[Website] Add blog post for Arrow 21.0.0

_posts/2025-07-16-21.0.0-release.md

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+---
+layout: post
+title: "Apache Arrow 21.0.0 Release"
+date: "2025-07-16 00:00:00"
+author: pmc
+categories: [release]
+---
+<!--
+{% comment %}
+Licensed to the Apache Software Foundation (ASF) under one or more
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+(the "License"); you may not use this file except in compliance with
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+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
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+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
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+{% endcomment %}
+-->
+
+The Apache Arrow team is pleased to announce the 21.0.0 release. This release
+covers over 2 months of development work and includes [**1234 resolved
+issues**][1] on [**1234 distinct commits**][2] from [**1234 distinct
+contributors**][2]. See the [Install Page](https://arrow.apache.org/install/) to
+learn how to get the libraries for your platform.
+
+The release notes below are not exhaustive and only expose selected highlights
+of the release. Many other bugfixes and improvements have been made: we refer
+you to the [complete changelog][3].
+
+## Community
+
+Since the 20.0.0 release, Alenka Frim has been invited to join the Project
+Management Committee (PMC).
+
+Thanks for your contributions and participation in the project!
+
+## Release Highlights
+
+## Columnar Format
+
+## Arrow Flight RPC Notes
+
+## C++ Notes
+
+### Acero
+
+### Compute
+
+The Cast function is now able to reorder fields when casting from one
+struct type to another; the fields are matched by name, not by index (GH-45028).
+
+Many compute kernels have been moved into a separate, optional, shared library
+(GH-25025). This allows reducing the Arrow C++ distribution size when the
+compute functionality is not being used by the application. Note that some
+compute functions, such as the Cast function, will still be built for internal
+use in various Arrow components.
+
+Better half-float support has been added to some compute functions:
+`is_nan`, `is_inf`, `is_finite`, `negate`, `negate_checked`, `sign` (GH-45083);
+`if_else`, `case_when`, `coalesce`, `choose`, `replace_with_mask`, `fill_null_forward`,
+`fill_null_backward` (GH-37027); `run_end_encode`, `run_end_decode` (GH-46285).
+
+Better decimal32 and decimal64 support has been added to some compute functions:
+`run_end_encode`, `run_end_decode` (GH-46285).
+
+A new function `utf8_zero_fill` acts like Python's `str.zfill` method by providing
+a left-padding function that preserves the optional leading plus/minus sign (GH-46683).
+
+Decimal sum aggregation now produces a decimal result with an increased precision
+in order to reduce the risk of overflowing the result type (GH-35166).
+
+### CSV
+
+Reading Duration columns is now supported (GH-40278).
+
+### Dataset
+
+It is now possible to preserve order when writing a dataset multi-threaded.
+The feature is disabled by default (GH-26818).
+
+### Filesystems
+
+The S3 filesystem can optionally be built into a separate DLL (GH-40343).
+
+### Parquet
+
+#### Encryption
+
+A new `SecureString` class must now be used to communicate sensitive data (such as
+secret keys) with Parquet encryption APIs. This class automatically wipes its
+contents from memory when destroyed, unlike regular `std::string` (GH-31603).
+
+#### Type support
+
+The new VARIANT logical type is supported at a low level, and an extension type
+`parquet.variant` is added to reflect such columns when reading them to Arrow
+(GH-45937).
+
+The UUID logical type is automatically converted to/from the `arrow.uuid`
+canonical extension type when reading or writing Parquet data, respectively.
+
+The GEOMETRY and GEOGRAPHY logical types are supported (GH-45522). They are
+automatically converted to/from the corresponding GeoArrow extension type,
+if it has been registered by GeoArrow. Geospatial column statistics are also
+supported.
+
+It is now possible to read BYTE_ARRAY columns directly as LargeBinary or BinaryView,
+without any intermediate conversion from Binary. Similarly, those types can be
+written directly to Parquet (GH-43041). This allows bypassing the 2 GiB data
+per chunk limitation of the Binary type, and can also improve performance.
+This also applies to String types when a Parquet column has the STRING logical type.
+
+Similarly, LIST columns can now be read directly as LargeList rather than List.
+This allows bypassing the 2^31 values per chunk limitation of regular List types
+(GH-46676).
+
+#### Other Parquet improvements
+
+A new feature named Content-Defined Chunking improves deduplication of Parquet
+files with mostly identical contents, by choosing data page boundaries based on
+actual contents rather than a number of values. For that, it uses a rolling hash
+function, and the min and max chunk size can be chosen. The feature is disabled by
+default and can be enabled on a per-file basis in the Parquet `WriterProperties`
+(GH-45750).
