英文:
How can I configure Alpakka Slick to enable streaming from my database in Java?
问题
我正在尝试在Java应用程序中使用Akka Streams和Postgres使用Slick查询来流式传输结果：
Source mySource = Slick.source(
slickSession,
"SELECT * from message where started_instant is null",
(this::createQueueItemFromSlickRow));
在Slick文档中明确指出，为了在Postgres中进行流式传输，需要设置额外的参数。然而，我只能找到Scala示例，并且在Alpakka文档中没有关于如何配置这些参数的信息。
目前，一旦流已经开始，我无法处理数据库中新加入的对象。
英文:
I am trying to stream results from a query using Slick within a Java application, using Akka Streams and Postgres:
Source mySource = Slick.source(
slickSession,
"SELECT * from message where started_instant is null",
(this::createQueueItemFromSlickRow));
In the Slick documentation it is clear that in order to stream with Postgres, additional parameters need to be set. However, I can only find Scala examples, and there is no information in the Alpakka documentation on how to configure this.
As it is, I can't process any new objects that come into the database after the stream has started.
答案1
得分: 1
正如您所述，Slick文档中指出：
> 注意：某些数据库系统可能需要以特定方式设置会话参数，以支持在客户端内存中不缓存所有数据的流式传输。例如，PostgreSQL需要同时使用.withStatementParameters(rsType = ResultSetType.ForwardOnly, rsConcurrency = ResultSetConcurrency.ReadOnly, fetchSize = n)（其中n是所需的页面大小）和.transactionally以实现正确的流式传输。
Alpakka的Slick支持最近在其Java DSL中公开了PreparedStatement<sup>1</sup>，您可以使用它来设置上述参数，如果您不能使用Scala API的话。例如：
PreparedStatement statement =
connection.prepareStatement(
"MY SQL STATEMENT",
ResultSet.TYPE_FORWARD_ONLY, // java.sql.ResultSet
ResultSet.CONCUR_READ_ONLY);
查看SlickTest.java类和上述拉取请求以获取更多详细信息。
<sup>1</sup>这个更改是即将发布的Alpakka 2.0.2版本的一部分（在撰写本文时，版本2.0.1是当前版本）。
英文:
As you stated, the Slick documentation notes the following:
> Note: Some database systems may require session parameters to be set in a certain way to support streaming without caching all data at once in memory on the client side. For example, PostgreSQL requires both .withStatementParameters(rsType = ResultSetType.ForwardOnly, rsConcurrency = ResultSetConcurrency.ReadOnly, fetchSize = n) (with the desired page size n) and .transactionally for proper streaming.
Alpakka's Slick support recently exposed the PreparedStatement in its Java DSL<sup>1</sup>, which you can use to set the aforementioned parameters if you cannot use the Scala API. For example:
PreparedStatement statement =
connection.prepareStatement(
"MY SQL STATEMENT",
ResultSet.TYPE_FORWARD_ONLY, // java.sql.ResultSet
ResultSet.CONCUR_READ_ONLY);
Check out the SlickTest.java class and the above pull request for more details.
<sup>1</sup>This change is part of the upcoming Alpakka 2.0.2 release. (Version 2.0.1 is the current release at the time of this writing.)
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