Arrow Flight RPC

Arrow Flight is an RPC framework for high-performance data services based on Arrow data, and is built on top of gRPC and the IPC format.

Flight is organized around streams of Arrow record batches, being either downloaded from or uploaded to another service. A set of metadata methods offers discovery and introspection of streams, as well as the ability to implement application-specific methods.

Methods and message wire formats are defined by Protobuf, enabling interoperability with clients that may support gRPC and Arrow separately, but not Flight. However, Flight implementations include further optimizations to avoid overhead in usage of Protobuf (mostly around avoiding excessive memory copies).

RPC Methods and Request Patterns

Flight defines a set of RPC methods for uploading/downloading data, retrieving metadata about a data stream, listing available data streams, and for implementing application-specific RPC methods. A Flight service implements some subset of these methods, while a Flight client can call any of these methods.

Data streams are identified by descriptors (the FlightDescriptor message), which are either a path or an arbitrary binary command. For instance, the descriptor may encode a SQL query, a path to a file on a distributed file system, or even a pickled Python object; the application can use this message as it sees fit.

Thus, one Flight client can connect to any service and perform basic operations. To facilitate this, Flight services are expected to support some common request patterns, described next. Of course, applications may ignore compatibility and simply treat the Flight RPC methods as low-level building blocks for their own purposes.

See Protocol Buffer Definitions for full details on the methods and messages involved.

Downloading Data

A client that wishes to download the data would:

Retrieving data via DoGet.

1. Construct or acquire a FlightDescriptor for the data set they are interested in.

   A client may know what descriptor they want already, or they may use methods like ListFlights to discover them.

2. Call GetFlightInfo(FlightDescriptor) to get a FlightInfo message.

   Flight does not require that data live on the same server as metadata. Hence, FlightInfo contains details on where the data is located, so the client can go fetch the data from an appropriate server. This is encoded as a series of FlightEndpoint messages inside FlightInfo. Each endpoint represents some location that contains a subset of the response data.

   An endpoint contains a list of locations (server addresses) where this data can be retrieved from, and a Ticket, an opaque binary token that the server will use to identify the data being requested.

   If FlightInfo.ordered is true, this signals there is some order between data from different endpoints. Clients should produce the same results as if the data returned from each of the endpoints was concatenated, in order, from front to back.

   If FlightInfo.ordered is false, the client may return data from any of the endpoints in arbitrary order. Data from any specific endpoint must be returned in order, but the data from different endpoints may be interleaved to allow parallel fetches.

   Note that since some clients may ignore FlightInfo.ordered, if ordering is important and client support cannot be ensured, servers should return a single endpoint.

   The response also contains other metadata, like the schema, and optionally an estimate of the dataset size.

3. Consume each endpoint returned by the server.

   To consume an endpoint, the client should connect to one of the locations in the endpoint, then call DoGet(Ticket) with the ticket in the endpoint. This will give the client a stream of Arrow record batches.

   If the server wishes to indicate that the data is on the local server and not a different location, then it can return an empty list of locations. The client can then reuse the existing connection to the original server to fetch data. Otherwise, the client must connect to one of the indicated locations.

   The server may list “itself” as a location alongside other server locations. Normally this requires the server to know its public address, but it may also use the special URI string arrow-flight-reuse-connection://? to tell clients that they may reuse an existing connection to the same server, without having to be able to name itself. See Connection Reuse below.

   In this way, the locations inside an endpoint can also be thought of as performing look-aside load balancing or service discovery functions. And the endpoints can represent data that is partitioned or otherwise distributed.

   The client must consume all endpoints to retrieve the complete data set. The client can consume endpoints in any order, or even in parallel, or distribute the endpoints among multiple machines for consumption; this is up to the application to implement. The client can also use FlightInfo.ordered. See the previous item for details of FlightInfo.ordered.

   Each endpoint may have expiration time (FlightEndpoint.expiration_time). If an endpoint has expiration time, the client can get data multiple times by DoGet until the expiration time is reached. Otherwise, it is application-defined whether DoGet requests may be retried. The expiration time is represented as google.protobuf.Timestamp.

