Chapter 2. Using the Cascade DataHub
- Table of Contents
- 2.1. Running the Cascade DataHub
- 2.2. Using the Cascade DataHub Viewer
- 2.3. Features
2.1. Running the Cascade DataHub
This section describes the steps necessary to start the Cascade DataHub.
Ensure /usr/cogent/bin is in your PATH (check this using the set command).
Start the Cascade QueueServer and Cascade NameServer (no ampersand '&' is required) using the qserve and nserve commands:
[sh]$ qserve [sh]$ nserve
The qserve program must always be started before nserve. In QNX, qserve and nserve should both be started before TCP/IP to avoid a connection race condition.
Start the Cascade DataHub using the datahub command with the appropriate arguments, (no ampersand '&' is required). For example:
[sh]$ datahub The example above would start the datahub in the default domain (called "default"). Another example is: [sh]$ datahub -d test This second example starts the datahub in an application domain called "test".
Since these commands are generally given together you may wish to group them in a batch file and call that batch file from the /etc/config/sysinit.<node> file.
Once these command are run you can check which tasks are running by using the nsnames command. For example:
[sh]$ nsnames
Task Queue Domain Node TaskID
sc/nserve sc/nserve 1 72
/db/default datahub_3c58 default 1 15367
Note that the datahub Queue and the Task ID's shown in the example will be different for every user.
2.1.1. Configuration
The Cascade DataHub does not require you to configure the names or types of data points you will be using in your system. If a program writes a data value to the datahub and the point does not exist, the datahub will create the point. If a program registers for exceptions for named points that currently do not exist in the datahub then the datahub will create those points and register the client program at the same time. If a program tries to read the value of a point that doesn't exist in the datahub, then the datahub will create the point and return a default zero value with a zero confidence to the client program.
Typically, the Cascade DataHub is started before other application modules. Then, other tasks are started that communicate with the datahub (to request exceptions, send data, or both). Memory (RAM) is allocated for the points as they are created by the datahub.
2.1.2. Testing the Cascade DataHub installation
Cogent has provided three programs that you can use to test your Cascade DataHub installation.
2.1.2.1. Writing data to the datahub
Cogent provides you with the writept utility so that you can write new data values into the datahub. Its syntax is described in writept, and its source code is listed in the Writing data to the Cascade DataHub appendix of the Cogent API manual.
2.1.2.2. Reading data from the datahub
The readpt utility allows you to read data values from the datahub. See readpt for details about the syntax for readpt, and see the source code for this program in Reading from the Cascade DataHub appendix of the Cogent API manual.
2.1.2.3. Registering for exceptions with the datahub
The waiter utility registers for exceptions with a datahub and displays any new point values that it receives. The source code for this example will help you develop applications that effectively utilize the datahub. See waiter for details about the syntax for waiter, and see its source code in Registering for exceptions from the Cascade DataHub appendix of the Cogent API manual.
| If you are using Cascade Connect with the Cascade DataHub then you should start the Cascade DataHub before you initiate the Cascade Connect connection from the MS-Windows computer. This allows Cascade Connect to identify the Cascade DataHub as a client on start up, and to register for exceptions with the datahub. |
2.1.2.4. Testing the Cascade DataHub
Run the appropriate commands (qserve, nserve, and datahub), as described above.
In a shell, type the command:
[sh]$ writept test1 25
This will create the point 'test1' in the datahub and assign it the value 25. writept does not return any value.
At the shell, type the command:
[sh]$ readpt test1 Point: test1 Value: 25 Conf: 100 Lock: 0 Secur: 0These tests establish that you can read and write data points to the Cascade DataHub.