Overview of Real-Time Database Management System Design for Power
System SCADA System
Jian Wu, Yong Cheng, and Noel N. Schulz
Department of Electrical and Computer Engineering
Mississippi State University
schulz@ece.msstate.edu
database. The data and its processing of a real-time
SCADA database have harsh time constraints. The
accuracy not only relies on the business logical result, but
also is decided by whether the logical derivation completes
in certain interval. Therefore, we need to combine real-time
theory with traditional database technology to realize a
real-time database in a SCADA system.
Abstract
A Supervision Control and Data Acquisition (SCADA)
system is a communication and control system used for
monitoring, operation and maintenance of energy
infrastructure grids. Compared with traditional
applications, a SCADA system has a harsh deadline for
critical tasks. There is special time constraint for the real
time database used in a SCADA system. The real time
database in SCADA extends traditional database to include
in-memory database. Such real time database management
are designed to operate in the harsh environment of realtime systems, with strict requirements for resource
utilization, and are ready to provide the performance and
reliability required by real-life applications. In this paper,
the main principle of real time database has been
introduced. Its implementation in power system SCADA is
discussed and a sample database is briefly introduced.
Key words:
Database
SCADA,
Real-time,
Database,
2. Technical Issues in Real-time Database
Implementation
Like traditional database management systems, real-time
database systems (RTDBMS) serve as repositories for data
and provide efficient storage, load and manipulation of
data. They must be equipped with basic functions of
general DBMS. Moreover, to meet the temporal
requirements of the managed data, timing constraints on
transactions, and performance, the real-time database
should support on-line query, location, deletion, and
insertion. An in-memory database is often used to achieve
the speed requirement. Functionalities of a real-time
database can be summarized as below [3,4]:
• Provides platform to share data with entire Energy
Management System (EMS)
• Provides open database interface, realizes the
function of database such as fast load, input,
deletion, inquiry, load, and access;
• Provides snapshot, history data storage, copy and
reutilization;
• Provides validity check;
• Provides security protection
To realize the functionalities of a real-time database,
there are several technical issues in real-time database
implementation as elaborated below.
In-memory
1. Introduction
A Supervision Control and Data Acquisition (SCADA)
system is a communication and control system used for
monitoring, operation and maintenance of energy
infrastructure grids. Compared with traditional applications,
a SCADA system has a harsh deadline for critical tasks. It
has different characteristics: the system must maintain
massive sharing data and control data; also each real-time
task of a SCADA system has a rigorous time requirement,
while the data for analysis and processing is time-variant.
Real time data has a short life cycle; for example, all the
data of remote measurements and remote signals must be
updated every 5 seconds and the decision or derivation
based on obsolete data is invalid [1,2].
Therefore, there is special time constraint for the
database used in a SCADA system. The database should be
able to process persistent static data, maintain the integrity
and consistency of them, should be able to deal with the
dynamic data within its time constraints during processing
and ensure the concurrence and efficiency of data access
[3,4]. This kind database is classified as a real time
0-4244-0169-0/06/$20.00 © 2006 IEEE.
2.1 Real-time Database Model Design and
Implementation
A relational database is good at data representation and
management. When a relational database is used in a
SCADA system, which is a large-scale complicated realtime system with the functions of data management,
monitoring, analysis and decision, and some shortcomings
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are exposed. For instance since different venders of power
system applications developed real-time database
independently, there is no standard data representation,
which causes integration problems. The common
Information Model (CIM) proposed by the Electric Power
Research Institute (EPRI) can be used to address this issue
[5].
3. Data Object of Dispatch System
Classification
Before building a data schema in real time database it’s
necessary to know the data object characteristic. The data
in SCADA system can be divided into three types of data
objects: Real time Data Object, Static Data Object and
Derived Data Object [1,2,3].