   If the expiration time is short, the client may be able to extend the expiration time by RenewFlightEndpoint action. The client need to use DoAction with RenewFlightEndpoint action type to extend the expiration time. Action.body must be RenewFlightEndpointRequest that has FlightEndpoint to be renewed.

   The client may be able to cancel the returned FlightInfo by CancelFlightInfo action. The client need to use DoAction with CancelFlightInfo action type to cancel the FlightInfo.

Downloading Data by Running a Heavy Query

A client may need to request a heavy query to download data. However, GetFlightInfo doesn’t return until the query completes, so the client is blocked. In this situation, the client can use PollFlightInfo instead of GetFlightInfo:

Polling a long-running query by PollFlightInfo.

1. Construct or acquire a FlightDescriptor, as before.

2. Call PollFlightInfo(FlightDescriptor) to get a PollInfo message.

   A server should respond as quickly as possible on the first call. So the client shouldn’t wait for the first PollInfo response.

   If the query isn’t finished, PollInfo.flight_descriptor has a FlightDescriptor. The client should use the descriptor (not the original FlightDescriptor) to call the next PollFlightInfo(). A server should recognize a PollInfo.flight_descriptor that is not necessarily the latest in case the client misses an update in between.

   If the query is finished, PollInfo.flight_descriptor is unset.

   PollInfo.info is the currently available results so far. It’s a complete FlightInfo each time not just the delta between the previous and current FlightInfo. A server should only append to the endpoints in PollInfo.info each time. So the client can run DoGet(Ticket) with the Ticket in the PollInfo.info even when the query isn’t finished yet. FlightInfo.ordered is also valid.

   A server should not respond until the result would be different from last time. That way, the client can “long poll” for updates without constantly making requests. Clients can set a short timeout to avoid blocking calls if desired.

   PollInfo.progress may be set. It represents progress of the query. If it’s set, the value must be in [0.0, 1.0]. The value is not necessarily monotonic or nondecreasing. A server may respond by only updating the PollInfo.progress value, though it shouldn’t spam the client with updates.

   PollInfo.timestamp is the expiration time for this request. After this passes, a server might not accept the poll descriptor anymore and the query may be cancelled. This may be updated on a call to PollFlightInfo. The expiration time is represented as google.protobuf.Timestamp.

   A client may be able to cancel the query by the CancelFlightInfo action.

   A server should return an error status instead of a response if the query fails. The client should not poll the request except for TIMED_OUT and UNAVAILABLE, which may not originate from the server.

3. Consume each endpoint returned by the server, as before.

Uploading Data

To upload data, a client would:

Uploading data via DoPut.

1. Construct or acquire a FlightDescriptor, as before.

2. Call DoPut(FlightData) and upload a stream of Arrow record batches.

   The FlightDescriptor is included with the first message so the server can identify the dataset.

DoPut allows the server to send response messages back to the client with custom metadata. This can be used to implement things like resumable writes (e.g. the server can periodically send a message indicating how many rows have been committed so far).

Exchanging Data

Some use cases may require uploading and downloading data within a single call. While this can be emulated with multiple calls, this may be difficult if the application is stateful. For instance, the application may wish to implement a call where the client uploads data and the server responds with a transformation of that data; this would require being stateful if implemented using DoGet and DoPut. Instead, DoExchange allows this to be implemented as a single call. A client would:

Complex data flow with DoExchange.

1. Construct or acquire a FlightDescriptor, as before.

2. Call DoExchange(FlightData).

   The FlightDescriptor is included with the first message, as with DoPut. At this point, both the client and the server may simultaneously stream data to the other side.

Authentication

Flight supports a variety of authentication methods that applications can customize for their needs.

“Handshake” authentication

This is implemented in two parts. At connection time, the client calls the Handshake RPC method, and the application-defined authentication handler can exchange any number of messages with its counterpart on the server. The handler then provides a binary token. The Flight client will then include this token in the headers of all future calls, which is validated by the server authentication handler.

Applications may use any part of this; for instance, they may ignore the initial handshake and send an externally acquired token (e.g. a bearer token) on each call, or they may establish trust during the handshake and not validate a token for each call, treating the connection as stateful (a “login” pattern).

Warning

Unless a token is validated on every call, this pattern is not secure, especially in the presence of a layer 7 load balancer, as is common with gRPC, or if gRPC transparently reconnects the client.

