Dissociated IPC Protocol#

Warning

Experimental: The Dissociated IPC Protocol is experimental in its current form. Based on feedback and usage the protocol definition may change until it is fully standardized.

Rationale#

The Arrow IPC format describes a protocol for transferring Arrow data as a stream of record batches. This protocol expects a continuous stream of bytes divided into discrete messages (using a length prefix and continuation indicator). Each discrete message consists of two portions:

  • A Flatbuffers header message

  • A series of bytes consisting of the flattened and packed body buffers (some message types, like Schema messages, do not have this section) - This is referred to as the message body in the IPC format spec.

For most cases, the existing IPC format as it currently exists is sufficiently efficient:

  • Receiving data in the IPC format allows zero-copy utilization of the body buffer bytes, no deserialization is required to form Arrow Arrays

  • An IPC file format can be memory-mapped because it is location agnostic and the bytes of the file are exactly what is expected in memory.

However, there are use cases that aren’t handled by this:

  • Constructing the IPC record batch message requires allocating a contiguous chunk of bytes and copying all of the data buffers into it, packed together back-to-back. This pessimizes the common case of wrapping existing, directly consumable data into an IPC message.

  • Even if Arrow data is located in a memory accessible across process boundaries or transports (such as UCX), there is no standard way to specify that shared location to consumers which could take advantage of it.

  • Arrow data located on a non-CPU device (such as a GPU) cannot be sent using Arrow IPC without having to copy the data back to the host device or copying the Flatbuffers metadata bytes into device memory.

    • By the same token, receiving IPC messages into device memory would require performing a copy of the Flatbuffers metadata back to the host CPU device. This is due to the fact that the IPC stream interleaves data and metadata across a single stream.

This protocol attempts to solve these use cases in an efficient manner.

Goals#

  • Define a generic protocol for passing Arrow IPC data, not tied to any particular transport, that also allows for utilizing non-CPU device memory, shared memory, and newer “high performance” transports such as UCX or libfabric.

    • This allows for the data in the body to be kept on non-CPU devices (like GPUs) without expensive device-to-host copies.

  • Allow for using Flight RPC purely for control flow by separating the stream of IPC metadata from IPC body bytes

Definitions#

IPC Metadata

The Flatbuffers message bytes that encompass the header of an Arrow IPC message

Tag

A little-endian uint64 value used for flow control and used in determining how to interpret the body of a message. Specific bits can be masked to allow identifying messages by only a portion of the tag, leaving the rest of the bits to be used for control flow or other message metadata. Some transports, such as UCX, have built-in support for such tag values and will provide them in CPU memory regardless of whether or not the body of the message may reside on a non-CPU device.

Sequence Number

A little-endian, 4-byte unsigned integer starting at 0 for a stream, indicating the sequence order of messages. It is also used to identify specific messages to tie the IPC metadata header to its corresponding body since the metadata and body can be sent across separate pipes/streams/transports.

If a sequence number reaches UINT32_MAX, it should be allowed to roll over as it is unlikely there would be enough unprocessed messages waiting to be processed that would cause an overlap of sequence numbers.

The sequence number serves two purposes: To identify corresponding metadata and tagged body data messages and to ensure we do not rely on messages having to arrive in order. A client should use the sequence number to correctly order messages as they arrive for processing.

The Protocol#

A reference example implementation utilizing libcudf and UCX can be found in the arrow-experiments repo.

Requirements#

A transport implementing this protocol MUST provide two pieces of functionality:

  • Message sending

    • Delimited messages (like gRPC) as opposed to non-delimited streams (like plain TCP without further framing).

    • Alternatively, a framing mechanism like the encapsulated message format for the IPC protocol can be used while leaving out the body bytes.

  • Tagged message sending

    • Sending a message that has an attached little-endian, unsigned 64-bit integral tag for control flow. A tag like this allows control flow to operate on a message whose body is on a non-CPU device without requiring the message itself to get copied off of the device.

