17.11.2020 03:02, Vladislav Shpilevoy пишет:
> Raft is being moved to a separate library in src/lib. It means,
> it can't depend on anything from box/, including global
> replication parameters such as replication_synchro_quorum.
>
> The patch makes raft stop using replication_synchro_quorum.
>
> Instead, it has a new option 'election_quorum'. Note, that this is
> just Raft API. Box API still uses replication_synchro_quorum. But
> it is used to calculate the final quorum in src/box/raft, not
> in src/box/raftlib. And to pass it to the base Raft
> implementation.
>
> Part of #5303
> ---
>   src/box/box.cc         |  2 +-
>   src/box/raft.c         | 52 +++++++++++++++++++++++++++++++
>   src/box/raft.h         |  8 +++++
>   src/box/raftlib.c      | 70 ++++++++----------------------------------
>   src/box/raftlib.h      | 10 +++---
>   src/box/replication.cc |  3 ++
>   6 files changed, 83 insertions(+), 62 deletions(-)
>
> diff --git a/src/box/box.cc b/src/box/box.cc
> index 25673ed42..cc0d7b81d 100644
> --- a/src/box/box.cc
> +++ b/src/box/box.cc
> @@ -921,7 +921,7 @@ box_set_replication_synchro_quorum(void)
>   		return -1;
>   	replication_synchro_quorum = value;
>   	txn_limbo_on_parameters_change(&txn_limbo);
> -	raft_cfg_election_quorum(box_raft());
> +	box_raft_reconsider_election_quorum();
>   	return 0;
>   }
>   
> diff --git a/src/box/raft.c b/src/box/raft.c
> index f289a6993..af6e71e0b 100644
> --- a/src/box/raft.c
> +++ b/src/box/raft.c
> @@ -30,6 +30,7 @@
>    */
>   #include "box.h"
>   #include "raft.h"
> +#include "replication.h"
>   
>   struct raft box_raft_global = {
>   	/*
> @@ -62,6 +63,57 @@ box_raft_on_update_f(struct trigger *trigger, void *event)
>   	return 0;
>   }
>   
> +void
> +box_raft_reconsider_election_quorum(void)

Suggestion: maybe use "rewrite"/"reset" instead of "reconsider"?
Or plain "update"?

Other than that, LGTM.

> +{
> +	/*
> +	 * When the instance is started first time, it does not have an ID, so
> +	 * the registered count is 0. But the quorum can never be 0. At least
> +	 * the current instance should participate in the quorum.
> +	 */
> +	int max = MAX(replicaset.registered_count, 1);
> +	/**
> +	 * Election quorum is not strictly equal to synchronous replication
> +	 * quorum. Sometimes it can be lowered. That is about bootstrap.
> +	 *
> +	 * The problem with bootstrap is that when the replicaset boots, all the
> +	 * instances can't write to WAL and can't recover from their initial
> +	 * snapshot. They need one node which will boot first, and then they
> +	 * will replicate from it.
> +	 *
> +	 * This one node should boot from its zero snapshot, create replicaset
> +	 * UUID, register self with ID 1 in _cluster space, and then register
> +	 * all the other instances here. To do that the node must be writable.
> +	 * It should have read_only = false, connection quorum satisfied, and be
> +	 * a Raft leader if Raft is enabled.
> +	 *
> +	 * To be elected a Raft leader it needs to perform election. But it
> +	 * can't be done before at least synchronous quorum of the replicas is
> +	 * bootstrapped. And they can't be bootstrapped because wait for a
> +	 * leader to initialize _cluster. Cyclic dependency.
> +	 *
> +	 * This is resolved by truncation of the election quorum to the number
> +	 * of registered replicas, if their count is less than synchronous
> +	 * quorum. That helps to elect a first leader.
> +	 *
> +	 * It may seem that the first node could just declare itself a leader
> +	 * and then strictly follow the protocol from now on, but that won't
> +	 * work, because if the first node will restart after it is booted, but
> +	 * before quorum of replicas is booted, the cluster will stuck again.
> +	 *
> +	 * The current solution is totally safe because
> +	 *
> +	 * - after all the cluster will have node count >= quorum, if user used
> +	 *   a correct config (God help him if he didn't);
> +	 *
> +	 * - synchronous replication quorum is untouched - it is not truncated.
> +	 *   Only leader election quorum is affected. So synchronous data won't
> +	 *   be lost.
