Use LazyThreadSafetyMode.PublicationOnly to avoid caching exception when creating rabbitmq connection

[RayMMond]

Problem

abp/framework/src/Volo.Abp.RabbitMQ/Volo/Abp/RabbitMQ/ConnectionPool.cs

Lines 28 to 37 in 6b32974

	return Connections.GetOrAdd(
	connectionName, () => new Lazy<IConnection>(() =>
	{
	var connection = Options.Connections.GetOrDefault(connectionName);
	var hostnames = connection.HostName.TrimEnd(';').Split(';');
	// Handle Rabbit MQ Cluster.
	return hostnames.Length == 1 ? connection.CreateConnection() : connection.CreateConnection(hostnames);
	})
	).Value;

If connection factory failed and throw a exception (timeout...), Lazy<IConnection> will cache exception and rethrow it everytime when you access it's Value Property.

Solution

According to this link:

In PublicationOnly thread safety mode, multiple threads can invoke the initialization logic but the first thread to complete the initialization successfully sets the value of the Lazy instance.

Maybe we should add LazyThreadSafetyMode.PublicationOnly to Lazy<T>'s ctor ?

 return Connections.GetOrAdd( 
     connectionName, () => new Lazy<IConnection>(() => 
     { 
         var connection = Options.Connections.GetOrDefault(connectionName); 
         var hostnames = connection.HostName.TrimEnd(';').Split(';'); 
         // Handle Rabbit MQ Cluster. 
         return hostnames.Length == 1 ? connection.CreateConnection() : connection.CreateConnection(hostnames); 
  
     }, LazyThreadSafetyMode.PublicationOnly) 
 ).Value; 

[RayMMond]

And this is another approach:

            connectionName ??= RabbitMqConnections.DefaultConnectionName;

            var lazyConnection = Connections.GetOrAdd(connectionName, () =>
                new Lazy<IConnection>(() =>
                {
                    var connection = Options.Connections.GetOrDefault(connectionName);
                    var hostnames = connection.HostName.TrimEnd(';').Split(';');
                    // Handle Rabbit MQ Cluster.
                    return hostnames.Length == 1
                        ? connection.CreateConnection()
                        : connection.CreateConnection(hostnames);
                }));

            if (!lazyConnection.IsValueCreated)
            {
                Connections.TryRemove(connectionName, out _);
            }

            return lazyConnection.Value;

[realLiangshiwei]

We can't use LazyThreadSafetyMode.PublicationOnly, because it allows all threads to execute valueFactory, It will open a lot of connections.

I will try find a better way.

[realLiangshiwei]

And this is another approach:

            connectionName ??= RabbitMqConnections.DefaultConnectionName;

            var lazyConnection = Connections.GetOrAdd(connectionName, () =>
                new Lazy<IConnection>(() =>
                {
                    var connection = Options.Connections.GetOrDefault(connectionName);
                    var hostnames = connection.HostName.TrimEnd(';').Split(';');
                    // Handle Rabbit MQ Cluster.
                    return hostnames.Length == 1
                        ? connection.CreateConnection()
                        : connection.CreateConnection(hostnames);
                }));

            if (!lazyConnection.IsValueCreated)
            {
                Connections.TryRemove(connectionName, out _);
            }

            return lazyConnection.Value;

This way has a problem, Valuefactory will be executed directly only when calling lazyConnection.Value;, IsValueCreated is always false.
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