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.. _format_columnar:

Arrow Columnar Format

Version: 1.5

.. seealso:: :ref:Additions to the Arrow columnar format since version 1.0.0 <post-1-0-0-format-versions>

The Arrow columnar format includes a language-agnostic in-memory data structure specification, metadata serialization, and a protocol for serialization and generic data transport.

This document is intended to provide adequate detail to create a new implementation of the columnar format without the aid of an existing implementation. We utilize Google's Flatbuffers_ project for metadata serialization, so it will be necessary to refer to the project's Flatbuffers protocol definition files_ while reading this document.

The columnar format has some key features:

Data adjacency for sequential access (scans)
O(1) (constant-time) random access [#f1]_
SIMD and vectorization-friendly
Relocatable without "pointer swizzling", allowing for true zero-copy access in shared memory

The Arrow columnar format provides analytical performance and data locality guarantees in exchange for comparatively more expensive mutation operations. This document is concerned only with in-memory data representation and serialization details; issues such as coordinating mutation of data structures are left to be handled by implementations.

.. [#f1] Except for the :ref:run-end-encoded-layout where random access is O(log n).

Terminology

Since different projects have used different words to describe various concepts, here is a small glossary to help disambiguate.

Array or Vector: a sequence of values with known length all having the same type. These terms are used interchangeably in different Arrow implementations, but we use "array" in this document.
Slot: a single logical value in an array of some particular data type
Buffer or Contiguous memory region: a sequential virtual address space with a given length. Any byte can be reached via a single pointer offset less than the region's length.
Physical Layout: The underlying memory layout for an array without taking into account any value semantics. For example, a 32-bit signed integer array and 32-bit floating point array have the same layout.
Data type: An application-facing semantic value type that is implemented using some physical layout. For example, Decimal128 values are stored as 16 bytes in a fixed-size binary layout. A timestamp may be stored as 64-bit fixed-size layout.
Primitive type: a data type having no child types. This includes such types as fixed bit-width, variable-size binary, and null types.
Nested type: a data type whose full structure depends on one or more other child types. Two fully-specified nested types are equal if and only if their child types are equal. For example, List<U> is distinct from List<V> iff U and V are different types.
Parent and child arrays: names to express relationships between physical value arrays in a nested type structure. For example, a List<T>-type parent array has a T-type array as its child (see more on lists below).
Parametric type: a type which requires additional parameters for full determination of its semantics. For example, all nested types are parametric by construction. A timestamp is also parametric as it needs a unit (such as microseconds) and a timezone.

.. _data_types:

Data Types

The file Schema.fbs_ defines built-in data types supported by the Arrow columnar format. Each data type uses a well-defined physical layout.

Schema.fbs_ is the authoritative source for the description of the standard Arrow data types. However, we also provide the below table for convenience:

(1) Type parameters listed in italics denote a data type's child types.
(2) The bit width parameter of a Time type is technically redundant as each unit mandates a single bit width.
(3) Whether a Union type uses the Sparse or Dense layout is denoted by its mode parameter.
(4) The index type of a Dictionary type can only be an integer type, preferably signed, with width 8 to 64 bits.
(5) The run end type of a Run-End Encoded type can only be a signed integer type with width 16 to 64 bits.

.. note:: Sometimes the term "logical type" is used to denote the Arrow data types and distinguish them from their respective physical layouts. However, unlike other type systems such as Apache Parquet <https://parquet.apache.org/>__'s, the Arrow type system doesn't have separate notions of physical types and logical types.

The Arrow type system separately provides :ref:extension types <format_metadata_extension_types>, which allow annotating standard Arrow data types with richer application-facing semantics (for example defining a "JSON" type laid upon the standard String data type).

.. _format_layout:

Physical Memory Layout

Arrays are defined by a few pieces of metadata and data:

A data type.
A sequence of buffers.
A length as a 64-bit signed integer. Implementations are permitted to be limited to 32-bit lengths, see more on this below.
A null count as a 64-bit signed integer.
An optional dictionary, for dictionary-encoded arrays.

Nested arrays additionally have a sequence of one or more sets of these items, called the child arrays.

Each data type has a well-defined physical layout. Here are the different physical layouts defined by Arrow:

Primitive (fixed-size): a sequence of values each having the same byte or bit width
Variable-size Binary: a sequence of values each having a variable byte length. Two variants of this layout are supported using 32-bit and 64-bit length encoding.
View of Variable-size Binary: a sequence of values each having a variable byte length. In contrast to Variable-size Binary, the values of this layout are distributed across potentially multiple buffers instead of densely and sequentially packed in a single buffer.
Fixed-size List: a nested layout where each value has the same number of elements taken from a child data type.
Variable-size List: a nested layout where each value is a variable-length sequence of values taken from a child data type. Two variants of this layout are supported using 32-bit and 64-bit length encoding.
View of Variable-size List: a nested layout where each value is a variable-length sequence of values taken from a child data type. This layout differs from Variable-size List by having an additional buffer containing the sizes of each list value. This removes a constraint on the offsets buffer — it does not need to be in order.
Struct: a nested layout consisting of a collection of named child fields each having the same length but possibly different types.
Sparse and Dense Union: a nested layout representing a sequence of values, each of which can have type chosen from a collection of child array types.
Dictionary-Encoded: a layout consisting of a sequence of integers (any bit-width) which represent indexes into a dictionary which could be of any type.
Run-End Encoded (REE): a nested layout consisting of two child arrays, one representing values, and one representing the logical index where the run of a corresponding value ends.
Null: a sequence of all null values.

The Arrow columnar memory layout only applies to data and not metadata. Implementations are free to represent metadata in-memory in whichever form is convenient for them. We handle metadata serialization in an implementation-independent way using Flatbuffers_, detailed below.

Buffer Alignment and Padding

Implementations are recommended to allocate memory on aligned addresses (multiple of 8- or 64-bytes) and pad (overallocate) to a length that is a multiple of 8 or 64 bytes. When serializing Arrow data for interprocess communication, these alignment and padding requirements are enforced. If possible, we suggest that you prefer using 64-byte alignment and padding. Unless otherwise noted, padded bytes do not need to have a specific value.

The alignment requirement follows best practices for optimized memory access:

Elements in numeric arrays will be guaranteed to be retrieved via aligned access.
On some architectures alignment can help limit partially used cache lines.

The recommendation for 64 byte alignment comes from the Intel performance guide_ that recommends alignment of memory to match SIMD register width. The specific padding length was chosen because it matches the largest SIMD instruction registers available on widely deployed x86 architecture (Intel AVX-512).

The recommended padding of 64 bytes allows for using SIMD_ instructions consistently in loops without additional conditional checks. This should allow for simpler, efficient and CPU cache-friendly code. In other words, we can load the entire 64-byte buffer into a 512-bit wide SIMD register and get data-level parallelism on all the columnar values packed into the 64-byte buffer. Guaranteed padding can also allow certain compilers to generate more optimized code directly (e.g. One can safely use Intel's -qopt-assume-safe-padding).

