jihuayu commented on issue #1402:
URL:
https://github.com/apache/incubator-kvrocks/issues/1402#issuecomment-1575519968
SCAN commond is frequently used in Redis clients.It's important to ensure
SCAN commond is correct for Redis toolchains to work with Kvrocks.
Therefore, I believe this issue needs to be resolved.
After carefully reading the Redis documentation, I found that cursors have
the following features:
1. The cursor is a 64-bit unsigned integer, with 0 meaning the end of the
iteration.
2. There is no state server side, the full state is captured by the cursor
3. During the iteration process, a given element may be returned multiple
times.
4. As long as the number of elements is not constantly increasing, the
iteration will eventually end.
5. Use a corrupted cursor is undefined behavior.
Therefore, I think we can use a **ring array** to store the key names for
the most recently used cursors.
When SCAN end_cursor is "keyname1" we push {key:keyname1,cursor:1276} to
cursor-ring.
Next SCAN use cursor 1276, we foreach cursor-ring from the newest item, and
get the it's key "keyname1". I call this process cursor translation.
Then we can use "keyname1" to seek rocksdb iterator.
If client call many SCAN command, cursor-ring will be filled and the oldest
cursor will be discarded.
Now the if client SCAN use a corrupted cursor(the cursor don't in
cursor-ring), we will return element from begin.
Based on the above design, we will have the following additional costs.
1. Less than 2KB of memory is used to store the cursor ring.
2. Each SCAN request requires one cursor translation and one cursor mapping
save. (Note: Ensuring thread safety is required for cursor mapping save)
Because SCAN command is a slow command, so the additional cursor translation
has a minimal impact.
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