Stateless prepared statements wrap DoPutPreparedStatementResult with Any which is differs from Go implementation

[matthewmturner]

Describe the bug

Originally posted in the arrow repo - we are seeing differences in the Go and Rust implementation of stateless prepared statements. Specifically, the rust implementation wraps the DoPutPreparedStatementResult in Any where the go implementation does not - which makes the two incompatible. From the conversation there it looks like they think the Rust implementation should be updated to no longer use Any.

To Reproduce

Make a stateless prepared statement request with parameter binding from Go FlightSQL Client to Rust Flight SQL Server.

Expected behavior

Compatibility between Rust and Go FlightSQL implementations.

Additional context

[alamb]

cc @erratic-pattern

[alamb]

I think we should follow the Go implementation as they are likely to have a better understanding of what is correct in protobuf

Any clients can wrap the ticket in Any if they want.

[erratic-pattern]

I've replied to the arrow repo issue here apache/arrow#41556 (comment) .

It's not clear to me yet whether this needs to be fixed in the Rust client or the Go client, as it seems other implementations are also using the Any wrapper and Go is currently the odd one out. I suggest we wait for a broader consensus on how the result should be represented before making any changes.

[alamb]

as it seems other implementations are also using the Any wrapper and Go is currently the odd one out.

What other implementations are using the Any wrapper? Is it java?

[erratic-pattern]

as it seems other implementations are also using the Any wrapper and Go is currently the odd one out.

What other implementations are using the Any wrapper? Is it java?

C++

Also, as I mentioned in my comment, it seems to be an established convention to Any wrap messages that are encoded into PutResult.app_metadata. This is what the arrow-rs implementation currently does. If other implementations are not currently doing this, then it suggests a deeper problem with language implementations having divergent behavior. I think we need to wait and see what others say about this.

[matthewmturner]

In my mind the only thing that matters is conformance to the spec, regardless of language convention. If the spec is ambiguous on this point then its not surprising we ended up here (I have not had a chance to review the spec so im not sure). If it's not ambiguous then my preferred approach would be to have all implementations update to conform with the spec, and then the spec can be revisited / updated if needed after that. If it is ambiguous on this point then I think the community will need to align on the appropriate next steps (formalizing conventions / updating spec / etc.)

[matthewmturner]

Following up on this - I have reviewed the Flight and FlightSQL spec and from what I can tell it is prescribed when Any should be used (ref ActionBeginTransactionResult, ActionBeginSavepointResult, and a few others) but I don't see this mentioned for DoPut and PutResult.

DoPut and PutResult definition.

/*
   * Push a stream to the flight service associated with a particular
   * flight stream. This allows a client of a flight service to upload a stream
   * of data. Depending on the particular flight service, a client consumer
   * could be allowed to upload a single stream per descriptor or an unlimited
   * number. In the latter, the service might implement a 'seal' action that
   * can be applied to a descriptor once all streams are uploaded.
   */
  rpc DoPut(stream FlightData) returns (stream PutResult) {}

/**
 * The response message associated with the submission of a DoPut.
 */
message PutResult {
  bytes app_metadata = 1;
}

ActionBeginTransactionResult definition (notice it mentions the result should be wrapped in Any

/*
 * The result of a "BeginTransaction" action.
 *
 * The transaction can be manipulated with the "EndTransaction" action, or
 * automatically via server timeout. If the transaction times out, then it is
 * automatically rolled back.
 *
 * The result should be wrapped in a google.protobuf.Any message.
 */
message ActionBeginTransactionResult {
  option (experimental) = true;

  // Opaque handle for the transaction on the server.
  bytes transaction_id = 1;
}

Of course let me know if I am mistaken here but based on this my preference would be to update the Rust implementation by removing the Any wrapper as I believe it would be more in line with the spec. Separate from that, we could then revisit whether in the spec DoPut results should be wrapped in Any.

[matthewmturner]

I can put together a PR if there is agreement on this.

[matthewmturner]

hi @alamb @erratic-pattern do you have thoughts on the above?

[alamb]

I believe @erratic-pattern has some thoughts he should be sharing shortly

[erratic-pattern]

I think this issue arises via a combination of factors:

• some message types explicitly document that they should be Any wrapped
• others do not document that they should be Any wrapped, but also don't recommend against it explicitly
• client implementors therefore assumed, in absense of explicit documentation, that message types should be consistent with each other, but each implementor comes to a divergent conclusion about what that consistent message handling actually should be.

My only preference is toward documenting what goes in app_metadata explicitly, or in the absence of such documentation, have consistent behavior between message types. This would eliminate future ambiguity and make it unlikely for similar issues to happen.

The problem with the current consensus of removing the Any wrapping on this one particular message type is that there will be no consistency between different message types going forward. Each message type sort of arbitrarily specifies whether or not it has Any wrapping, and there is no particular rhyme or reason for why this should or shouldn't be the case.