Array lengths

Array lengths are represented in the Arrow metadata as a 64-bit signed integer. An implementation of Arrow is considered valid even if it only supports lengths up to the maximum 32-bit signed integer, though. If using Arrow in a multi-language environment, we recommend limiting lengths to 2 :sup:31 - 1 elements or less. Larger data sets can be represented using multiple array chunks.

Null count

The number of null value slots is a property of the physical array and considered part of the data structure. The null count is represented in the Arrow metadata as a 64-bit signed integer, as it may be as large as the array length.

Validity bitmaps

Any value in an array may be semantically null, whether primitive or nested type.

All array types, with the exception of union types (more on these later), utilize a dedicated memory buffer, known as the validity (or "null") bitmap, to encode the nullness or non-nullness of each value slot. The validity bitmap must be large enough to have at least 1 bit for each array slot.

Whether any array slot is valid (non-null) is encoded in the respective bits of this bitmap. A 1 (set bit) for index j indicates that the value is not null, while a 0 (bit not set) indicates that it is null. Bitmaps are to be initialized to be all unset at allocation time (this includes padding): ::

is_valid[j] -> bitmap[j / 8] & (1 << (j % 8))

We use least-significant bit (LSB) numbering_ (also known as bit-endianness). This means that within a group of 8 bits, we read right-to-left: ::

values = [0, 1, null, 2, null, 3]

bitmap
j mod 8   7  6  5  4  3  2  1  0
          0  0  1  0  1  0  1  1

Arrays having a 0 null count may choose to not allocate the validity bitmap; how this is represented depends on the implementation (for example, a C++ implementation may represent such an "absent" validity bitmap using a NULL pointer). Implementations may choose to always allocate a validity bitmap anyway as a matter of convenience. Consumers of Arrow arrays should be ready to handle those two possibilities.

Nested type arrays (except for union types as noted above) have their own top-level validity bitmap and null count, regardless of the null count and valid bits of their child arrays.

Array slots which are null are not required to have a particular value; any "masked" memory can have any value and need not be zeroed, though implementations frequently choose to zero memory for null values.

Fixed-size Primitive Layout

A primitive value array represents an array of values each having the same physical slot width typically measured in bytes, though the spec also provides for bit-packed types (e.g. boolean values encoded in bits).

Internally, the array contains a contiguous memory buffer whose total size is at least as large as the slot width multiplied by the array length. For bit-packed types, the size is rounded up to the nearest byte.

The associated validity bitmap is contiguously allocated (as described above) but does not need to be adjacent in memory to the values buffer.

Example Layout: Int32 Array

For example a primitive array of int32s: ::

[1, null, 2, 4, 8]

Would look like: ::

* Length: 5, Null count: 1
* Validity bitmap buffer:

  | Byte 0 (validity bitmap) | Bytes 1-63            |
  |--------------------------|-----------------------|
  | 00011101                 | 0 (padding)           |

* Value Buffer:

  | Bytes 0-3   | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-19 | Bytes 20-63           |
  |-------------|-------------|-------------|-------------|-------------|-----------------------|
  | 1           | unspecified | 2           | 4           | 8           | unspecified (padding) |

Example Layout: Non-null int32 Array

[1, 2, 3, 4, 8] has two possible layouts: ::

* Length: 5, Null count: 0
* Validity bitmap buffer:

  | Byte 0 (validity bitmap) | Bytes 1-63            |
  |--------------------------|-----------------------|
  | 00011111                 | 0 (padding)           |

* Value Buffer:

  | Bytes 0-3   | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-19 | Bytes 20-63           |
  |-------------|-------------|-------------|-------------|-------------|-----------------------|
  | 1           | 2           | 3           | 4           | 8           | unspecified (padding) |

or with the bitmap elided: ::

* Length 5, Null count: 0
* Validity bitmap buffer: Not required
* Value Buffer:

  | Bytes 0-3   | Bytes 4-7   | Bytes 8-11  | bytes 12-15 | bytes 16-19 | Bytes 20-63           |
  |-------------|-------------|-------------|-------------|-------------|-----------------------|
  | 1           | 2           | 3           | 4           | 8           | unspecified (padding) |

Variable-size Binary Layout

Each value in this layout consists of 0 or more bytes. While primitive arrays have a single values buffer, variable-size binary have an offsets buffer and data buffer.

The offsets buffer contains length + 1 signed integers (either 32-bit or 64-bit, depending on the data type), which encode the start position of each slot in the data buffer. The length of the value in each slot is computed using the difference between the offset at that slot's index and the subsequent offset. For example, the position and length of slot j is computed as:

slot_position = offsets[j]
slot_length = offsets[j + 1] - offsets[j]  // (for 0 <= j < length)

It should be noted that a null value may have a positive slot length. That is, a null value may occupy a non-empty memory space in the data buffer. When this is true, the content of the corresponding memory space is undefined.

Offsets must be monotonically increasing, that is offsets[j+1] >= offsets[j] for 0 <= j < length, even for null slots. This property ensures the location for all values is valid and well defined.

Generally the first slot in the offsets array is 0, and the last slot is the length of the values array. When serializing this layout, we recommend normalizing the offsets to start at 0.

Example Layout: VarBinary

['joe', null, null, 'mark']

will be represented as follows: ::

Length: 4, Null count: 2
Validity bitmap buffer:

Byte 0 (validity bitmap) Bytes 1-63
00001001 0 (padding)
Offsets buffer:

Bytes 0-19 Bytes 20-63
0, 3, 3, 3, 7 unspecified (padding)
Value buffer:

Byte 0 (validity bitmap)	Bytes 1-63
00001001	0 (padding)

Bytes 0-19	Bytes 20-63
0, 3, 3, 3, 7	unspecified (padding)

| Bytes 0-6      | Bytes 7-63            |

|----------------|-----------------------|
| joemark        | unspecified (padding) |

.. _variable-size-binary-view-layout:

Variable-size Binary View Layout

.. note:: New in Arrow Columnar Format 1.4

Each value in this layout consists of 0 or more bytes. These bytes' locations are indicated using a views buffer, which may point to one of potentially several data buffers or may contain the characters inline.

The views buffer contains length view structures with the following layout:

* Short strings, length <= 12
  | Bytes 0-3  | Bytes 4-15                            |
  |------------|---------------------------------------|
  | length     | data (padded with 0)                  |

* Long strings, length > 12
  | Bytes 0-3  | Bytes 4-7  | Bytes 8-11 | Bytes 12-15 |
  |------------|------------|------------|-------------|
  | length     | prefix     | buf. index | offset      |

In both the long and short string cases, the first four bytes encode the length of the string and can be used to determine how the rest of the view should be interpreted.

