Allow passing location hint as a property for better co-location

[sradevski]

Currently containers get provisioned in arbitrary locations, which causes significant latency issues if eg. the container communicates with another service or database in a distant region, as mentioned here and here.

Ideally we have very explicit control of where the durable object and container get provisioned, but even having some degree of confidence in them being provisioned in a region near the requested one (basically what locationHint achieves for durable objects) would make sandboxes much more usable.

My understanding might be wrong, but if a locationHint field is exposed that is passed to the durable object, both the durable object and container will likely get provisioned in the vicinity of the requested location, but let me know if my understanding is wrong or if there are plans to control the placement through better mechanisms.

[whoiskatrin]

@sradevski This is partially wrong. locationHint would influence DO placement, but the container placement is a separate decision by the platform. Today they're often in different locations. We are working on fixing this, but it's not guaranteed yet.

How container placement actually works

• Container placement is not driven by DO location. When ctx.container.start() is called:
• The platform selects the nearest location with a pre-fetched image — nearest to the originating request, not the DO
• There's a finite set of pre-warmed locations, so the container may end up far from both the user and the DO
• Once a container is running, subsequent requests route to that same location regardless of origin
• If the container sleeps and restarts, it may land in a completely different location