Even if we go and explicitly document each and every message type with its unambiguous Any wrapping behavior, I don't see any clear reason for different message types to have different Any wrapping behavior in the first place. It seem arbitrary and based on historical precedent rather than explicitly designed with a purpose in mind.

This is a "path of least resistance" approach to making the minimal possible spec change, but ultimately makes the specification messy and thus harder to implement and maintain in the long term.

So my recommendation would be to:

1. Have a clear rationale for or against Any wrapping of message types
2. make all bytes-encoded message types across the specification have consistent Any wrapping behavior, and document this on the bytes field itself as well as the message types, except when there is a clear justification for why one message type should deviate. Document all such justification.

[erratic-pattern]

An additional remark on the 1st point above (Have a clear rationale for or against Any wrapping of message types), I don't see a clear justification for or against Any wrapping from a technical standpoint. I think we just need to choose one and stick with it across the spec.

My preference would be no Any wrapping because it is simpler. However, I think the best choice would be whichever change results in the smallest amount of breaking code in the current implementations. We should look at all clients across all messages types and see what they are currently doing, then choose the Any wrapping behavior that results in the fewest changes.

Depending on what clients currently do, this could be a much larger and more breaking change than just changing one particular message type.

But, for example, if only one client is the odd one out in terms of Any wrapping semantics, we can avoid a lot of breakage by simply changing that one client to match the behavior of the others. But to do that we need a clear picture of what each client is doing across all message types.

[matthewmturner]

@erratic-pattern thank you for the feedback. i understand and agree with your point - documenting explicitly what goes in app_metadata in particular.

That being said, I think we need to distinguish what should be done now to get the different implementations in a state where they are compatible with each other (or explicitly deciding we are okay with a transitory state of being incompatible) and what the next steps are to update the spec to have better defined behavior with regards to Any / bytes encoding - which will require work from the whole arrow community.

While we could hold off on making changes until there is agreement on the spec changes I think that leaves us in a quite bad spot as the current implementations are not compatible.

My preference is to interpret the spec literally (i.e. Any is allowed where it is specified) and have all implementations align on this (even if it is not perfect). Then, update the spec to be explicit and the implementations can be updated to stay in conformance with that (at least we will be consistent with regards to following the spec to the letter). I dont think there should be room for implementations to make any assumptions about how to handle the spec (maybe thats unreasonable, im not sure).

The alternate route of course being that we leave things as they are in the current state, which is incompatible, and then work with the community to determine what changes are needed before making any updates. It's not clear to me that this could be done in a short period of time which is why my preference is to get conformance first and then update spec. But, we probably need opinions from other implementations as well - maybe its doable.

CC @lidavidm and @zeroshade for their views

[tustvold]

My 2 cents is that type-erased payloads should always be Any. Protobuf is designed to ignore message content that it doesn't expect to allow for API evolution, and a consequence of this is decoding a message as an incorrect type does not necessarily lead to an error. By ensuring such payloads are consistently wrapped in Any this form of silent data corruption can be avoided, payloads are at least somewhat self-describing, we preserve some form of type safety and clients can potentially support multiple different response payloads.

Now I agree it is unfortunate that FlightSQL was designed in this way, gRPC provides a type-safe DSL and the layering of FlightSQL on top of Flight sacrificed many of these benefits, but the least we can do is ensure that users get a sensible runtime error....

This comment from @zeroshade on the linked PR makes me more confident of this

That said, it does appear that the Go implementation is outnumbered here as the C++ implementation does assume an Any wrapper around the DoPutPreparedStatementResult, but it just seems odd to me as it breaks the existing convention that we had, leaving it to be kind of arbitrary and not documented anywhere.

We should double-check what the Java driver does, however, given I know some people are using JDBC against the arrow-rs, I suspect it is also expecting the payload to be Any wrapped.

[erratic-pattern]

Thank you @tustvold for the additional technical context on why Any is useful here. I had forgotten that you can't catch decoding errors as a form of message versioning, due to the possibility of false positives when decoding. I think that puts me in favor of Any wrapping here.

The more important part in my mind is that behavior is consistent and not arbitrary per payload type. I don't think this should ever be the case in the specification, temporarily or long term.

@matthewmturner:

That being said, I think we need to distinguish what should be done now to get the different implementations in a state where they are compatible with each other (or explicitly deciding we are okay with a transitory state of being incompatible) and what the next steps are to update the spec to have better defined behavior with regards to Any / bytes encoding - which will require work from the whole arrow community.

While we could hold off on making changes until there is agreement on the spec changes I think that leaves us in a quite bad spot as the current implementations are not compatible.

I am not sure whether or not we need a stopgap measure such as this. If the change is as simple as updating the Go implementation and clarifying the documentation in the spec, then what you're suggesting would actually be more work than moving forward with the more ideal change. We would need to break the conventions used by all other implementations, add a lot of temporary documentation to the spec, and then remove it all in the "final" iteration.