In the short string case the string's bytes are inlined — stored inside the view itself, in the twelve bytes which follow the length. Any remaining bytes after the string itself are padded with 0.

In the long string case, a buffer index indicates which data buffer stores the data bytes and an offset indicates where in that buffer the data bytes begin. Buffer index 0 refers to the first data buffer, IE the first buffer after the validity buffer and the views buffer. The half-open range [offset, offset + length) must be entirely contained within the indicated buffer. A copy of the first four bytes of the string is stored inline in the prefix, after the length. This prefix enables a profitable fast path for string comparisons, which are frequently determined within the first four bytes.

All integers (length, buffer index, and offset) are signed.

This layout is adapted from TU Munich's UmbraDB_.

Note that this layout uses one additional buffer to store the variadic buffer lengths in the :ref:Arrow C data interface <c-data-interface-binary-view-arrays>.

.. _variable-size-list-layout:

Variable-size List Layout

List is a nested type which is semantically similar to variable-size binary. There are two list layout variations — "list" and "list-view" — and each variation can be delimited by either 32-bit or 64-bit offsets integers.

List Layout


The List layout is defined by two buffers, a validity bitmap and an offsets
buffer, and a child array. The offsets are the same as in the
variable-size binary case, and both 32-bit and 64-bit signed integer
offsets are supported options for the offsets. Rather than referencing
an additional data buffer, instead these offsets reference the child
array.

Similar to the layout of variable-size binary, a null value may
correspond to a **non-empty** segment in the child array. When this is
true, the content of the corresponding segment can be arbitrary.

A list type is specified like ``List<T>``, where ``T`` is any type
(primitive or nested). In these examples we use 32-bit offsets where
the 64-bit offset version would be denoted by ``LargeList<T>``.

**Example Layout: ``List<Int8>`` Array**

We illustrate an example of ``List<Int8>`` with length 4 having values::

    [[12, -7, 25], null, [0, -127, 127, 50], []]

will have the following representation: ::

    * Length: 4, Null count: 1
    * Validity bitmap buffer:

      | Byte 0 (validity bitmap) | Bytes 1-63            |
      |--------------------------|-----------------------|
      | 00001101                 | 0 (padding)           |

    * Offsets buffer (int32)

      | Bytes 0-3  | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-19 | Bytes 20-63           |
      |------------|-------------|-------------|-------------|-------------|-----------------------|
      | 0          | 3           | 3           | 7           | 7           | unspecified (padding) |

    * Values array (Int8Array):
      * Length: 7,  Null count: 0
      * Validity bitmap buffer: Not required
      * Values buffer (int8)

        | Bytes 0-6                    | Bytes 7-63            |
        |------------------------------|-----------------------|
        | 12, -7, 25, 0, -127, 127, 50 | unspecified (padding) |

**Example Layout: ``List<List<Int8>>``**

``[[[1, 2], [3, 4]], [[5, 6, 7], null, [8]], [[9, 10]]]``

will be represented as follows: ::

    * Length 3
    * Nulls count: 0
    * Validity bitmap buffer: Not required
    * Offsets buffer (int32)

      | Bytes 0-3  | Bytes 4-7  | Bytes 8-11 | Bytes 12-15 | Bytes 16-63           |
      |------------|------------|------------|-------------|-----------------------|
      | 0          |  2         |  5         |  6          | unspecified (padding) |

    * Values array (`List<Int8>`)
      * Length: 6, Null count: 1
      * Validity bitmap buffer:

        | Byte 0 (validity bitmap) | Bytes 1-63  |
        |--------------------------|-------------|
        | 00110111                 | 0 (padding) |

      * Offsets buffer (int32)

        | Bytes 0-27           | Bytes 28-63           |
        |----------------------|-----------------------|
        | 0, 2, 4, 7, 7, 8, 10 | unspecified (padding) |

      * Values array (Int8):
        * Length: 10, Null count: 0
        * Validity bitmap buffer: Not required

          | Bytes 0-9                     | Bytes 10-63           |
          |-------------------------------|-----------------------|
          | 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 | unspecified (padding) |

.. _listview-layout:

ListView Layout

.. note:: New in Arrow Columnar Format 1.4

The ListView layout is defined by three buffers: a validity bitmap, an offsets buffer, and an additional sizes buffer. Sizes and offsets have the identical bit width and both 32-bit and 64-bit signed integer options are supported.

As in the List layout, the offsets encode the start position of each slot in the child array. In contrast to the List layout, list lengths are stored explicitly in the sizes buffer instead of inferred. This allows offsets to be out of order. Elements of the child array do not have to be stored in the same order they logically appear in the list elements of the parent array.

Every list-view value, including null values, has to guarantee the following invariants: ::

0 <= offsets[i] <= length of the child array
0 <= offsets[i] + size[i] <= length of the child array

A list-view type is specified like ListView<T>, where T is any type (primitive or nested). In these examples we use 32-bit offsets and sizes where the 64-bit version would be denoted by LargeListView<T>.

Example Layout: ListView<Int8> Array

We illustrate an example of ListView<Int8> with length 4 having values::

[[12, -7, 25], null, [0, -127, 127, 50], []]

It may have the following representation: ::

* Length: 4, Null count: 1
* Validity bitmap buffer:

  | Byte 0 (validity bitmap) | Bytes 1-63            |
  |--------------------------|-----------------------|
  | 00001101                 | 0 (padding)           |

* Offsets buffer (int32)

  | Bytes 0-3  | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-63           |
  |------------|-------------|-------------|-------------|-----------------------|
  | 0          | 7           | 3           | 0           | unspecified (padding) |

* Sizes buffer (int32)

  | Bytes 0-3  | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-63           |
  |------------|-------------|-------------|-------------|-----------------------|
  | 3          | 0           | 4           | 0           | unspecified (padding) |

* Values array (Int8Array):
  * Length: 7,  Null count: 0
  * Validity bitmap buffer: Not required
  * Values buffer (int8)

    | Bytes 0-6                    | Bytes 7-63            |
    |------------------------------|-----------------------|
    | 12, -7, 25, 0, -127, 127, 50 | unspecified (padding) |

Example Layout: ListView<Int8> Array

We continue with the ListView<Int8> type, but this instance illustrates out of order offsets and sharing of child array values. It is an array with length 5 having logical values::

[[12, -7, 25], null, [0, -127, 127, 50], [], [50, 12]]

It may have the following representation: ::

* Length: 5, Null count: 1
* Validity bitmap buffer:

  | Byte 0 (validity bitmap) | Bytes 1-63            |
  |--------------------------|-----------------------|
  | 00011101                 | 0 (padding)           |

* Offsets buffer (int32)

  | Bytes 0-3  | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-19 | Bytes 20-63           |
  |------------|-------------|-------------|-------------|-------------|-----------------------|
  | 4          | 7           | 0           | 0           | 3           | unspecified (padding) |

* Sizes buffer (int32)

  | Bytes 0-3  | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-19 | Bytes 20-63           |
  |------------|-------------|-------------|-------------|-------------|-----------------------|
  | 3          | 0           | 4           | 0           | 2           | unspecified (padding) |

* Values array (Int8Array):
  * Length: 7,  Null count: 0
  * Validity bitmap buffer: Not required
  * Values buffer (int8)

    | Bytes 0-6                    | Bytes 7-63            |
    |------------------------------|-----------------------|
    | 0, -127, 127, 50, 12, -7, 25 | unspecified (padding) |

Fixed-Size List Layout

Fixed-Size List is a nested type in which each array slot contains a fixed-size sequence of values all having the same type.

