AVEVA Enterprise SCADA PubSub API Reference

PubSubConnection Class

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PubSubConnection Class

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Last UpdatedApr 08, 2026
4 minute read

HMI and SCADA

A PubSub connection enables an application to publish information associated with a topic string and to subscribe to topic strings, thereby receiving published information on those topics. Other publish-related and subscribe-related functionality, such as security restrictions, are also supported by the PubSub connection.

Syntax

[CLSCompliant(false)]
public sealed class PubSubConnection : IPubSubConnection, ISharedPubSubConnection

[CLSCompliant(false)]
public ref class PubSubConnection sealed : public IPubSubConnection, ISharedPubSubConnection

Remarks

The PubSub connection is thread-safe. However, it should be noted that Disconnect is not supported from callbacks as this may cause a deadlock.

If an application exists in the system "main", it may publish on a topic such as "main.myappinfo". All system primitive data types are supported but not complex objects. Another application can now subscribe to "main.myappinfo" and receive this information. If another piece of data was published on "main.myappinfo.info2", the subscribing application would also receive this as it is subscribed to the parent topic "main.myappinfo". There are two types of subscription supported: blocking and non-blocking subscriptions. With callback subscriptions, the application creates a callback PubSubCallbackMethod delegate. When a topic arrives on the callback, this delegate is called. With blocking subscriptions, the application must call to retrieve a message. will block until a message is available or a timeout expires. is used for blocking subscriptions, and SubscribeCB is used for callback subscriptions.

There are several methods for connecting to PubSub.

Note: Unless otherwise indicated, PubSub will attempt to maintain the connection and reconnect automatically.

The following connection syntax is supported:

Connect: A unique .Net PubSubConnection with a shared engine connection. Not used with GlobalConnection. No auto reconnect.

Connect(String,Int32): A unique .Net PubSubConnection object with a unique engine connection. Not used with GlobalConnection. No auto reconnect.

: Open the connection synchronously. Will block until the connection succeeds or a timeout expires. The PubSubConnection object can be shared or non-shared.

There is a global PubSub connection available for use. This connection is shared between objects, threads, and even different libraries, including libraries written in other languages, for example, C. It can be easily accessed and reduces the load on the PubSub system if used throughout the application, but it should not be cleaned up or disconnected as this may affect other code. The following example shows a global connection being used:

PubSubConnection.GlobalConnection.Publish("main.mytopic.i", i, PubSubHeaderData.PS_NORMAL_DATA);

Shared connections to the engine are supported through the GlobalConnection member and through the Connect function when no parameters are provided. At the .NET level, these functions provide different functionality, GlobalConnection is a shared .NET PubSub object, whereas Connect is used to connect a unique PubSubConnection object up to a shared connection to the PubSub system. These functions operate simiarly to a telephone system in a home or office (ignoring the fact that everyone can hear each other's conversations). A group of people speaking with a speaker phone would be similar to the GlobalConnection, one connection object shared by many. A group of people with separate handsets but the same telephone connection would be similar to the Connect function, many connection objects, one connection. A group of people with separate handsets and different telephone numbers in a conference call would be similar to non-shared PubSub connections, many connection objects, many connections. This final case can be achieved by using a non-global connection and either , or Connect(String,Int32) where the string is an arbitrary application name and the int is the message buffer size that PubSub should set aside for this connection.

Note: Shared connections are also sharing subscriptions and delivery queues. If callbacks are used for subscriptions, then message delivery can be made thread or object specific by having different callbacks for each thread or object; however, if blocking receive is used, then the threads must be aware that they may receive messages subscribed to by other threads and may not receive messages that they have subscribed to because they may be delivered to another thread calling . This scenario may be useful if an application wants to use thread-pooling for handling incoming messages. With thread pooling, several receive threads are busily processing messages and do not care where the subscriptions came from as they are simply trying to drain the queue as fast as possible. If the subscriptions are thread or object specific, then the application has two choices: do not use a global connection, or use callbacks.

PubSub supports an asynchronous connection model as well as synchronous. With asynchronous connections the application tells the PubSub system to maintain a connection as much as it can. PubSub will inform the application when the connection comes alive and goes away using the topic "pubsub.status.connection". Subscriptions can be done any time and will be maintained, but calls require a fully connected PubSub connection or else they will throw an exception.

Example

Using a Global Connection

PubSubConnection.GlobalConnection.OpenConnection(); int i = 5; PubSubConnection.GlobalConnection.Publish("main.mytopic.i", i, PubSubHeaderData.PS_NORMAL_DATA);

PubSubConnection.GlobalConnection.OpenConnection();
int i = 5;
PubSubConnection.GlobalConnection.Publish("main.mytopic.i", i, PubSubHeaderData.PS_NORMAL_DATA);

Using a Shared Connection

PubSubConnection PS = new PubSubConnection(); PS.Connect(); int i = 5; PS.Publish("main.mytopic.i", i, PubSubHeaderData.PS_NORMAL_DATA);

PubSubConnection PS = new PubSubConnection();
PS.Connect();
int i = 5;
PS.Publish("main.mytopic.i", i, PubSubHeaderData.PS_NORMAL_DATA);

Using a Unique Connection

PubSubConnection PS = new PubSubConnection(); PS.Connect("myApp", 50000); // Could also use PS.OpenConnection("myApp"); int i = 5; PS.Publish("main.mytopic.i", i, PubSubHeaderData.PS_NORMAL_DATA);

PubSubConnection PS = new PubSubConnection();
PS.Connect("myApp", 50000);
// Could also use PS.OpenConnection("myApp");
int i = 5;
PS.Publish("main.mytopic.i", i, PubSubHeaderData.PS_NORMAL_DATA);

AVEVA Enterprise SCADA PubSub API Reference