31e0315710
Fix cdc writing unnecesary entries to it's log, like for example when Alternator deletes an item which in reality doesn't exist.
Originally @wps0 tackled this issue. This patch is an extension of his work. His work involved adding `should_skip` function to cdc, which would process a `mutation` object and decide, wherever changes in the object should be added to cdc log or not.
The issue with his approach is that `mutation` object might contain changes for more than one row. If - for example - the `mutation` object contains two changes, delete of non-existing row and create of non-existing row, `should_skip` function will detect changes in second item and allow whole `mutation` (BOTH items) to be added. For example (using python's boto3) running this on empty table:
```
with table.batch_writer() as batch:
batch.put_item({'p': 'p', 'c': 'c0'})
batch.delete_item(Key={'p': 'p', 'c': 'c1'})
```
will emit two events ("put" event and "delete" event), even though the item with `c` set to `c1` does not exist (thus can't be deleted). Note, that both entries in batch write must use the same partition key, otherwise upper layer with split them into separate `mutation` objects and the issue will not happen.
The solution is to do similar processing, but consider each change separated from others. This is tricky to implement due to a way cdc works. When cdc processes `mutation` object (containing X changes), it emits cdc entries in phases. Phase 1 - emit `preimage` (old state) for each change (if requested). Phase 2 - for each change emit actual "diff" (update / delete and so on). Phase 3 - emit `postimage` (new state).
We will know if change needs to be skipped during phase 2. By that time phase 1 is completed and preimage for the change is emited. At that moment we set a flag that the change (identified by clustering key value) needs to be skipped - we add a clustering key to a `ignore-rows` set (`_alternator_clustering_keys_to_ignore` variable) and continue normally. Once all phases finish we add a `postprocess` phase (`clean_up_noop_rows` function). It will go through generated cdc mutations and skip all modifications, for which clustering key is in `ignore-rows` set. After skipping we need to do a "cleanup" operation - each generated cdc mutation contain index (incremented by one), if we skipped some parts, the index is not consecutive anymore, so we reindex final changes.
There's a special case worth mentioning - Alternator tables without clustering keys. At that point `mutation` object passed to cdc can contain exactly one change (since different partition keys are splitted by upper layers and Alternator will never emit `mutation` object containing two (or more) changes with the same primary key. Here, when we decide the change is to be skipped we add empty `bytes` object to `ignore-rows` set. When checking `ignore-rows` set, we check if it's empty or not (we don't check for presence of empty `bytes` object).
Note: there might be some confusion between this patch and #28452 patch. Both started from the same error observation and use similar tests for validation, as both are easily triggered by BatchWrite commands (both needs `mutation` object passed to cdc to contain more than one single change). This issue tho is about wrong data written in cdc log and is fixed at cdc, where #28452 is about wrong way of parsing correct cdc data and is fixed at Alternator side of things. Note, that we need #28452 to truly verify (otherwise we will emit correct cdc entries, but Alternator will incorrectly parse them).
Note: to benefit / notice this patch you need `alternator_streams_increased_compatibility` flag turned on.
Note: rework is quite "broad" and covers a lot of ground - every operation, that might result in a no-change to the database state should be tested. An additional test was added - trying to remove a column from non-existing item, as well as trying to remove non-existing column from existing item.
Fixes: #28368
Fixes: SCYLLADB-1528
Fixes: SCYLLADB-538
Closes scylladb/scylladb#28544
* github.com:scylladb/scylladb:
alternator: remove unnecesary code
alternator: fix Alternator writing unnecesary cdc entries
alternator: add failing tests for Streams
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