|
@@ -0,0 +1,429 @@
|
|
|
|
|
+---
|
|
|
|
|
+simd: '0046'
|
|
|
|
|
+title: Optimistic cluster restart automation
|
|
|
|
|
+authors:
|
|
|
|
|
+ - Wen Xu (Anza)
|
|
|
|
|
+category: Standard
|
|
|
|
|
+type: Core
|
|
|
|
|
+status: Implemented
|
|
|
|
|
+created: 2023-04-07
|
|
|
|
|
+feature: N/A (gated by command line flag instead)
|
|
|
|
|
+development:
|
|
|
|
|
+ - Anza - implemented
|
|
|
|
|
+ - Firedancer - implemented
|
|
|
|
|
+---
|
|
|
|
|
+
|
|
|
|
|
+## Summary
|
|
|
|
|
+
|
|
|
|
|
+During a cluster restart following an outage, make validators enter a separate
|
|
|
|
|
+recovery protocol that uses Gossip to exchange local status and automatically
|
|
|
|
|
+reach consensus on the block to restart from. Proceed to restart if validators
|
|
|
|
|
+in the restart can reach agreement, or print debug information and halt
|
|
|
|
|
+otherwise. To distinguish the new restart process from other operations, we
|
|
|
|
|
+call the new process "Wen restart".
|
|
|
|
|
+
|
|
|
|
|
+## New Terminology
|
|
|
|
|
+
|
|
|
|
|
+* `cluster restart`: When there is an outage such that the whole cluster
|
|
|
|
|
+stalls, human may need to restart most of the validators with a sane state so
|
|
|
|
|
+that the cluster can continue to function. This is different from sporadic
|
|
|
|
|
+single validator restart which does not impact the cluster. See
|
|
|
|
|
+[`cluster restart`](https://docs.solana.com/running-validator/restart-cluster)
|
|
|
|
|
+for details.
|
|
|
|
|
+
|
|
|
|
|
+* `cluster restart slot`: In current `cluster restart` scheme, human normally
|
|
|
|
|
+decide on one block for all validators to restart from. This is very often the
|
|
|
|
|
+highest `optimistically confirmed block`, because `optimistically confirmed
|
|
|
|
|
+block` should never be rolled back. But it's also okay to start from a child of
|
|
|
|
|
+the highest `optimistically confirmed block` as long as consensus can be
|
|
|
|
|
+reached.
|
|
|
|
|
+
|
|
|
|
|
+* `optimistically confirmed block`: a block which gets the votes from the
|
|
|
|
|
+majority of the validators in a cluster (> 2/3 stake). Our algorithm tries to
|
|
|
|
|
+guarantee that an optimistically confirmed block will never be rolled back.
|
|
|
|
|
+
|
|
|
|
|
+* `wen restart phase`: During the proposed optimistic `cluster restart`
|
|
|
|
|
+automation process, the validators in restart will first spend some time to
|
|
|
|
|
+exchange information, repair missing blocks, and finally reach consensus. The
|
|
|
|
|
+validators only continue normal block production and voting after consensus is
|
|
|
|
|
+reached. We call this preparation phase where block production and voting are
|
|
|
|
|
+paused the `wen restart phase`.
|
|
|
|
|
+
|
|
|
|
|
+* `wen restart shred version`: right now we update `shred_version` during a
|
|
|
|
|
+`cluster restart`, it is used to verify received shreds and filter Gossip
|
|
|
|
|
+peers. In the proposed optimistic `cluster restart` plan, we introduce a new
|
|
|
|
|
+temporary shred version in the `wen restart phase` so validators in restart
|
|
|
|
|
+don't interfere with those not in restart. Currently this `wen restart shred
|
|
|
|
|
+version` is calculated using `(current_shred_version + 1) % 0xffff`.
|
|
|
|
|
+
|
|
|
|
|
+* `RESTART_STAKE_THRESHOLD`: We need enough validators to participate in a
|
|
|
|
|
+restart so they can make decision for the whole cluster. If everything works
|
|
|
|
|
+perfect, we only need 2/3 of the total stake. However, validators could die
|
|
|
|
|
+or perform abnormally, so we currently set the `RESTART_STAKE_THRESHOLD` at
|
|
|
|
|
+80%, which is the same as what we use now for `--wait_for_supermajority`.