URI Specification#

When providing a URI to a consumer to contact for use with this protocol (such as via the Location URI for Flight), the URI should specify a scheme like ucx: or fabric:, that is easily identifiable. In addition, the URI should encode the following URI query parameters:

Note

As this protocol matures, this document will get updated with commonly recognized transport schemes that get used with it.

  • want_data - REQUIRED - uint64 integer value

    • This value should be used to tag an initial message to the server to initiate a data transfer. The body of the initiating message should be an opaque binary identifier of the data stream being requested (like the Ticket in the Flight RPC protocol)

  • free_data - OPTIONAL - uint64 integer value

    • If the server might send messages using offsets / addresses for remote memory accessing or shared memory locations, the URI should include this parameter. This value is used to tag messages sent from the client to the data server, containing specific offsets / addresses which were provided that are no longer required by the client (i.e. any operations that directly reference those memory locations, such as copying the remote data into local memory, have been completed).

  • remote_handle - OPTIONAL - base64-encoded string

    • When working with shared memory or remote memory, this value indicates any required handle or identifier that is necessary for accessing the memory.

      • Using UCX, this would be an rkey value

      • With CUDA IPC, this would be the value of the base GPU pointer or memory handle, and subsequent addresses would be offsets from this base pointer.

Handling of Backpressure#

Currently this proposal does not specify any way to manage the backpressure of messages to throttle for memory and bandwidth reasons. For now, this will be transport-defined rather than lock into something sub-optimal.

As usage among different transports and libraries grows, common patterns will emerge that will allow for a generic, but efficient, way to handle backpressure across different use cases.

Note

While the protocol itself is transport agnostic, the current usage and examples only have been tested using UCX and libfabric transports so far, but that’s all.

Protocol Description#

There are two possibilities that can occur:

  1. The streams of metadata and body data are sent across separate connections

%% 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 %% %% http://www.apache.org/licenses/LICENSE-2.0 %% %% 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. sequenceDiagram participant D as Data Stream participant C as Client participant M as Metadata Stream activate C C-->>+M: TaggedMessage(server.want_data, bytes=ID_of_desired_data) C-->>+D: TaggedMessage(server.want_data, bytes=ID_of_desired_data) M-->>C: Message(bytes([1]) + le_bytes(sequence_number) + schema_metadata) loop each batch par M-->>C: Message(bytes([1]) + le_bytes(sequence_number) + batch_metadata) and alt D-->>C: TaggedMessage((bytes[0] << 55) | le_bytes(sequence_number),<br/>bytes=batch_data) else D-->>C: TaggedMessage((bytes[1] << 55) | le_bytes(sequence_number),<br/>bytes=uint64_pairs) end end end M-->>C: Message(bytes([0]) + le_bytes(sequence_number)) deactivate M loop C-->>D: TaggedMessage(server.free_data, bytes=uint64_list) end deactivate D deactivate C

  1. The streams of metadata and body data are sent simultaneously across the same connection

%% 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 %% %% http://www.apache.org/licenses/LICENSE-2.0 %% %% 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. sequenceDiagram participant C as Client participant S as Server activate C C-->>+S: TaggedMessage(server.want_data, bytes=ID_of_desired_data) S-->>C: Message(bytes([1]) + le_bytes(sequence_number) + schema_metadata) par loop each chunk S-->>C: Message(bytes([1]) + le_bytes(sequence_number) + batch_metadata) end S-->>C: Message(bytes([0]) + le_bytes(sequence_number)) and loop each chunk alt S-->>C: TaggedMessage((bytes[0] << 55) | le_bytes(sequence_number),<br/>bytes=batch_data) else S-->>C: TaggedMessage((bytes[1] << 55) | le_bytes(sequence_number),<br/>bytes=uint64_pairs) end end end loop C-->>S: TaggedMessage(server.free_data, bytes=uint64_list) end deactivate S deactivate C

Server Sequence#

There can be either a single server handling both the IPC Metadata stream and the Body data streams, or separate servers for handling the IPC Metadata and the body data. This allows for streaming of data across either a single transport pipe or two pipes if desired.

