On Thu, May 2, 2019 at 7:20 AM Robert Bradshaw <rober...@google.com> wrote:
> On Sat, Apr 27, 2019 at 1:14 AM Lukasz Cwik <lc...@google.com> wrote:
> >
> > We should stick with URN + payload + artifact metadata[1] where the only
> mandatory one that all SDKs and expansion services understand is the
> "bytes" artifact type. This allows us to add optional URNs for file://,
> http://, Maven, PyPi, ... in the future. I would make the artifact
> staging service use the same URN + payload mechanism to get compatibility
> of artifacts across the different services and also have the artifact
> staging service be able to be queried for the list of artifact types it
> supports.
>
> +1
>
> > Finally, we would need to have environments enumerate the artifact types
> that they support.
>
> Meaning at runtime, or as another field statically set in the proto?
>
I don't believe runners/SDKs should have to know what artifacts each
environment supports at runtime and instead have environments enumerate
them explicitly in the proto. I have been thinking about a more general
"capabilities" block on environments which allow them to enumerate URNs
that the environment understands. This would include artifact type URNs,
PTransform URNs, coder URNs, ... I haven't proposed anything specific down
this line yet because I was wondering how environment resources (CPU, min
memory, hardware like GPU, AWS/GCP/Azure/... machine types) should/could
tie into this.
> > Having everyone have the same "artifact" representation would be
> beneficial since:
> > a) Python environments could install dependencies from a
> requirements.txt file (something that the Google Cloud Dataflow Python
> docker container allows for today)
> > b) It provides an extensible and versioned mechanism for SDKs,
> environments, and artifact staging/retrieval services to support additional
> artifact types
> > c) Allow for expressing a canonical representation of an artifact like a
> Maven package so a runner could merge environments that the runner deems
> compatible.
> >
> > The flow I could see is:
> > 1) (optional) query artifact staging service for supported artifact types
> > 2) SDK request expansion service to expand transform passing in a list
> of artifact types the SDK and artifact staging service support, the
> expansion service returns a list of artifact types limited to those
> supported types + any supported by the environment
>
> The crux of the issue seems to be how the expansion service returns
> the artifacts themselves. Is this going with the approach that the
> caller of the expansion service must host an artifact staging service?
>
The caller would not need to host an artifact staging service (but would
become effectively a proxy service, see my comment below for more details)
as I would have expected this to be part of the expansion service response.
> There is also the question here is how the returned artifacts get
> attached to the various environments, or whether they get implicitly
> applied to all returned stages (which need not have a consistent
> environment)?
>
I would suggest returning additional information that says what artifact is
for which environment. Applying all artifacts to all environments is likely
to cause issues since some environments may not understand certain artifact
types or may get conflicting versions of artifacts. I would see this
happening since an expansion service that aggregates other expansion
services seems likely, for example:
/-> ExpansionSerivce(Python)
ExpansionService(Aggregator) --> ExpansionService(Java)
\-> ExpansionSerivce(Go)
> 3) SDK converts any artifact types that the artifact staging service or
> environment doesn't understand, e.g. pulls down Maven dependencies and
> converts them to "bytes" artifacts
>
> Here I think we're conflating two things. The "type" of an artifact is
> both (1) how to fetch the bytes and (2) how to interpret them (e.g. is
> this a jar file, or a pip tarball, or just some data needed by a DoFn,
> or ...) Only (1) can be freely transmuted.
Your right. Thinking about this some more, general artifact conversion is
unlikely to be practical because how to interpret an artifact is
environment dependent. For example, a requirements.txt used to install pip
packages for a Python docker container depends on the filesystem layout of
that specific docker container. One could simulate doing a pip install on
the same filesystem, see the diff and then of all the packages in
requirements.txt but this quickly becomes impractical.
> > 4) SDK sends artifacts to artifact staging service
> > 5) Artifact staging service converts any artifacts to types that the
> environment understands
> > 6) Environment is started and gets artifacts from the artifact retrieval
> service.
