Re: IO related waits

On 9/16/24 20:55, veem v wrote:
>
>
> On Tue, 17 Sept 2024 at 03:41, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com
> <mailto:adrian(dot)klaver(at)aklaver(dot)com>> wrote:
>
>
> Are you referring to this?:
>
> https://nightlies.apache.org/flink/flink-docs-release-1.20/docs/dev/datastream/operators/asyncio/ <https://nightlies.apache.org/flink/flink-docs-release-1.20/docs/dev/datastream/operators/asyncio/>
>
> If not then you will need to be more specific.
>
>
> Yes, I was referring to this one. So what can be the caveats in this
> approach, considering transactions meant to be ACID compliant as
> financial transactions.Additionally I was not aware of the parameter
> "synchronous_commit" in DB side which will mimic the synchronous commit.
>
> Would both of these mimic the same asynchronous behaviour and achieves
> the same, which means the client data load throughput will increase
> because the DB will not wait for those data to be written to the WAL and
> give a confirmation back to the client and also the client will not wait
> for the DB to give a confirmation back on the data to be persisted in
> the DB or not?. Also, as in the backend the flushing of the WAL to the
> disk has to happen anyway(just that it will be delayed now), so can this
> method cause contention in the database storage side if the speed in
> which the data gets ingested from the client is not getting written to
> the disk , and if it can someway impact the data consistency for the
> read queries?

This is not something that I am that familiar with. I suspect though
this is more complicated then you think. From the link above:

" Prerequisites #

As illustrated in the section above, implementing proper asynchronous
I/O to a database (or key/value store) requires a client to that
database that supports asynchronous requests. Many popular databases
offer such a client.

In the absence of such a client, one can try and turn a synchronous
client into a limited concurrent client by creating multiple clients and
handling the synchronous calls with a thread pool. However, this
approach is usually less efficient than a proper asynchronous client.
"

Which means you need to on Flink end:

1) Use Flink async I/O .

2) Find a client that supports async or fake it by using multiple
synchronous clients.

On Postgres end there is this:

https://www.postgresql.org/docs/current/wal-async-commit.html

That will return a success signal to the client quicker if
synchronous_commit is set to off. Though the point of the Flink async
I/O is not to wait for the response before moving on, so I am not sure
how much synchronous_commit = off would help.

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com