Header-based/middleware-based authentication

Clients may include custom headers with calls. Custom middleware can then be implemented to validate and accept/reject calls on the server side.

Mutual TLS (mTLS)

The client provides a certificate during connection establishment which is verified by the server. The application does not need to implement any authentication code, but must provision and distribute certificates.

This may only be available in certain implementations, and is only available when TLS is also enabled.

Some Flight implementations may expose the underlying gRPC API as well, in which case any authentication method supported by gRPC is available.

Location URIs

Flight is primarily defined in terms of its Protobuf and gRPC specification below, but Arrow implementations may also support alternative transports (see Flight RPC). Clients and servers need to know which transport to use for a given URI in a Location, so Flight implementations should use the following URI schemes for the given transports:

Transport	URI Scheme
gRPC (plaintext)	grpc: or grpc+tcp:
gRPC (TLS)	grpc+tls:
gRPC (Unix domain socket)	grpc+unix:
(reuse connection)	arrow-flight-reuse-connection:
UCX (plaintext) (1)	ucx:

Notes:

• (1) Flight UCX transport has been deprecated on the 19.0.0 release. The Dissociated IPC Protocol section proposes an alternative solution.

Connection Reuse

“Reuse connection” above is not a particular transport. Instead, it means that the client may try to execute DoGet against the same server (and through the same connection) that it originally obtained the FlightInfo from (i.e., that it called GetFlightInfo against). This is interpreted the same way as when no specific Location are returned.

This allows the server to return “itself” as one possible location to fetch data without having to know its own public address, which can be useful in deployments where knowing this would be difficult or impossible. For example, a developer may forward a remote service in a cloud environment to their local machine; in this case, the remote service would have no way to know the local hostname and port that it is being accessed over.

For compatibility reasons, the URI should always be arrow-flight-reuse-connection://?, with the trailing empty query string. Java’s URI implementation does not accept scheme: or scheme://, and C++’s implementation does not accept an empty string, so the obvious candidates are not compatible. The chosen representation can be parsed by both implementations, as well as Go’s net/url and Python’s urllib.parse.

Error Handling

Arrow Flight defines its own set of error codes. The implementation differs between languages (e.g. in C++, Unimplemented is a general Arrow error status while it’s a Flight-specific exception in Java), but the following set is exposed:

Error Code	Description
UNKNOWN	An unknown error. The default if no other error applies.
INTERNAL	An error internal to the service implementation occurred.
INVALID_ARGUMENT	The client passed an invalid argument to the RPC.
TIMED_OUT	The operation exceeded a timeout or deadline.
NOT_FOUND	The requested resource (action, data stream) was not found.
ALREADY_EXISTS	The resource already exists.
CANCELLED	The operation was cancelled (either by the client or the server).
UNAUTHENTICATED	The client is not authenticated.
UNAUTHORIZED	The client is authenticated, but does not have permissions for the requested operation.
UNIMPLEMENTED	The RPC is not implemented.
UNAVAILABLE	The server is not available. May be emitted by the client for connectivity reasons.

External Resources

• https://arrow.apache.org/blog/2019/10/13/introducing-arrow-flight/

• https://arrow.apache.org/blog/2018/10/09/0.11.0-release/

• https://www.slideshare.net/JacquesNadeau5/apache-arrow-flight-overview

Protocol Buffer Definitions

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 * <p>
 * http://www.apache.org/licenses/LICENSE-2.0
 * <p>
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

syntax = "proto3";
import "google/protobuf/timestamp.proto";

option java_package = "org.apache.arrow.flight.impl";
option go_package = "github.com/apache/arrow-go/arrow/flight/gen/flight";
option csharp_namespace = "Apache.Arrow.Flight.Protocol";

package arrow.flight.protocol;

/*
 * A flight service is an endpoint for retrieving or storing Arrow data. A
 * flight service can expose one or more predefined endpoints that can be
 * accessed using the Arrow Flight Protocol. Additionally, a flight service
 * can expose a set of actions that are available.
 */
service FlightService {

  /*
   * Handshake between client and server. Depending on the server, the
   * handshake may be required to determine the token that should be used for
   * future operations. Both request and response are streams to allow multiple
   * round-trips depending on auth mechanism.
   */
  rpc Handshake(stream HandshakeRequest) returns (stream HandshakeResponse) {}