A fixed size list type is specified like FixedSizeList<T>[N], where T is any type (primitive or nested) and N is a 32-bit signed integer representing the length of the lists.

A fixed size list array is represented by a values array, which is a child array of type T. T may also be a nested type. The value in slot j of a fixed size list array is stored in an N-long slice of the values array, starting at an offset of j * N.

Example Layout: FixedSizeList<byte>[4] Array

Here we illustrate FixedSizeList<byte>[4].

For an array of length 4 with respective values: ::

[[192, 168, 0, 12], null, [192, 168, 0, 25], [192, 168, 0, 1]]

will have the following representation: ::

* Length: 4, Null count: 1
* Validity bitmap buffer:

  | Byte 0 (validity bitmap) | Bytes 1-63            |
  |--------------------------|-----------------------|
  | 00001101                 | 0 (padding)           |

* Values array (byte array):
  * Length: 16,  Null count: 0
  * validity bitmap buffer: Not required

    | Bytes 0-3       | Bytes 4-7   | Bytes 8-15                      |
    |-----------------|-------------|---------------------------------|
    | 192, 168, 0, 12 | unspecified | 192, 168, 0, 25, 192, 168, 0, 1 |

Struct Layout

A struct is a nested type parameterized by an ordered sequence of types (which can all be distinct), called its fields. Each field must have a UTF8-encoded name, and these field names are part of the type metadata.

Physically, a struct array has one child array for each field. The child arrays are independent and need not be adjacent to each other in memory. A struct array also has a validity bitmap to encode top-level validity information.

For example, the struct (field names shown here as strings for illustration purposes)::

Struct <
  name: VarBinary
  age: Int32
>

has two child arrays, one VarBinary array (using variable-size binary layout) and one 4-byte primitive value array having Int32 logical type.

Example Layout: Struct<VarBinary, Int32>

The layout for [{'joe', 1}, {null, 2}, null, {'mark', 4}], having child arrays ['joe', null, 'alice', 'mark'] and [1, 2, null, 4] would be: ::

* Length: 4, Null count: 1
* Validity bitmap buffer:

  | Byte 0 (validity bitmap) | Bytes 1-63            |
  |--------------------------|-----------------------|
  | 00001011                 | 0 (padding)           |

* Children arrays:
  * field-0 array (`VarBinary`):
    * Length: 4, Null count: 1
    * Validity bitmap buffer:

      | Byte 0 (validity bitmap) | Bytes 1-63            |
      |--------------------------|-----------------------|
      | 00001101                 | 0 (padding)           |

    * Offsets buffer:

      | Bytes 0-19     | Bytes 20-63           |
      |----------------|-----------------------|
      | 0, 3, 3, 8, 12 | unspecified (padding) |

     * Value buffer:

      | Bytes 0-11     | Bytes 12-63           |
      |----------------|-----------------------|
      | joealicemark   | unspecified (padding) |

  * field-1 array (int32 array):
    * Length: 4, Null count: 1
    * Validity bitmap buffer:

      | Byte 0 (validity bitmap) | Bytes 1-63            |
      |--------------------------|-----------------------|
      | 00001011                 | 0 (padding)           |

    * Value Buffer:

      | Bytes 0-3   | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-63           |
      |-------------|-------------|-------------|-------------|-----------------------|
      | 1           | 2           | unspecified | 4           | unspecified (padding) |

Struct Validity


A struct array has its own validity bitmap that is independent of its
child arrays' validity bitmaps. The validity bitmap for the struct
array might indicate a null when one or more of its child arrays has
a non-null value in its corresponding slot; or conversely, a child
array might indicate a null in its validity bitmap while the struct array's
validity bitmap shows a non-null value.

Therefore, to know whether a particular child entry is valid, one must
take the logical AND of the corresponding bits in the two validity bitmaps
(the struct array's and the child array's).

This is illustrated in the example above, one of the child arrays has a
valid entry ``'alice'`` for the null struct but it is "hidden" by the
struct array's validity bitmap. However, when treated independently,
corresponding entries of the children array will be non-null.

Union Layout
------------

A union is defined by an ordered sequence of types; each slot in the
union can have a value chosen from these types. The types are named
like a struct's fields, and the names are part of the type metadata.

Unlike other data types, unions do not have their own validity bitmap. Instead,
the nullness of each slot is determined exclusively by the child arrays which
are composed to create the union.

We define two distinct union types, "dense" and "sparse", that are
optimized for different use cases.

Dense Union
~~~~~~~~~~~

Dense union represents a mixed-type array with 5 bytes of overhead for
each value. Its physical layout is as follows:

* One child array for each type
* Types buffer: A buffer of 8-bit signed integers. Each type in the
  union has a corresponding type id whose values are found in this
  buffer. A union with more than 128 possible types can be modeled as
  a union of unions.
* Offsets buffer: A buffer of signed Int32 values indicating the
  relative offset into the respective child array for the type in a
  given slot. The respective offsets for each child value array must
  be in order / increasing.

**Example Layout: ``DenseUnion<f: Float32, i: Int32>``**

For the union array: ::

    [{f=1.2}, null, {f=3.4}, {i=5}]

will have the following layout: ::

    * Length: 4, Null count: 0
    * Types buffer:

      | Byte 0   | Byte 1      | Byte 2   | Byte 3   | Bytes 4-63            |
      |----------|-------------|----------|----------|-----------------------|
      | 0        | 0           | 0        | 1        | unspecified (padding) |

    * Offset buffer:

      | Bytes 0-3 | Bytes 4-7   | Bytes 8-11 | Bytes 12-15 | Bytes 16-63           |
      |-----------|-------------|------------|-------------|-----------------------|
      | 0         | 1           | 2          | 0           | unspecified (padding) |

    * Children arrays:
      * Field-0 array (f: Float32):
        * Length: 3, Null count: 1
        * Validity bitmap buffer: 00000101

        * Value Buffer:

          | Bytes 0-11     | Bytes 12-63           |
          |----------------|-----------------------|
          | 1.2, null, 3.4 | unspecified (padding) |


      * Field-1 array (i: Int32):
        * Length: 1, Null count: 0
        * Validity bitmap buffer: Not required

        * Value Buffer:

          | Bytes 0-3 | Bytes 4-63            |
          |-----------|-----------------------|
          | 5         | unspecified (padding) |

Sparse Union
~~~~~~~~~~~~

A sparse union has the same structure as a dense union, with the omission of
the offsets array. In this case, the child arrays are each equal in length to
the length of the union.