> >
> > On Wed, Apr 24, 2019 at 4:44 AM Robert Bradshaw <rober...@google.com>
> wrote:
> >>
> >> On Wed, Apr 24, 2019 at 12:21 PM Maximilian Michels <m...@apache.org>
> wrote:
> >> >
> >> > Good idea to let the client expose an artifact staging service that
> the
> >> > ExpansionService could use to stage artifacts. This solves two
> problems:
> >> >
> >> > (1) The Expansion Service not being able to access the Job Server
> >> > artifact staging service
> >> > (2) The client not having access to the dependencies returned by the
> >> > Expansion Server
> >> >
> >> > The downside is that it adds an additional indirection. The
> alternative
> >> > to let the client handle staging the artifacts returned by the
> Expansion
> >> > Server is more transparent and easier to implement.
> >>
> >> The other downside is that it may not always be possible for the
> >> expansion service to connect to the artifact staging service (e.g.
> >> when constructing a pipeline locally against a remote expansion
> >> service).
> >
> > Just to make sure, your saying the expansion service would return all
> the artifacts (bytes, urls, ...) as part of the response since the
> expansion service wouldn't be able to connect to the SDK that is running
> locally either.
>
> Yes. Well, more I'm asking how the expansion service would return any
> artifacts.
>
> What we have is
>
> Runner <--- SDK ---> Expansion service.
>
> Where the unidirectional arrow means "instantiates a connection with"
> and the other direction (and missing arrows) may not be possible.
I believe the ExpansionService Expand request should become a
unidirectional stream back to the caller so that artifacts could be sent
back to the SDK (effectively mirroring the artifact staging service API).
So the expansion response would stream back a bunch artifact data messages
and also the expansion response containing PTransform information.
> >> > Ideally, the Expansion Service won't return any dependencies because
> the
> >> > environment already contains the required dependencies. We could make
> it
> >> > a requirement for the expansion to be performed inside an environment.
> >> > Then we would already ensure during expansion time that the runtime
> >> > dependencies are available.
> >>
> >> Yes, it's cleanest if the expansion service provides an environment
> >> without all the dependencies provided. Interesting idea to make this a
> >> property of the expansion service itself.
> >
> > I had thought this too but an opaque docker container that was built on
> top of a base Beam docker container would be very difficult for a runner to
> introspect and check to see if its compatible to allow for fusion across
> PTransforms. I think artifacts need to be communicated in their canonical
> representation.
>
> It's clean (from the specification point of view), but doesn't allow
> for good introspection/fusion (aside from one being a base of another,
> perhaps).
>
> >> > > In this case, the runner would (as
> >> > > requested by its configuration) be free to merge environments it
> >> > > deemed compatible, including swapping out beam-java-X for
> >> > > beam-java-embedded if it considers itself compatible with the
> >> > > dependency list.
> >> >
> >> > Could you explain how that would work in practice?
> >>
> >> Say one has a pipeline with environments
> >>
> >> A: beam-java-sdk-2.12-docker
> >> B: beam-java-sdk-2.12-docker + dep1
> >> C: beam-java-sdk-2.12-docker + dep2
> >> D: beam-java-sdk-2.12-docker + dep3
> >>
> >> A runner could (conceivably) be intelligent enough to know that dep1
> >> and dep2 are indeed compatible, and run A, B, and C in a single
> >> beam-java-sdk-2.12-docker + dep1 + dep2 environment (with the
> >> corresponding fusion and lower overhead benefits). If a certain
> >> pipeline option is set, it might further note that dep1 and dep2 are
> >> compatible with its own workers, which are build against sdk-2.12, and
> >> choose to run these in embedded + dep1 + dep2 environment.
> >
> > We have been talking about the expansion service and cross language
> transforms a lot lately but I believe it will initially come at the cost of
> poor fusion of transforms since "merging" environments that are compatible
> is a difficult problem since it brings up many of the dependency management
> issues (e.g. diamond dependency issues).
>
> I agree. I think expansion services offering "kitchen-sink"
> containers, when possible, can go far here. If we could at least
> recognize when one environment/set of deps is a superset of another,
> that could be an easy case that would yield a lot of benefit as well.
>
+1