2.2 Real-time Task Scheduling and its
Concurrency Controls
3.1 Real-time Data Object
In addition to maintaining database consistency as in
conventional databases, real-time database systems must
also handle transactions within time constraints. Scheduling
real-time transactions is far more complex than traditional
real-time scheduling. The system must guarantee the most
time-critical task to be executed as early as possible. That
is, the system should be able to schedule the transactions
execution sequence according to the priority [2].
Real-time data such as remote measurement and remote
signal reflects the status of a power system. The common
characteristic of the data is the strict time constraint. The
data is written into the real-time database periodically. A
real-time data object is associated with a time stamp and
life cycle. They are only valid for the responding sampling
time. The task of query the sample from Remote Terminal
Unit (RTU) and database written is managed by real-time
task dispatch management as stated in next section. Once
the value of real time data object is written into the
database, it can not be changed by other task. New sample
data will be stored in a real-time database as a new data
object [5].
2.2 Data Storage and Sharing Memory
Management
Traditional disk database operation is based on disk I/O.
But the disk I/O time delay and its uncertainty sometimes is
fatal to the real-time task. Therefore another essential issue
of real-time databases is to eliminate time delay and
uncertainty. This needs the support of "database in
memory". To ensure real-time tasks access the database fast
and accurately, two problems should be solved [6].
(1) Guarantees all the key tasks of load and store
happen in memory only.
(2) Realizes data sharing among tasks.
3.2 Static Data Object
A static data object is constant. It is a special data type in
real-time databases, and its value does not change with
time. The time stamp associated with the data is system
creating time or the renewal time. This type of data, such as
parameters of transmission line and transformer impedance,
is spread wide in SCADA systems.
2.3 Database Security and Recovery
3.3 Derived Data Object
If the database system crashes, a traditional database
system uses a log to restore the database to what it was at
the time of the crash, but real-time database restoration is
more complex due to the facts below [3]:
(1) The process of restoration blocks the real-time task
for accessing the database and causes real-time task
overtime. It is possible for active real-time task to
access transient data and make the wrong decision;
(2) In the real-time database much data is temporary and
volatile. Sometimes the influence of data
inconsistency and incorrect is "temporary" and "noncontinuous".
For example, incorrect remote
measurement will affect the state estimation
happened at same time interval only but not after
this interval. Therefore, the recover time of different
types of crashed data is dependent upon the
application that used the data.
A derived data object is calculated from a group of real
time Data Objects and other data objects. Therefore, the
derived data object also has a time stamp and life cycle.
The time stamp of a derived data object is the derivation
time. The life cycle of this type data is the time interval
between this derivation time and the next derivation time.
The value of a derived data object in a database may be
renewed. Its value may be preserved or not depending on
its functionality. This type of data includes power grid
network model, magnitude and angle of bus voltage, and
branch current obtained from state estimation in EMS. The
data could be the solution from one module, or the raw data
of some task.
4. Real-time Database System Structure
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each other. It is necessary to coordinate activities of realtime tasks. The task dispatch management should provide
the following functions to share database resources and
coordinate conflict [8]:
•Start and stop task at fixed time;
•Allow setup conditions to trigger task;
•Specify execution sequence among tasks;
•Constrain the instance number of global task.
In order to support the functionalities above, a real time
database can be an integration of traditional relational
database and in-memory database. The general architecture
is shown in Figure 1[1,2]. It includes the real-time task
dispatch management, in-memory database, I/O dispatch
and the relational database.
R e a l-tim e a p p lic a t io n
4.2 I/O dispatch
R e a l-tim e ta sk d isp a tc h
m a na gem en t
The I/O dispatch is responsible for data synchronization
of in-memory database with relational database. The inmemory database has a one-to-one relation with relational
database. The data in a relational database needs to be
automatically synchronized with in-memory database. It is
important to decide when to write the data in an in-memory
database to a relational database and when to export the
data in a relational database to in-memory database. There
is no general solution. One possible way is to provide
methods of periodical update and forced update in dual
direction between a relational database and an in-memory
database. For different applications, the two methods can
be used for different objects according to their
characteristics.
