Zarr core specification (version 3.0)#

Editor’s draft 25 May 2022

Specification URI:

  • Alistair Miles (@alimanfoo), Wellcome Sanger Institute

  • Jonathan Striebel (@jstriebel), Scalable Minds

  • Jeremy Maitin-Shepard (@jbms), Google

Corresponding ZEP:

ZEP 1 — Zarr specification version 3

Issue tracking:

GitHub issues

Suggest an edit for this spec:

GitHub editor

Suggest extensions or other changes as a Zarr Enhancement Proposal (ZEP):

ZEP 0 — Purpose and process

Copyright 2019-Present Zarr core development team. This work is licensed under a Creative Commons Attribution 3.0 Unported License.


This specification defines the Zarr format for N-dimensional typed arrays.

Status of this document#


This document is a draft for review and subject to changes. It will become final when the Zarr Enhancement Proposal (ZEP) 1 is approved via the ZEP process.


This specification defines a format for multidimensional array data. This type of data is common in scientific and numerical computing applications. Many domains are facing computational challenges as increasingly large volumes of data are being generated, for example, via high resolution microscopy, remote sensing imagery, genome sequencing or numerical simulation. The primary motivation for the development of Zarr has been to help address this challenge by enabling the storage of large multidimensional arrays in a way that is compatible with parallel and/or distributed computing applications.

This specification is intended to supersede the Zarr storage specification version 2 (Zarr v2). The Zarr v2 specification has been implemented in several programming languages and has been used successfully to store and analyse large scientific datasets from a variety of domains. However, as experience has been gained, it has become clear that there are several opportunities for modest but useful improvements to be made in the format, and for establishing a foundation that allows for greater interoperability, whilst also enabling a variety of more advanced and specialised features to be explored and developed.

This specification also draws heavily on the N5 API and file-system specification, which was developed in parallel to Zarr v2 and has many of the same design goals and features. This specification defines a core set of features at the intersection of both Zarr v2 and N5, and so aims to provide a common target that can be fully implemented across multiple programming environments and serve a wide range of applications.

In particular, we highlight the following areas motivating the development of this specification.

Distributed storage#

The Zarr v2 specification was originally developed and implemented for use with local filesystem storage only. It then became clear that the same format could also be used with distributed storage systems, including cloud object stores such as Amazon S3, Google Cloud Storage or Azure Blob Storage. However, distributed storage systems have a number of important differences from local file systems, both in terms of the features they support and their performance characteristics. For example, cloud stores have much greater latency per request than local file systems, and this means that certain operations such as exploring a hierarchy of arrays using the Zarr v2 format can be unacceptably slow. Workarounds have been developed, such as the use of metadata consolidation, but there are opportunities for modifications to the core format that address these issues directly and work more performantly across a range of underlying storage systems with varying features and latency characteristics. For example, this specification aims to minimise the number of storage requests required when opening and exploring a hierarchy of arrays.


While the Zarr v2 and N5 specifications have each been implemented in multiple programming languages, there is currently not feature parity across all implementations. This is in part because the feature set includes some features that are not easily translated or supported across different programming languages. This specification aims to define a set of core features that are useful and sufficient to address a significant fraction of use cases, but are also straightforward to implement fully across different programming languages. Additional functionality can then be layered via extensions, some of which may aim for wide adoption, some of which may be more specialised and have more limited implementation.


The development of systems for storage of very large array-like data is a very active area of research and development, and there are many possibilities that remain to be explored. A goal of this specification is to define a format with a number of clear extension points and mechanisms, in order to provide a framework for freely building on and exploring these possibilities. We aim to make this possible, whilst also providing pathways for a graceful degradation of functionality where possible, in order to retain interoperability. We also aim to provide a framework for community-defined extensions, which can be developed and published independently without requiring centralised coordination of all specifications.

See extension points below.

Stability Policy#

This core specification adheres to a MAJOR.MINOR version number format. When incrementing the minor version, only additional features can be added. Breaking changes require to increment the major version.

A zarr implementation that provides the read and write API by implementing a specification X.Y can be considered compatible with all datasets which only use features contained in version X.Y.

For example, spec X.1 adds core feature “foo” compared to X.0. Assuming implementation A implements X.0 and implementation B implements X.1. Data using feature “foo” can only be read with implementation A. B fails to open it, as the key “foo” is unknown.

Data not using “foo” can be used with both implementations, even if it’s written with implementation B.

Therefore, data is only marked with the respective major version, unknown features are auto-discovered via the metadata document.

Notably, this excludes extension points such as codecs, data types, chunk grids and storage transformers from the compatibility of the core specification, as well as store support. However, versioned extension points and stores are also expected to follow this stability policy.

Document conventions#

Conformance requirements are expressed with a combination of descriptive assertions and [RFC2119] terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in [RFC2119]. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. Examples in this specification are introduced with the words “for example”.

Concepts and terminology#

This section introduces and defines some key terms and explains the conceptual model underpinning the Zarr format.

The following figure illustrates the first part of the terminology:



A Zarr hierarchy is a tree structure, where each node in the tree is either a group or an array. Group nodes may have children but array nodes may not. All nodes in a hierarchy have a name and a path. The root of a Zarr hierarchy may be either a group or an array. In the latter case, the hierarchy consists of just the single array.


A group is a node in a hierarchy that may have child nodes.


An array is a node in a hierarchy. An array is a data structure with zero or more dimensions whose lengths define the shape of the array. An array contains zero or more data elements. All elements in an array conform to the same data type. An array may not have child nodes.


Each node in a hierarchy has a name, which is a string of characters with some additional constraints defined in the section on node names below. Two sibling nodes cannot have the same name. The root node does not have a name and is the empty string "".


Each node in a hierarchy has a path which uniquely identifies that node and defines its location within the hierarchy. The path is a string, formed by joining together the “/” character, followed by the name of each ancestor node separated by the “/” character, followed by the name of the node itself. For example, the path “/foo/bar” identifies a node named “bar”, whose parent is named “foo”, whose parent is the root of the hierarchy. The path “/” identifies the root node.