  /*
   * Get a list of available streams given a particular criteria. Most flight
   * services will expose one or more streams that are readily available for
   * retrieval. This api allows listing the streams available for
   * consumption. A user can also provide a criteria. The criteria can limit
   * the subset of streams that can be listed via this interface. Each flight
   * service allows its own definition of how to consume criteria.
   */
  rpc ListFlights(Criteria) returns (stream FlightInfo) {}

  /*
   * For a given FlightDescriptor, get information about how the flight can be
   * consumed. This is a useful interface if the consumer of the interface
   * already can identify the specific flight to consume. This interface can
   * also allow a consumer to generate a flight stream through a specified
   * descriptor. For example, a flight descriptor might be something that
   * includes a SQL statement or a Pickled Python operation that will be
   * executed. In those cases, the descriptor will not be previously available
   * within the list of available streams provided by ListFlights but will be
   * available for consumption for the duration defined by the specific flight
   * service.
   */
  rpc GetFlightInfo(FlightDescriptor) returns (FlightInfo) {}

  /*
   * For a given FlightDescriptor, start a query and get information
   * to poll its execution status. This is a useful interface if the
   * query may be a long-running query. The first PollFlightInfo call
   * should return as quickly as possible. (GetFlightInfo doesn't
   * return until the query is complete.)
   *
   * A client can consume any available results before
   * the query is completed. See PollInfo.info for details.
   *
   * A client can poll the updated query status by calling
   * PollFlightInfo() with PollInfo.flight_descriptor. A server
   * should not respond until the result would be different from last
   * time. That way, the client can "long poll" for updates
   * without constantly making requests. Clients can set a short timeout
   * to avoid blocking calls if desired.
   *
   * A client can't use PollInfo.flight_descriptor after
   * PollInfo.expiration_time passes. A server might not accept the
   * retry descriptor anymore and the query may be cancelled.
   *
   * A client may use the CancelFlightInfo action with
   * PollInfo.info to cancel the running query.
   */
  rpc PollFlightInfo(FlightDescriptor) returns (PollInfo) {}

  /*
   * For a given FlightDescriptor, get the Schema as described in Schema.fbs::Schema
   * This is used when a consumer needs the Schema of flight stream. Similar to
   * GetFlightInfo this interface may generate a new flight that was not previously
   * available in ListFlights.
   */
   rpc GetSchema(FlightDescriptor) returns (SchemaResult) {}

  /*
   * Retrieve a single stream associated with a particular descriptor
   * associated with the referenced ticket. A Flight can be composed of one or
   * more streams where each stream can be retrieved using a separate opaque
   * ticket that the flight service uses for managing a collection of streams.
   */
  rpc DoGet(Ticket) returns (stream FlightData) {}

  /*
   * 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) {}

  /*
   * Open a bidirectional data channel for a given descriptor. This
   * allows clients to send and receive arbitrary Arrow data and
   * application-specific metadata in a single logical stream. In
   * contrast to DoGet/DoPut, this is more suited for clients
   * offloading computation (rather than storage) to a Flight service.
   */
  rpc DoExchange(stream FlightData) returns (stream FlightData) {}

  /*
   * Flight services can support an arbitrary number of simple actions in
   * addition to the possible ListFlights, GetFlightInfo, DoGet, DoPut
   * operations that are potentially available. DoAction allows a flight client
   * to do a specific action against a flight service. An action includes
   * opaque request and response objects that are specific to the type action
   * being undertaken.
   */
  rpc DoAction(Action) returns (stream Result) {}

  /*
   * A flight service exposes all of the available action types that it has
   * along with descriptions. This allows different flight consumers to
   * understand the capabilities of the flight service.
   */
  rpc ListActions(Empty) returns (stream ActionType) {}
}

/*
 * The request that a client provides to a server on handshake.
 */
message HandshakeRequest {

  /*
   * A defined protocol version
   */
  uint64 protocol_version = 1;

  /*
   * Arbitrary auth/handshake info.
   */
  bytes payload = 2;
}

message HandshakeResponse {

  /*
   * A defined protocol version
   */
  uint64 protocol_version = 1;