|
|
|
|
|
+
|
|
|
|
|
+## Motivation
|
|
|
|
|
+
|
|
|
|
|
+Currently during a `cluster restart`, validator operators need to decide the
|
|
|
|
|
+highest optimistically confirmed slot, then restart the validators with new
|
|
|
|
|
+command-line arguments.
|
|
|
|
|
+
|
|
|
|
|
+The current process involves a lot of human intervention, if people make a
|
|
|
|
|
+mistake in deciding the highest optimistically confirmed slot, it is
|
|
|
|
|
+detrimental to the viability of the ecosystem.
|
|
|
|
|
+
|
|
|
|
|
+We aim to automate the negotiation of highest optimistically confirmed slot and
|
|
|
|
|
+the distribution of all blocks on that fork, so that we can lower the
|
|
|
|
|
+possibility of human mistakes in the `cluster restart` process. This also
|
|
|
|
|
+reduces the burden on validator operators, because they don't have to stay
|
|
|
|
|
+around while the validators automatically try to reach consensus, the validator
|
|
|
|
|
+will halt and print debug information if anything goes wrong, and operators can
|
|
|
|
|
+set up their own monitoring accordingly.
|
|
|
|
|
+
|
|
|
|
|
+However, there are many ways an automatic restart can go wrong, mostly due to
|
|
|
|
|
+unforseen situations or software bugs. To make things really safe, we apply
|
|
|
|
|
+multiple checks during the restart, if any check fails, the automatic restart
|
|
|
|
|
+is halted and debugging info printed, waiting for human intervention. Therefore
|
|
|
|
|
+we say this is an optimistic cluster restart procedure.
|
|
|
|
|
+
|
|
|
|
|
+## Alternatives Considered
|
|
|
|
|
+
|
|
|
|
|
+### Automatically detect outage and perform `cluster restart`
|
|
|
|
|
+
|
|
|
|
|
+The reaction time of a human in case of emergency is measured in minutes,
|
|
|
|
|
+while a `cluster restart` where human initiate validator restarts takes hours.
|
|
|
|
|
+We considered various approaches to automatically detect outage and perform
|
|
|
|
|
+`cluster restart`, which can reduce recovery speed to minutes or even seconds.
|
|
|
|
|
+
|
|
|
|
|
+However, automatically restarting the whole cluster seems risky. Because
|
|
|
|
|
+if the recovery process itself doesn't work, it might be some time before
|
|
|
|
|
+we can get human's attention. And it doesn't solve the cases where new binary
|
|
|
|
|
+is needed. So for now we still plan to have human in the loop.
|
|
|
|
|
+
|
|
|
|
|
+After we gain more experience with the restart approach in this proposal, we
|
|
|
|
|
+may slowly try to make the process more automatic to improve reliability.
|
|
|
|
|
+
|
|
|
|
|
+### Use Gossip and consensus to figure out restart slot before the restart
|
|
|
|
|
+
|
|
|
|
|
+The main difference between this and the current restart proposal is this
|
|
|
|
|
+alternative tries to make the cluster automatically enter restart preparation
|
|
|
|
|
+phase without human intervention.
|
|
|
|
|
+
|
|
|
|
|
+While getting humans out of the loop improves recovery speed, there are
|
|
|
|
|
+concerns about recovery Gossip messages interfering with normal Gossip
|
|
|
|
|
+messages, and automatically start a new message in Gossip seems risky.
|
|
|
|
|
+
|
|
|
|
|
+### Automatically reduce block production in an outage
|
|
|
|
|
+
|
|
|
|
|
+Right now we have vote-only mode, a validator will only pack vote transactions
|
|
|
|
|
+into new blocks if the tower distance (last_vote - local_root) is greater than
|
|
|
|
|
+400 slots.
|
|
|
|
|
+
|
|
|
|
|
+Unfortunately in the previous outages vote-only mode isn't enough to save the
|
|
|
|
|
+cluster. There are proposals of more aggressive block production reduction to
|
|
|
|
|
+save the cluster. For example, a leader could produce only one block in four
|
|
|
|
|
+consecutive slots allocated to it.