While a sparse union may use significantly more space compared with a
dense union, it has some advantages that may be desirable in certain
use cases:

* A sparse union is more amenable to vectorized expression evaluation in some use cases.
* Equal-length arrays can be interpreted as a union by only defining the types array.

**Example layout: ``SparseUnion<i: Int32, f: Float32, s: VarBinary>``**

For the union array: ::

    [{i=5}, {f=1.2}, {s='joe'}, {f=3.4}, {i=4}, {s='mark'}]

will have the following layout: ::

    * Length: 6, Null count: 0
    * Types buffer:

     | Byte 0     | Byte 1      | Byte 2      | Byte 3      | Byte 4      | Byte 5       | Bytes  6-63           |
     |------------|-------------|-------------|-------------|-------------|--------------|-----------------------|
     | 0          | 1           | 2           | 1           | 0           | 2            | unspecified (padding) |

    * Children arrays:

      * i (Int32):
        * Length: 6, Null count: 4
        * Validity bitmap buffer:

          | Byte 0 (validity bitmap) | Bytes 1-63            |
          |--------------------------|-----------------------|
          | 00010001                 | 0 (padding)           |

        * Value buffer:

          | Bytes 0-3   | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-19 | Bytes 20-23  | Bytes 24-63           |
          |-------------|-------------|-------------|-------------|-------------|--------------|-----------------------|
          | 5           | unspecified | unspecified | unspecified | 4           |  unspecified | unspecified (padding) |

      * f (Float32):
        * Length: 6, Null count: 4
        * Validity bitmap buffer:

          | Byte 0 (validity bitmap) | Bytes 1-63            |
          |--------------------------|-----------------------|
          | 00001010                 | 0 (padding)           |

        * Value buffer:

          | Bytes 0-3    | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-19 | Bytes 20-23 | Bytes 24-63           |
          |--------------|-------------|-------------|-------------|-------------|-------------|-----------------------|
          | unspecified  | 1.2         | unspecified | 3.4         | unspecified | unspecified | unspecified (padding) |

      * s (`VarBinary`)
        * Length: 6, Null count: 4
        * Validity bitmap buffer:

          | Byte 0 (validity bitmap) | Bytes 1-63            |
          |--------------------------|-----------------------|
          | 00100100                 | 0 (padding)           |

        * Offsets buffer (Int32)

          | Bytes 0-3  | Bytes 4-7   | Bytes 8-11  | Bytes 12-15 | Bytes 16-19 | Bytes 20-23 | Bytes 24-27 | Bytes 28-63            |
          |------------|-------------|-------------|-------------|-------------|-------------|-------------|------------------------|
          | 0          | 0           | 0           | 3           | 3           | 3           | 7           | unspecified (padding)  |

        * Values buffer:

          | Bytes 0-6  | Bytes 7-63            |
          |------------|-----------------------|
          | joemark    | unspecified (padding) |

Only the slot in the array corresponding to the type index is considered. All
"unselected" values are ignored and could be any semantically correct array
value.

Null Layout
-----------

We provide a simplified memory-efficient layout for the Null data type
where all values are null. In this case no memory buffers are
allocated.

.. _dictionary-encoded-layout:

Dictionary-encoded Layout
-------------------------

Dictionary encoding is a data representation technique to represent
values by integers referencing a **dictionary** usually consisting of
unique values. It can be effective when you have data with many
repeated values.

Any array can be dictionary-encoded. The dictionary is stored as an optional
property of an array. When a field is dictionary encoded, the values are
represented by an array of non-negative integers representing the index of the
value in the dictionary. The memory layout for a dictionary-encoded array is
the same as that of a primitive integer layout. The dictionary is handled as a
separate columnar array with its own respective layout.

As an example, you could have the following data: ::

    type: VarBinary

    ['foo', 'bar', 'foo', 'bar', null, 'baz']

In dictionary-encoded form, this could appear as:

::

    data VarBinary (dictionary-encoded)
       index_type: Int32
       values: [0, 1, 0, 1, null, 2]

    dictionary
       type: VarBinary
       values: ['foo', 'bar', 'baz']

Note that a dictionary is permitted to contain duplicate values or
nulls:

::

    data VarBinary (dictionary-encoded)
       index_type: Int32
       values: [0, 1, 3, 1, 4, 2]

    dictionary
       type: VarBinary
       values: ['foo', 'bar', 'baz', 'foo', null]

The null count of such arrays is dictated only by the validity bitmap
of its indices, irrespective of any null values in the dictionary.

Since unsigned integers can be more difficult to work with in some cases
(e.g. in the JVM), we recommend preferring signed integers over unsigned
integers for representing dictionary indices. Additionally, we recommend
avoiding using 64-bit unsigned integer indices unless they are required by an
application.

We discuss dictionary encoding as it relates to serialization further
below.

.. _run-end-encoded-layout:

Run-End Encoded Layout
----------------------

.. note:: New in Arrow Columnar Format 1.3

Run-end encoding (REE) is a variation of run-length encoding (RLE). These
encodings are well-suited for representing data containing sequences of the
same value, called runs. In run-end encoding, each run is represented as a
value and an integer giving the index in the array where the run ends.

Any array can be run-end encoded. A run-end encoded array has no buffers
by itself, but has two child arrays. The first child array, called the run ends array,
holds either 16, 32, or 64-bit signed integers. The actual values of each run
are held in the second child array.
For the purposes of determining field names and schemas, these child arrays
are prescribed the standard names of **run_ends** and **values** respectively.

The values in the first child array represent the accumulated length of all runs
from the first to the current one, i.e. the logical index where the
current run ends. This allows relatively efficient random access from a logical
index using binary search. The length of an individual run can be determined by
subtracting two adjacent values. (Contrast this with run-length encoding, in
which the lengths of the runs are represented directly, and in which random
access is less efficient.)

.. note::
   Because the ``run_ends`` child array cannot have nulls, it's reasonable
   to consider why the ``run_ends`` are a child array instead of just a
   buffer, like the offsets for a :ref:`variable-size-list-layout`. This
   layout was considered, but it was decided to use the child arrays.

   Child arrays allow us to keep the "logical length" (the decoded length)
   associated with the parent array and the "physical length" (the number
   of run ends) associated with the child arrays.  If ``run_ends`` was a
   buffer in the parent array then the size of the buffer would be unrelated
   to the length of the array and this would be confusing.


A run must have a length of at least 1. This means the values in the
run ends array all are positive and in strictly ascending order. A run end cannot be
null.

The REE parent has no validity bitmap, and it's null count field should always be 0.
Null values are encoded as runs with the value null.

