How to debug redis

[ngjaying]

@rwadowski In my smoke testing with the redis store for qos 2, I got some errors like below randomly:

time="2021-08-26 17:45:11" level=info msg="Cannot save checkpoint 1629971111464 due to storage error: save checkpoint err: list at KV:TS:rule1 key should be non empty" file="checkpoint/coordinator.go:257" rule=rule1

I believe the cause is in internal/pkg/ts/redis/redisTs.go Set function:

reply, err := conn.Do(AddToSortedSet, t.key, key, b)
if err != nil {
	return err
}
length, err := redis.Int(reply, err)
if err != nil {
	return err
}
if length == 0 {
	return fmt.Errorf("list at %s key should be non empty", t.key)
}
t.last = key
return nil

The reply is 0 sometimes. How can we know why it has no error but return a 0?

[rwadowski]

@ngjaying There is nothing about case when given score already exists in the sorted set in the documentation. Is it possible to save checkpoint with same timestamp ?

I made small test

127.0.0.1:6379> ZADD test-set 1 "one"
(integer) 1
127.0.0.1:6379> ZADD test-set 1 "one"
(integer) 0
127.0.0.1:6379> ZADD test-set 1 "one1"
(integer) 1
127.0.0.1:6379> ZADD test-set 1 "one1"
(integer) 0
127.0.0.1:6379> ZRANGE test-test 0 -1
(empty list or set)
127.0.0.1:6379> ZRANGE test-set 0 -1
1) "one"
2) "one1"

It looks like that zero value is returned in the case of the same score

[rwadowski]

Does logic require return an error in case of duplicate ?

[rwadowski]

No, there should be no duplicate. How to check if that score(timestamp) exist in redis

func (t ts) Last(value interface{}) (int64, error) which uses ZREVRANGE key 0 0 WITHSCORES

[ngjaying]

"Does logic require return an error in case of duplicate ?" No. And it seems there should be no duplicate. Especially in the set method, you even check the last timestamp.

if key <= t.last {
		return false, nil
	}

[rwadowski]

Is it possible to call this interface from many threads ?

[rwadowski]

So maybe we should just allow to overwrite ? It is nosql after all