  /*
   * Arbitrary auth/handshake info.
   */
  bytes payload = 2;
}

/*
 * A message for doing simple auth.
 */
message BasicAuth {
  string username = 2;
  string password = 3;
}

message Empty {}

/*
 * Describes an available action, including both the name used for execution
 * along with a short description of the purpose of the action.
 */
message ActionType {
  string type = 1;
  string description = 2;
}

/*
 * A service specific expression that can be used to return a limited set
 * of available Arrow Flight streams.
 */
message Criteria {
  bytes expression = 1;
}

/*
 * An opaque action specific for the service.
 */
message Action {
  string type = 1;
  bytes body = 2;
}

/*
 * An opaque result returned after executing an action.
 */
message Result {
  bytes body = 1;
}

/*
 * Wrap the result of a getSchema call
 */
message SchemaResult {
  // The schema of the dataset in its IPC form:
  //   4 bytes - an optional IPC_CONTINUATION_TOKEN prefix
  //   4 bytes - the byte length of the payload
  //   a flatbuffer Message whose header is the Schema
  bytes schema = 1;
}

/*
 * The name or tag for a Flight. May be used as a way to retrieve or generate
 * a flight or be used to expose a set of previously defined flights.
 */
message FlightDescriptor {

  /*
   * Describes what type of descriptor is defined.
   */
  enum DescriptorType {

    // Protobuf pattern, not used.
    UNKNOWN = 0;

    /*
     * A named path that identifies a dataset. A path is composed of a string
     * or list of strings describing a particular dataset. This is conceptually
     *  similar to a path inside a filesystem.
     */
    PATH = 1;

    /*
     * An opaque command to generate a dataset.
     */
    CMD = 2;
  }

  DescriptorType type = 1;

  /*
   * Opaque value used to express a command. Should only be defined when
   * type = CMD.
   */
  bytes cmd = 2;

  /*
   * List of strings identifying a particular dataset. Should only be defined
   * when type = PATH.
   */
  repeated string path = 3;
}

/*
 * The access coordinates for retrieval of a dataset. With a FlightInfo, a
 * consumer is able to determine how to retrieve a dataset.
 */
message FlightInfo {
  // The schema of the dataset in its IPC form:
  //   4 bytes - an optional IPC_CONTINUATION_TOKEN prefix
  //   4 bytes - the byte length of the payload
  //   a flatbuffer Message whose header is the Schema
  bytes schema = 1;

  /*
   * The descriptor associated with this info.
   */
  FlightDescriptor flight_descriptor = 2;

  /*
   * A list of endpoints associated with the flight. To consume the
   * whole flight, all endpoints (and hence all Tickets) must be
   * consumed. Endpoints can be consumed in any order.
   *
   * In other words, an application can use multiple endpoints to
   * represent partitioned data.
   *
   * If the returned data has an ordering, an application can use
   * "FlightInfo.ordered = true" or should return the all data in a
   * single endpoint. Otherwise, there is no ordering defined on
   * endpoints or the data within.
   *
   * A client can read ordered data by reading data from returned
   * endpoints, in order, from front to back.
   *
   * Note that a client may ignore "FlightInfo.ordered = true". If an
   * ordering is important for an application, an application must
   * choose one of them:
   *
   * * An application requires that all clients must read data in
   *   returned endpoints order.
   * * An application must return the all data in a single endpoint.
   */
  repeated FlightEndpoint endpoint = 3;

  // Set these to -1 if unknown.
  int64 total_records = 4;
  int64 total_bytes = 5;

  /*
   * FlightEndpoints are in the same order as the data.
   */
  bool ordered = 6;

  /*
   * Application-defined metadata.
   *
   * There is no inherent or required relationship between this
   * and the app_metadata fields in the FlightEndpoints or resulting
   * FlightData messages. Since this metadata is application-defined,
   * a given application could define there to be a relationship,
   * but there is none required by the spec.
   */
  bytes app_metadata = 7;
}

