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implement provisioner (#1473)
* sketch out provisioner basics
* handle provisionable data
* stub out select_inherent_data
* split runtime APIs into sub-chapters to improve linkability
* explain SignedAvailabilityBitfield semantics
* add internal link to further documentation
* some more work figuring out how the provisioner can do its thing
* fix broken link
* don't import enum variants where it's one layer deep
* make request_availability_cores a free fn in util
* document more precisely what should happen on block production
* finish first-draft implementation of provisioner
* start working on the full and proper backed candidate selection rule
* Pass number of block under construction via RequestInherentData
* Revert "Pass number of block under construction via RequestInherentData"
This reverts commit 850fe62cc0dfb04252580c21a985962000e693c8.
That initially looked like the better approach--it spent the time
budget for fetching the block number in the proposer, instead of
the provisioner, and that felt more appropriate--but it turns out
not to be obvious how to get the block number of the block under
construction from within the proposer. The Chain API may be less
ideal, but it should be easier to implement.
* wip: get the block under production from the Chain API
* add ChainApiMessage to AllMessages
* don't break the run loop if a provisionable data channel closes
* clone only those backed candidates which are coherent
* propagate chain_api subsystem through various locations
* add delegated_subsystem! macro to ease delegating subsystems
Unfortunately, it doesn't work right:
```
error[E0446]: private type `CandidateBackingJob` in public interface
--> node/core/backing/src/lib.rs:775:1
|
86 | struct CandidateBackingJob {
| - `CandidateBackingJob` declared as private
...
775 | delegated_subsystem!(CandidateBackingJob as CandidateBackingSubsystem);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't leak private type
```
I'm not sure precisely what's going wrong, here; I suspect the problem is
the use of `$job as JobTrait>::RunArgs` and `::ToJob`; the failure would be
that it's not reifying the types to verify that the actual types are public,
but instead referring to them via `CandidateBackingJob`, which is in fact private;
that privacy is the point.
Going to see if I can generic my way out of this, but we may be headed for a
quick revert here.
* fix delegated_subsystem
The invocation is a bit more verbose than I'd prefer, but it's also
more explicit about what types need to be public. I'll take it as a win.
* add provisioning subsystem; reduce public interface of provisioner
* deny missing docs in provisioner
* refactor core selection per code review suggestion
This is twice as much code when measured by line, but IMO it is
in fact somewhat clearer to read, so overall a win.
Also adds an improved rule for selecting availability bitfields,
which (unlike the previous implementation) guarantees that the
appropriate postconditions hold there.
* fix bad merge double-declaration
* update guide with (hopefully) complete provisioner candidate selection procedure
* clarify candidate selection algorithm
* Revert "clarify candidate selection algorithm"
This reverts commit c68a02ac9cf42b3a4a28eb197d38633a40d0e3e6.
* clarify candidate selection algorithm
* update provisioner to implement candidate selection per the guide
* add test that no more than one bitfield is selected per validator
* add test that each selected bitfield corresponds to an occupied core
* add test that more set bits win conflicts
* add macro for specializing runtime requests; specailize all runtime requests
* add tests harness for select_candidates tests
* add first real select_candidates test, fix test_harness
* add mock overseer and test that success is possible
* add test that the candidate selection algorithm picks the right ones
* make candidate selection test somewhat more stringent
This commit is contained in:
Peter Goodspeed-Niklaus
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@@ -14,82 +14,88 @@ It aims to achieve these tasks with these goals in mind:
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- Validator assignments should not be gameable. Malicious cartels should not be able to manipulate the scheduler to assign themselves as desired.
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- High or close to optimal throughput of parachains and parathreads. Work among validator groups should be balanced.
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## Availability Cores
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The Scheduler manages resource allocation using the concept of "Availability Cores". There will be one availability core for each parachain, and a fixed number of cores used for multiplexing parathreads. Validators will be partitioned into groups, with the same number of groups as availability cores. Validator groups will be assigned to different availability cores over time.
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An availability core can exist in either one of two states at the beginning or end of a block: free or occupied. A free availability core can have a parachain or parathread assigned to it for the potential to have a backed candidate included. After backing, the core enters the occupied state as the backed candidate is pending availability. There is an important distinction: a core is not considered occupied until it is in charge of a block pending availability, although the implementation may treat scheduled cores the same as occupied ones for brevity. A core exits the occupied state when the candidate is no longer pending availability - either on timeout or on availability. A core starting in the occupied state can move to the free state and back to occupied all within a single block, as availability bitfields are processed before backed candidates. At the end of the block, there is a possible timeout on availability which can move the core back to the free state if occupied.
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Cores are treated as an ordered list and are typically referred to by their index in that list.
