I think this depends on the implementation of the OutputFormat. For instance, an HBase, Cassandra or ES OF will handle most operations as idempotent when the scheme is designed appropriately.
You are (rightly) focusing on FileOF's, which also depend on the semantics of their implementation. MR always required an atomic rename of the DFS, and only moved output files into place once the task commits its output. Also I think it unreasonable to bring exactly once considerations into the discussion because nothing provides this right now without a multi-stage commit protocol. Such a protocol would be provided at the framework level and to the best of my knowledge it's semantic expectations on the output handler are undefined. My original question comes from wanting to use the LocalCollectionOF to test a streaming flow that sinks to Kafka, without rewriting the flow in test code. So in this case you're right that it does apply to tests. I don't think correctness of tests is a trivial concern though. As for RollingFileSink, I've not seen this conversation so I cannot comment. Per my earlier examples, I think it's not correct to assume all OF implementations are file-based. On Monday, February 8, 2016, Aljoscha Krettek <aljos...@apache.org> wrote: > Hi, > one problem that I see with OutputFormat is that they are not made for a > streaming world. By this, I mean that they don’t handle failure well and > don’t consider fault-torelant streaming, i.e. exactly once semantics. For > example, what would be expected to happen if a job with a FileOutputFormat > fails and needs to recover. Now, there might be some garbage left in the > files that would get emitted again after restoring to a checkpoint, thus > leading to duplicate results. > > Having OutputFormats in a Streaming programs can work well in toy examples > and tests but can be dangerous in real-world jobs. I once talked with > Robert about this and we came up with the idea (I think it was mostly him) > of generalizing the RollingFileSink (which is fault-tolerance aware) so > that it can easily be used with something akin to OutputFormats. > > What do you think? > > -Aljoscha > > On 08 Feb 2016, at 19:40, Nick Dimiduk <ndimi...@apache.org > <javascript:;>> wrote: > > > > On Mon, Feb 8, 2016 at 9:51 AM, Maximilian Michels <m...@apache.org > <javascript:;>> wrote: > > Changing the class hierarchy would break backwards-compatibility of the > API. However, we could add another method to DataStream to easily use > OutputFormats in streaming. > > > > Indeed, that's why I suggested deprecating one and moving toward a > consolidated class hierarchy. It won't happen overnight, but this can be > managed pretty easily with some adapter code like this and some additional > overrides in the public APIs. > > > > How did you write your adapter? I came up with the one below. > > > > Our implementations are similar. This one is working fine with my test > code. > > > > https://gist.github.com/ndimiduk/18820fcd78412c6b4fc3 > > > > On Mon, Feb 8, 2016 at 6:07 PM, Nick Dimiduk <ndimi...@apache.org > <javascript:;>> wrote: > > In my case, I have my application code that is calling addSink, for > which I'm writing a test that needs to use LocalCollectionOutputFormat. > Having two separate class hierarchies is not helpful, hence the adapter. > Much of this code already exists in the implementation of FileSinkFunction, > so the project already supports it in a limited way. > > > > On Mon, Feb 8, 2016 at 4:16 AM, Maximilian Michels <m...@apache.org > <javascript:;>> wrote: > > Hi Nick, > > > > SinkFunction just implements user-defined functions on incoming > > elements. OutputFormat offers more lifecycle methods. Thus it is a > > more powerful interface. The OutputFormat originally comes from the > > batch API, whereas the SinkFunction originates from streaming. Those > > were more separate code paths in the past. Ultimately, it would make > > sense to have only the OutputFormat interface but I think we have to > > keep it to not break the API. > > > > If you need the lifecycle methods in streaming, there is > > RichSinkFunction, which implements OutputFormat and SinkFunction. In > > addition, it gives you access to the RuntimeContext. You can pass this > > directly to the "addSink(sinkFunction)" API method. > > > > Cheers, > > Max > > > > On Mon, Feb 8, 2016 at 7:14 AM, Nick Dimiduk <ndimi...@apache.org > <javascript:;>> wrote: > > > Heya, > > > > > > Is there a plan to consolidate these two interfaces? They appear to > provide > > > identical functionality, differing only in lifecycle management. I > found > > > myself writing an adaptor so I can consume an OutputFormat where a > > > SinkFunction is expected; there's not much to it. This seems like code > that > > > Flink should ship. > > > > > > Maybe one interface or the other can be deprecated for 1.0 API? > > > > > > Thanks, > > > Nick > > > > > > > >