A path always starts with / and cannot end with /, because node names cannot contain /.


An array has a fixed number of zero or more dimensions. Each dimension has an integer length. This specification only considers the case where the lengths of all dimensions are finite. However, extensions may be defined which allow a dimension to have an infinite or variable length.


The shape of an array is the tuple of dimension lengths. For example, if an array has 2 dimensions, where the length of the first dimension is 100 and the length of the second dimension is 20, then the shape of the array is (100, 20). A shape can be the empty tuple in the case of zero-dimension arrays (scalar)


An array contains zero or more elements. Each element can be identified by a tuple of integer coordinates, one for each dimension of the array. If all dimensions of an array have finite length, then the number of elements in the array is given by the product of the dimension lengths.

Data type

A data type defines the set of possible values that an array may contain, and a default binary representation (i.e., sequence of bytes) for each possible value. For example, the 32-bit signed integer data type defines binary representations for all integers in the range −2,147,483,648 to 2,147,483,647. This specification only defines a limited set of data types, but extensions may define other data types.


An array is divided into a set of chunks, where each chunk is a hyperrectangle defined by a tuple of intervals, one for each dimension of the array. The chunk shape is the tuple of interval lengths, and the chunk size (i.e., number of elements contained within the chunk) is the product of its interval lengths.

The chunk shape elements are non-zero when the corresponding dimensions of the arrays are of non-zero length.


The chunks of an array are organised into a grid. This specification only considers the case where all chunks have the same chunk shape and the chunks form a regular grid. However, extensions may define other grid types such as rectilinear grids.

Metadata document

Each array in a hierarchy is represented by a metadata document, which is a machine-readable document containing essential processing information about the node. For example, an array metadata document will specify the number of dimensions, shape, data type, grid, and codec for that array.

Groups can have an optional metadata document which provides extra information about a group.


The metadata documents and encoded chunk data for all nodes in a hierarchy are held in a store as raw bytes. To enable a variety of different store types to be used, this specification defines an Abstract store interface which is a common set of operations that stores may provide. For example, a directory in a file system can be a zarr store, where keys are file names, values are file contents, and files can be read, written, listed or deleted via the operating system. Equally, an S3 bucket can provide this interface, where keys are resource names, values are resource contents, and resources can be read, written or deleted via HTTP.

The following figure illustrates the codec, store and storage transformer terminology for a use case of reading from an array:



An array may be associated with a list of codecs. Each codec specifies a bidirectional transform (an encode transform and a decode transform).

Each codec has an encoded representation and a decoded representation; each of these two representations are defined to be either:

  • a multi-dimensional array of some shape and data type, or

  • a byte string.

Logically, a codec c must define three properties:

  • c.compute_encoded_representation_type(decoded_representation_type), a procedure that determines the encoded representation based on the decoded representation and any codec parameters. In the case of a decoded representation that is a multi-dimensional array, the shape and data type of the encoded representation must be computable based only on the shape and data type, but not the actual element values, of the encoded representation. If the decoded_representation_type is not supported, this algorithm must fail with an error.

  • c.encode(decoded_value), a procedure that computes the encoded representation, and is used when writing an array.

  • c.decode(encoded_value, decoded_representation_type), a procedure that computes the decoded representation, and is used when reading an array.

If more than one codec is specified for an array, each codec is applied sequentially; when encoding, the encoded output of codec i serves as the decoded input of codec i+1, and similarly when decoding, the decoded output of codec i+1 serves as the encoded input to codec i.

Storage transformer

To provide performance enhancements or other optimizations, storage transformers may intercept and alter the storage keys and bytes of an array before they reach the underlying physical storage. Upon retrieval, the original keys and bytes are restored within the transformer. Any number of predefined storage transformers can be registered and stacked. In contrast to codecs, storage transformers can act on the a complete array, rather than individual chunks. See the storage transformers details below.

Node names#

The root node does not have a name and is the empty string "". Except for the root node, each node in a hierarchy must have a name, which is a string of unicode code points. The following constraints apply to node names:

  • must not be the empty string ("")

  • must not include the character "/"

  • must not be a string composed only of period characters, e.g. "." or ".."

  • must not start with the reserved prefix "__"

To ensure consistent behaviour across different storage systems and programming languages, we recommend users to only use characters in the sets a-z, A-Z, 0-9, -, _, ..

Node names are case sensitive, e.g., the names “foo” and “FOO” are not identical.

When using non-ASCII Unicode characters, we recommend users to use case-folded NFKC-normalized strings following the General Security Profile for Identifiers of the Unicode Security Mechanisms (Unicode Technical Standard #39). This follows the Recommendations for Programmers (B) of the Unicode Security Considerations (Unicode Technical Report #36).


A storage transformer for unicode normalization might be added later, see zarr-developers/zarr-specs#201.


The underlying store might pose additional restriction on node names, such as the following:


If a store requires an explicit byte string representation the default representation is the UTF-8 encoded Unicode string.


The prefix __zarr is reserved for core zarr data, and extensions can use other files and folders starting with __.

Data types#

A data type describes the set of possible binary values that an array element may take, along with some information about how the values should be interpreted.

This core specification defines a limited set of data types to represent boolean values, integers, and floating point numbers. Extensions may define additional data types. All of the data types defined here have a fixed size, in the sense that all values require the same number of bytes. However, extensions may define variable sized data types.

Note that the Zarr specification is intended to enable communication of data between a variety of computing environments. The native byte order may differ between machines used to write and read the data.

Each data type is associated with an identifier, which can be used in metadata documents to refer to the data type. For the data types defined in this specification, the identifier is a simple ASCII string. However, extensions may use any JSON value to identify a data type.

Core data types#

Data types#


Numerical type

Default binary representation



Single byte, with false encoded as \\x00 and true encoded as \\x01.