I will spend some time looking across language implementations to see what the predominant convention currently is, and that will give us a better idea the scope of change that needs to be made here.

My preference is to interpret the spec literally (i.e. Any is allowed where it is specified) and have all implementations align on this (even if it is not perfect). Then, update the spec to be explicit and the implementations can be updated to stay in conformance with that (at least we will be consistent with regards to following the spec to the letter). I dont think there should be room for implementations to make any assumptions about how to handle the spec (maybe thats unreasonable, im not sure).

I think the problem here is that there is ambiguity and absence on what to do here in the spec, which is why we are in a situation where we have divergent implementations. I think enforcing a literal interpretation of the spec, even temporarily, could potentially do more harm than good, especially if there is a predominant implied convention being used across implementations. In an ideal world the spec would be the definitive source of truth, but that is rarely the case. I think it is better to update the spec to document such implied conventions as they are discovered. The purpose of a system is what it does.

[matthewmturner]

I'm not trying to minimize the amount of work (I don't consider these large scale changes within the arrow libs although i acknowledge it could potentially have larger impacts on users of the libs) - I just want to 1. get us into a workable state where implementations are compatible and 2. get it right (update spec and all implementations to be aligned). If we can quickly get alignment across the ecosystem then great but im concerned were going to end up in a holding pattern where were stuck with incompatible implementations. But I guess we'll see what your analysis and the feedback from others comes out to be.

[zeroshade]

I think in the current situation I think that unless we can get alignment across the ecosystem fairly quickly, it might make the most sense for us first to get the Go implementation into alignment with the others using Any to align everything to be compatible with each other. Then we can have the larger discussion on what it should be, clarify the spec, and make any other necessary changes.

What does everyone else think?

[matthewmturner]

@zeroshade Im definitely supportive of that - appreciate your flexibility.

[lidavidm]

• Any sounds good to me if we're all agreed.
• We need to seriously tackle improved integration testing, and ideally provide a conformance test that isn't tied into the integration test framework and can be used by projects developing on Flight SQL.
• In the medium term I would like us to consider one of the root causes which is Flight RPC's insistence that you stuff everything into DoAction. Instead I would like to explore having Flight SQL be a proper gRPC service definition instead of relying on undocumented convention for the input/return types. The C++ implementation has held us back here but I don't think it's fair to let it drag the other implementations around.

[zeroshade]

@lidavidm I plan on scheduling my team to try to tackle exposing the gRPC innards to the c++ implementation next quarter.

[matthewmturner]

@lidavidm @zeroshade @erratic-pattern thank you all for the feedback and helping get this to a short term productive outcome despite having to diverge from some prior language specific conventions.

likely by tomorrow i should have time to create issues for both updating the go implementation and starting conversation on updating the spec. im not as familiar with the go arrow implementation but if needed i can put together a PR for that.

[matthewmturner]

Actually there is already issue in the main arrow repo so i dont think we need a new one there.

[zeroshade]

@matthewmturner @erratic-pattern @tustvold @alamb @lidavidm

Looks like I was slightly wrong. I just dug into the C++ code a bit more to confirm precisely where I needed to make the changes on the Go side to make sure we match and it looks like Rust is the odd-implementation out. Looking at https://github.com/apache/arrow/blob/main/cpp/src/arrow/flight/sql/client.cc#L223, https://github.com/apache/arrow/blob/main/cpp/src/arrow/flight/sql/client.cc#L250, and https://github.com/apache/arrow/blob/main/cpp/src/arrow/flight/sql/client.cc#L358 it appears that the C++ implementation matches the Go implemenation: Expecting that the DoPutUpdateResult object is serialized into the metadata member of the PutResult object.

So it looks like the quick solution would be for Rust to conform to what C++ and Go do, and then we can work on improving the spec and integration testing.

[tustvold]

I would suggest we confirm what the other implementations do, especially Java given the importance of JDBC, along with what the desired end state is, before we make breaking changes to any implementations. We do not want to be in a position where we make a breaking change only to revert it down the line.

[lidavidm]

Java hasn't yet implemented it but the PR also did not wrap in Any.

[lidavidm]

apache/arrow#41237 CC @stevelorddremio

[stevelorddremio]

@lidavidm is correct. Currently the draft Java implementation does not wrap the result in Any.
I will hold off pushing out further changes, but would welcome a consensus before I continue.
My thoughts are unless there is a reason to wrap the metadata with Any, we should not do it.

[alamb]

Am I correct in summarizing the consensus:

1. implementations should not wrap the result in Any.
2. This would require changing the arrow-rs implementation (only) to match Go, C++ and the (draft) Java

Did I get that right?

[tustvold]

I think that is an accurate description of the implementation state, although I would observe the somewhat arbitrary way some places wrap in Any and some don't is perhaps worth fixing, and if so it would be unfortunate to change the Rust implementation only to then change it back