When Inflights to a particular node is full, i.e. MaxInflightMsgs for the
append messages flow is saturated, it is still necessary to continue sending
MsgApp to ensure progress. Currently this is achieved by "forgetting" the first
in-flight message in the window, which frees up quota for one new MsgApp.
This new message is constructed in such a way that it potentially has multiple
entries, or a large entry. The effect of this is that the in-flight limitations
can be exceeded arbitrarily, for as long as the flow to this node continues
being saturated. In particular, if a follower is stuck, the leader will keep
sending entries to it.
This commit makes the MsgApp empty when Inflights is saturated, and prevents
the described leakage of Entries to slow followers.
Signed-off-by: Pavel Kalinnikov <pavel@cockroachlabs.com>
This change makes the etcd package compatible with the existing Go
ecosystem for module versioning.
Used this tool to update package imports:
https://github.com/KSubedi/gomove
Mechanically extract `progressTracker`, `Progress`, and `inflights`
to their own package named `tracker`. Add lots of comments in the
progress, and take the opportunity to rename and clarify various
fields.
Instead of having disjoint mappings of ID to *Progress for voters and
learners, use a map[id]struct{} for each and share a map of *Progress
among them.
This is easier to handle when joint quorums are introduced, at which
point a node may be a voting member of two quorums.
The Progress maps contain both the active configuration and information
about the replication status. By pulling it into its own component, this
becomes easier to unit test and also clarifies the code, which will see
changes as etcd-io/etcd#7625 is addressed.
More functionality will move into `prs` in self-contained follow-up commits.
Each progress has a inflighs sliding window. When the progress
is in replicate state, inflights will control the sending speed
of the leader.
The leader can have at most maxInflight number of inflight
messages for each replicate progress. Receving a appResp moves
forward the sliding window. Heartbeat response free one
slot if the window is full.
