lihaosky commented on code in PR #26610:
URL: https://github.com/apache/flink/pull/26610#discussion_r2138839903
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docs/content/docs/dev/table/functions/udfs.md:
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@@ -1017,6 +1018,139 @@ If you intend to implement or call functions in Python,
please refer to the [Pyt
{{< top >}}
+Asynchronous Scalar Functions
+----------------
+
+When interacting with external systems (for example when enriching stream
events with data stored in a database), one needs to take care that network or
other latency does not dominate the streaming application’s running time.
+
+Naively accessing data in the external database, for example using a
`ScalarFunction`, typically means **synchronous** interaction: A request is
sent to the database and the `ScalarFunction` waits until the response has been
received. In many cases, this waiting makes up the vast majority of the
function’s time.
+
+To address this inefficiency, there is an `AsyncScalarFunction`. Asynchronous
interaction with the database means that a single function instance can handle
many requests concurrently and receive the responses concurrently. That way,
the waiting time can be overlaid with sending other requests and receiving
responses. At the very least, the waiting time is amortized over multiple
requests. This leads in most cased to much higher streaming throughput.
+
+{{< img src="/fig/async_io.svg" width="50%" >}}
+
+#### Defining an AsyncScalarFunction
+
+A user-defined asynchronous scalar function maps zero, one, or multiple scalar
values to a new scalar value, but does it asynchronously. Any data type listed
in the [data types section]({{< ref "docs/dev/table/types" >}}) can be used as
a parameter or return type of an evaluation method.
+
+In order to define an asynchronous scalar function, one has to extend the base
class `AsyncScalarFunction` in `org.apache.flink.table.functions` and implement
one or more evaluation methods named `eval(...)`. The first argument must be a
`CompletableFuture<...>` which is used to return the result, with subsequent
arguments being the parameters passed to the function.
Review Comment:
Add `eval` overloading to example as well?
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