition of thedatabase branch must be copied.
The extended syntax introduces the concept of “class” as a definition of a new structured data type that can beused as any other available data type. This means that a class instance can be used just in the same way as a native
type while defining attributes inside new classes or points.
Each new class has to be derived from another one, from which it inherits all the attributes. Inside a class definition (or a point instantiation) it is also possible to:• redefine attributes to assign new initial values• add new attributes• overload structured attributes
Methods to be used from C++ applications can be implemented developing a C++ twin class, using specificclasses provided in CCS.

7THE TCS DATABASEEvery TCS module defines its own database branch on the workstation’s database, following a standard structure. This is automatically imposed by using the database classes coupled with the C++ classes provided by theevent handling application framework: for every C++ class accessing the database, there is a twin database classcontaining the required points and attributes.
The use of inheritance lets the programmer specialize the database class according to the changes in the C++class.
The application “owner” of the database branch is the only one with write access. All the other applications
can read database values by accessing them directly or, better, through the provided C++ access classes.
The following figure shows a simplified view of a TCS database:

TCS

foc

msw

list

prs

actionadc
aoguidem2
track

controltrk

asm
controldata

axes
environment
positionsiteConstants
times

Cassegrain
NasmithA
NasmithB
altAxisazAxis

setup
act
ltm1
ltm2
ltm3

Fig.5 - Structure of a TCS real-time database

8STATUSThe real-time database preprocessor and the event handling tool-kit have been developed during the secondhalf of ‘94 and the first half of ‘95 and have been extensively used in many VLT applications.
In particular they have been used for the development of TCS part 1 (which includes Presetting, Tracking andMode Switching modules), released in September ‘95.
TCS part 1 will be tested at the end of this year on the NTT (New Technology Telescope) in LaSilla as part of
the upgrade of its control software. The NTT will use as much as possible the same software as the VLT, in orderto reduce maintenance resources and to test it before the VLT itself is available. In the case of the TCS, the software has been explicitly designed for this purpose, isolating in specific subclasses all the functionality that is different in the two telescopes. In every case where specific behaviour must be implemented, a base class provides
whatever can be in common and two specific sub-classes (one for NTT and one for VLT) implement the part thatis unique to each system.
In the first half of ‘96, there will be tests on a pre-assembled telescope at the manufacturer’s site in Milan,Italy. It will be a test of the telescope structure, drive system and encoders and also of TCS part 1.
As a by-product of the development of TSC part 1, a number of general classes have been developed andadded to the set provided by CCS. These classes are now used by other applications.

9CONCLUSION

The adoption of an object oriented design methodology, with the essential support and guide to implementation provided by the C++ application framework, has allowed the development of a Telescope Control System thatis intrinsically more maintainable and extendible compared with traditional techniques.
The initial effort spent in designing the general event-driven architecture and implementing the support toolkit has been already paid back by the fact that the software can be easily adapted to other systems (like the NTTand, in perspective, the auxiliary telescopes). Moreover, the same general concepts are now being applied to othercomponents of the VLT control system, like instrument software.
This approach provides a number of benefits:• Different applications, developed by different teams, share the same architecture, making easier maintenance in the future.
• Standards and conventions are not only stated “on paper” but automatically enforced by the use ofstandard classes.

•

10Important code components are better tested because they are extensively used in many applications.

ACKNOWLEDGMENTS

The author wishes to thank all colleagues in the VLT Software Group , and any other ESO staff, who havecontributed to the concepts, ideas and software reported in this paper.

11REFERENCES[1] G.Chiozzi - CCS Event Handling Tool-kit User Manual - VLT-MAN-ESO-17210-0771, European Southern
Observatory, 1995
[2] G.Chiozzi - CCS On Line Database Loader User Manual - VLT-MAN-ESO-17210-0707, European SouthernObservatory, 1995
[3] B.Gilli - Workstation environment for the VLT - Proceedings of SPIE, vol.2199, pp.1026-1033, 1994
[4] B.Gustafsson - VLT Local Control Unit Real Time Environment - Proceedings of SPIE, vol.2199, pp.10141025, 1994
[5] G.Raffi - Status of ESO/VLT Control Software - These proceedings[6] G.Raffi - Control Software for the ESO VLT - Proc. Int. Conf. on Accelerator and Large Experimental Physics
Control Systems, Tsukuba, Japan, 1991, KEK Proc. 92-15[7] C.O.Pak, S.Kurokawa, T.Katoh - Eds. Proc. Int. Conf. on Accelerator and Large Experimental Physics ControlSystems, Tsukuba, Japan, 1991, KEK Proc. 92-15[8] K.Wirenstrand - VLT Telescope Control Software, an overview - Proceedings of SPIE, vol. 2479, pp.129-139,
1995
NOTE: The postscript files of the ESO documents, included those mentioned in this paper, are available
through anonymous ftp at ftp.eso.org.