Currently, when a replica's view update backlog is full, the write is still
sent by the coordinator to all replicas. Because of the backlog, the write
fails on the replica, causing inconsistency that needs to be fixed by repair.
To avoid these inconsistencies, this patch adds a check on the coordinator
for overloaded replicas. As a result, a write may be rejected before being
sent to any replicas and later retried by the user, when the replica is no
longer overloaded.
Fixesscylladb/scylladb#17426
Currently, a view update backlog may reach an invalid state, when
its max is 0 and its relative_size() is NaN as a result. This can
be achieved either by constructing the backlog with a 0 max or by
modifying the max of an existing backlog. In particular, this
happens when creating the backlog using the default constructor.
In this patch the the default constructor is deleted and a check
is added to make sure that the max is different than 0 is added
to its constructor - if the check fails, we construct an empty
backlog instead, to handle the possibility of getting an invalid
backlog sent from a node with a version that's missing this check.
Additionally, we make the backlogs members private, exposing them
only through const getters.
In order to prevent overload caused by too many view updates,
their number is limited by delaying client responses.
The amount of time to delay for is calculated based on the
fullness of the view update backlog.
Currently this is done in the function calculate_delay,
used by abstract_write_response_handler.
In the following commits I will introduce another throttling
mechanism that uses the same equation to calculate wait time,
so it would be good to reuse the exsiting function.
Let's make the function globally accessible.
Signed-off-by: Jan Ciolek <jan.ciolek@scylladb.com>
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
The view update backlog represents the pending view data that a base replica
maintains. It is the maximum of the memory backlog - how much memory pending
view updates are consuming - and the disk backlog - how much view hints are
consuming. The size of a backlog is relative to its maximum size.
We will use this class to represent a base replica's view update
backlog at the coordinator.
Signed-off-by: Duarte Nunes <duarte@scylladb.com>