Integer in [-2^7, 2^7-1]

1 byte two’s complement


Integer in [-2^15, 2^15-1]

2-byte little endian two’s complement


Integer in [-2^31, 2^31-1]

4-byte little endian two’s complement


Integer in [-2^63, 2^63-1]

8-byte little endian two’s complement


Integer in [0, 2^8-1]

1 byte


Integer in [0, 2^16-1]

2-byte little endian


Integer in [0, 2^32-1]

4-byte little endian


Integer in [0, 2^64-1]

8-byte little endian

float16 (optionally supported)

IEEE 754 half-precision floating point: sign bit, 5 bits exponent, 10 bits mantissa

2-byte little endian IEEE 754 binary16


IEEE 754 single-precision floating point: sign bit, 8 bits exponent, 23 bits mantissa

4-byte little endian IEEE 754 binary32


IEEE 754 double-precision floating point: sign bit, 11 bits exponent, 52 bits mantissa

8-byte little endian IEEE 754 binary64


real and complex components are each IEEE 754 single-precision floating point

2 consecutive 4-byte little endian IEEE 754 binary32 values


real and complex components are each IEEE 754 double-precision floating point

2 consecutive 8-byte little endian IEEE 754 binary64 values

r* (Optional)

raw bits, use for extension type fallbacks

variable, given by *, is limited to be a multiple of 8.

Additionally to these base types, an implementation should also handle the raw/opaque pass-through type designated by the lower-case letter r followed by the number of bits, multiple of 8. For example, r8, r16, and r24 should be understood as fall-back types of respectively 1, 2, and 3 byte length.

Zarr v3 is limited to type sizes that are a multiple of 8 bits but may support other type sizes in later versions of this specification.


While the default binary representation is little endian, the endian codec may be specified to use big endian encoding instead.


We are explicitly looking for more feedback and prototypes of code using the r*, raw bits, for various endianness and whether the spec could be made clearer.


Currently only fixed size elements are supported as a core data type. There are many request for variable length element encoding. There are many ways to encode variable length and we want to keep flexibility. While we seem to agree that for random access the most likely contender is to have two arrays, one with the actual variable length data and one with fixed size (pointer + length) to the variable size data, we do not want to commit to such a structure. See zarr-developers/zarr-specs#62.

Chunk grids#

A chunk grid defines a set of chunks which contain the elements of an array. The chunks of a grid form a tessellation of the array space, which is a space defined by the dimensionality and shape of the array. This means that every element of the array is a member of one chunk, and there are no gaps or overlaps between chunks.

In general there are different possible types of grids. The core specification defines the regular grid type, where all chunks are hyperrectangles of the same shape. Extensions may define other grid types, such as rectilinear grids where chunks are still hyperrectangles but do not all share the same shape.

A grid type must also define rules for constructing an identifier for each chunk that is unique within the grid, which is a string of ASCII characters that can be used to construct keys to save and retrieve chunk data in a store, see also the Storage section.

Regular grids#

A regular grid is a type of grid where an array is divided into chunks such that each chunk is a hyperrectangle of the same shape. The dimensionality of the grid is the same as the dimensionality of the array. Each chunk in the grid can be addressed by a tuple of positive integers (k, j, i, …) corresponding to the indices of the chunk along each dimension.

The origin element of a chunk has coordinates in the array space (k * dz, j * dy, i * dx, …) where (dz, dy, dx, …) are the chunk sizes along each dimension. Thus the origin element of the chunk at grid index (0, 0, 0, …) is at coordinate (0, 0, 0, …) in the array space, i.e., the grid is aligned with the origin of the array. If the length of any array dimension is not perfectly divisible by the chunk length along the same dimension, then the grid will overhang the edge of the array space.

The shape of the chunk grid will be (ceil(z / dz), ceil(y / dy), ceil(x / dx), …) where (z, y, x, …) is the array shape, “/” is the division operator and “ceil” is the ceiling function. For example, if a 3 dimensional array has shape (10, 200, 3000), and has chunk shape (5, 20, 400), then the shape of the chunk grid will be (2, 10, 8), meaning that there will be 2 chunks along the first dimension, 10 along the second dimension, and 8 along the third dimension.

Regular Grid Example#

Array Shape

Chunk Shape

Chunk Grid Shape


(10, 200, 3000)

(5, 20, 400)

(2, 10, 8)

The grid does overhang the edge of the array on the 3rd dimension.

An element of an array with coordinates (c, b, a, …) will occur within the chunk at grid index (c // dz, b // dy, a // dx, …), where “//” is the floor division operator. The element will have coordinates (c % dz, b % dy, a % dx, …) within that chunk, where “%” is the modulo operator. For example, if a 3 dimensional array has shape (10, 200, 3000), and has chunk shape (5, 20, 400), then the element of the array with coordinates (7, 150, 900) is contained within the chunk at grid index (1, 7, 2) and has coordinates (2, 10, 100) within that chunk.

The store key corresponding to a given grid cell is determined based on the chunk_key_encoding member of the Array metadata.

Note that this specification does not consider the case where the chunk grid and the array space are not aligned at the origin vertices of the array and the chunk at grid index (0, 0, 0, …). However, extensions may define variations on the regular grid type such that the grid indices may include negative integers, and the origin element of the array may occur at an arbitrary position within any chunk, which is required to allow arrays to be extended by an arbitrary length in a “negative” direction along any dimension.


Chunks at the border of an array always have the full chunk size, even when the array only covers parts of it. For example, having an array with "shape": [30, 30] and "chunk_shape": [16, 16], the chunk 0,1 would also contain unused values for the indices 0-16, 30-31. When writing such chunks it is recommended to use the current fill value for elements outside the bounds of the array.

Chunk encoding#

Chunks are encoded into a binary representation for storage in a store, using the chain of codecs specified by the codecs metadata field.

