HostID Truncation can cause collisions

[mathewc]

Currently when generating a default host ID we use the host name (slot host name) and we truncate to 32 characters max (code here). This ensures that the generated ID conforms to the core SDK length restrictions (code here).

This truncation can of course open the possibility for naming collisions, particularly in the case of slots. In slot scenarios, if the site name is over 32 characters long and a slot is created that starts with the same 32 characters and is only disambiguated in later characters, both the production and the slot site will be using the same host ID. This can lead to issues. For example, TimerTrigger uses the host ID as a component of the blob lease path. In this case the timer function will only be able to run in one of the sites because they're competing for the same lock. Similarly, customers often have apps deployed to different regions using the same long naming path, varying only in later name components (e.g. region/environment), and can run into this.

More information on can be found in AZFD0004: Host ID collision and Host ID Collisions.

[paulbatum]

This is a dangerous change to make for v1 but we should fix it in v2. This would require some updates to the scale controller as I believe it has this logic duplicated.

[paulbatum]

I've confirmed the scale controller will need an update, but it should be straightforward once we know what the new logic for generating the host ID is.

[paulbatum]

We had to punt this due to higher priority issues and the need to coordinate the change across multiple components. Revisit again in V3.

[mathewc]

As a workaround for customers running into this issue, in Functions v2, you can set an explicit HostID in app settings, using a different ID for each environment. The app setting name to use is AzureFunctionsWebHost:hostId. For Functions v1 you can specify the ID in host.json via the hostId property. The host ID value should be unique for all apps/slots you're running. The important thing is that the IDs are under 32 characters. The restrictions for HostIds that the value must satisfy are here. Another way to generate an ID would be to take a GUID, remove the dashes and make it lower case, e.g. 1835D7B5-5C98-4790-815D-072CC94C6F71 => 1835d7b55c984790815d072cc94c6f71

[ankitkumarr]

Had a customer issue last week, where this truncation caused collision between multiple function apps sharing the same storage account.

Fabio Cavalcante (@fabiocav), is this already planned for V3?

[fabiocav]

Deferring this work as it would have scale controller dependencies and does not align with the timing.