Support lazy decoding query response rows or not decoding,users can get the netty ByteBuf of row list directly

[jianliu]

Describe the feature

Im working on database L7 proxy which providee transparent database-sharding scenarios like Vitess\Apache shardingSphere,now we use jdbc in proxy which makes proxy decode all databases query response rows and block query thread.

I noticed that vertx uses netty handle the query,but the decoder decodes all rows directly,is it possible to support new features like lazy decoding rows or not,and users can get the netty ByteBuf which can make the proxy use ByteBuf (zero-copy) to send the query rows directlty to the client.

Use cases

A list of use cases this feature will enable and the value it creates.

• L7 Proxy transport data packets without decoding rows.

value:
Not only client can use this client, Proxy could use it to improve performance

Contribution

Hope the official support.
I`d like to try support this feature for mysql client.

[vietj]

it sounds like something not trivial to achieve, have you investigated whether it's actually possible with the client ?

[jianliu]

I saw the code of verx-sql-client and mysql implements,
The easiest way to support this is to add a lazyDecodeRow option, and interface Row to add a getByteBuf entry , RowResultDecoder store ByteBuf reference in memory,and lazy decode or not decode

[vietj]

I can see an issue with lazy evaluation because Netty uses referenced counted memory and this would imply to retain the byte buf portion until the row value is accessed. If the code does not evalutate the lazy value then it would create a memory leak.

[jianliu]

This is a good question and I think there are a ways to avoid it.If the row is decoded,the reference counted will be decremented directly,
If the row is not decoded,we can decrement the count after use the ByteBuf.Normal clients should not open the lazyDecodeRow,and if use lazyDecodeRow,users should pay attention to the release things

[jianliu]

Or give an entry to handle a row,and release it in framework,like this:

accumulator.accept(container,row);
if (lazyDecodeRow) {
ReferenceCountUtil.release(row);
}

[vietj]

I think what we could tr is make the SPI for interacting with the database as commands "public" (currently it's in io.vertx.sqlclient.impl.command package), e.g you can send SimpleQueryCommand and set a handler that would handle the lazy row. The current API in front of the command will always decode the full row, but you would be given the control over the row decoding

[jianliu]

Sounds great, adding an SPI to Command is a way to support this without affecting the current API.

[jianliu]

Could you give a timeline for the development of this feature

[vietj]

after Vert.x 4.3 seems reasonnable, in the next couple of months. We are currently wrapping 4.3 release

[vietj]

I added it to 4.3.0 but it will likely be bumped to 4.3.x until it is fixed, but this way it is on our radar

[jianliu]

nice, thank you very much.