Before this PR, writes to the previous CDC generations would
always be rejected. After this PR, they will be accepted if the
write's timestamp is greater than `now - generation_leeway`.
This change was proposed around 3 years ago. The motivation was
to improve user experience. If a client generates timestamps by
itself and its clock is desynchronized with the clock of the node
the client is connected to, there could be a period during
generation switching when writes fail. We didn't consider this
problem critical because the client could simply retry a failed
write with a higher timestamp. Eventually, it would succeed. This
approach is safe because these failed writes cannot have any side
effects. However, it can be inconvenient. Writing to previous
generations was proposed to improve it.
The idea was rejected 3 years ago. Recently, it turned out that
there is a case when the client cannot retry a write with the
increased timestamp. It happens when a table uses CDC and LWT,
which makes timestamps permanent. Once Paxos commits an entry
with a given timestamp, Scylla will keep trying to apply that entry
until it succeeds, with the same timestamp. Applying the entry
involves writing to the CDC log table. If it fails, we get stuck.
It's a major bug with an unknown perfect solution.
Allowing writes to previous generations for `generation_leeway` is
a probabilistic fix that should solve the problem in practice.
Apart from this change, this PR adds tests for it and updates
the documentation.
This PR is sufficient to enable writes to the previous generations
only in the gossiper-based topology. The Raft-based topology
needs some adjustments in loading and cleaning CDC generations.
These changes won't interfere with the changes introduced in this
PR, so they are left for a follow-up.
Fixesscylladb/scylladb#7251Fixesscylladb/scylladb#15260Closesscylladb/scylladb#17134
* github.com:scylladb/scylladb:
docs: using-scylla: cdc: remove info about failing writes to old generations
docs: dev: cdc: document writing to previous CDC generations
test: add test_writes_to_previous_cdc_generations
cdc: generation: allow increasing generation_leeway through error injection
cdc: metadata: allow sending writes to the previous generations
(cherry picked from commit 9bb4482ad0)
Backport note: in tests, replaced `servers_add` with loop of `server_add`
We change the type of IDs in CDC_GENERATIONS_V3 to timeuuid to
give them a time-based order. We also change how we initialize
them so that the new CDC generation always has the highest ID.
This is the last step to enabling the efficient clearing of
obsolete CDC generation data.
Additionally, we change the types of current_cdc_generation_uuid,
new_cdc_generation_data_uuid and the second values of the elements
in unpublished_cdc_generations to timeuuid, so that they match id
in CDC_GENERATIONS_V3.
After moving the creation of uuid out of
make_new_generation_description, this function only calls the
topology_description_generator's constructor and its generate
method. We could remove this function, but we instead simplify
the code by removing the topology_description_generator class.
We can do this refactor because make_new_generation_description
is the only place using it. We inline its generate method into
make_new_generation_description and turn its private methods into
static functions.
We make CDC_GENERATIONS_V3 single-partition by adding the key
column and changing the clustering key from range_end to
(id, range_end). This is the first step to enabling the efficient
clearing of obsolete CDC generation data, which we need to prevent
Raft-topology snapshots from endlessly growing as we introduce new
generations over time. The next step is to change the type of the id
column to timeuuid. We do it in the following commits.
After making CDC_GENERATIONS_V3 single-partition, there is no easy
way of preserving the num_ranges column. As it is used only for
sanity checking, we remove it to simplify the implementation.
Update the `Generation switching` section: most of the existing
description landed in `Gossiper-based topology changes` subsection, and
a new subsection was added to describe Raft group 0 based topology
changes. Marked as WIP - we expect further development in this area
soon.
The existing gossiper-based description was also updated a bit.
