service-bus | PHP Service Bus implementation | Pub Sub library

Yes; the Service Bus guarantee is at-least-once. Duplicates are possible when publishing fails with an ambiguous state and is retried. Using the duplicate detection feature can help to guard against this, but requires that your application take responsibility for assigning unique MessageId values.

Duplicates can also occur if a message is received and is not settled. Once its lock has expired, the message is eligible to be received again. Applications receiving messages should ensure that processing is idempotent and any needed detection or guards for duplicates are in place.

The article you linked to has a big purple Note block right after the first paragraph. It states two things. The second statement is:

You can't enable or disable duplicate detection after the queue or topic is created. You can only do so at the time of creating the queue or topic.

This means you cannot update an existing Queue to allow for Duplicate Message Detection.

EDIT:
A small addition to the above: there is an --enable-duplicate-detection parameter for the az servicebus queue update Azure CLI command. Trying to use this, however, returns a BadRequest with SubCode 40000 and a specific error message.

Looking at Service Bus Resource Manager exceptions - Error: Bad Request, it states the following:

Error message Description Recommendation SubCode=40000. The value for the 'requiresDuplicateDetection' property of an existing Queue(or Topic) cannot be changed. The value for the 'requiresDuplicateDetection' property of an existing Queue(or Topic) cannot be changed. Duplicate detection must be enabled/disabled at the time of entity creation. The duplicate detection configuration parameter cannot be changed after creation.

EDIT:
According to the exceptions documentation linked to above (under Bad Request, second last item in the table), RequiresSessions also needs to be specified during creation:

Support for sessions should be enabled at the time of entity creation. Once created, you cannot enable/disable sessions on an existing entity (queue or subscription)

I have a blog post where all three filter types are explained

1. Boolean filters
2. Correlation filters
3. SQL filters

There are several differences between correlation and SQL filters. Just to list a few

• Correlation filters are intended to operation on a few system properties such as ContentType, MessageId, ReplyTo, ReplyToSessionId, SessionId, To, and CorrelationId. SQL filters are not constrained to system properties only and can operation on user/applicaiton properties/headers.
• Within a correlation filter, multiple conditions are AND-ed. SQL filter is much more flexible and can use AND as well as OR operations.
• The cost of evaluating filter expression is minimal and almost immediate w/o extra compute required. SQL filters can require more compute but offer more flexibility.

When defining multiple rules for a subscription, the rules are OR-ed. I.e. it's enough for a single rule to be evaluated as truthy for the message to be sent to a subscription.

What I don't understand, is when the message is sent, how does it know which subscription to go to?

This is accomplished through topic filters. Each message that gets sent to a Topic is "kind of broadcasted" (for the lack of better term) to every Subscription and the Subscription only accepts a message when that message matches one of the filter rules specified for that Subscription.

You can learn more about it here: https://docs.microsoft.com/en-us/azure/service-bus-messaging/topic-filters.

The trigger creates and manages the ServiceBusClient and any senders/receivers that are used. Since the trigger is already handing messages to your function, normally, you wouldn't need to bind to the receiver directly for use in the body of the function; for message settlement and related activities, you'd make use of the ServiceBusMessageActions as demonstrated here.

I don't believe that it is currently possible to bind directly to a ServiceBusReceiver via the trigger; it appears we redirect that to the message actions implicitly (src) - but I haven't tried to verify that.

It appears that your network environment is not allowing traffic for ports 5671 and 5672. This is often resolved by configuring the ServiceBusClient to use the web sockets transport which uses port 443 (default HTTPS port):

For throttling, each call to SendAsync or ReceiveAsync counts once against the limit. In the case of sending, each batch is packaged as a single AMQP message and will consume a single credit.

As you don't want downgrading from .Net 5.0 version, You can use the Azure.Messaging.ServiceBus client library instead of using Microsoft.Azure.ServiceBus.

Try the below code to receive/consume a message from service bus using the Azure.Messaging.ServiceBus client library: