Details
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New Feature
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Status: Closed
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Major
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Resolution: Abandoned
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1.5.0
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None
Description
Motivation
There are some scenarios drive us to introduce this ElasticBloomFilter, one is Stream Join, another is Data Deduplication, and some special user cases...This has given us a great experience, for example, we implemented the Runtime Filter Join base on it, and it gives us a great performance improvement. With this feature, It diff us from the "normal stream join", allows us to improve performance while reducing resource consumption by about half!!!
I will list the two most typical user cases that optimized by the ElasticBloomFilter: one is "Runtime Filter Join" in detail, another is "Data Dedeplication" in brief.
Scenario 1: Runtime Filter Join
In general, stream join is one of the most performance cost task. For every record from both side, we need to query the state from the other side, this will lead to poor performance when the state size if huge. So, in production, we always need to spend a lot slots to handle stream join. But, indeed, we can improve this in somehow, there a phenomenon of stream join can be found in production. That's the “joined ratio” of the stream join is often very low, for example.
- stream join in promotion analysis: Job need to join the promotion log with the action(click, view, buy) log with the promotion_id to analysis the effect of the promotion.
- stream join in AD(advertising) attribution: Job need to join the AD click log with the item payment log on the click_id to find which click of which AD that brings the payment to do attribution.
- stream join in click log analysis of doc: Job need to join viewed log(doc viewed by users) with the click log (doc clicked by users) to analysis the reason of the click and the property of the users.
- ….so on
All these cases have one common property, that is the joined ratio is very low. Here is a example to describe it, we have 10000 records from the left stream, and 10000 records from the right stream, and we execute select * from leftStream l join rightStream r on l.id = r.id , we only got 100 record from the result, that is the case for low joined ratio, this is an example for inner join, but it can also apply to left & right join.
there are more example I can come up with low joined ratio…but the point I want to raise up is that the low joined ratio of stream join in production is a very common phenomenon(maybe even the almost common phenomenon in some companies, at least in our company that is the case).
How to improve this?
We can see from the above case, 10000 record join 10000 record and we only got 100 result, that means, we query the state 20000 times (10000 for the left stream and 10000 for the right stream) but only 100 of them are meaningful!!! If we could reduce the useless query times, then we can definitely improve the performance of stream join.
the way we used to improve this is to introduce the Runtime Filter Join, the mainly ideal is that, we build a filter for the state on each side (left stream & right stream). When we need to query the state on that side we first check the corresponding filter whether the key is possible in the state, if the filter say "not, it impossible in the State", then we stop querying the state, if it say "hmm, it maybe in state", then we need to query the state. As you can see, the best choose of the filter is Bloom Filter, it has all the feature that we want: extremely good performance, non-existence of false negative.
The simplest pseudo code for Runtime Filter Join(the comments is based on RocksDBBackend)
void performJoinNormally(Record recordFromLeftStream) {
Iterator<Record> rightIterator = rigthStreamState.iterator();
// perform the `seek()` on the RocksDB, and iterator one by one,
// this is an expensive operation especially when the key can't be found in RocksDB.
for (Record recordFromRightState : rightIterator) {
……...
}
}
void performRuntimeFilterJoin(Record recordFromLeftStream) {
Iterator<Record> rightIterator = EMPTY_ITERATOR;
if (rigthStreamfilter.containsCurrentKey()) {
rightIterator = rigthStreamState.iterator();
}
// perform the `seek()` only when filter.containsCurrentKey() return true
for (Record recordFromRightState : rightIterator) {
.......
}
// add the current key into the filter of left stream.
leftStreamFilter.addCurrentKey();
}
Scenario 2: Data Deduplication
We have implemented two general functions based on the ElasticBloomFilter. They are count(distinct x) and select distinct x, y, z from table. Unlike the Runtime Filter Join the result of this two functions is approximate, not exactly. There are used in the scenario where we don't need a 100% accurate result, for example, to count the number of visiting users in each online store. In general, we don't need a 100% accurate result in this case(indeed we can't give a 100% accurate result, because there could be error when collecting user_id from different devices), if we could get a 98% accurate result with only 1/2 resource, that could be very nice.
void countDistinctNormally(Key key, Iterator<Record> records) {
// query 1 times
final long oldVal = valState.get();
long val = oldVal;
// query records.size() times
for (Record record : records) {
if (mapState.get(record) == null) {
++val;
mapState.put(record);
}
}
if (val != oldVal) {
valState.update(val);
}
}
void countDistinctBF(Key key, Iterator<Record> records) {
// query 1 times
final long oldVal = valState.get();
long val = oldVal;
for (Record record : records) {
if (!bfState.contains(record)) {
++val;
bfState.add(record);
}
}
if (val != oldVal) {
valState.update(val);
}
}
I believe there would be more user cases in stream world that could be optimized by the Bloom Filter(as what it had done in the big data world)...
Required features and challenges
There are a few challenges with using bloom filter in flink. Firstly, it need to be held as operator state because it need to support 1) fault-tolerant, and as well as 2) rescaling. Beside, because we need to support rescaling, so we need to create bloom filter for each key group to store data fails into it, so another challenge is how to 3) handle data skewed(The amount of data that falls into different groups could be very different )? Imagine that we create a BF on each key group for the incoming data, and we are able to estimate the total amount of data, then the question is what the size should we create for the BF that on each key group? It is so tricky and even impossible to estimate the amount of data on each key group. After that, because that Bloom Filter need to live in the memory to get the extremely performance, so we need a 4) TTL policy to recycle memory, otherwise we will get OOM finally. So, as a brief summarize we need to at lest fullfill the follow features:
- Fault tolerant(checkpoint & restoring)
- Rescaling
- Handle data skewed
- TTL policy
Design doc: design doc
Attachments
Issue Links
- blocks
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- Closed
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- Closed
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FLINK-8918 Introduce Runtime Filter Join
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- Closed
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- is duplicated by
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- Closed
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- links to