As an example, you could have the following data: ::

    type: Float32
    [1.0, 1.0, 1.0, 1.0, null, null, 2.0]

In Run-end-encoded form, this could appear as:

::

    * Length: 7, Null count: 0
    * Child Arrays:

      * run_ends (Int32):
        * Length: 3, Null count: 0 (Run Ends cannot be null)
        * Validity bitmap buffer: Not required (if it exists, it should be all 1s)
        * Values buffer

          | Bytes 0-3   | Bytes 4-7   | Bytes 8-11  | Bytes 12-63           |
          |-------------|-------------|-------------|-----------------------|
          | 4           | 6           | 7           | unspecified (padding) |

      * values (Float32):
        * Length: 3, Null count: 1
        * Validity bitmap buffer:

          | Byte 0 (validity bitmap) | Bytes 1-63            |
          |--------------------------|-----------------------|
          | 00000101                 | 0 (padding)           |

        * Values buffer

          | Bytes 0-3   | Bytes 4-7   | Bytes 8-11  | Bytes 12-63           |
          |-------------|-------------|-------------|-----------------------|
          | 1.0         | unspecified | 2.0         | unspecified (padding) |


Buffer Listing for Each Layout
------------------------------

For the avoidance of ambiguity, we provide listing the order and type
of memory buffers for each layout.

.. csv-table:: Buffer Layouts
   :header: "Layout Type", "Buffer 0", "Buffer 1", "Buffer 2", "Variadic Buffers"
   :widths: 30, 20, 20, 20, 20

   "Primitive",validity,data,,
   "Variable Binary",validity,offsets,data,
   "Variable Binary View",validity,views,,data
   "List",validity,offsets,,
   "List View",validity,offsets,sizes,
   "Fixed-size List",validity,,,
   "Struct",validity,,,
   "Sparse Union",type ids,,,
   "Dense Union",type ids,offsets,,
   "Null",,,,
   "Dictionary-encoded",validity,data (indices),,
   "Run-end encoded",,,,

.. _format-ipc:

Serialization and Interprocess Communication (IPC)
==================================================

The primitive unit of serialized data in the columnar format is the
"record batch". Semantically, a record batch is an ordered collection
of arrays, known as its **fields**, each having the same length as one
another but potentially different data types. A record batch's field
names and types collectively form the batch's **schema**.

In this section we define a protocol for serializing record batches
into a stream of binary payloads and reconstructing record batches
from these payloads without need for memory copying.

The columnar IPC protocol utilizes a one-way stream of binary messages
of these types:

* Schema
* RecordBatch
* DictionaryBatch

We specify a so-called *encapsulated IPC message* format which
includes a serialized Flatbuffer type along with an optional message
body. We define this message format before describing how to serialize
each constituent IPC message type.

.. _ipc-message-format:

Encapsulated message format
---------------------------

For simple streaming and file-based serialization, we define a
"encapsulated" message format for interprocess communication. Such
messages can be "deserialized" into in-memory Arrow array objects by
examining only the message metadata without any need to copy or move
any of the actual data.

The encapsulated binary message format is as follows:

* A 32-bit continuation indicator. The value ``0xFFFFFFFF`` indicates
  a valid message. This component was introduced in version 0.15.0 in
  part to address the 8-byte alignment requirement of Flatbuffers
* A 32-bit little-endian length prefix indicating the metadata size
* The message metadata as using the ``Message`` type defined in
  `Message.fbs`_
* Padding bytes to an 8-byte boundary
* The message body, whose length must be a multiple of 8 bytes

Schematically, we have: ::

    <continuation: 0xFFFFFFFF>
    <metadata_size: int32>
    <metadata_flatbuffer: bytes>
    <padding>
    <message body>

The complete serialized message must be a multiple of 8 bytes so that messages
can be relocated between streams. Otherwise the amount of padding between the
metadata and the message body could be non-deterministic.

The ``metadata_size`` includes the size of the ``Message`` plus
padding. The ``metadata_flatbuffer`` contains a serialized ``Message``
Flatbuffer value, which internally includes:

* A version number
* A particular message value (one of ``Schema``, ``RecordBatch``, or
  ``DictionaryBatch``)
* The size of the message body
* A ``custom_metadata`` field for any application-supplied metadata

When read from an input stream, generally the ``Message`` metadata is
initially parsed and validated to obtain the body size. Then the body
can be read.

Schema message
--------------

The Flatbuffers files `Schema.fbs`_ contains the definitions for all
built-in data types and the ``Schema`` metadata type which represents
the schema of a given record batch. A schema consists of an ordered
sequence of fields, each having a name and type. A serialized ``Schema``
does not contain any data buffers, only type metadata.

The ``Field`` Flatbuffers type contains the metadata for a single
array. This includes:

* The field's name
* The field's data type
* Whether the field is semantically nullable. While this has no
  bearing on the array's physical layout, many systems distinguish
  nullable and non-nullable fields and we want to allow them to
  preserve this metadata to enable faithful schema round trips.
* A collection of child ``Field`` values, for nested types
* A ``dictionary`` property indicating whether the field is
  dictionary-encoded or not. If it is, a dictionary "id" is assigned
  to allow matching a subsequent dictionary IPC message with the
  appropriate field.

We additionally provide both schema-level and field-level
``custom_metadata`` attributes allowing for systems to insert their
own application defined metadata to customize behavior.

.. _ipc-recordbatch-message:

RecordBatch message
-------------------

A RecordBatch message contains the actual data buffers corresponding
to the physical memory layout determined by a schema. The metadata for
this message provides the location and size of each buffer, permitting
Array data structures to be reconstructed using pointer arithmetic and
thus no memory copying.

The serialized form of the record batch is the following:

* The ``data header``, defined as the ``RecordBatch`` type in
  `Message.fbs`_.
* The ``body``, a flat sequence of memory buffers written end-to-end
  with appropriate padding to ensure a minimum of 8-byte alignment

The data header contains the following:

* The length and null count for each flattened field in the record
  batch
* The memory offset and length of each constituent ``Buffer`` in the
  record batch's body

Fields and buffers are flattened by a pre-order depth-first traversal
of the fields in the record batch. For example, let's consider the
schema ::

    col1: Struct<a: Int32, b: List<item: Int64>, c: Float64>
    col2: Utf8

The flattened version of this is: ::

    FieldNode 0: Struct name='col1'
    FieldNode 1: Int32 name='a'
    FieldNode 2: List name='b'
    FieldNode 3: Int64 name='item'
    FieldNode 4: Float64 name='c'
    FieldNode 5: Utf8 name='col2'

For the buffers produced, we would have the following (refer to the
table above): ::

    buffer 0: field 0 validity
    buffer 1: field 1 validity
    buffer 2: field 1 values
    buffer 3: field 2 validity
    buffer 4: field 2 offsets
    buffer 5: field 3 validity
    buffer 6: field 3 values
    buffer 7: field 4 validity
    buffer 8: field 4 values
    buffer 9: field 5 validity
    buffer 10: field 5 offsets
    buffer 11: field 5 data

The ``Buffer`` Flatbuffers value describes the location and size of a
piece of memory. Generally these are interpreted relative to the
**encapsulated message format** defined below.

