The use case is to alert when queue contains broken entries. Especially
important when ClickHouse breaks backwards compatibility between
versions and log entries written by newer versions aren't parseable by
old versions.
```
Code: 27, e.displayText() = DB::Exception: Cannot parse input: expected 'quorum: ' before: 'merge_type: 2\n'
```
This was introduced in https://github.com/ClickHouse/ClickHouse/pull/8602.
The idea was to avoid data re-appearing in ClickHouse after DROP/DETACH
PARTITION. This problem was only present in MergeTree engine and I don't
understand why we need to do the same in ReplicatedMergeTree.
For ReplicatedMergeTree the state of truth is stored in ZK, deleting
things from filesystem just introduces inconsistencies and this is the
main source for errors like "No active replica has part X or covering
part".
The resulting problem is fixed by
https://github.com/ClickHouse/ClickHouse/pull/25820, but in my opinion
we would better avoid introducing the ZK/FS inconsistency in the first
place.
When does this inconsistency appear? Often the sequence is like this:
0. Write 2 parts to ZK [all_0_0_0, all_1_1_0]
1. A merge gets scheduled
2. New part replaces old parts [new: all_0_1_1, old: all_0_0_0, all_1_1_0]
3. Replica gets shutdown and old parts are removed from filesystem
4. Replica comes back online, metadata about all parts is still stored in ZK for this new replica.
5. Other replica after cleanup thread runs will have only [all_0_1_1] in
ZK
5. User triggers a DROP_RANGE after a while (drop range is for all_0_1_9999*)
6. Each replica deletes from ZK only [all_0_1_1]. The replica that got
restarted uses its in-memory state to choose nodes to delete from ZK.
7. Restart the replica again. It will now think that there are 2 parts
that it lost and needs to fetch them [all_0_0_0, all_1_1_0].
`clearOldPartsAndRemoveFromZK` which is triggered from cleanup thread
runs cleanup sequence correctly, it first removes things from ZK and
then from filesystem. I don't see much benefit of triggering it on
shutdown and would rather have it called only from a single place.
---
This is a very, very edge case situation but it proves that the current
"fix" (https://github.com/ClickHouse/ClickHouse/pull/25820) isn't
complete.
```
create table test(
v UInt64
)
engine=ReplicatedMergeTree('/clickhouse/test', 'one')
order by v
settings old_parts_lifetime = 30;
create table test2(
v UInt64
)
engine=ReplicatedMergeTree('/clickhouse/test', 'two')
order by v
settings old_parts_lifetime = 30;
create table test3(
v UInt64
)
engine=ReplicatedMergeTree('/clickhouse/test', 'three')
order by v
settings old_parts_lifetime = 30;
insert into table test values (1), (2), (3);
insert into table test values (4);
optimize table test final;
detach table test;
detach table test2;
alter table test3 drop partition tuple();
attach table test;
attach table test2;
```
```
(CONNECTED [localhost:9181]) /> ls /clickhouse/test/replicas/one/parts
all_0_0_0
all_1_1_0
(CONNECTED [localhost:9181]) /> ls /clickhouse/test/replicas/two/parts
all_0_0_0
all_1_1_0
(CONNECTED [localhost:9181]) /> ls /clickhouse/test/replicas/three/parts
```
```
detach table test;
attach table test;
```
`test` will now figure out that parts exist only in ZK and will issue `GET_PART`
after first removing parts from ZK.
`test2` will receive fetch for unknown parts and will trigger part checks itself.
Because `test` doesn't have the parts anymore in ZK `test2` will mark them as LostForever.
It will also not insert empty parts, because the partition is empty.
`test` is left with `GET_PART` in the queue and stuck.
```
SELECT
table,
type,
replica_name,
new_part_name,
last_exception
FROM system.replication_queue
Query id: 74c5aa00-048d-4bc1-a2ea-6f69501c11a0
Row 1:
──────
table: test
type: GET_PART
replica_name: one
new_part_name: all_0_0_0
last_exception: Code: 234. DB::Exception: No active replica has part all_0_0_0 or covering part. (NO_REPLICA_HAS_PART) (version 21.9.1.1)
Row 2:
──────
table: test
type: GET_PART
replica_name: one
new_part_name: all_1_1_0
last_exception: Code: 234. DB::Exception: No active replica has part all_1_1_0 or covering part. (NO_REPLICA_HAS_PART) (version 21.9.1.1)
```