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```dot process
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digraph {
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label = "Availability Core State Machine\n\n\n";
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labelloc = "t";
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label = "Availability Core State Machine\n\n\n";
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labelloc = "t";
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{ rank=same vg1 vg2 }
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{ rank=same vg1 vg2 }
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vg1 [label = "Free" shape=rectangle]
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vg2 [label = "Occupied" shape=rectangle]
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vg1 [label = "Free" shape=rectangle]
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vg2 [label = "Occupied" shape=rectangle]
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vg1 -> vg2 [label = "Assignment & Backing" ]
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vg2 -> vg1 [label = "Availability or Timeout" ]
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vg1 -> vg2 [label = "Assignment & Backing" ]
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vg2 -> vg1 [label = "Availability or Timeout" ]
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}
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```
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```dot process
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digraph {
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label = "Availability Core Transitions within Block\n\n\n";
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labelloc = "t";
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splines="line";
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label = "Availability Core Transitions within Block\n\n\n";
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labelloc = "t";
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splines="line";
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subgraph cluster_left {
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label = "";
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labelloc = "t";
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subgraph cluster_left {
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label = "";
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labelloc = "t";
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fr1 [label = "Free" shape=rectangle]
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fr2 [label = "Free" shape=rectangle]
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occ [label = "Occupied" shape=rectangle]
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fr1 [label = "Free" shape=rectangle]
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fr2 [label = "Free" shape=rectangle]
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occ [label = "Occupied" shape=rectangle]
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fr1 -> fr2 [label = "No Backing"]
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fr1 -> occ [label = "Backing"]
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fr1 -> fr2 [label = "No Backing"]
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fr1 -> occ [label = "Backing"]
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{ rank=same fr2 occ }
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}
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{ rank=same fr2 occ }
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}
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subgraph cluster_right {
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label = "";
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labelloc = "t";
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subgraph cluster_right {
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label = "";
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labelloc = "t";
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occ2 [label = "Occupied" shape=rectangle]
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fr3 [label = "Free" shape=rectangle]
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fr4 [label = "Free" shape=rectangle]
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occ3 [label = "Occupied" shape=rectangle]
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occ4 [label = "Occupied" shape=rectangle]
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occ2 [label = "Occupied" shape=rectangle]
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fr3 [label = "Free" shape=rectangle]
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fr4 [label = "Free" shape=rectangle]
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occ3 [label = "Occupied" shape=rectangle]
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occ4 [label = "Occupied" shape=rectangle]
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occ2 -> fr3 [label = "Availability"]
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occ2 -> occ3 [label = "No availability"]
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fr3 -> fr4 [label = "No backing"]
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fr3 -> occ4 [label = "Backing"]
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occ3 -> occ4 [label = "(no change)"]
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occ3 -> fr3 [label = "Availability Timeout"]
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occ2 -> fr3 [label = "Availability"]
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occ2 -> occ3 [label = "No availability"]
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fr3 -> fr4 [label = "No backing"]
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fr3 -> occ4 [label = "Backing"]
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occ3 -> occ4 [label = "(no change)"]
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occ3 -> fr3 [label = "Availability Timeout"]
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{ rank=same; fr3[group=g1]; occ3[group=g2] }
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{ rank=same; fr4[group=g1]; occ4[group=g2] }
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}
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{ rank=same; fr3[group=g1]; occ3[group=g2] }
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{ rank=same; fr4[group=g1]; occ4[group=g2] }
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}
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}
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```
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## Validator Groups
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Validator group assignments do not need to change very quickly. The security benefits of fast rotation is redundant with the challenge mechanism in the [Validity module](validity.md). Because of this, we only divide validators into groups at the beginning of the session and do not shuffle membership during the session. However, we do take steps to ensure that no particular validator group has dominance over a single parachain or parathread-multiplexer for an entire session to provide better guarantees of liveness.
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Validator groups rotate across availability cores in a round-robin fashion, with rotation occurring at fixed intervals. The i'th group will be assigned to the `(i+k)%n`'th core at any point in time, where `k` is the number of rotations that have occurred in the session, and `n` is the number of cores. This makes upcoming rotations within the same session predictable.
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When a rotation occurs, validator groups are still responsible for distributing availability chunks for any previous cores that are still occupied and pending availability. In practice, rotation and availability-timeout frequencies should be set so this will only be the core they have just been rotated from. It is possible that a validator group is rotated onto a core which is currently occupied. In this case, the validator group will have nothing to do until the previously-assigned group finishes their availability work and frees the core or the availability process times out. Depending on if the core is for a parachain or parathread, a different timeout `t` from the [`HostConfiguration`](../types/runtime.md#host-configuration) will apply. Availability timeouts should only be triggered in the first `t-1` blocks after the beginning of a rotation.
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## Claims
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Parathreads operate on a system of claims. Collators participate in auctions to stake a claim on authoring the next block of a parathread, although the auction mechanism is beyond the scope of the scheduler. The scheduler guarantees that they'll be given at least a certain number of attempts to author a candidate that is backed. Attempts that fail during the availability phase are not counted, since ensuring availability at that stage is the responsibility of the backing validators, not of the collator. When a claim is accepted, it is placed into a queue of claims, and each claim is assigned to a particular parathread-multiplexing core in advance. Given that the current assignments of validator groups to cores are known, and the upcoming assignments are predictable, it is possible for parathread collators to know who they should be talking to now and how they should begin establishing connections with as a fallback.
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With this information, the Node-side can be aware of which parathreads have a good chance of being includable within the relay-chain block and can focus any additional resources on backing candidates from those parathreads. Furthermore, Node-side code is aware of which validator group will be responsible for that thread. If the necessary conditions are reached for core reassignment, those candidates can be backed within the same block as the core being freed.
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Parathread claims, when scheduled onto a free core, may not result in a block pending availability. This may be due to collator error, networking timeout, or censorship by the validator group. In this case, the claims should be retried a certain number of times to give the collator a fair shot.
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Cores are treated as an ordered list of cores and are typically referred to by their index in that list.
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## Storage
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Utility structs:
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