Each ClearSCADA server can send polls to other servers or clients that are connected to the server via a TCP/IP connection. The poll is a request for a response, and is used to check that there are no open connections that should be closed.
During normal operation, the ClearSCADA server has open connections to any other servers in the arrangement and each of the clients. If a client or server is shutdown, the TCP/IP connection to the ClearSCADA server should be closed. However, sometimes there may be conditions on the network that result in the TCP/IP connection remaining open even though the client or server has been shutdown. As each open connection uses a ClearSCADA license, this can result in users with valid licenses not being able to log on due to other connections remaining open (if the license limit has been reached).
The ClearSCADA server uses the Socket Keep Alive feature to check that the servers and clients at the other end of the TCP/IP connections are still using the connection. If the clients or other servers respond to a poll that is sent, the TCP/IP connection is being used and is intentionally open. If there is no response, the ClearSCADA server will send further polls (up to 5) to check that the connection is definitely not being used. If there are still no responses, the server will close the connection.
You can use the Socket Keep Alive settings to define whether the Send Poll feature is enabled or disabled.
If you enable the feature, you can also define:
- How long the server waits between each poll.
- How long the server waits before sending another poll (if there was no reply to the first poll).
To alter the Socket Keep Alive settings:
- Access the ClearSCADA Server Configuration Tool.
- Expand the Global Parameters branch of the tree structure.
- Select the Advanced entry to display the Advanced section.
- Define the settings by using the check box and fields in the Socket Keep Alive box:
- Send Poll—Select the check box to enable the feature or clear it to disable the feature. If you disable the feature, your system may have open connections to clients or servers that have shutdown which could result in licensed users being unable to log on (if the license limit has been reached).
- Time between polls—Define the number of seconds between polls being sent to the client at the other end of the TCP/IP connection. The default setting of 300 is usually appropriate.
- If there are system users that have valid licenses but are unable to log on, the Time between polls setting may be too slow. By decreasing the number of seconds, you can instruct the server to sends polls more quickly.
- Before you decrease the Time between polls, you should assess your network, as lower Time between polls settings can result in increased traffic, which may have an impact on your network.
- If you do need to change the Time between polls, we recommend that the value is no less than 60 seconds.
- Interval—Define how often the server attempts to verify that the TCP/IP connection is being used. Typically, the default setting of 1 second is appropriate.
- Apply the changes to the server.
Now that you have defined the socket keep alive setting, you can proceed with other aspects of the server configuration.