Cool !!!
I guess we can leverage this in IOs with SDF.
Thanks
Regards
JB
On 14/05/2018 23:48, Eugene Kirpichov wrote:
Hi folks,
Wanted to give a heads up about the existence of a commonly requested
feature and its first successful production usage.
The feature is the Wait.on() transform [1] , and the first successful
production usage is in Spanner [2] .
The Wait.on() transform allows you to "do this, then that" - in the
sense that a.apply(Wait.on(signal)) re-emits PCollection "a", but only
after the PCollection "signal" is "done" in the same window (i.e. when
no more elements can arrive into the same window of "signal"). The
PCollection "signal" is typically a collection of results of some
operation - so Wait.on(signal) allows you to wait until that operation
is done. It transparently works correctly in streaming pipelines too.
This may sound a little convoluted, so the example from documentation
should help.
PCollection<Void> firstWriteResults = data.apply(ParDo.of(...write to
first database...));
data.apply(Wait.on(firstWriteResults))
// Windows of this intermediate PCollection will be processed no
earlier than when
// the respective window of firstWriteResults closes.
.apply(ParDo.of(...write to second database...));
This is indeed what Spanner folks have done, and AFAIK they intend this
for importing multiple dependent database tables - e.g. first import a
parent table; when it's done, import the child table - all within one
pipeline. You can see example code in the tests [3].
Please note that this kind of stuff requires support from the IO
connector - IO.write() has to return a result that can be waited on. The
code of SpannerIO is a great example; another example is FileIO.write().
People have expressed wishes for similar support in Bigtable and
BigQuery connectors but it's not there yet. It would be really cool if
somebody added it to these connectors or others (I think there was a
recent thread discussing how to add it to BigQueryIO).
[1]
https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Wait.java
[2] https://github.com/apache/beam/pull/4264
[3]
https://github.com/apache/beam/blob/a3ce091b3bbebf724c63be910bd3bc4cede4d11f/sdks/java/io/google-cloud-platform/src/test/java/org/apache/beam/sdk/io/gcp/spanner/SpannerWriteIT.java#L158