Determination of encoded representations#

To encode or decode a chunk, the encoded and decoded representations for each codec in the chain must first be determined as follows:

  1. The initial decoded representation, decoded_representation[0] is a multi-dimensional array with the same data type as the zarr array, and shape equal to the chunk shape.

  2. For each codec i, the encoded representation is equal to the decoded representation decoded_representation[i+1] of the next codec, and is computed from codecs[i].compute_encoded_representation_type(decoded_representation[i]). If compute_encoded_representation_type fails because of an incompatible decoded representation, an implementation should indicate an error.

Conversion between multi-dimensional array and byte string representations#

Some codecs operate directly on multi-dimensional arrays of elements, e.g. encoding a 3-d array as a multi-channel jpeg image. Other codecs operate at the byte level, e.g. gzip compression. If a codec that operates at the byte level receives as input an array that is not a 1-dimensional uint8 array, it may convert the input array to a byte string by concatenating the default binary representations of each element in lexicographical order (C order). Similarly, if a codec that expects a multi-dimensional array as input instead receives a byte string, it may decode each element in lexicographical order according to the default binary representation of each element.


To encode elements in a different order than the default lexicographical order (C order/row major), the transpose codec may be specified.

Encoding procedure#

Based on the computed decoded_representations list, a chunk is encoded using the following procedure:

  1. The initial encoded chunk EC[0] of the type specified by decoded_representation[0] is equal to the chunk array A (with a shape equal to the chunk shape, and data type equal to the zarr array data type).

  2. For each codec codecs[i] in codecs, EC[i+1] := codecs[i].encode(EC[i]).

  3. The final encoded chunk representation EC_final is always a byte string. If EC[codecs.length] is a byte string, then EC_final := EC[codecs.length]. Otherwise, EC_final is converted from EC[codecs.length].

  4. EC_final is written to the store.

Decoding procedure#

Based on the computed decoded_representations list, a chunk is encoded using the following procedure:

  1. The encoded chunk representation EC_final is read from the store.

  2. If codecs[codecs.length] is a byte string, EC[codecs.length] := EC_final. Otherwise, EC[codecs.length] is converted from EC_final.

  3. For each codec codecs[i] in codecs, iterating in reverse order, EC[i] := codecs[i].decode(EC[i+1], decoded_representation[i]).

  4. The chunk array A is equal to EC[0].

Specifying codecs#

To allow for flexibility to define and implement new codecs, this specification does not define any codecs, nor restrict the set of codecs that may be used. Each codec must be defined via a separate specification. In order to refer to codecs in array metadata documents, each codec must have a unique identifier, which is a URI that dereferences to a human-readable specification of the codec. A codec specification must declare the codec identifier, and describe (or cite documents that describe) the encoding and decoding algorithms and the format of the encoded data.

A codec may have configuration parameters which modify the behaviour of the codec in some way. For example, a compression codec may have a compression level parameter, which is an integer that affects the resulting compression ratio of the data. Configuration parameters must be declared in the codec specification, including a definition of how configuration parameters are represented as JSON.

The Zarr core development team maintains a repository of codec specifications, which are hosted alongside this specification in the zarr-specs GitHub repository, and which are published on the zarr-specs documentation Web site. For ease of discovery, it is recommended that codec specifications are contributed to the zarr-specs GitHub repository. However, codec specifications may be maintained by any group or organisation and published in any location on the Web. For further details of the process for contributing a codec specification to the zarr-specs GitHub repository, see the Zarr community process specification.

Further details of how codecs are configured for an array are given in the section below on Array metadata.


This section defines the structure of metadata documents for Zarr hierarchies, which consists of two types of metadata documents: array metadata documents, and group metadata documents. Both types of metadata documents are stored under the key zarr.json within the prefix of the array or group. Each type of metadata document is described in the following subsections.

Metadata documents are defined here using the JSON type system defined in [RFC8259]. In this section, the terms “value”, “number”, “string” and “object” are used to denote the types as defined in [RFC8259]. The term “array” is also used as defined in [RFC8259], except where qualified as “Zarr array”. Following [RFC8259], this section also describes an object as a set of name/value pairs. This section also defines how metadata documents are encoded for storage.

Array metadata#

Each Zarr array in a hierarchy must have an array metadata document, named zarr.json. This document must contain a single object with the following mandatory names:


An integer defining the version of the storage specification to which the array store adheres, must be 3 here.


A string defining the type of hierarchy node element, must be array here.


An array of integers providing the length of each dimension of the Zarr array. For example, a value [10, 20] indicates a two-dimensional Zarr array, where the first dimension has length 10 and the second dimension has length 20.


The data type of the Zarr array. If the data type is defined in this specification, then the value must be the data type identifier provided as a string. For example, "float64" for little-endian 64-bit floating point number.

The data_type value is an extension point and may be defined by a data type extension. If the data type is defined by an extension, then the value must be an object containing the names name and optionally configuration and fallback. The name is required and its value must refer to a v3 data type specification. configuration is optional and its value is defined by the extension. The fallback is optional and, if provided, its value must be one or a list of the data type identifiers defined in this specification or an extension. Fallback extension datatypes are specified as an object with name and (optionally) configuration.

If an implementation does not recognise the extension or specific data type, but a fallback is present, then the implementation may proceed using the first known fallback value as the data type. For fixed-sized data types, if there is no more specific fallback available, a raw number of bytes using the raw type (r*) should be given.

The default list of fallbacks to put into the metadata should by defined by the data type extension, but it may be overridden by the user. Note for implementations: Silently using a fallback without explicit approval might cause problems for users, please consider options to (de-)activate fallback behavior and/or appropriate warnings.


The chunk grid of the Zarr array. If the chunk grid is a regular chunk grid as defined in this specification, then the value must be an object with the names name and configuration. The value of name must be the string "regular", and the value of configuration an object with the member chunk_shape. chunk_shape must be an array of integers providing the lengths of the chunk along each dimension of the array. For example, {"type": "regular", "configuration": {"chunk_shape": [2, 5]}} means a regular grid where the chunks have length 2 along the first dimension and length 5 along the second dimension.

