java.sql2 DynamicMultiOperation with interlaced exceptions

Lukas Eder lukas.eder at
Fri Oct 13 07:33:42 UTC 2017


2017-10-13 0:34 GMT+02:00 Douglas Surber <douglas.surber at>:

> Lukas,
> Thanks for your comments. My reaction however is that this is an example
> of trying to shoehorn the database access problem into a reactive stream
> solution. For example bind variables are absolutely not a stream of any
> kind. They are best considered a map.

What about:

- Lobs, arrays
- Table valued parameters
- Cursor binds to stored procedures (e.g. in Oracle: SYS_REFCURSOR IN
- XML or JSON document streams
- Batch statements with bind variable batches
- Statement batches (not batch statements) where a batch of statements,
each with individual bind values, is sent to the server (although it would
be fair to say that bind variables wouldn't be supported in this case)

I could definitely see those as being "published" to a stream. Drivers /
servers could decide on their own if they want to run a statement with all
the binds complete, or separate the sending of binds from the statement
execution. Specifically when loading large amounts of data into the
database using a batched INSERT statement, this could be valuable.

I remember the numerous times in previous jobs where I had to go through
hoops to send PDF lobs into the database from some client, making sure the
client doesn't completely shut down because it is blocking on the database
(through all the dozens of layers in between). That stuff is really hard to
get right, and I was really hoping, the new API would solve this as well,
reactive or "simply "asynchronous...

If managing backpressure and asynchronicity of data sent to the server is
valuable *enough* is one question. But I don't think you can say that bind
variables are *absolutely not* a stream of any kind. Large LOBs were sent
to servers through a synchronous

> However rather that require construction of a Map the API allows passing
> each key/value pair separately. This is acceptable as the number of
> key/value pairs, the number of parameters, is part of the code (at least
> for the use cases we are targeting). Representing bind values as a stream
> is inappropriate. Sure lots of these concepts can be represented as
> publishing and subscribing, but that doesn’t make it the best idea.

Sure, perhaps you're right. I don't know the concrete use-cases you're
targeting. What I'm suggesting, though, is that a driver spec could cover
all use-cases:

- Fully reactive
- "simply" asynchronous

The lowest API level would be fully reactive. On top of that, there could
be convenience API that either delegates to the reactive API, simplifying
the API interaction for the user through CompletionStage /

The two places where back pressure are needed are controlling the rate at
> which the client submits Operations and controlling the rate at which the
> implementation produces rows. In a large majority of cases neither of these
> matter.

Yes of course. The "classic" JDBC API will be sufficient for most
applications anyway.

> Only in a very, very few cases will a client produce enough Operations
> fast enough that controlling the rate is of benefit. I would be perfectly
> happy if the API did not include any support for back pressure for
> Operation submission. A very few of the target use cases might have a query
> or two that produce enough rows that back pressure would be valuable. But
> even in those cases the overwhelming majority of queries would produce such
> a limited number of rows that there would be no benefit to back pressure.
> So only a small fraction of the queries in the target use cases would
> benefit from back pressure.
> Multi-result SQL could be represented as a stream of results. This is a
> SQL use case that I have no experience with. I would guess that the number
> of results is generally small. While any one result might produce a lot of
> rows (the second case in the previous paragraph) the number of results is
> probably small and the benefit of back pressure on the results (not the
> rows) is minimal.

You're making a lot of assumptions here :) I just hope you're right, and
I'm a bit afraid you might not be, in case of which the API you're
designing and which will be in the JDK for the next 20 years might be a lot
less useful than it could be. The discussions on this list have only just
started after years of quasi silence on the topic. I'm sure there are going
to be many opinions that have not been covered yet.

I'd love to learn more about your use-cases and their sources to better
understand the context of this work, though. Surely, I'm missing something
that is already obvious to you.

I’m no fan of CompletableFuture. In all honesty I dislike it quite a bit
> for reasons that would distract from this discussion. The Class Library
> Team strongly encouraged us to use CompletableFuture. I recall there was a
> reason not to use CompletionStage but I don’t remember what it was. I’ll
> review that decision. On the face of it I’d prefer CompletionStage for the
> reasons you give and others.

Interesting, thanks for sharing. Would be really interesting to know that
reason not to use CompletionStage.

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