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https://issues.apache.org/jira/browse/FLINK-17723?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
John Lonergan updated FLINK-17723:
----------------------------------
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.
==
Another supporting example.
When I see code like this ...
FileOutputFormat
{code:java}
public void close() throws IOException {
final FSDataOutputStream s = this.stream;
if (s != null) {
this.stream = null;
s.close();
}
}
{code}
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.
was:
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.
==
Another supporting example.
When I see code like this ...
FileOutputFormat
{code:java}
public void close() throws IOException {
final FSDataOutputStream s = this.stream;
if (s != null) {
this.stream = null;
s.close();
}
}
{code}
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?
I find if worrying that I see a lot of code in the project that is similarly
uncertain and inconsistent.
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.
> Written design for flink threading model and guarantees made to the various
> structual components
> ------------------------------------------------------------------------------------------------
>
> Key: FLINK-17723
> URL: https://issues.apache.org/jira/browse/FLINK-17723
> Project: Flink
> Issue Type: Improvement
> Components: Documentation
> Reporter: John Lonergan
> Priority: Major
>
> 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.
> ==
> Another supporting example.
> When I see code like this ...
> FileOutputFormat
>
> {code:java}
> public void close() throws IOException {
> final FSDataOutputStream s = this.stream;
> if (s != null) {
> this.stream = null;
> s.close();
> }
> }
> {code}
> 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.
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