This patch allows to leave snapshot_trailing amount of entries
when a state machine is snapshotted and raft log entries are dropped.
Those entries can be used to catch up nodes that are slow without
requiring snapshot transfer. The value is part of the configuration
and can be changed.
The patch implements periodic taking of a snapshot and trimming of
the raft log.
In raft the only way the log of already committed entries can be shorten
is by taking a snapshot of the state machine and dropping log entries
included in the snapshot from the raft log. To not let log to grow too
large the patch takes the snapshot periodically after applying N number
of entries where N can be configured by setting snapshot_threshold
value in raft's configuration.
This patch adds the logic that detects that a follower misses data from
a snapshot and initiate snapshot transfer in that case. Upon receiving
the snapshot the follower stores it locally and applies it to its state
machine. The code assumes that the snapshot is already exists on a
leader.
For convenience making Raft tests, use declarative structures.
Servers are set up and initialized and then updates are processed.
For now, updates are just adding entries to leader and change of leader.
Updates and leader changes can be specified to run after initial test setup.
An example test for 3 nodes, node 0 starting as leader having two entries
0 and 1 for term 1, and with current term 2, then adding 12 entries,
changing leader to node 1, and adding 12 more entries. The test will
automatically add more entries to the last leader until the test limit
of total_values (default 100).
{.name = "test_name", .nodes = 3, .initial_term = 2,
.initial_states = {{.le = {{1,0},{1,1}}},
.updates = {entries{12},new_leader{1},entries{12}},},
Leader is isolated before change via is_leader returning false.
Initial leader (default server 0) will be set with this method, too.
Signed-off-by: Alejo Sanchez <alejo.sanchez@scylladb.com>
Send more that one entry in single append_entry message but
limit one packets size according to append_request_threshold parameter.
Message-Id: <20201007142602.GA2496906@scylladb.com>
Server address UUID 0 is not a valid server id since there is code that
assumes if server_id is 0 the value is not set (e.g _voted_for).
Prevent users from manually setting this invalid value.
Signed-off-by: Alejo Sanchez <alejo.sanchez@scylladb.com>
When AppendEntries/InstallSnapshot term is the same as the current
server's, and the current servers' leader is not set, we should
assign it to avoid starting election if the current leader becomes idle.
Restructure the code accordingly - change candidate state to Follower
upon InstallSnapshot.
It was erroneously replaced by the logic based on time which caused us
to send one probe per tick which is not an intention at all. There can
be one outstanding probe message but the moment it gets a reply next
one should be sent without waiting for a tick.
This patch introduces partial RAFT implementation. It has only log
replication and leader election support. Snapshotting and configuration
change along with other, smaller features are not yet implemented.
The approach taken by this implementation is to have a deterministic
state machine coded in raft::fsm. What makes the FSM deterministic is
that it does not do any IO by itself. It only takes an input (which may
be a networking message, time tick or new append message), changes its
state and produce an output. The output contains the state that has
to be persisted, messages that need to be sent and entries that may
be applied (in that order). The input and output of the FSM is handled
by raft::server class. It uses raft::rpc interface to send and receive
messages and raft::storage interface to implement persistence.
This commit introduce public raft interfaces. raft::server represents
single raft server instance. raft::state_machine represents a user
defined state machine. raft::rpc, raft::rpc_client and raft::storage are
used to allow implementing custom networking and storage layers.
A shared failure detector interface defines keep-alive semantics,
required for efficient implementation of thousands of raft groups.
