FaxianZhao opened a new pull request #7675: [FLINK-8297] [flink-rocksdb] A plan store elements of ListState as multiple key-values in rocksdb URL: https://github.com/apache/flink/pull/7675 ## What is the purpose of the change Enable storing lists not fitting to memory per single key. ## Brief change log Ref from PR #5185. I draft RocksDBLargeListState, it inherit AbstractRocksDBMapState which is similar RocksDBMapState. Dual to AbstractRocksDBMapState and RocksDBListState implement the same interface InternalKvState, and they meet an java generics conflict. So I create an new interface InternalLargeListState, I think we can refine the interface hierarchy to resolve this issue. RocksDBLargeListState store values to KV structure. We store the lexicographically max key as KV value in the default key(#KeyGroup#Key#Namespace). If the value is instance of StreamRecord, we treat its timestamp as a part of key, and itself as value. **It will sort elements by timestamp instead of insertion order, but I think its behavior is acceptable. We can also involve TtlValue.** Otherwise, treat current timestamp(maybe proc time) as a part of key, and itself as value. The rocksdb store structure. ``` -----------Key------------------- --------------------Value--------- #KeyGroup#Key#Namespace #KeyGroup#Key#Namespace#ts3 (max lexicographically key) #KeyGroup#Key#Namespace#ts1 value1,value2,value3 #KeyGroup#Key#Namespace#ts2 value4 #KeyGroup#Key#Namespace#ts3 value5,value6 ``` Advantage: 1. We can seek to the lexicographically max key, then reverse iterate the list. 2. We can iterate the values with the event time / proc time, it good at monotonous scenario. Like TimeEvictor. 3. We could make RocksDBListState and RocksDBMapState also inherit AbstractRocksDBMapState. Disadvantage: 1. We couldn't simple replace RocksDBListState as RocksDBLargeListState, because of the above interface's generics conflict. 2. It will add 8 bytes cost to store extended timestamp in key part. 3. For the timestamp base elements, it will sort elements by timestamp instead of insertion order. This behavior is not algn with others ListState. 4. If the values has the same timestamp, the behavior almost same as RocksDBListState. I have a solution for this, but have not implement it yet. We can add order suffix for each timestamp. Like below. ``` -----------Key--------------------- --------------------Value--------- #KeyGroup#Key#Namespace #KeyGroup#Key#Namespace#ts3#2 (Max lexicographically key) #KeyGroup#Key#Namespace#ts1 3 (The values' count which has the same timestamp ts1) #KeyGroup#Key#Namespace#ts1#1 value1 #KeyGroup#Key#Namespace#ts1#2 value2 #KeyGroup#Key#Namespace#ts1#3 value3 #KeyGroup#Key#Namespace#ts2 1 #KeyGroup#Key#Namespace#ts2#1 value4 #KeyGroup#Key#Namespace#ts3 2 #KeyGroup#Key#Namespace#ts3#1 value5 #KeyGroup#Key#Namespace#ts3#2 value6 ``` I have try to add a murmur3 hash in the key part to resolve timestamp conflict, but it will crash the element order. ## Verifying this change This change added tests and can be verified as follows: - Added test in KVStateRequestSerializerRocksDBTest, the behavior is same as KVStateRequestSerializerRocksDBTest#testListSerialization - Because of the class reference issue, added test in RocksDBStateBackendTest, but implement them in StateBackendTestBase. The test behavior ref from StateBackendTestBase.testListState** ## Does this pull request potentially affect one of the following parts: - Dependencies (does it add or upgrade a dependency): no - The public API, i.e., is any changed class annotated with `@Public(Evolving)`: yes - The serializers: no - The runtime per-record code paths (performance sensitive): no - Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no - The S3 file system connector: no ## Documentation - Does this pull request introduce a new feature? yes - If yes, how is the feature documented? JavaDocs
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