I n -m e m o r y d a ta b a se
D a ta b a se m o d e l m a n a g e m e n t
R e a l-tim e re s o u r c e m a n a g e m e n t
D a ta o p e r a tio n
N e tw o rk c o m m u n ic a tio n
5. In-Memory Database
I /O D isp a tc h
An in-memory database is the core part of a real-time
database. It functions cover the database data model, data
operation, real-time resource management, and network
communication. The traditional database is a disk-based
database. Its processing time is indeterministic, because it
involves the disk access, data transfer between internal and
external memory, buffer management, waiting list and lock
management. This property makes traditional database
system unable to achieve the requirements of real-time
transactions that are of high efficiency and time
deterministic. Therefore, the introduction of in-memory
database makes it possible to have most transactions
happen in the memory, which avoids disk I/O during realtime transactions, and reduces uncertainty and enhances
efficiency [6,8].
R e la tio n a l D a ta b a se
Figure 1. Real time database system structure
The real-time task dispatch management provides
coordination between activities of real-time tasks to access
the real time database. The in-memory database holds the
main effective range of data in the memory to avoid disk
I/O during real-time business implementation and enhances
implementation efficiency. It is the core part of the
database. The detailed elaboration will be presented in
section 5. The relational database in this architecture can
be used as interface for further development, as well as
storage medium for in-memory database. The I/O dispatch
is responsible for data synchronization of memory database
and relational database, through which the real-time
database can be seamlessly integrated with traditional
database.
5.1 Data Model of Database
An in-memory database, as the extension of a relational
database, has similarity with a relational database. A table
is the key part of an in-memory database. A table describes
a group data object entity of the same type. While a table is
two-dimensional in relational database, a table in an inmemory database can be multi-dimensional. Although a
two-dimensional table is deceptively simple to design and
operate, this simplicity requires complex joins of many
4.1 Real-time Task Dispatch Management
In SCADA, there are nesting, communication and
cooperation relations among real-time tasks. The derived
data object of a task is possibly the input data object of
another task. The data object and the task are dependent on
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tables when doing data analysis over history or over one
property [5].
The introduction of a multi-dimensional table structure
allows an in-memory database to process data over history
or over one property and save storage space in memory.
Furthermore it speeds up users’ access to data from any
dimension and facilitates data analysis.
to get partially correct but prompt data, rather than accurate
but obsolete data. Thus, such risk is tolerable [1,2].
5.3.2 Standard I/O
To make up for the insufficiency of a fast access
interface, the database can provide another access interface- standard I/O. This access interface needs to provide
database security and data integrity check. It also needs to
provide an effective network access mechanism and strict
concurrency control. As the manipulation of any data in a
database must interact with a database server, this kind of
visit mechanism has reduced efficiency compared with a
fast access interface. This kind of mechanism can be used
in a distributed application, human-machine interface
(HMI), and interface with other applications [1,2].
5.2 Real-time Resource Management
5.2.1 Classification of Data Storage Form
There are two types of data storage in a database, storage
in in-memory database, and storage in the disk. In a
SCADA system the amount of data is extremely huge. Not
all data must be put into an in-memory database. Data
storage form can be decided according to the following data
characteristics [3]:
• Timeliness: in a real-time database each data
object has a lifecycle; the data with a short
lifecycle must be preserved in an in-memory
database.
• Effectiveness: the data with frequent access must
be stored in the memory database.
• Crucial nature: In order to guarantee the system
efficiency, the crucial data should be preserved in
the in-memory database.
5.4 Communication
The basic requests of real-time database network
communication are high efficiency and reliability. Many
mechanisms such as the client/server interaction, streaming
transmission, and creation of a mirror database at other
sites can be used to realize data distribution. These
mechanisms have different advantages and disadvantages
and are applicable to different application area [7,8,9].