|
|
|
|
|
+
|
|
|
|
|
+However, this only solves the problem in specific type of outage, and it seems
|
|
|
|
|
+risky to aggressively reduce block production, so we are not proceeding with
|
|
|
|
|
+this proposal for now.
|
|
|
|
|
+
|
|
|
|
|
+## Detailed Design
|
|
|
|
|
+
|
|
|
|
|
+The new protocol tries to make all restarting validators get the same data
|
|
|
|
|
+blocks and the same set of last votes, so that they will with high probability
|
|
|
|
|
+converge on the same canonical fork and proceed.
|
|
|
|
|
+
|
|
|
|
|
+When the cluster is in need of a restart, we assume validators holding at least
|
|
|
|
|
+`RESTART_STAKE_THRESHOLD` percentage of stakes will enter the restart mode.
|
|
|
|
|
+Then the following steps will happen:
|
|
|
|
|
+
|
|
|
|
|
+1. The operator restarts the validator into the `wen restart phase` at boot,
|
|
|
|
|
+where it will not make new blocks or vote. The validator propagates its local
|
|
|
|
|
+voted fork information to all other validators in restart.
|
|
|
|
|
+
|
|
|
|
|
+2. While aggregating local vote information from all others in restart, the
|
|
|
|
|
+validator repairs all blocks which could potentially have been optimistically
|
|
|
|
|
+confirmed.
|
|
|
|
|
+
|
|
|
|
|
+3. After enough validators are in restart and repair is complete, the validator
|
|
|
|
|
+counts votes on each fork and computes local heaviest fork.
|
|
|
|
|
+
|
|
|
|
|
+4. A coordinator which is configured on everyone's command line sends out its
|
|
|
|
|
+heaviest fork to everyone.
|
|
|
|
|
+
|
|
|
|
|
+5. Each validator verifies that the coordinator's choice is reasonable:
|
|
|
|
|
+
|
|
|
|
|
+ 1. If yes, proceed and restart
|
|
|
|
|
+
|
|
|
|
|
+ 2. If no, print out what it thinks is wrong, halt and wait for human
|
|
|
|
|
+
|
|
|
|
|
+See each step explained in details below.
|
|
|
|
|
+
|
|
|
|
|
+We assume that as most 5% of the validators in restart can be malicious or
|
|
|
|
|
+contains bugs, this number is consistent with other algorithms in the consensus
|
|
|
|
|
+protocol. We call these `non-conforming` validators.
|
|
|
|
|
+
|
|
|
|
|
+### Wen restart phase
|
|
|
|
|
+
|
|
|
|
|
+1. **Gossip last vote and ancestors on that fork**
|
|
|
|
|
+
|
|
|
|
|
+ The main goal of this step is to propagate most recent ancestors on the last
|
|
|
|
|
+ voted fork to all others in restart.
|
|
|
|
|
+
|
|
|
|
|
+ We use a new Gossip message `RestartLastVotedForkSlots`, its fields are:
|
|
|
|
|
+
|
|
|
|
|
+ * `last_voted_slot`: `u64` the slot last voted, this also serves as
|
|
|
|
|
+ last_slot for the bit vector.
|
|
|
|
|
+ * `last_voted_hash`: `Hash` the bank hash of the slot last voted slot.
|
|
|
|
|
+ * `ancestors`: `Run-length encoding` compressed bit vector representing the
|
|
|
|
|
+ slots on sender's last voted fork. the least significant bit is always
|
|
|
|
|
+ `last_voted_slot`, most significant bit is `last_voted_slot-65535`.
|
|
|
|
|
+
|
|
|
|
|
+ The max distance between oldest ancestor slot and last voted slot is hard
|
|
|
|
|
+ coded at 65535, because that's 400ms * 65535 = 7.3 hours, we assume that
|
|
|
|
|
+ most validator administrators would have noticed an outage within 7 hours,
|
|
|
|
|
+ and the optimistic confirmation must have halted within 64k slots of the
|
|
|
|
|
+ last confirmed block. Also 65535 bits nicely fits into u16, which makes
|
|
|
|
|
+ encoding more compact. If a validator restarts after 7 hours past the
|
|
|
|
|
+ outage, it cannot join the restart this way. If enough validators failed to
|
|
|
|
|
+ restart within 7 hours, then we fallback to the manual, interactive
|
|
|
|
|
+ `cluster restart` method.