Metadata Stream Sequence#

The standing state of the server is waiting for a tagged message with a specific <want_data> tag value to initiate a transfer. This <want_data> value is defined by the server and propagated to any clients via the URI they are provided. This protocol does not prescribe any particular value so that it will not interfere with any other existing protocols that rely on tag values. The body of that message will contain an opaque, binary identifier to indicate a particular dataset / data stream to send.

Note

For instance, the ticket that was passed with a FlightInfo message would be the body of this message. Because it is opaque, it can be anything the server wants to use. The URI and identifier do not need to be given to the client via Flight RPC, but could come across from any transport or protocol desired.

Upon receiving a <want_data> request, the server should respond by sending a stream of messages consisting of the following:

block-beta columns 8 block:P["\n\n\n\nPrefix"]:5 T["Message type\nByte 0"] S["Sequence number\nBytes 1-4"] end H["Flatbuffer bytes\nRest of the message"]:3

  • A 5-byte prefix

    • The first byte of the message indicates the type of message, currently there are only two allowed message types (more types may get added in the future):

      1. End of Stream

      2. Flatbuffers IPC Metadata Message

    • the next 4-bytes are a little-endian, unsigned 32-bit integer indicating the sequence number of the message. The first message in the stream (MUST always be a schema message) MUST have a sequence number of 0. Each subsequent message MUST increment the number by 1.

  • The full Flatbuffers bytes of an Arrow IPC header

As defined in the Arrow IPC format, each metadata message can represent a chunk of data or dictionaries for use by the stream of data.

After sending the last metadata message, the server MUST indicate the end of the stream by sending a message consisting of exactly 5 bytes:

  • The first byte is 0, indicating an End of Stream message

  • The last 4 bytes are the sequence number (4-byte, unsigned integer in little-endian byte order)

Data Stream Sequence#

If a single server is handling both the data and metadata streams, then the data messages should begin being sent to the client in parallel with the metadata messages. Otherwise, as with the metadata sequence, the standing state of the server is to wait for a tagged message with the <want_data> tag value, whose body indicates the dataset / data stream to send to the client.

For each IPC message in the stream of data, a tagged message MUST be sent on the data stream if that message has a body (i.e. a Record Batch or Dictionary message). The tag for each message should be structured as follows:

block-beta columns 8 S["Sequence number\nBytes 0-3"]:4 U["Unused (Reserved)\nBytes 4-6"]:3 T["Message type\nByte 7"]:1

  • The least significant 4-bytes (bits 0 - 31) of the tag should be the unsigned 32-bit, little-endian sequence number of the message.

  • The most significant byte (bits 56 - 63) of the tag indicates the message body type as an 8-bit unsigned integer. Currently only two message types are specified, but more can be added as needed to expand the protocol:

    1. The body contains the raw body buffer bytes as a packed buffer (i.e. the standard IPC format body bytes)

    2. The body contains a series of unsigned, little-endian 64-bit integer pairs to represent either shared or remote memory, schematically structured as

      • The first two integers (e.g. the first 16 bytes) represent the total size (in bytes) of all buffers and the number of buffers in this message (and thus the number of following pairs of uint64)

      • Each subsequent pair of uint64 values are an address / offset followed the length of that particular buffer.

  • All unspecified bits (bits 32 - 55) of the tag are reserved for future use by potential updates to this protocol. For now they MUST be 0.

Note

Any shared/remote memory addresses that are sent across MUST be kept alive by the server until a corresponding tagged <free_data> message is received. If the client disconnects before sending any <free_data> messages, it can be assumed to be safe to clean up the memory if desired by the server.

After sending the last tagged IPC body message, the server should maintain the connection and wait for tagged <free_data> messages. The structure of these <free_data> messages is simple: one or more unsigned, little-endian 64-bit integers which indicate the addresses/offsets that can be freed.

Once there are no more outstanding addresses to be freed, the work for this stream is complete.

