Question: The requirement is that we have an Azure Service Bus topic which we want to set up as Push Subscriber in Google PubSub Topic. This way any messages published to Google PubSub Topic will be pushed to Azure SB Topic without any intermediate layer involved.

On Paper this should work because messages can be published to Azure SB Topic using API framework and Google PubSub can also configure API as Push Subscribers. 

I have gone through following articles but couldn't make this linking workout.

* Azure SB as API: <https://learn.microsoft.com/en-us/rest/api/servicebus/send-message-to-queue>
* Google PubSub Push Subscriptions: <https://cloud.google.com/pubsub/docs/push>

Has anyone done this kind of linking before?

Thanks in Advance

Answer: 
It would be nice to create a pub/sub push subscription, which pushes to Azure Event Hub. I ran into the same problem when configuring this setup.

Pub/sub push subscriptions currently do not support custom Authorization headers, which are needed as per the Event Hub [documentation](https://learn.microsoft.com/en-us/rest/api/eventhub/send-event).

```
POST https://your-namespace.servicebus.windows.net/your-event-hub/messages?timeout=60&api-version=2014-01 HTTP/1.1  
Authorization: SharedAccessSignature sr=your-namespace.servicebus.windows.net&sig=your-sas-key&se=1403736877&skn=RootManageSharedAccessKey  
Content-Type: application/atom+xml;type=entry;charset=utf-8  
Host: your-namespace.servicebus.windows.net  

{ "DeviceId":"dev-01", "Temperature":"37.0" }

```

So the only two options I see here:

1. Push setup: Create a cloud function or dataflow job in Google Cloud. Push pub/sub events to this endpoint and then pass on the event to Azure Event hub with the appropriate headers.
2. Pull setup: Poll a pub/sub pull subscription from the Azure side with an Azure function or WebJob.

Both options require extra compute resources, so definitely not the preferred way of doing this. I would always try a push setup first, since then you don't have t o continuously have a polling job running in the background.

I have hopes that pub/sub push subscriptions will support custom headers anywhere in the future. Lately some other useful features have been added like: Dead Lettering, Message Ordering and Partitioning (Lite Topics). Hopefully they will add custom headers as well.
I have a workaround on this problem.

1- you can create cloud function on google cloud that can push data for you on azure service bus.

2- you can develop web job on azure that will run continuously and check google pub/sub topic with the help of provided connection string and google pub/sub supporting libraries. 

From above mentioned solution you can get data from google cloud and push to your azure service bus.