The ``size`` field of ``Buffer`` is not required to account for padding
bytes. Since this metadata can be used to communicate in-memory pointer
addresses between libraries, it is recommended to set ``size`` to the actual
memory size rather than the padded size.

.. _variadic-buffers:

Variadic buffers
----------------

.. note:: New in Arrow Columnar Format 1.4

Some types such as Utf8View are represented using a variable number of buffers.
For each such Field in the pre-ordered flattened logical schema, there will be
an entry in ``variadicBufferCounts`` to indicate the number of variadic buffers
which belong to that Field in the current RecordBatch.

For example, consider the schema ::

    col1: Struct<a: Int32, b: BinaryView, c: Float64>
    col2: Utf8View

This has two fields with variadic buffers, so ``variadicBufferCounts`` will
have two entries in each RecordBatch. For a RecordBatch of this schema with
``variadicBufferCounts = [3, 2]``, the flattened buffers would be::

    buffer 0:  col1    validity
    buffer 1:  col1.a  validity
    buffer 2:  col1.a  values
    buffer 3:  col1.b  validity
    buffer 4:  col1.b  views
    buffer 5:  col1.b  data
    buffer 6:  col1.b  data
    buffer 7:  col1.b  data
    buffer 8:  col1.c  validity
    buffer 9:  col1.c  values
    buffer 10: col2    validity
    buffer 11: col2    views
    buffer 12: col2    data
    buffer 13: col2    data


Compression
-----------

There are three different options for compression of record batch
body buffers: Buffers can be uncompressed, buffers can be
compressed with the ``lz4`` compression codec, or buffers can be
compressed with the ``zstd`` compression codec. Buffers in the
flat sequence of a message body must be compressed separately using
the same codec. Specific buffers in the sequence of compressed
buffers may be left uncompressed (for example if compressing those
specific buffers would not appreciably reduce their size).

The compression type used is defined in the ``data header``
of the :ref:`ipc-recordbatch-message` in the optional ``compression``
field with the default being uncompressed.

.. note::

   ``lz4`` compression codec means the
   `LZ4 frame format <https://github.com/lz4/lz4/blob/dev/doc/lz4_Frame_format.md>`_
   and should not to be confused with
   `"raw" (also called "block") format <https://github.com/lz4/lz4/blob/dev/doc/lz4_Block_format.md>`_.

The difference between compressed and uncompressed buffers in the
serialized form is as follows:

* If the buffers in the :ref:`ipc-recordbatch-message` are **compressed**

  - the ``data header`` includes the length and memory offset
    of each **compressed buffer** in the record batch's body together
    with the compression type

  - the ``body`` includes a flat sequence of **compressed buffers**
    together with the **length of the uncompressed buffer** as a 64-bit
    little-endian signed integer stored in the first 8 bytes of each
    buffer in the sequence. This uncompressed length can be set to ``-1`` to indicate
    that that specific buffer is left uncompressed.

* If the buffers in the :ref:`ipc-recordbatch-message` are **uncompressed**

  - the ``data header`` includes the length and memory offset
    of each **uncompressed buffer** in the record batch's body

  - the ``body`` includes a flat sequence of **uncompressed buffers**.

.. note::

   Some Arrow implementations lack support for producing and consuming
   IPC data with compressed buffers using one or either of the codecs
   listed above. See :doc:`../status` for details.

   Some applications might apply compression in the protocol they use
   to store or transport Arrow IPC data. (For example, an HTTP server
   might serve gzip-compressed Arrow IPC streams.) Applications that
   already use compression in their storage or transport protocols
   should avoid using buffer compression. Double compression typically
   worsens performance and does not substantially improve compression
   ratios.

Byte Order (`Endianness`_)
---------------------------

The Arrow format is little endian by default.

Serialized Schema metadata has an endianness field indicating
endianness of RecordBatches. Typically this is the endianness of the
system where the RecordBatch was generated. The main use case is
exchanging RecordBatches between systems with the same Endianness.  At
first we will return an error when trying to read a Schema with an
endianness that does not match the underlying system. The reference
implementation is focused on Little Endian and provides tests for
it. Eventually we may provide automatic conversion via byte swapping.

IPC Streaming Format
--------------------

We provide a streaming protocol or "format" for record batches. It is
presented as a sequence of encapsulated messages, each of which
follows the format above. The schema comes first in the stream, and it
is the same for all of the record batches that follow. If any fields
in the schema are dictionary-encoded, one or more ``DictionaryBatch``
messages will be included. ``DictionaryBatch`` and ``RecordBatch``
messages may be interleaved, but before any dictionary key is used in
a ``RecordBatch`` it should be defined in a ``DictionaryBatch``. ::

    <SCHEMA>
    <DICTIONARY 0>
    ...
    <DICTIONARY k - 1>
    <RECORD BATCH 0>
    ...
    <DICTIONARY x DELTA>
    ...
    <DICTIONARY y DELTA>
    ...
    <RECORD BATCH n - 1>
    <EOS [optional]: 0xFFFFFFFF 0x00000000>

.. note:: An edge-case for interleaved dictionary and record batches occurs
   when the record batches contain dictionary encoded arrays that are
   completely null. In this case, the dictionary for the encoded column might
   appear after the first record batch.

When a stream reader implementation is reading a stream, after each
message, it may read the next 8 bytes to determine both if the stream
continues and the size of the message metadata that follows. Once the
message flatbuffer is read, you can then read the message body.

The stream writer can signal end-of-stream (EOS) either by writing 8 bytes
containing the 4-byte continuation indicator (``0xFFFFFFFF``) followed by 0
metadata length (``0x00000000``) or closing the stream interface. We
recommend the ".arrows" file extension for the streaming format although
in many cases these streams will not ever be stored as files.

IPC File Format
---------------

We define a "file format" supporting random access that is an extension of
the stream format. The file starts and ends with a magic string ``ARROW1``
(plus padding). What follows in the file is identical to the stream format.
At the end of the file, we write a *footer* containing a redundant copy of
the schema (which is a part of the streaming format) plus memory offsets and
sizes for each of the data blocks in the file. This enables random access to
any record batch in the file. See `File.fbs`_ for the precise details of the
file footer.

