Details
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Improvement
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Status: Closed
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Minor
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Resolution: Abandoned
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None
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None
Description
I enjoy using Flink but ...
Do we have a written design for the threading model including the guarantees made by the core framework in terms of threading and concurrency.
Looking at various existing components such as JDBC and file sinks and other non-core facilities.
Having some difficulty understanding the intended design.
Want to understand the assumptions I can make about when certain functions will be called (for example JDBCOutputFormat open vs flush vs writeRecord vs close) and whether this will always be from the same thread or some other thread, or whether they might be called concurrently, in order to verify the correctness of the code.
What guarantees are there?
Does a certain reference need a volatile or even a synchronisation or not.
What's the design for threading?
If the intended design is not written down then we have to infer it from the code and we will definitiely come to different conclusions and thus bugs and leaks. and other avoidable horrors.
It's really hard writing good MT code and a strong design is necessary to provide a framework for the code.
Some info here https://flink.apache.org/contributing/code-style-and-quality-common.html#concurrency-and-threading , but this isn't a design and doesn't say how it's meant to work. However that page does agree that writing MT code is very hard and this just underlines the need for a strong and detailed design for this aspect.
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Another supporting example.
When I see code like this ...
FileOutputFormat
public void close() throws IOException { final FSDataOutputStream s = this.stream; if (s != null) { this.stream = null; s.close(); } }
My feeling is that someone else wasn't sure what the right approach was.
I can only guess that the author was concerned that someone else was going to call the function concurrently, or mess with the class state by some other means. And, if that were true then would this code even be MT safe - who knows? Ought there be a volatile in there or a plain old sync? Or perhaps none of the caution is needed at all (framework guarantees preventing the need)?
Or take a look at the extensive sychronisation efforts in https://github.com/apache/flink/blob/master/flink-connectors/flink-hadoop-compatibility/src/main/java/org/apache/flink/api/java/hadoop/mapred/HadoopOutputFormatBase.java is this code correct? Not to mention that fact that this close() method might throw an NPE if there is any possiblity that 'this.outputCommitter' might not have been initialised in open OR is the framework can ever call close() without open() having completed.
I find if worrying that I see a lot of code in the project that is similarly uncertain and inconsistent syncronisation and resource management.
I would have hoped that the underlying core framework provided guarantees that avoided the need to have extensive synchronisation effort in derived or auxiliary classes.
What's the design.