/*
 * The information to process a long-running query.
 */
message PollInfo {
  /*
   * The currently available results.
   *
   * If "flight_descriptor" is not specified, the query is complete
   * and "info" specifies all results. Otherwise, "info" contains
   * partial query results.
   *
   * Note that each PollInfo response contains a complete
   * FlightInfo (not just the delta between the previous and current
   * FlightInfo).
   *
   * Subsequent PollInfo responses may only append new endpoints to
   * info.
   *
   * Clients can begin fetching results via DoGet(Ticket) with the
   * ticket in the info before the query is
   * completed. FlightInfo.ordered is also valid.
   */
  FlightInfo info = 1;

  /*
   * The descriptor the client should use on the next try.
   * If unset, the query is complete.
   */
  FlightDescriptor flight_descriptor = 2;

  /*
   * Query progress. If known, must be in [0.0, 1.0] but need not be
   * monotonic or nondecreasing. If unknown, do not set.
   */
  optional double progress = 3;

  /*
   * Expiration time for this request. After this passes, the server
   * might not accept the retry descriptor anymore (and the query may
   * be cancelled). This may be updated on a call to PollFlightInfo.
   */
  google.protobuf.Timestamp expiration_time = 4;
}

/*
 * The request of the CancelFlightInfo action.
 *
 * The request should be stored in Action.body.
 */
message CancelFlightInfoRequest {
  FlightInfo info = 1;
}

/*
 * The result of a cancel operation.
 *
 * This is used by CancelFlightInfoResult.status.
 */
enum CancelStatus {
  // The cancellation status is unknown. Servers should avoid using
  // this value (send a NOT_FOUND error if the requested query is
  // not known). Clients can retry the request.
  CANCEL_STATUS_UNSPECIFIED = 0;
  // The cancellation request is complete. Subsequent requests with
  // the same payload may return CANCELLED or a NOT_FOUND error.
  CANCEL_STATUS_CANCELLED = 1;
  // The cancellation request is in progress. The client may retry
  // the cancellation request.
  CANCEL_STATUS_CANCELLING = 2;
  // The query is not cancellable. The client should not retry the
  // cancellation request.
  CANCEL_STATUS_NOT_CANCELLABLE = 3;
}

/*
 * The result of the CancelFlightInfo action.
 *
 * The result should be stored in Result.body.
 */
message CancelFlightInfoResult {
  CancelStatus status = 1;
}

/*
 * An opaque identifier that the service can use to retrieve a particular
 * portion of a stream.
 *
 * Tickets are meant to be single use. It is an error/application-defined
 * behavior to reuse a ticket.
 */
message Ticket {
  bytes ticket = 1;
}

/*
 * A location where a Flight service will accept retrieval of a particular
 * stream given a ticket.
 */
message Location {
  string uri = 1;
}

/*
 * A particular stream or split associated with a flight.
 */
message FlightEndpoint {

  /*
   * Token used to retrieve this stream.
   */
  Ticket ticket = 1;

  /*
   * A list of URIs where this ticket can be redeemed via DoGet().
   *
   * If the list is empty, the expectation is that the ticket can only
   * be redeemed on the current service where the ticket was
   * generated.
   *
   * If the list is not empty, the expectation is that the ticket can be
   * redeemed at any of the locations, and that the data returned will be
   * equivalent. In this case, the ticket may only be redeemed at one of the
   * given locations, and not (necessarily) on the current service. If one
   * of the given locations is "arrow-flight-reuse-connection://?", the
   * client may redeem the ticket on the service where the ticket was
   * generated (i.e., the same as above), in addition to the other
   * locations. (This URI was chosen to maximize compatibility, as 'scheme:'
   * or 'scheme://' are not accepted by Java's java.net.URI.)
   *
   * In other words, an application can use multiple locations to
   * represent redundant and/or load balanced services.
   */
  repeated Location location = 2;

  /*
   * Expiration time of this stream. If present, clients may assume
   * they can retry DoGet requests. Otherwise, it is
   * application-defined whether DoGet requests may be retried.
   */
  google.protobuf.Timestamp expiration_time = 3;

  /*
   * Application-defined metadata.
   *
   * There is no inherent or required relationship between this
   * and the app_metadata fields in the FlightInfo or resulting
   * FlightData messages. Since this metadata is application-defined,
   * a given application could define there to be a relationship,
   * but there is none required by the spec.
   */
  bytes app_metadata = 4;
}