The chunk_grid value is an extension point and may be defined by an extension. If the chunk grid type is defined by an extension, then name must be a string referring to a v3 chunk grid specification. The configuration is optional and defined by the extension.


The mapping from chunk grid cell coordinates to keys in the underlying store.

The value must be an object with required string member name, specifying the encoding type, and optional member configuration specifying encoding type-dependent parameters; the configuration value must be an object if it is specified.

The following encodings are defined:

  • default

    The configuration object may contain one optional member, separator, which must be either "/" or ".". If not specified, separator defaults to "/".

    The key for a chunk with grid index (k, j, i, …) is formed by taking the initial prefix c, and appending for each dimension:

    • the separator character, followed by,

    • the ASCII decimal string representation of the chunk index within that dimension.

    For example, in a 3 dimensional array, with a separator of / the identifier for the chunk at grid index (1, 23, 45) is the string "c/1/23/45". With a separator of ., the identifier is the string "c.1.23.45".


    A main difference with spec v2 is that the default chunk separator changed from . to /, as in N5. This decreases the maximum number of items in hierarchical stores like directory stores.


    Arrays may have 0 dimensions (when for example representing scalars), in which case the coordinate of a chunk is the empty tuple, and the chunk key will consist of the string c.

  • v2

    The configuration object may contain one optional member, separator, which must be either "/" or ".". If not specified, separator defaults to ".".

    The identifier for chunk with at least one dimension is formed by concatenating for each dimension:

    • the ASCII decimal string representation of the chunk index within that dimension, followed by

    • the separator character, except that it is omitted for the last dimension.

    For example, in a 3 dimensional array, with a separator of . the identifier for the chunk at grid index (1, 23, 45) is the string "1.23.45". With a separator of /, the identifier is the string "1/23/45".

    For chunk grids with 0 dimensions, the single chunk has the key "0".


    This encoding is intended only to allow existing v2 arrays to be converted to v3 without having to rename chunks. It is not recommended to be used when writing new arrays.


Provides an element value to use for uninitialised portions of the Zarr array.

If the data type of the Zarr array is Boolean then the value must be the literal false or true. If the data type is one of the integer data types defined in this specification, then the value must be a number with no fraction or exponent part and must be within the range of the data type.

For any data type, the fill_value is required. The literal null is not permitted. The fill value needs to be defined so that the data is independent of implementation details. Internally implementations may provide a default fill_value, but that must be converted to a fixed value in the stored metadata.

If the data_type of an array is defined in a data_type extension, then said extension is responsible for interpreting the value of fill_value and return a suitable type that can be used.

For core data types for which fill values are not permitted in JSON or for which decimal representation could be lossy, a string representing of the binary (starting with 0b) or hexadecimal value (starting with 0x) is accepted. This string must include all leading or trailing zeroes necessary to match the given type size. The string values "NaN", "+Infinity" and "-Infinity" are also understood for floating point data types.

The following members are optional:


The value must be an object. The object may contain any key/value pairs, where the key must be a string and the value can be an arbitrary JSON literal. Intended to allow storage of arbitrary user metadata.


An extension to store user attributes in a separate document is being discussed in zarr-developers/zarr-specs#72.


A proposal to specify metadata conventions (ZEP 4) is being discussed in zarr-developers/zeps#28.


Specifies a list of codecs to be used for encoding and decoding chunks. The value must be an array of objects, each object containing a member with name whose value is a string referring to a v3 codec specification. The codec object may also contain a configuration object which consists of the parameter names and values as defined by the corresponding codec specification. An absent codecs member is equivalent to specifying an empty list of codecs.

storage_transformers (array)#

Specifies a stack of storage transformers. Each value in the list must be an object containing the names name and optionally configuration. The name is required and the value must be a string referring to the extension. The object may also contain a configuration object which consists of the parameter names and values as defined by the corresponding storage transformer specification. When the storage_transformers name is absent no storage transformer is used, same for an empty list.


Specifies dimension names, e.g. ["x", "y", "z"]. If specified, must be an array of strings or null objects with the same length as shape. An unnamed dimension is indicated by the null object. If dimension_names is not specified, all dimensions are unnamed.

For compatibility with zarr implementations and applications that support using dimension names to uniquely identify dimensions, it is recommended but not required that all non-null dimension names are distinct (no two dimensions have the same non-empty name).

This specification also does not place any restrictions on the use of the same dimension name across multiple arrays within the same zarr hierarchy, but extensions or specific applications may do so.

The array metadata object must not contain any other names. Those are reserved for future versions of this specification. An implementation must fail to open zarr hierarchies, groups or arrays with unknown metadata fields, with the exception of objects with a "must_understand": false key-value pair.

For example, the array metadata JSON document below defines a two-dimensional array of 64-bit little-endian floating point numbers, with 10000 rows and 1000 columns, divided into a regular chunk grid where each chunk has 1000 rows and 100 columns, and thus there will be 100 chunks in total arranged into a 10 by 10 grid. Within each chunk the binary values are laid out in C contiguous order. Each chunk is compressed using gzip compression prior to storage:

    "zarr_format": 3,
    "node_type": "array",
    "shape": [10000, 1000],
    "dimension_names": ["rows", "columns"],
    "data_type": "float64",
    "chunk_grid": {
        "name": "regular",
        "configuration": {
            "chunk_shape": [1000, 100]
    "chunk_key_encoding": {
        "name": "default",
        "configuration": {
            "separator": "/"
    "codecs": [{
        "type": "gzip",
        "configuration": {
            "level": 1
    "fill_value": "NaN",
    "extensions": [],
    "attributes": {
        "foo": 42,
        "bar": "apples",
        "baz": [1, 2, 3, 4]

The following example illustrates an array with the same shape and chunking as above, but using a (currently made up) extension data type:

    "zarr_format": 3,
    "node_type": "array",
    "shape": [10000, 1000],
    "data_type": {
        "name": "datetime",
        "configuration": {
            "unit": "ns"
        "fallback": "int64"
    "chunk_grid": {
        "name": "regular",
        "configuration": {
            "chunk_shape": [1000, 100]
    "chunk_key_encoding": {
        "name": "default",
        "configuration": {
            "separator": "/"
    "codecs": [{
        "name": "gzip",
        "configuration": {
            "level": 1
    "fill_value": null,


Comparison with zarr spec v2:

  • dtype has been renamed to data_type,

  • chunks has been replaced with chunk_grid,

  • dimension_separator has been replaced with chunk_key_encoding,

  • order has been replaced by the transpose codec,

  • the separate filters and compressor fields been combined into the single codecs field.