> +	 */
> +	int quorum = MIN(replication_synchro_quorum, max);
> +	raft_cfg_election_quorum(box_raft(), quorum);
> +}
> +
>   void
>   box_raft_init(void)
>   {
> diff --git a/src/box/raft.h b/src/box/raft.h
> index fe0f073dc..09297273f 100644
> --- a/src/box/raft.h
> +++ b/src/box/raft.h
> @@ -48,6 +48,14 @@ box_raft(void)
>   	return &box_raft_global;
>   }
>   
> +/**
> + * Let the global raft know that the election quorum could change. It happens
> + * when configuration is updated, and when new nodes are added or old are
> + * deleted from the cluster.
> + */
> +void
> +box_raft_reconsider_election_quorum(void);
> +
>   void
>   box_raft_init(void);
>   
> diff --git a/src/box/raftlib.c b/src/box/raftlib.c
> index c156d6f46..0657fa85a 100644
> --- a/src/box/raftlib.c
> +++ b/src/box/raftlib.c
> @@ -130,50 +130,6 @@ raft_can_vote_for(const struct raft *raft, const struct vclock *v)
>   	return cmp == 0 || cmp == 1;
>   }
>   
> -/**
> - * Election quorum is not strictly equal to synchronous replication quorum.
> - * Sometimes it can be lowered. That is about bootstrap.
> - *
> - * The problem with bootstrap is that when the replicaset boots, all the
> - * instances can't write to WAL and can't recover from their initial snapshot.
> - * They need one node which will boot first, and then they will replicate from
> - * it.
> - *
> - * This one node should boot from its zero snapshot, create replicaset UUID,
> - * register self with ID 1 in _cluster space, and then register all the other
> - * instances here. To do that the node must be writable. It should have
> - * read_only = false, connection quorum satisfied, and be a Raft leader if Raft
> - * is enabled.
> - *
> - * To be elected a Raft leader it needs to perform election. But it can't be
> - * done before at least synchronous quorum of the replicas is bootstrapped. And
> - * they can't be bootstrapped because wait for a leader to initialize _cluster.
> - * Cyclic dependency.
> - *
> - * This is resolved by truncation of the election quorum to the number of
> - * registered replicas, if their count is less than synchronous quorum. That
> - * helps to elect a first leader.
> - *
> - * It may seem that the first node could just declare itself a leader and then
> - * strictly follow the protocol from now on, but that won't work, because if the
> - * first node will restart after it is booted, but before quorum of replicas is
> - * booted, the cluster will stuck again.
> - *
> - * The current solution is totally safe because
> - *
> - * - after all the cluster will have node count >= quorum, if user used a
> - *   correct config (God help him if he didn't);
> - *
> - * - synchronous replication quorum is untouched - it is not truncated. Only
> - *   leader election quorum is affected. So synchronous data won't be lost.
> - */
> -static inline int
> -raft_election_quorum(const struct raft *raft)
> -{
> -	(void)raft;
> -	return MIN(replication_synchro_quorum, replicaset.registered_count);
> -}
> -
>   /**
>    * Wakeup the Raft worker fiber in order to do some async work. If the fiber
>    * does not exist yet, it is created.
> @@ -427,13 +383,12 @@ raft_process_msg(struct raft *raft, const struct raft_request *req,
>   			 * and now was answered by some other instance.