+
+The `EncodedStatistics` of a column chunk are publicly exposed in `ColumnChunkMetaData`
+and can be read faster than if decoded as `Statistics` (GH-46462).
+
+SIMD optimizations for the BYTE_STREAM_SPLIT have been improved (GH-46788).
+
+Reading FIXED_LEN_BYTE_ARRAY data has been made significantly faster (up to 3x
+faster on some benchmarks). This benefits logical types such as FLOAT16 (GH-43891).
+
+### Miscellaneous C++ changes
+
+The `ARROW_USE_PRECOMPILED_HEADERS` build option was removed, as `CMAKE_UNITY_BUILD`
+usually provides more benefits while requiring less maintenance.
+
+New data creation helpers `ArrayFromJSONString`, `ChunkedArrayFromJSONString`,
+`DictArrayFromJSONString`, `ScalarFromJSONString` and `DictScalarFromJSONString`
+are now exposed publicly. While not as high-performing as `BufferBuilder` and
+the concrete `ArrayBuilder` subclasses, they allow easy creation of test
+or example data, for example:
+```c++
+  ARROW_ASSIGN_OR_RAISE(
+      auto string_array,
+      arrow::ArrayFromJSONString(arrow::utf8(), R"(["Hello", "World", null])"));
+  ARROW_ASSIGN_OR_RAISE(
+      auto list_array,
+      arrow::ArrayFromJSONString(arrow::list(arrow::int32()),
+                                 "[[1, null, 2], [], [3]]"));
+```
+
+Some APIs were changed to accept `std::string_view` instead of `const std::string&`.
+Most uses of those APIs should not be affected (GH-46551).
+
+A new pretty-print option allows limiting element size when printing string
+or binary data (GH-46403).
+
+It is now possible to export `Tensor` data using
+[DLPack](https://dmlc.github.io/dlpack/latest/) (GH-39294).
+
+Half-float arrays can be properly diff'ed and pretty-printed (GH-36753).
+
+Some header files in `arrow/util` that were not supposed to be exposed are
+now made internal (GH-46459).
+
+## C# Notes
+
+- TODO: Note about future repo move plan. "This is the final release of the C# implementation of Arrow as part of the monorepo...etc.
+
+## Java, JavaScript, Go, and Rust Notes
+
+The Java, JavaScript, Go, and Rust Go projects have moved to separate repositories outside
+the main Arrow [monorepo](https://github.com/apache/arrow).
+
+- For notes on the latest release of the [Java
+implementation](https://github.com/apache/arrow-java), see the latest [Arrow
+Java changelog][7].
+- For notes on the latest release of the [JavaScript
+implementation](https://github.com/apache/arrow-js), see the latest [Arrow
+JavaScript changelog][8].
+- For notes on the latest release of the [Rust
+  implementation](https://github.com/apache/arrow-rs) see the latest [Arrow Rust
+  changelog][5].
+- For notes on the latest release of the [Go
+implementation](https://github.com/apache/arrow-go), see the latest [Arrow Go
+changelog][6].
+
+## Linux Packaging Notes
+
+We added support for AlmaLinux 10. You can use AlmaLinux 10 packages on Red Hat Enterprise Linux 10 like distributions too.
+
+We dropped support for CentOS Stream 8 because it reached EOL on 2024-05-31.
+
+## Python Notes
+
+## Ruby and C GLib Notes
+
+Here are updates both for Ruby and C GLib:
+
+- Added support for fixed shape tensor extension data type.
+- Added support for UUID extension data type.
+- Added support for fixed size list data type.
+- Added support for [the Arrow C data interface](https://arrow.apache.org/docs/format/CDataInterface.html) for chunked array.
+* Added support for distinct count in array statistics.
+
+### Ruby
+
+There is no update only for Ruby.
+
+### C GLib
+
+Here are updates only for C GLib:
+
+- You must call `garrow_compute_initialize()` explicitly before you use computation related features.
+
+
+[1]: https://github.com/apache/arrow/milestone/69?closed=1
+[2]: {{ site.baseurl }}/release/21.0.0.html#contributors
+[3]: {{ site.baseurl }}/release/21.0.0.html#changelog
+[4]: {{ site.baseurl }}/docs/r/news/
+[5]: <https://github.com/apache/arrow-rs/blob/main/CHANGELOG.md>
+[6]: <https://github.com/apache/arrow-go/releases>
+[7]: <https://github.com/apache/arrow-java/releases>
+[8]: <https://github.com/apache/arrow-js/releases>