/*
 * The request of the RenewFlightEndpoint action.
 *
 * The request should be stored in Action.body.
 */
message RenewFlightEndpointRequest {
  FlightEndpoint endpoint = 1;
}

/*
 * A batch of Arrow data as part of a stream of batches.
 */
message FlightData {

  /*
   * The descriptor of the data. This is only relevant when a client is
   * starting a new DoPut stream.
   */
  FlightDescriptor flight_descriptor = 1;

  /*
   * Header for message data as described in Message.fbs::Message.
   */
  bytes data_header = 2;

  /*
   * Application-defined metadata.
   */
  bytes app_metadata = 3;

  /*
   * The actual batch of Arrow data. Preferably handled with minimal-copies
   * coming last in the definition to help with sidecar patterns (it is
   * expected that some implementations will fetch this field off the wire
   * with specialized code to avoid extra memory copies).
   */
  bytes data_body = 1000;
}

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

/*
 * EXPERIMENTAL: Union of possible value types for a Session Option to be set to.
 *
 * By convention, an attempt to set a valueless SessionOptionValue should
 * attempt to unset or clear the named option value on the server.
 */
message SessionOptionValue {
  message StringListValue {
    repeated string values = 1;
  }

  oneof option_value {
    string string_value = 1;
    bool bool_value = 2;
    sfixed64 int64_value = 3;
    double double_value = 4;
    StringListValue string_list_value = 5;
  }
}

/*
 * EXPERIMENTAL: A request to set session options for an existing or new (implicit)
 * server session.
 *
 * Sessions are persisted and referenced via a transport-level state management, typically
 * RFC 6265 HTTP cookies when using an HTTP transport.  The suggested cookie name or state
 * context key is 'arrow_flight_session_id', although implementations may freely choose their
 * own name.
 *
 * Session creation (if one does not already exist) is implied by this RPC request, however
 * server implementations may choose to initiate a session that also contains client-provided
 * session options at any other time, e.g. on authentication, or when any other call is made
 * and the server wishes to use a session to persist any state (or lack thereof).
 */
message SetSessionOptionsRequest {
  map<string, SessionOptionValue> session_options = 1;
}

/*
 * EXPERIMENTAL: The results (individually) of setting a set of session options.
 *
 * Option names should only be present in the response if they were not successfully
 * set on the server; that is, a response without an Error for a name provided in the
 * SetSessionOptionsRequest implies that the named option value was set successfully.
 */
message SetSessionOptionsResult {
  enum ErrorValue {
    // Protobuf deserialization fallback value: The status is unknown or unrecognized.
    // Servers should avoid using this value. The request may be retried by the client.
    UNSPECIFIED = 0;
    // The given session option name is invalid.
    INVALID_NAME = 1;
    // The session option value or type is invalid.
    INVALID_VALUE = 2;
    // The session option cannot be set.
    ERROR = 3;
  }

  message Error {
    ErrorValue value = 1;
  }

  map<string, Error> errors = 1;
}

/*
 * EXPERIMENTAL: A request to access the session options for the current server session.
 *
 * The existing session is referenced via a cookie header or similar (see
 * SetSessionOptionsRequest above); it is an error to make this request with a missing,
 * invalid, or expired session cookie header or other implementation-defined session
 * reference token.
 */
message GetSessionOptionsRequest {
}

/*
 * EXPERIMENTAL: The result containing the current server session options.
 */
message GetSessionOptionsResult {
    map<string, SessionOptionValue> session_options = 1;
}

/*
 * Request message for the "Close Session" action.
 *
 * The exiting session is referenced via a cookie header.
 */
message CloseSessionRequest {
}

/*
 * The result of closing a session.
 */
message CloseSessionResult {
  enum Status {
    // Protobuf deserialization fallback value: The session close status is unknown or
    // not recognized. Servers should avoid using this value (send a NOT_FOUND error if
    // the requested session is not known or expired). Clients can retry the request.
    UNSPECIFIED = 0;
    // The session close request is complete. Subsequent requests with
    // the same session produce a NOT_FOUND error.
    CLOSED = 1;
    // The session close request is in progress. The client may retry
    // the close request.
    CLOSING = 2;
    // The session is not closeable. The client should not retry the
    // close request.
    NOT_CLOSEABLE = 3;
  }

  Status status = 1;
}