|
|
|
|
|
+
|
|
|
|
|
+ When a validator enters restart, it uses `wen restart shred version` to
|
|
|
|
|
+ avoid interfering with those outside the restart. To be extra cautious, we
|
|
|
|
|
+ will also filter out `RestartLastVotedForkSlots` and `RestartHeaviestFork`
|
|
|
|
|
+ (described later) in Gossip if a validator is not in `wen restart phase`.
|
|
|
|
|
+ There is a slight chance that the `wen restart shred version` would collide
|
|
|
|
|
+ with the shred version after the `wen restart phase`, but with the filtering
|
|
|
|
|
+ described above it should not be a problem.
|
|
|
|
|
+
|
|
|
|
|
+ When a validator receives `RestartLastVotedForkSlots` from someone else, it
|
|
|
|
|
+ will discard all slots smaller than the local root. Because the local root
|
|
|
|
|
+ should be an `optimistic confirmed` slot, it does not need to keep any slot
|
|
|
|
|
+ older than local root.
|
|
|
|
|
+
|
|
|
|
|
+2. **Repair ledgers up to the restart slot**
|
|
|
|
|
+
|
|
|
|
|
+ The main goal of this step is to repair all blocks which could potentially
|
|
|
|
|
+ be optimistically confirmed.
|
|
|
|
|
+
|
|
|
|
|
+ We need to prevent false negative at all costs, because we can't rollback an
|
|
|
|
|
+ `optimistically confirmed block`. However, false positive is okay. Because
|
|
|
|
|
+ when we select the heaviest fork in the next step, we should see all the
|
|
|
|
|
+ potential candidates for optimistically confirmed slots, there we can count
|
|
|
|
|
+ the votes and remove some false positive cases.
|
|
|
|
|
+
|
|
|
|
|
+ However, it's also overkill to repair every block presented by others. When
|
|
|
|
|
+ `RestartLastVotedForkSlots` messages are being received and aggregated, a
|
|
|
|
|
+ validator can categorize blocks missing locally into 2 categories: must-have
|
|
|
|
|
+ and ignored.
|
|
|
|
|
+
|
|
|
|
|
+ We repairs all blocks with no less than 42% stake. The number is
|
|
|
|
|
+ `67% - 5% - stake_on_validators_not_in_restart`. We require that at least 80%
|
|
|
|
|
+ join the restart, any block with less than 67% - (100 - 80)% - 5% = 42% can
|
|
|
|
|
+ never be optimistically confirmed before the restart.
|
|
|
|
|
+
|
|
|
|
|
+ It's possible that different validators see different 80%, so their
|
|
|
|
|
+ must-have blocks might be different, but there will be another repair round
|
|
|
|
|
+ in the final step so this is fine. Whenever some block gets to 42%, repair
|
|
|
|
|
+ could be started, because when more validators join the restart, this number
|
|
|
|
|
+ will only go up but will never go down.
|
|
|
|
|
+
|
|
|
|
|
+ When a validator gets `RestartLastVotedForkSlots` from 80% of the stake, and
|
|
|
|
|
+ all those "must-have" blocks are repaired, it can proceed to next step.
|
|
|
|
|
+
|
|
|
|
|
+3. **Calculate heaviest fork**
|
|
|
|
|
+
|
|
|
|
|
+ After receiving `RestartLastVotedForkSlots` from the validators holding
|
|
|
|
|
+ stake more than `RESTART_STAKE_THRESHOLD` and repairing slots in "must-have"
|
|
|
|
|
+ category, pick the heaviest fork like this:
|
|
|
|
|
+
|
|
|
|
|
+ 1. Calculate the threshold for a block to be on the heaviest fork, the
|
|
|
|
|
+ heaviest fork should have all blocks with possibility to be optimistically
|
|
|
|
|
+ confirmed. The number is `67% - 5% - stake_on_validators_not_in_restart`.
|
|
|
|
|
+
|
|
|
|
|
+ For example, if 80% validators are in restart, the number would be
|
|
|
|
|
+ `67% - 5% - (100-80)% = 42%`. If 90% validators are in restart, the number
|
|
|
|
|
+ would be `67% - 5% - (100-90)% = 52%`.