Group metadata#

A Zarr group metadata object must contain the following mandatory key:


An integer defining the version of the storage specification to which the array store adheres, must be 3 here.


A string defining the type of hierarchy node element, must be group here.

Optional keys:


The value must be an object. The object may contain any key/value pairs, where the key must be a string and the value can be an arbitrary JSON literal. Intended to allow storage of arbitrary user metadata.

For example, the JSON document below defines an explicit group:

    "zarr_format": 3,
    "node_type": "group",
    "attributes": {
        "spam": "ham",
        "eggs": 42,

The group metadata object must not contain any other names. Those are reserved for future versions of this specification. An implementation must fail to open zarr hierarchies or groups with unknown metadata fields, with the exception of objects with a "must_understand": false key-value pair.


A group does not need a metadata document to exist. (See implicit groups.)


A Zarr store is a system that can be used to store and retrieve data from a Zarr hierarchy. For a store to be compatible with this specification, it must support a set of operations defined in the Abstract store interface subsection. The store interface can be implemented using a variety of underlying storage technologies, described in the subsection on Store implementations.

Additionally, a store should specify a canonical URI format that can be used to identify nodes in this store. Implementations should use the specified formats when opening a Zarr hierarchy to automatically determine the appropriate store.

Abstract store interface#

The store interface is intended to be simple to implement using a variety of different underlying storage technologies. It is defined in a general way here, but it should be straightforward to translate into a software interface in any given programming language. The goal is that an implementation of this specification could be modular and allow for different store implementations to be used.

The store interface defines a set of operations involving keys and values. In the context of this interface, a key is any string containing only characters in the ranges a-z, A-Z, 0-9, or in the set /.-_, where the final character is not a / character. In general, a value is a sequence of bytes. Specific stores may choose more specific storage formats, which must be stated in the specification of the respective store. E.g. a database store might encode values of *.json keys with a database-native json type.

It is assumed that the store holds (key, value) pairs, with only one such pair for any given key. I.e., a store is a mapping from keys to values. It is also assumed that keys are case sensitive, i.e., the keys “foo” and “FOO” are different.

To read and write partial values, a range specifies two integers range_start and range_length, that specify a part of the value starting at byte range_start (inclusive) and having a length of range_length bytes. range_length may be none, indicating all available data until the end of the referenced value. For example range [0, none] specifies the full value. Stores that do not support partial access can still fulfill partial requests by first extracting the full value and then returning a subset of bytes.

The store interface also defines some operations involving prefixes. In the context of this interface, a prefix is a string containing only characters that are valid for use in keys and ending with a trailing / character.

The store operations are grouped into three sets of capabilities: readable, writeable and listable. It is not necessary for a store implementation to support all of these capabilities.

A readable store supports the following operations:

get - Retrieve the value associated with a given key.

Parameters: key
Output: value

get_partial_values - Retrieve possibly partial values from given key_ranges.

Parameters: key_ranges: ordered set of key, range pairs,
a key may occur multiple times with different ranges
Output: list of values, in the order of the key_ranges, may contain none
for missing keys

A writeable store supports the following operations:

set - Store a (key, value) pair.

Parameters: key, value
Output: none

set_partial_values - Store values at a given key, starting at byte range_start.

Parameters: key_start_values: set of key,
range_start, value triples, a key may occur multiple
times with different range_starts, range_starts with
length of the respective value must not specify overlapping
ranges for the same key
Output: none

erase - Erase the given key/value pair from the store.

Parameters: key
Output: none

erase_values - Erase the given key/value pairs from the store.

Parameters: keys: set of keys
Output: none

erase_prefix - Erase all keys with the given prefix from the store:

Parameter: prefix
Output: none


Some KV stores do allow creation and update of keys, but not deletion. For example, Zip archives do not allow removal of content without recreating the full archive.

Inability to delete can affect ability to rename keys as well, as a rename is often a sequence or atomic combination of a deletion and a creation.

A listable store supports any one or more of the following operations:

list - Retrieve all keys in the store.

Parameters: none
Output: set of keys

list_prefix - Retrieve all keys with a given prefix.

Parameters: prefix
Output: set of keys with the given prefix,

For example, if a store contains the keys “a/b”, “a/c/d” and “e/f/g”, then list_prefix("a/") would return “a/b” and “a/c/d”.

Note: the behavior of list_prefix is undefined if prefix does not end with a trailing slash / and the store can assume there is at least one key that starts with prefix.

list_dir - Retrieve all keys and prefixes with a given prefix and which do not contain the character “/” after the given prefix.

Parameters: prefix
Output: set of keys and set of prefixes

For example, if a store contains the keys “a/b”, “a/c”, “a/d/e”, “a/f/g”, then list_dir("a/") would return keys “a/b” and “a/c” and prefixes “a/d/” and “a/f/”. list_dir("b/") would return the empty set.

Note that because keys are case sensitive, it is assumed that the operations set("foo", a) and set("FOO", b) will result in two separate (key, value) pairs being stored. Subsequently get("foo") will return a and get("FOO") will return b.