>   			 */
>   			assert(raft->volatile_vote == instance_id);
> -			int quorum = raft_election_quorum(raft);
>   			bool was_set = bit_set(&raft->vote_mask, source);
>   			raft->vote_count += !was_set;
> -			if (raft->vote_count < quorum) {
> +			if (raft->vote_count < raft->election_quorum) {
>   				say_info("RAFT: accepted vote for self, vote "
>   					 "count is %d/%d", raft->vote_count,
> -					 quorum);
> +					 raft->election_quorum);
>   				break;
>   			}
>   			raft_sm_become_leader(raft);
> @@ -594,7 +549,7 @@ end_dump:
>   			raft_sm_wait_leader_dead(raft);
>   		} else if (raft->vote == instance_id) {
>   			/* Just wrote own vote. */
> -			if (raft_election_quorum(raft) == 1)
> +			if (raft->election_quorum == 1)
>   				raft_sm_become_leader(raft);
>   			else
>   				raft_sm_become_candidate(raft);
> @@ -692,7 +647,7 @@ raft_sm_become_leader(struct raft *raft)
>   {
>   	assert(raft->state != RAFT_STATE_LEADER);
>   	say_info("RAFT: enter leader state with quorum %d",
> -		 raft_election_quorum(raft));
> +		 raft->election_quorum);
>   	assert(raft->leader == 0);
>   	assert(raft->is_candidate);
>   	assert(!raft->is_write_in_progress);
> @@ -730,7 +685,7 @@ raft_sm_become_candidate(struct raft *raft)
>   	assert(raft->vote == instance_id);
>   	assert(raft->is_candidate);
>   	assert(!raft->is_write_in_progress);
> -	assert(raft_election_quorum(raft) > 1);
> +	assert(raft->election_quorum > 1);
>   	raft->state = RAFT_STATE_CANDIDATE;
>   	raft->vote_count = 1;
>   	raft->vote_mask = 0;
> @@ -999,14 +954,14 @@ raft_cfg_election_timeout(struct raft *raft, double timeout)
>   }
>   
>   void
> -raft_cfg_election_quorum(struct raft *raft)
> +raft_cfg_election_quorum(struct raft *raft, int election_quorum)
>   {
> -	if (raft->state != RAFT_STATE_CANDIDATE ||
> -	    raft->state == RAFT_STATE_LEADER)
> -		return;
> -	if (raft->vote_count < raft_election_quorum(raft))
> -		return;
> -	raft_sm_become_leader(raft);
> +	/* At least self is always a part of the quorum. */
> +	assert(election_quorum > 0);
> +	raft->election_quorum = election_quorum;
> +	if (raft->state == RAFT_STATE_CANDIDATE &&
> +	    raft->vote_count >= raft->election_quorum)
> +		raft_sm_become_leader(raft);
>   }
>   
>   void
> @@ -1077,6 +1032,7 @@ raft_create(struct raft *raft)
>   		.state = RAFT_STATE_FOLLOWER,
>   		.volatile_term = 1,
>   		.term =	1,
> +		.election_quorum = 1,
>   		.election_timeout = 5,
>   		.death_timeout = 5,
>   	};
> diff --git a/src/box/raftlib.h b/src/box/raftlib.h
> index b33a20326..c9c13136e 100644
> --- a/src/box/raftlib.h
> +++ b/src/box/raftlib.h
> @@ -150,6 +150,8 @@ struct raft {
>   	vclock_map_t vote_mask;
>   	/** Number of votes for this instance. Valid only in candidate state. */
>   	int vote_count;
> +	/** Number of votes necessary for successful election. */
> +	int election_quorum;
>   	/** State machine timed event trigger. */
>   	struct ev_timer timer;
>   	/** Worker fiber to execute blocking tasks like IO. */
> @@ -225,12 +227,12 @@ void
>   raft_cfg_election_timeout(struct raft *raft, double timeout);
>   
>   /**
> - * Configure Raft leader election quorum. There is no a separate option.
> - * Instead, synchronous replication quorum is used. Since Raft is tightly bound
> - * with synchronous replication.
> + * Configure Raft leader election quorum. That may trigger immediate election,
> + * if the quorum is lowered, and this instance is a candidate having enough
> + * votes for the new quorum.
>    */
>   void
> -raft_cfg_election_quorum(struct raft *raft);
> +raft_cfg_election_quorum(struct raft *raft, int election_quorum);
>   
>   /**
>    * Configure Raft leader death timeout. I.e. number of seconds without
> diff --git a/src/box/replication.cc b/src/box/replication.cc
> index 65512cf0f..19d7f6beb 100644
> --- a/src/box/replication.cc
> +++ b/src/box/replication.cc
> @@ -39,6 +39,7 @@
>   #include "box.h"
>   #include "gc.h"
>   #include "error.h"
> +#include "raft.h"
>   #include "relay.h"
>   #include "sio.h"
>   
> @@ -250,6 +251,7 @@ replica_set_id(struct replica *replica, uint32_t replica_id)
>   	say_info("assigned id %d to replica %s",
>   		 replica->id, tt_uuid_str(&replica->uuid));
>   	replica->anon = false;
> +	box_raft_reconsider_election_quorum();
>   }
>   
>   void
> @@ -298,6 +300,7 @@ replica_clear_id(struct replica *replica)
>   		assert(!replica->anon);
>   		replica_delete(replica);
>   	}
> +	box_raft_reconsider_election_quorum();
>   }
>   
>   void

-- 
Serge Petrenko