|
|
|
|
|
+
|
|
|
|
|
+ 2. Sort all blocks over the threshold by slot number, and verify that they
|
|
|
|
|
+ form a single chain. The first block in the list should be the local root.
|
|
|
|
|
+
|
|
|
|
|
+ If any block does not satisfy above constraint, print the first offending
|
|
|
|
|
+ block and exit.
|
|
|
|
|
+
|
|
|
|
|
+ The list should not be empty, it should contain at least the local root.
|
|
|
|
|
+
|
|
|
|
|
+ To see why the above algorithm is safe, we will prove that:
|
|
|
|
|
+
|
|
|
|
|
+ 1. Any block optimistically confirmed before the restart will always be
|
|
|
|
|
+ on the list:
|
|
|
|
|
+
|
|
|
|
|
+ Assume block A is one such block, it would have `67%` stake, discounting
|
|
|
|
|
+ `5%` non-conforming and people not participating in wen_restart, it should
|
|
|
|
|
+ have at least `67% - 5% - stake_on_validators_not_in_restart` stake, so it
|
|
|
|
|
+ should pass the threshold and be in the list.
|
|
|
|
|
+
|
|
|
|
|
+ 2. Any block in the list should only have at most one child in the list:
|
|
|
|
|
+
|
|
|
|
|
+ Let's use `X` to denote `stake_on_validators_not_in_restart` for brevity.
|
|
|
|
|
+ Assuming a block has child `A` and `B` both on the list, the children's
|
|
|
|
|
+ combined stake would be `2 * (67% - 5% - X)`. Because we only allow one
|
|
|
|
|
+ RestartHeaviestFork per pubkey, every validator should select either `A`
|
|
|
|
|
+ or `B`, it's easy to find and filter out vialators who selected both. So the
|
|
|
|
|
+ children's total stake should be less than `100% - X`. We can calculate that
|
|
|
|
|
+ if `124% - 2 * X < 100% - X`, then `X > 24%`, this is not possible when we
|
|
|
|
|
+ have at least 80% of the validators in restart. So we prove any block in the
|
|
|
|
|
+ list can have at most one child in the list by contradiction.
|
|
|
|
|
+
|
|
|
|
|
+ 3. If a block not optimistically confirmed before the restart is on the
|
|
|
|
|
+ list, it can only be at the end of the list and none of its siblings are
|
|
|
|
|
+ on the list.
|
|
|
|
|
+
|
|
|
|
|
+ Let's say block D is the first not optimistically confirmed block on the
|
|
|
|
|
+ list, its parent E is confirmed and on the list. We know from above point
|
|
|
|
|
+ that E can only have 1 child on the list, therefore D must be at the end
|
|
|
|
|
+ of the list while its siblings are not on the list.
|
|
|
|
|
+
|
|
|
|
|
+ Even if the last block D on the list may not be optimistically confirmed,
|
|
|
|
|
+ it already has at least `42% - 5% = 37%` stake. Say F is its sibling with
|
|
|
|
|
+ the most stake, F can only have less than `42%` stake because it's not on
|
|
|
|
|
+ the list. So picking D over F is equal to the case where `5%` stake
|
|
|
|
|
+ switched from fork F to fork D, 80% of the cluster can switch to fork D
|
|
|
|
|
+ if that turns out to be the heaviest fork.
|
|
|
|
|
+
|
|
|
|
|
+ After picking the appropriate slot, replay the block and all its ancestors
|
|
|
|
|
+ to get the bankhash for the picked slot.
|
|
|
|
|
+
|
|
|
|
|
+4. **Verify the heaviest fork of the coordinator**
|
|
|
|
|
+
|
|
|
|
|
+ There will be one coordinator specified on the command line of everyone's
|
|
|
|
|
+ command line. Even though everyone will calculate its own heaviest fork in
|
|
|
|
|
+ previous step, only the coordinator's heaviest fork will be checked and
|
|
|
|
|
+ optionally accepted by others.
|
|
|
|
|
+
|
|
|
|
|
+ We use a new Gossip message `RestartHeaviestFork`, its fields are:
|
|
|
|
|
+
|
|
|
|
|
+ * `slot`: `u64` slot of the picked block.
