Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
This patch implements RAFT extension that allows to perform linearisable
reads by accessing local state machine. The extension is described
in section 6.4 of the PhD. To sum it up to perform a read barrier on
a follower it needs to asks a leader the last committed index that it
knows about. The leader must make sure that it is still a leader before
answering by communicating with a quorum. When follower gets the index
back it waits for it to be applied and by that completes read_barrier
invocation.
The patch adds three new RPC: read_barrier, read_barrier_reply and
execute_read_barrier_on_leader. The last one is the one a follower uses
to ask a leader about safe index it can read. First two are used by a
leader to communicate with a quorum.
If the leader becomes a non-voter after a configuration change,
step down and become a follower.
Non-voting members are an extension to Raft, so the protocol spec does
not define whether they can be leaders. I can not think of a reason
why they can't, yet I also can not think of a reason why it's useful,
so let's forbid this.
We already do not allow non-voters to become candidates, and
they ignore timeout_now RPC (leadership transfer), so they
already can not be elected.
The tracker maintains a separate pointer to current leader progress,
but all this complexity is not needed because the tracker already have
find() function that can either find a leader's progress by id or return
null. Removing the tracking simplifies code and make going out of sync
(which is always a possibility if a state is maintained in two different
places) impossible.
When probes are sent over a slow network, the leader would send
multiple probes to a lagging follower before it would get a
reject response to the first probe back. After getting a reject, the
leader will be able to correctly position `next_idx` for that
follower and switch to pipeline mode. Then, an out of order reject
to a now irrelevant probe could crash the leader, since it would
effectively request it to "rewind" its `match_idx` for that
follower, and the code asserts this never happens.
We fix the problem by strengthening `is_stray_reject`. The check that
was previously only made in `PIPELINE` case
(`rejected.non_matching_idx <= match_idx`) is now always performed and
we add a new check: `rejected.last_idx < match_idx`. We also strengthen
the assert.
The commit improves the documentation by explaining that
`is_stray_reject` may return false negatives. We also precisely state
the preconditions and postconditions of `is_stray_reject`, give a more
precise definition of `progress.match_idx`, argue how the
postconditions of `is_stray_reject` follow from its preconditions
and Raft invariants, and argue why the (strengthened) assert
must always pass.
Message-Id: <20210423173117.32939-1-kbraun@scylladb.com>
Current leader code check for most nodes to be alive, but this is
incorrect since some nodes may be non-voting and hence should not
cause a leader to stepdown if dead. It also incorrect with joint config
since quorum is calculated differently there. Fix it by introducing
activity_tracker class that knows how to handle all the above details.
Section 3.10 of the PhD describes two cases for which the extension can
be helpful:
1. Sometimes the leader must step down. For example, it may need to reboot
for maintenance, or it may be removed from the cluster. When it steps
down, the cluster will be idle for an election timeout until another
server times out and wins an election. This brief unavailability can be
avoided by having the leader transfer its leadership to another server
before it steps down.
2. In some cases, one or more servers may be more suitable to lead the
cluster than others. For example, a server with high load would not make
a good leader, or in a WAN deployment, servers in a primary datacenter
may be preferred in order to minimize the latency between clients and
the leader. Other consensus algorithms may be able to accommodate these
preferences during leader election, but Raft needs a server with a
sufficiently up-to-date log to become leader, which might not be the
most preferred one. Instead, a leader in Raft can periodically check
to see whether one of its available followers would be more suitable,
and if so, transfer its leadership to that server. (If only human leaders
were so graceful.)
The patch here implements the extension and employs it automatically
when a leader removes itself from a cluster.
Set follower's next_idx to snapshot index + 1 when switching
it to snapshot mode. If snapshot transfer succeeds, that's the
best match for the follower's next replication index. If it fails,
the leader will send a new probe to find out the follower position
again and re-try sending a possibly newer snapshot.
The change helps reduce protocol state managed outside FSM.
This patch adds a support for non-voting members. Non voting member is a
member which vote is not counted for leader election purposes and commit
index calculation purposes and it cannot become a leader. But otherwise
it is a normal raft node. The state is needed to let new nodes to catch
up their log without disturbing a cluster.
All kind of transitions are allowed. A node may be added as a voting member
directly or it may be added as non-voting and then changed to be voting
one through additional configuration change. A node can be demoted from
voting to non-voting member through a configuration change as well.
Message-Id: <20210304101158.1237480-2-gleb@scylladb.com>
While a duplicate vote from the same server is not possible by a
conforming Raft implementation, Raft assumptions on network permit
duplicates.
So, in theory, it is possible that a vote message is delivered
multiple times.
The current voting implementation does reject votes from non-members,
but doesn't check for duplicate votes.
Keep track of who has voted yet, and reject duplicate votes.
A unit test follows.
