In order to achieve a better fault tolerance
WinCC OA Managers
can connect to other WinCC OA
Managers running on different computers via redundant network
connections (this means connecting through two network connections
(= IP connection)).
Redundant network connections can be used for remote
user interfaces, for redundant systems (see also Redundancy,
basics), between distributed systems (Connections of the Dist
Manager - see also Distributed
systems, basics) or for any manager running on a different
computer as its "server" (e.g. drivers or API Managers
running on a separate computer).
The used network connections have to be defined
in the hosts
file. Under Windows the hosts
file is located in the directory <SystemRoot>/system32/drivers/etc/,
under Linux in the directory /etc/.
Basically you have to notice, that the names of the redundant
network connections in the hosts file must not be
the same as the hostname. On that score aliases for the IP addresses
are defined in the hosts
file.
 Caution
Note that:
- the entry for the localhost, 127.0.0.1 localhost, must be defined
in the host file
- all computers of a distributed/redundant system must have the
same IPv6 state (active or inactive). A mixed mode is not supported
- IPv6 entries are supported from Windows 7
- IPv6 entries are not allowed under Solaris
The following example will show you how the configuration
has to be done, in order to guarantee a proper functionality of
redundant network connections during operation.
 NOTE
If the hosts file contains a configuration
error, an error message is shown in the Logviewer (e.g. WCCOActrl (1), 2005.08.25 13:37:45.775,
SYS, WARNING, 0, , Possible mismatch
in host configuration: host1=eitst010xp host2=eitst0172k-1).
Settings in the hosts file
Configuration of a redundant system, which is equipped
with a redundant network connection between both servers (Redu
host 1 = server1, Redu host 2 = server2). Both computers have
two network controllers (main controller and secondary route).
IP addresses for server1: 1.1.1.11 (Main controller)
and 1.1.2.11 (Secondary route)
IP addresses for server2: 1.1.1.12 (Main controller)
and 1.1.2.12 (Secondary route)
The hosts
file has to look like the following on both computers:
1.1.1.11 server1
server1-1
1.1.2.11 server1-2
1.1.1.12 server2 server2-1
1.1.2.12 server2-2
When defining config entries (e.g. with redundancy)
the first connection has to be the connection, which has the same
IP address like the computer. According to the hosts
file shown above, the "data" entry in the config file
for a redundant project looks like the following (on the
computer "server1"):
data = "server1-1,server1-2$server2-1,server2-2"
The correct host name has to stand next to the
IP address of the main network controller. This guarantees that
CTRL functions like dataHost(), eventHost(),
... return always the correct host name. The aliases for
the IP connection stand next to the host name.
Caution
The entries with <Host name>-1 resp. -2
are aliases for the redundant network connections. The aliases
must not be the same as the host name. However you can also use
other alias names for the connections!
With a "PING"
command you can check the configuration:
All defined names in the hosts file have to return
the correct IP address (ping <Name>).
For example "ping server1-1"
must return "1.1.1.11"
("ping server1" also has to return "1.1.1.11"),
"ping server2-2" must return "1.1.2.12".
A "PING"
on the IP addresses must return the correct names, which are defined
first in the file (ping -a <IP address>).
For example "ping -a 1.1.1.11"
must return "server1"
(and not server1-1), "ping
-a 1.1.2.12" must return "server2-2".
The redundant network connection is configured
in the config file. Each entry that specifies a host name (or
IP address) to connect to (e.g. event, data or distPeer) has to
be changed so that the entry contains the names of both IP connections
of the remote host separated by a ",".
For example, if the server (called "server")
has two network controllers with IP connections called "server_n1";
and "server_n2" the
config entry looks like following:
event = "server_n1,server_n2"
 Example
1
Remote UI to a
single system (Host name "server",
IP connections called "server_n1"
and "server_n2")
The config file of the remote UI
must contain the following entries:
[general]
data = "server_n1,server_n2"
event = "server_n1,server_n2"
Example
2
Distributed system ("System1"
is a single system and "System2"
is a redundant system). System1 establishes a connection to System2
via a redundant network connection (Host names of System2: System2_host1
and System2_host2; IP connections of System2: Sys2_host1_n1, Sys2_host1_n2,
Sys2_host2_n1, Sys2_host2_n2).
The config file of System1, which connects to System2,
must contain the following entries:
[general]
distributed = 1
[dist]
distPeer = "Sys2_host1_n1,Sys2_host1_n2$Sys2_host2_n1,Sys2_host2_n2"
2 #Connect to both hosts of the
System2 via redundant network connection
Note
Note that if you define a server port you have
to define one port for all connections e.g. host1,host2:port.
If you do not define a port the default port is used.
Furthermore the user must have a HOME
directory on the local computer and no HARD mounts are allowed!
Test of Redundant Network Connections
When testing the redundant network
connections, please take the following two points into consideration:
Each network connection must
be tested separately. Make sure if one the network
connections is deactivated, each address for this connection
(host1-1 <---> host 2-1 or host 1-2 <---> host
2-2) is not accessible.
Message Processing and redundant Network Connections
in a distributed system
If you configure redundant network connections
in a distributed system, the messages are sent using two connections.
When the system receives a message, it accepts the message immediately.
If the system then receives the identical message through the
second connection, it discards the message. The system does not
wait until it receives the message through two network connections. |