|
|
|
|
|
+ * `hash`: `Hash` bank hash of the picked block.
|
|
|
|
|
+
|
|
|
|
|
+ After deciding the heaviest block, the coordinator Gossip
|
|
|
|
|
+ `RestartHeaviestFork(X.slot, X.hash)` out, where X is the block the
|
|
|
|
|
+ coordinator picked locally in previous step. The coordinator will stay up
|
|
|
|
|
+ until manually restarted by its operator.
|
|
|
|
|
+
|
|
|
|
|
+ For every non-coordinator validator, it will perform the following actions
|
|
|
|
|
+ on the heaviest fork sent by the coordinator:
|
|
|
|
|
+
|
|
|
|
|
+ 1. If the bank selected is missing locally, repair this slot and all slots
|
|
|
|
|
+ with higher stake.
|
|
|
|
|
+
|
|
|
|
|
+ 2. Check that the bankhash of selected slot matches the data locally.
|
|
|
|
|
+
|
|
|
|
|
+ 3. Verify that the selected fork contains local root, and that its local
|
|
|
|
|
+ heaviest fork slot is on the same fork as the coordinator's choice.
|
|
|
|
|
+
|
|
|
|
|
+ If any of the above repair or check fails, exit with error message, the
|
|
|
|
|
+ coordinator may have made a mistake and this needs manual intervention.
|
|
|
|
|
+
|
|
|
|
|
+ When exiting this step, no matter what a non-coordinator validator chooses,
|
|
|
|
|
+ it will send a `RestartHeaviestFork` back to leader to report its status.
|
|
|
|
|
+ This reporting is just for ease of aggregating the cluster's status at the
|
|
|
|
|
+ coordinator, it doesn't have other effects.
|
|
|
|
|
+
|
|
|
|
|
+5. **Generate incremental snapshot and exit**
|
|
|
|
|
+
|
|
|
|
|
+If the previous step succeeds, the validator immediately starts adding a hard
|
|
|
|
|
+fork at the designated slot and perform `set_root`. Then it will start
|
|
|
|
|
+generating an incremental snapshot at the agreed upon `cluster restart slot`.
|
|
|
|
|
+After snapshot generation completes, the `--wait_for_supermajority` args with
|
|
|
|
|
+correct shred version, restart slot, and expected bankhash will be printed to
|
|
|
|
|
+the logs.
|
|
|
|
|
+
|
|
|
|
|
+After the snapshot generation is complete, a non coordinator then exits with
|
|
|
|
|
+exit code `200` to indicate work is complete.
|
|
|
|
|
+
|
|
|
|
|
+A coordinator will stay up until restarted by the operator to make sure any
|
|
|
|
|
+late comers get the `RestartHeaviestFork` message. It also aggregates the
|
|
|
|
|
+`RestartHeaviestFork` messages sent by the non-coordinators to report on the
|
|
|
|
|
+status of the cluster.
|
|
|
|
|
+
|
|
|
|
|
+## Impact
|
|
|
|
|
+
|
|
|
|
|
+This proposal adds a new `wen restart` mode to validators, under this mode the
|
|
|
|
|
+validators will not participate in normal cluster activities. Compared to
|
|
|
|
|
+today's `cluster restart`, the new mode may mean more network bandwidth and
|
|
|
|
|
+memory on the restarting validators, but it guarantees the safety of
|
|
|
|
|
+optimistically confirmed user transactions, and validator operators don't need
|
|
|
|
|
+to manually generate and download snapshots during a `cluster restart`.
|
|
|
|
|
+
|
|
|
|
|
+## Security Considerations
|
|
|
|
|
+
|
|
|
|
|
+The two added Gossip messages `RestartLastVotedForkSlots` and
|
|
|
|
|
+`RestartHeaviestFork` will only be sent and processed when the validator is
|
|
|
|
|
+restarted in `wen restart` mode. So random validator restarting in the new
|
|
|
|
|
+mode will not clutter the Gossip CRDS table of a normal system.
|
|
|
|
|
+
|
|
|
|
|
+Non-conforming validators could send out wrong `RestartLastVotedForkSlots`
|
|
|
|
|
+messages to mess with `cluster restart`s, these should be included in the
|
|
|
|
|
+Slashing rules in the future.