Client Sequence#

A client for this protocol needs to concurrently handle both the data and metadata streams of messages which may either both come from the same server or different servers. Below is a flowchart showing how a client might handle the metadata and data streams:

%% 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 %% http://www.apache.org/licenses/LICENSE-2.0 %% 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. graph LR client((Client))-->c1{{Send #60;want_data#gt; Msg}} subgraph meta [Meta Message] direction LR m1[/Msg Type #40;byte 0#41;<br/>Seq Num #40;bytes 1-5#41;/]-- type 1 -->m2[[Process IPC Header]] m2-- IPC has body -->m3[Get Corresponding<br/>Tagged Msg] m2-- Schema Msg -->m4[/Store Schema/] m1-- type 0 -->e[Indicate End of Stream] end subgraph data [Data Stream] direction LR d1[Request Msg<br/>for Seq Num]-->d2{Most Significant<br/>Byte} d2-- 0 -->d3[Construct from<br/>Metadata and Body] d2-- 1 -->d4[Get shared/remote<br/>buffers] d4 -->d5[Construct from<br/>Metadata and buffers] d3 & d5 -->e2[Output Batch] end client -- recv untagged msg --> meta client -- get tagged msg --> data

  1. First the client sends a tagged message using the <want_data> value it was provided in the URI as the tag, and the opaque ID as the body.

    • If the metadata and data servers are separate, then a <want_data> message needs to be sent separately to each.

    • In either scenario, the metadata and data streams can be processed concurrently and/or asynchronously depending on the nature of the transports.

  2. For each untagged message the client receives in the metadata stream:

    • The first byte of the message indicates whether it is an End of Stream message (value 0) or a metadata message (value 1).

    • The next 4 bytes are the sequence number of the message, an unsigned 32-bit integer in little-endian byte order.

    • If it is not an End of Stream message, the remaining bytes are the IPC Flatbuffer bytes which can be interpreted as normal.

      • If the message has a body (i.e. Record Batch or Dictionary message) then the client should retrieve a tagged message from the Data Stream using the same sequence number.

    • If it is an End of Stream message, then it is safe to close the metadata connection if there are no gaps in the sequence numbers received.

  3. When a metadata message that requires a body is received, the tag mask of 0x00000000FFFFFFFF should be used alongside the sequence number to match the message regardless of the higher bytes (e.g. we only care about matching the lower 4 bytes to the sequence number)

    • Once recieved, the Most Significant Byte’s value determines how the client processes the body data:

      • If the most significant byte is 0: Then the body of the message is the raw IPC packed body buffers allowing it to easily be processed with the corresponding metadata header bytes.

      • If the most significant byte is 1: The body of the message will consist of a series of pairs of unsigned, 64-bit integers in little-endian byte order.

        • The first two integers represent 1) the total size of all the body buffers together to allow for easy allocation if an intermediate buffer is needed and 2) the number of buffers being sent (nbuf).

        • The rest of the message will be nbuf pairs of integers, one for each buffer. Each pair is 1) the address / offset of the buffer and 2) the length of that buffer. Memory can then be retrieved via shared or remote memory routines based on the underlying transport. These addresses / offsets MUST be retained so they can be sent back in <free_data> messages later, indicating to the server that the client no longer needs the shared memory.

  4. Once an End of Stream message is received, the client should process any remaining un-processed IPC metadata messages.

  5. After individual memory addresses / offsets are able to be freed by the remote server (in the case where it has sent these rather than the full body bytes), the client should send corresponding <free_data> messages to the server.

    • A single <free_data> message consists of an arbitrary number of unsigned 64-bit integer values, representing the addresses / offsets which can be freed. The reason for it being an arbitrary number is to allow a client to choose whether to send multiple messages to free multiple addresses or to coalesce multiple addresses into fewer messages to be freed (thus making the protocol less “chatty” if desired)

Continuing Development#

If you decide to try this protocol in your own environments and system, we’d love feedback and to learn about your use case. As this is an experimental protocol currently, we need real-world usage in order to facilitate improving it and finding the right generalizations to standardize on across transports.

Please chime in using the Arrow Developers Mailing list: https://arrow.apache.org/community/#mailing-lists

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