It is recommended that the implementation of the get_partial_values, set_partial_values and erase_values methods is made optional, providing fallbacks for them by default. However, it is recommended to supply those operations where possible for efficiency. Also, the get, set and erase can easily be mapped onto their partial_values counterparts. Therefore, it is also recommended to supply fallbacks for those if the partial_values operations can be implemented. An entity containing those fallbacks could be named StoreWithPartialAccess.

Store implementations#

(This subsection is not normative.)

A store implementation maps the abstract operations of the store interface onto concrete operations on some underlying storage system. This specification does not constrain or make any assumptions about the nature of the underlying storage system. Thus it is possible to implement the store interface in a variety of different ways.

For example, a store implementation might use a conventional file system as the underlying storage system, mapping keys onto file paths and values onto file contents. The get operation could then be implemented by reading a file, the set operation implemented by writing a file, and the list_dir operation implemented by listing a directory.

For example, a store implementation might use a key-value database such as BerkeleyDB or LMDB as the underlying storage system. In this case the implementation of get and set operations would be whatever native operations are provided by the database for getting and setting key/value pairs. Such a store implementation might natively support the list operation but might not support list_prefix or list_dir, although these could be implemented via list with post-processing of the returned keys.

For example, a store implementation might use a cloud object storage service such as Amazon S3, Azure Blob Storage, or Google Cloud Storage as the underlying storage system, mapping keys to object names and values to object contents. The store interface operations would then be implemented via concrete operations of the service’s REST API, i.e., via HTTP requests. E.g., the get operation could be implemented via an HTTP GET request to an object URL, the set operation could be implemented via an HTTP PUT request to an object URL, and the list operations could be implemented via an HTTP GET request to a bucket URL (i.e., listing a bucket).

The examples above are meant to be illustrative only, and other implementations are possible. This specification does not attempt to standardise any store implementations, however where a store implementation is expected to be widely used then it is recommended to create a store implementation spec and contribute it to the zarr-specs GitHub repository. For an example of a store implementation spec, see the File system store (version 1.0) specification.


This section describes how to translate high level operations to create, erase or modify Zarr hierarchies, groups or arrays, into low level operations on the key/value store interface defined above.

In this section a “hierarchy path” is a logical path which identifies a group or array node within a Zarr hierarchy, and a “storage key” is a key used to store and retrieve data via the store interface. There is a further distinction between “metadata keys” which are storage keys used to store metadata documents, and “chunk keys” which are storage keys used to store encoded chunks.

Note that any non-root hierarchy path will have ancestor paths that identify ancestor nodes in the hierarchy. For example, the path “/foo/bar” has ancestor paths “/foo” and “/”.

Storage keys#

The array or group metadata document for the root of a Zarr hierarchy is stored under the key zarr.json.

The metadata document of a non-root array or group with hierarchy path P is obtained by stripping the leading / of the path and appending /zarr.json. For example, the metadata document of an array or group with path /foo/bar is foo/bar/zarr.json.

All chunk or other data of an array is stored under the key prefix determined by its path. For a root array, the key prefix is obtained from the metadata document key by stripping the trailing “zarr.json”. For example, for a root array, the prefix is the empty string. For a non-root array with hierarchy path /foo/bar, the prefix is foo/bar/.

Metadata Storage Key example#


Path “P”

Key for Metadata at path P

Array (Root)



Group (Root)















Data Storage Key example#

Path P of array

Chunk grid indices

Data key


(1, 0)


It is recommended that the root of a Zarr fileset ends with .zarr to indicate the start of a hierarchy to users.


The following section describes possible operations of an implementation as a non-normative guide-line.

Let P be an arbitrary hierarchy path.

Let meta_key(P) be the metadata key for P, P/zarr.json.

Let data_key(P, j, i ...) be the data key for P for the chunk with grid coordinates (j, i, …).

Let “+” be the string concatenation operator.

Create a group

To create an explicit group at hierarchy path P, perform set(meta_key(P), value), where value is the serialization of a valid group metadata document.

Creating a group at path P implies the existence of groups at all ancestor paths of P.

Create an array

To create an array at hierarchy path P, perform set(meta_key(P), value), where value is the serialisation of a valid array metadata document.

Creating an array at path P implies the existence of groups at all ancestor paths of P.

Store chunk data in an array

To store chunk data in an array at path P and chunk coordinate (j, i, …), perform set(data_key(P, j, i, ...), value), where value is the serialisation of the corresponding chunk, encoded according to the information in the array metadata stored under the key meta_key(P).

Retrieve chunk data in an array

To retrieve chunk data in an array at path P and chunk coordinate (i, j, …), perform get(data_key(P, j, i, ...), value). The returned value is the serialisation of the corresponding chunk, encoded according to the array metadata stored at meta_key(P).

Discover children of a group

To discover the children of a group at hierarchy path P, perform list_dir(P + "/"). Any returned prefix Q not starting with __ indicates a child array or group. To determine whether the child is an array or group, the document meta_key(Q) must be checked.

For example, if a group is created at path “/foo/bar” and an array is created at path “/foo/baz/qux”, then the store will contain the keys “foo/bar/zarr.json” and “foo/baz/qux/zarr.json”. Groups at paths “/”, “/foo” and “/foo/baz” have not been explicitly created but are implied by their descendants. To list the children of the group at path “/foo”, perform list_dir("/foo/"), which will return the prefixes “foo/bar” and “foo/baz”. From this it can be inferred that child groups or arrays “/foo/bar” and “/foo/baz” are present.

If a store does not support any of the list operations then discovery of group children is not possible, and the contents of the hierarchy must be communicated by some other means, such as via an extension (see zarr-developers/zarr-specs#15) or via some out of band communication.

Discover all nodes in a hierarchy

To discover all nodes in a hierarchy, one should discover the children of the root of the hierarchy and then recursively list children of child groups.

For hierarchies without group storage transformers one may also call list_prefix("/"). All zarr.json keys represent either explicit groups or arrays. All intermediate prefixes ending in a / are implicit groups.

Erase a group or array

To erase an array at path P, erase the metadata document and array data for the array, erase_prefix(P + "/").

