The CloudEvent source gets overwritten by the emulator

[dzhemaptula-tomtom]

Describe the bug
The original source of the CloudEvent gets overwritten by the topic.

To Reproduce
Steps to reproduce the behavior:

1. Send any event with a custom source set on the cloud event
2. Read event

Expected behavior
The source should stay the same as received originally.

Additional context
Permalink to where this issue happens

As described above. In azure the original source from the cloud event is kept when forwarding it to the subscribed endpoints.

[jochenhebbrecht]

Hi,

I stumbled into the same problem as the topic poster today.

The CloudEvent I'm posting towards the EventGrid topic follows this format:

{
    "id": "5e3ab450-f4a5-40cc-b1ec-819b0bd1bc26",
👉  "source": "/my-source-identifier",
    "data": {
        "..."
    },
    "type": "my.type.of.event",
    "time": "2025-01-17T14:07:03.303787590Z",
    "specversion": "1.0",
    "datacontenttype": "application/json"
}

On the subscription endpoint, I suddenly see this event:

{
    "id": "5e3ab450-f4a5-40cc-b1ec-819b0bd1bc26",
👉  "source": "/subscriptions/00000000-0000-0000-0000-00000000/resourceGroups/my-resource-group/providers/Microsoft.EventGrid/topics/my-event-grid-topic",
    "data": {
        "..."
    },
    "type": "my.type.of.event",
    "time": "2025-01-17T14:07:03.303787590Z",
    "specversion": "1.0",
    "datacontenttype": "application/json"
}

On the subscription endpoint, we actually rely on the value in the source field. On Azure itself, the source remains the same and stable. The behaviour of the emulator is in this way unexpected and potentially wrong. A potential (quick?) fix would be to not override the source field if it's already there. An example patch:

diff --git a/src/EventGridEmulator/EventHandling/CloudEventHttpContextHandler.cs b/src/EventGridEmulator/EventHandling/CloudEventHttpContextHandler.cs
index 44bef91..22ef190 100644
--- a/src/EventGridEmulator/EventHandling/CloudEventHttpContextHandler.cs
+++ b/src/EventGridEmulator/EventHandling/CloudEventHttpContextHandler.cs
@@ -21,7 +21,10 @@ internal sealed class CloudEventHttpContextHandler : BaseEventHttpContextHandler
     {
         foreach (var @event in cloudEvents)
         {
-            @event.Source = $"{SubscriberConstants.DefaultTopicValue}{topicName}";
+            if (String.IsNullOrEmpty(@event.Source))
+            {
+                @event.Source = $"{SubscriberConstants.DefaultTopicValue}{topicName}";
+            }
         }
     }

[meziantou]

If the emulator behavior differ from EventGrid, then it should be fixed. Would you like to create a PR with the suggested fix?

[jochenhebbrecht]

Thanks for your reply @meziantou! According to the contributing guidelines on this repository, I noticed you don't accept external pull requests (yet). But I assume you allow a PR from a forked repository?

[meziantou]

You can create a PR from your fork. To accept the PR, and because of our compliance process, we'll create a new PR with your commits (you will still be author of the commit in the git history).

[dzhemaptula-tomtom]

Hey that's great! I already have a fork, I'll get to it soon. Thanks both for the updates!

[dzhemaptula-tomtom]

Opened #107 as a fix

[jochenhebbrecht]

Happy to announce that with version 0.4.5, I have no longer have the issue in which I lose the contents of the source field in the CloudEvent message 🥳