Schematically we have: ::

    <magic number "ARROW1">
    <empty padding bytes [to 8 byte boundary]>
    <STREAMING FORMAT with EOS>
    <FOOTER>
    <FOOTER SIZE: int32>
    <magic number "ARROW1">

In the file format, there is no requirement that dictionary keys
should be defined in a ``DictionaryBatch`` before they are used in a
``RecordBatch``, as long as the keys are defined somewhere in the
file. Further more, it is invalid to have more than one **non-delta**
dictionary batch per dictionary ID (i.e. dictionary replacement is not
supported). Delta dictionaries are applied in the order they appear in
the file footer. We recommend the ".arrow" extension for files created with
this format. Note that files created with this format are sometimes called
"Feather V2" or with the ".feather" extension, the name and the extension
derived from "Feather (V1)", which was a proof of concept early in
the Arrow project for language-agnostic fast data frame storage for
Python (pandas) and R.

Dictionary Messages
-------------------

Dictionaries are written in the stream and file formats as a sequence of record
batches, each having a single field. The complete semantic schema for a
sequence of record batches, therefore, consists of the schema along with all of
the dictionaries. The dictionary types are found in the schema, so it is
necessary to read the schema to first determine the dictionary types so that
the dictionaries can be properly interpreted: ::

    table DictionaryBatch {
      id: long;
      data: RecordBatch;
      isDelta: boolean = false;
    }

The dictionary ``id`` in the message metadata can be referenced one or more times
in the schema, so that dictionaries can even be used for multiple fields. See
the :ref:`dictionary-encoded-layout` section for more about the semantics of
dictionary-encoded data.

The dictionary ``isDelta`` flag allows existing dictionaries to be
expanded for future record batch materializations. A dictionary batch
with ``isDelta`` set indicates that its vector should be concatenated
with those of any previous batches with the same ``id``. In a stream
which encodes one column, the list of strings ``["A", "B", "C", "B",
"D", "C", "E", "A"]``, with a delta dictionary batch could take the
form: ::

    <SCHEMA>
    <DICTIONARY 0>
    (0) "A"
    (1) "B"
    (2) "C"

    <RECORD BATCH 0>
    0
    1
    2
    1

    <DICTIONARY 0 DELTA>
    (3) "D"
    (4) "E"

    <RECORD BATCH 1>
    3
    2
    4
    0
    EOS

Alternatively, if ``isDelta`` is set to false, then the dictionary
replaces the existing dictionary for the same ID.  Using the same
example as above, an alternate encoding could be: ::


    <SCHEMA>
    <DICTIONARY 0>
    (0) "A"
    (1) "B"
    (2) "C"

    <RECORD BATCH 0>
    0
    1
    2
    1

    <DICTIONARY 0>
    (0) "A"
    (1) "C"
    (2) "D"
    (3) "E"

    <RECORD BATCH 1>
    2
    1
    3
    0
    EOS

.. _format_metadata:

Custom Application Metadata
---------------------------

We provide a ``custom_metadata`` field at three levels to provide a
mechanism for developers to pass application-specific metadata in
Arrow protocol messages. This includes ``Field``, ``Schema``, and
``Message``.

The colon symbol ``:`` is to be used as a namespace separator. It can
be used multiple times in a key.

The ``ARROW`` pattern is a reserved namespace for internal Arrow use
in the ``custom_metadata`` fields. For example,
``ARROW:extension:name``.

.. _format_metadata_extension_types:

Extension Types
---------------

User-defined "extension" types can be defined setting certain
``KeyValue`` pairs in ``custom_metadata`` in the ``Field`` metadata
structure. These extension keys are:

* ``'ARROW:extension:name'`` for the string name identifying the
  custom data type. We recommend that you use a "namespace"-style
  prefix for extension type names to minimize the possibility of
  conflicts with multiple Arrow readers and writers in the same
  application. For example, use ``myorg.name_of_type`` instead of
  simply ``name_of_type``
* ``'ARROW:extension:metadata'`` for a serialized representation
  of the ``ExtensionType`` necessary to reconstruct the custom type

.. note::
   Extension names beginning with ``arrow.`` are reserved for
   :ref:`canonical extension types <format_canonical_extensions>`,
   they should not be used for third-party extension types.

This extension metadata can annotate any of the built-in Arrow logical
types. For example, Arrow specifies a canonical extension type that
represents a UUID as a ``FixedSizeBinary(16)``. Arrow implementations are
not required to support canonical extensions, so an implementation that
does not support this UUID type will simply interpret it as a
``FixedSizeBinary(16)`` and pass along the ``custom_metadata`` in
subsequent Arrow protocol messages.

Extension types may or may not use the
``'ARROW:extension:metadata'`` field. Let's consider some example
extension types:

* ``uuid`` represented as ``FixedSizeBinary(16)`` with empty metadata
* ``latitude-longitude`` represented as ``struct<latitude: double,
  longitude: double>``, and empty metadata
* ``tensor`` (multidimensional array) stored as ``Binary`` values and
  having serialized metadata indicating the data type and shape of
  each value. This could be JSON like ``{'type': 'int8', 'shape': [4,
  5]}`` for a 4x5 cell tensor.
* ``trading-time`` represented as ``Timestamp`` with serialized
  metadata indicating the market trading calendar the data corresponds
  to

.. seealso::
   :ref:`format_canonical_extensions`


Implementation guidelines
=========================

An execution engine (or framework, or UDF executor, or storage engine,
etc) can implement only a subset of the Arrow spec and/or extend it
given the following constraints:

Implementing a subset of the spec
---------------------------------

* **If only producing (and not consuming) arrow vectors**: Any subset
  of the vector spec and the corresponding metadata can be implemented.
* **If consuming and producing vectors**: There is a minimal subset of
  vectors to be supported.  Production of a subset of vectors and
  their corresponding metadata is always fine.  Consumption of vectors
  should at least convert the unsupported input vectors to the
  supported subset (for example Timestamp.millis to timestamp.micros
  or int32 to int64).

Extensibility
-------------

An execution engine implementor can also extend their memory
representation with their own vectors internally as long as they are
never exposed. Before sending data to another system expecting Arrow
data, these custom vectors should be converted to a type that exist in
the Arrow spec.

.. _Flatbuffers: http://github.com/google/flatbuffers
.. _Flatbuffers protocol definition files: https://github.com/apache/arrow/tree/main/format
.. _Schema.fbs: https://github.com/apache/arrow/blob/main/format/Schema.fbs
.. _Message.fbs: https://github.com/apache/arrow/blob/main/format/Message.fbs
.. _File.fbs: https://github.com/apache/arrow/blob/main/format/File.fbs
.. _least-significant bit (LSB) numbering: https://en.wikipedia.org/wiki/Bit_numbering
.. _Intel performance guide: https://web.archive.org/web/20151101074635/https://software.intel.com/en-us/articles/practical-intel-avx-optimization-on-2nd-generation-intel-core-processors
.. _Endianness: https://en.wikipedia.org/wiki/Endianness
.. _SIMD: https://www.intel.com/content/www/us/en/docs/cpp-compiler/developer-guide-reference/2021-8/simd-data-layout-templates.html
.. _Parquet: https://parquet.apache.org/docs/
.. _UmbraDB: https://db.in.tum.de/~freitag/papers/p29-neumann-cidr20.pdf