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"Receiving messages in order might increase latency."

It would be good to know what sort of overhead we are looking at here; very interested in this feature

From a max of 200 MB/s. To max of 1MB/s for each index.

The throughput limitation is 1MB/s per ordering key. The number of unique ordering keys allowed on a topic is limited only by what can be represented by a 1KB string, so very high throughput on a topic is still possible.

Disclaimer: I work on Cloud Pub/Sub and am the primary engineer on the ordering feature.

And do messages without an ordering key incur the same overhead?

In the unordered case, there should be no noticeable performance degradation. This was something that we looked into extensively from both the server side as well as with our client libraries.

Disclaimer: I work on Pub/Sub.

Stupid question, but is there an SLO/SLA around the max delay and bandwidth in the ordered case?

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