|
|
|
|
|
+
|
|
|
|
|
+### Handling oscillating votes
|
|
|
|
|
+
|
|
|
|
|
+Non-conforming validators could change their last votes back and forth, this
|
|
|
|
|
+could lead to instability in the system. We forbid any change of slot or hash
|
|
|
|
|
+in `RestartLastVotedForkSlots` or `RestartHeaviestFork`, everyone will stick
|
|
|
|
|
+with the first value received, and discrepancies will be recorded in the proto
|
|
|
|
|
+file for later slashing.
|
|
|
|
|
+
|
|
|
|
|
+### Handling multiple epochs
|
|
|
|
|
+
|
|
|
|
|
+Even though it's not very common that an outage happens across an epoch
|
|
|
|
|
+boundary, we do need to prepare for this rare case. Because the main purpose
|
|
|
|
|
+of `wen restart` is to make everyone reach aggrement, the following choices
|
|
|
|
|
+are made:
|
|
|
|
|
+
|
|
|
|
|
+* Every validator only handles 2 epochs, any validator will discard slots
|
|
|
|
|
+which belong to an epoch which is > 1 epoch away from its root. If a validator
|
|
|
|
|
+has very old root so it can't proceed, it will exit and report error. Since
|
|
|
|
|
+we assume an outage will be discovered within 7 hours and one epoch is roughly
|
|
|
|
|
+two days, handling 2 epochs should be enough.
|
|
|
|
|
+
|
|
|
|
|
+* The stake weight of each slot is calculated using the epoch the slot is in.
|
|
|
|
|
+Because right now epoch stakes are calculated 1 epoch ahead of time, and we
|
|
|
|
|
+only handle 2 epochs, the local root bank should have the epoch stakes for all
|
|
|
|
|
+epochs we need.
|
|
|
|
|
+
|
|
|
|
|
+* When aggregating `RestartLastVotedForkSlots`, for any epoch with validators
|
|
|
|
|
+voting for any slot in this epoch having at least 33% stake, calculate the
|
|
|
|
|
+stake of active validators in this epoch. Only exit this stage if all epochs
|
|
|
|
|
+reaching the above bar has > 80% stake. This is a bit restrictive, but it
|
|
|
|
|
+guarantees that whichever slot we select for HeaviestFork, we have enough
|
|
|
|
|
+validators in the restart. Note that the epoch containing local root should
|
|
|
|
|
+always be considered, because root should have > 33% stake.
|
|
|
|
|
+
|
|
|
|
|
+Now we prove this is safe, whenever there is a slot being optimistically
|
|
|
|
|
+confirmed in the new epoch, we will only exit the aggregating of
|
|
|
|
|
+`RestartLastVotedForkSlots` stage if > 80% in the new epoch joined:
|
|
|
|
|
+
|
|
|
|
|
+1. Assume slot `X` is optimistically confirmed in the new epoch, it has >67%
|
|
|
|
|
+stake in the new epoch.
|
|
|
|
|
+
|
|
|
|
|
+2. Our stake warmup/cooldown limit is at 9% currently, so at least
|
|
|
|
|
+67% - 9% = 58% of the stake were from the old epoch.
|
|
|
|
|
+
|
|
|
|
|
+3. We always have >80% stake of the old epoch, so at least
|
|
|
|
|
+58% - 20% = 38% of the stake were in restart. Excluding non-conforming
|
|
|
|
|
+stake, at least 38% - 5% = 33% should be in the restart and they
|
|
|
|
|
+should at least report they voted for `X` which is in the new epoch.
|
|
|
|
|
+
|
|
|
|
|
+4. According to the above rule we will require >80% stake in the new
|
|
|
|
|
+epoch as well.
|
|
|
|
|
+
|
|
|
|
|
+## Backwards Compatibility
|
|
|
|
|
+
|
|
|
|
|
+This change is backward compatible with previous versions, because validators
|
|
|
|
|
+only enter the new mode during new restart mode which is controlled by a
|
|
|
|
|
+command line argument. All current restart arguments like
|
|
|
|
|
+`--wait-for-supermajority` and `--expected-bank-hash` will be kept as is.
|
Wen