To erase an explicit or implicit group at path P: erase all nodes under this group and its metadata document - it should be sufficient to perform erase_prefix(P + "/")

Determine if a node exists

To determine if a node exists at path P, try in the following order

  • get(meta_key(P)) (success implies an array or explicit group at P);

  • list_dir(P + "/") (non-empty result set implies an implicit group at P).


For listable stores, list_dir(parent(P)) can be an alternative.

Storage transformers#

A Zarr storage transformer allows to change the zarr-compatible data before storing it. The stored transformed data is restored to its original state whenever data is requested by the Array. Storage transformers can be configured per array via the storage_transformers name in the array metadata. Storage transformers which do not change the storage layout (e.g. for caching) may be specified at runtime without adding them to the array metadata.


It is planned to add storage transformers also to groups in a later revision of this spec, see zarr-developers/zarr-specs#215.

A storage transformer serves the same abstract store interface as the store. However, it should not persistently store any information necessary to restore the original data, but instead propagates this to the next storage transformer or the final store. From the perspective of an array or a previous stage transformer both store and storage transformer follow the same protocol and can be interchanged regarding the protocol. The behaviour can still be different, e.g. requests may be cached or the form of the underlying data can change.

Storage transformers may be stacked to combine different functionalities:

graph LR Array --> t1 subgraph stack [Storage transformers] t1[Transformer 1] --> t2[...] --> t3[Transformer N] end t3 --> Store

A fixed set of storage providers is recommended for implementation with this specification:

Predefined storage transformers#

Extension points#

Different types of extensions can exist and they can be grouped as follows:





data type



chunk grid



chunk key encoding






storage transformer

storage_transformers (array)

If such extension points are used by groups or arrays, they are required, except for data types specifying fallback data types.

See zarr-developers/zarr-specs#49 for a list of potential extensions.

Specifications for new extensions are recommended to be published in the zarr-developers/zarr-specs repository via the ZEP process. If a specification is published decentralized (e.g. for initial experimentation or due to a very specialized scope), it must use a URL in the name key of its metadata, which identifies the publishing organization or individual, and should point to the specification of the extension.

Future versions of this specification may also add new core features by adding new top-level metadata keys. Such features are required by default. However, if the value of an unknown feature is an object containing the key-value pair "must_understand": false, it can be ignored.

Implementation Notes#

This section is non-normative and presents notes from implementers about cases that need to be carefully considered but do not strictly fall into the spec.

Explicit vs implicit group#

While this zarr spec v3 defines implicit and explicit groups, implementations may decide to create an explicit group for all implicit groups they encounter; in particular when using a hierarchical storage.

Erasure of an implicit group may automatically erase any empty parent. For example on a S3 store where the namespace is flat, erasure of the last key with a prefix will erase all the implicit group in the prefix.

Care must thus be taken when erasing an array or a group if the parent needs to be converted into an explicit group.

A race-condition arises if a client writes an array at path P, and another concurrently assumes P is an implicit group and writes subgroups or arrays into it. Implementations may choose to never use implicit groups to avoid this.


In general, arrays can be resized for writable (and if necessary deletable) stores. In the most basic case, two scenarios can be considered, shrinking along an array dimension, or increasing its size.

When shrinking, implementations can consider to delete chunks if the store allows this, or keep them. This should either be configurable, or be communicated to the user appropriately.

When increasing an array along a dimension, chunks may or may not have existed in the new area. For areas where no chunks existed previously, they implicitly have the fill value after updating the metadata, no new chunks need to be written in this case. Previous partial chunks will contain the fill value at the time of writing them by default. If there was chunk data in the new area which was not deleted when shrinking the array, this data will be shown by default. The latter case should be signalled to the user appropriately. An implementation can also allow the user to choose to delete previous data explicitly when increasing the array (by writing the fill value into partial chunks and deleting others), but this should not be the default behavior.

Comparison with Zarr v2#

This section is informative.

Below is a summary of the key differences between this specification (v3) and Zarr v2.

  • v3 has explicit support for extensions via defined extension points and mechanisms.

  • v3 allows for greater flexibility in how groups and arrays are created. In particular, v3 supports implicit groups, which are groups that do not have a metadata document but whose existence is implied by descendant nodes. This change enables multiple arrays to be created in parallel without generating race conditions for the metadata when creating parent groups.

  • The set of data types specified in v3 is less than in v2. Additional data types will be defined via extensions.



T. Bray, Ed. The JavaScript Object Notation (JSON) Data Interchange Format. December 2017. Best Current Practice. URL:


S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL:

Change log#

All notable and possibly implementation-affecting changes to this specification are documented in this section, grouped by the specification status and ordered by time.

Draft Changes#

  • Removed extensions field and clarified extension point behavior, changing the config format of data-types, chunk-grid, storage-transformers and codecs. PR #204

  • Changed format_version to the int 3, added key node_type to group and array metadata. PR #204

  • Restructured keys and removed entry-point metadata. PR #200

  • Added the dimension_names array metadata field. PR #162

  • Replaced chunk_memory_layout with transpose codec. PR #189

  • Allowed to have a list of fallback data types. PR #167

  • Removed the 255 character limit for paths. PR #175

  • Removed the /root prefix for paths. PR #175

    • meta/root.array.json is now meta/array.json

    • meta/root/foo/ is now meta/foo/

  • Moved the metadata_key_suffix entrypoint metadata key into metadata_encoding, which now just specifies “json” via the type key and is an extension point. PR #171

  • Changed data type names and changed endianness to be handled by a codec. PR #155

  • Replaced the compressor field in the array metadata with a codecs field that can specify a list of codecs. PR #153

  • Required fill_value in the array metadata to be defined. PR #145

  • Added array storage transformers which can be configured per array via the storage_transformers name in the array metadata. PR #134

  • The changelog is incomplete before 2022, please refer to the commits on GitHub.


Links: view spec; view source

@@TODO summary of changes since previous tag.