Ref T9253. Broadly, this realigns Allocator behavior to be more consistent and straightforward and amenable to intended future changes.
This attempts to make language more consistent: resources are "allocated" and leases are "acquired".
This prepares for (but does not implement) optimistic "slot locking", as discussed in D10304. Although I suspect some blueprints will need to perform other locking eventually, this does feel like a good fit for most of the locking blueprints need to do.
In particular, I've made the blueprint operations on $resource and $lease objects more purposeful: they need to invoke an activator on the appropriate object to be implemented correctly. Before they invoke this activator method, they configure the object. In a future diff, this configuration will include specifying slot locks that the lease or resource must acquire. So the API will be something like:
$lease ->setActivateWhenAcquired(true) ->needSlotLock('x') ->needSlotLock('y') ->acquireOnResource($resource);
In the common case where slot locks are a good fit, I think this should make correct blueprint implementation very straightforward.
This prepares for (but does not implement) resources and leases which need significant setup steps. I've basically carved out two modes:
- The "activate immediately" mode, as here, immediately opens the resource or activates the lease. This is appropriate if little or no setup is required. I expect many leases to operate in this mode, although I expect many resources will operate in the other mode.
- The "allocate now, activate later" mode, which is not fully implemented yet. This will queue setup workers when the allocator exits. Overall, this will work very similarly to Harbormaster.
- This new structure makes it acceptable for blueprints to sleep as long as they want during resource allocation and lease acquisition, so long as they are not waiting on anything which needs to be completed by the queue. Putting a sleep(15 * 60) in your EC2Blueprint to wait for EC2 to bring a machine up will perform worse than using delayed activation, but won't deadlock the queue or block any locks.
Overall, this flow is more similar to Harbormaster's flow. Having consistency between Harbormaster's model and Drydock's model is good, and I think Harbormaster's model is also simply much better than Drydock's (what exists today in Drydock was implemented a long time ago, and we had more support and infrastructure by the time Harbormaster was implemented, as well as a more clearly defined problem).
The particular strength of Harbormaster is that objects always (or almost always, at least) have a single, clearly defined writer. Ensuring objects have only one writer prevents races and makes reasoning about everything easier.
Drydock does not currently have a clearly defined single writer, but this moves us in that direction. We'll probably need more primitives eventually to flesh this out, like Harbormaster's command queue for messaging objects which you can't write to.
This blueprint was originally implemented in D13843. This makes a few changes to the blueprint itself:
- A bunch of code from that (e.g., interfaces) doesn't exist yet.
- I let the blueprint have multiple services. This simplifies the code a little and seems like it costs us nothing.
This also removes bin/drydock create-resource, which no longer makes sense to expose. It won't get locking, leasing, etc., correct, and can not be made correct.