5.2.2 Data Security and Restoration in Database
The real-time task cannot be interrupted during the
restoration process, which means that the data restoration
should not affect the operation of a real-time system. A
traditional database is usually restored by using rollback
segment. But it is difficult for this method to satisfy the
real-time request. A simple reliable restoration method is
to create a mirror database for essential data. A mirror
database can backup the entire database at local or other
points of the network. When the data is crashed at a local
site, data can be restored from the backup point, which
enhances the essential data security [7].
5.4.1 Client/Server Interaction
In the client/server interaction communication, a client
accesses the remote database (server) through the local
real-time database. For each access there is a series of
interactions between the client and the remote server to
establish the connection. This is a typical application of
client/server used for a majority of database management
systems. As the size of message is small and interaction
frequency is high, it is easy to cause network overburden,
low efficiency and server overload. The advantage of
client/server interaction is no data synchronization
necessary among databases and it’s applicable for small
amount of real time data sharing.
5.3 Database Operation
5.4.2 Streaming Transmission
Similar to client/server interactions, streaming
transmissions "deliver" the data to the client according to
request. But the basic principle of streaming transmission is
that the server will deliver the data to the client periodically
as requested by the client at the first connection. Thus, it
can avoid the client/server frequent connection
establishment process. Also, the server can reorganize
messages according to client request to increase the average
message size and reduce the frames passing through the
network. Thus transmission efficiency is enhanced.
5.3.1 Fast Access Interface
Fast data access is basic requirement of a real-time
database. In order to enhance operating efficiency of
system analysis and decision software, a real-time system
needs to provide a fast database access interface. Through
this interface, database access efficiency can be the same as
memory variable operation. One possible realization is to
map an entire partition of the database to the shared
memory. And return the result in a C structure through an
Application Program Interface (API). Such a mechanism
provides effective data manipulation ability and
circumambulates database security and data integrity check,
which introduces a certain degree of risk. However the
guideline of this interface design for a real-time database is
5.4.3 Creation of Mirror Database at Other Site
A different site mirror database can help to keep data
synchronization efficiently. It is applicable to special
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applications when massive data is transferred. The main
principle is to mirror some basic unit of the database, such
as the whole database, a partition of the database or tables
to other sites. This technique helps to quick start SCADA
on one node with the mirror database specified at other
node. Moreover, it can help to fast update realize the
database remotely.
This research work has been made possible through the
support of the DoD MURI Fund # N00014-04-1-0404.
9. References
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6. Real time database implementation
Since designs of EMS databases have revolved around
stringent real-time performance requirements as stated
above, specialized proprietary databases are used to adapt
to high transaction rates, high volumes of information, and
the diversity of data characteristic of the EMS environment.
At the same time, to make real time database plug
compatible application software, standard interface are
required to provide data access [10].
One of the leading real time databases is the PI system,
developed by OSIsoft. PI system is a large real-time
/historical database used for data collection, storage and
monitor. PI database uses swinging-door compression and
particular filtration technique to process the raw data before
entering the archive, and achieves efficient archive storage
without losing significant data. PI database platform also
provides standard data access through its Data Access
Package (DAP) including API, SDK, ODBC and OLEDB
[11].
7. Summary
The real time database in SCADA extends raditional
database to include in-memory database. Such RTDBMS
are designed to operate in the harsh environment of realtime systems, with strict requirements for resource
utilization, and are ready to provide the performance and
reliability required by real-life applications.
In implementation of the real-time database for a
dispatch automation system, we need to consider the use
standard data model--CIM for the data objects. An inmemory database is the core part of a real-time database.
Special mechanisms are required for real-time task
scheduling and concurrence control, data storage
management and crash recovery. Also speed interface
needs to be provided to handle data access.
Above all, till now real time database is not a
commercial one, In-memory database need to design
according to application, we need to consider the
management system design and also database schema
design. This paper discusses the technique issues to
implement a real-time database and provide a general
guideline for the key functions. Finally, a sample real time
database, PI database platform is briefly introduced.
8. Acknowledgements
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