Low-level file layout

The overall structure of a file is as follows (in order):

ASDF is a hybrid text and binary format. The header, tree and block index are text, (specifically, in UTF-8 with DOS or UNIX-style newlines), while the blocks are raw binary.

The low-level file layout is designed in such a way that the tree section can be edited by hand, possibly changing its size, without requiring changes in other parts of the file. While such an operation may invalidate the Block index, the format is designed so that if the block index is removed or invalid, it may be regenerated by “skipping along” the blocks in the file.

The same is not true for resizing a block, which has an explicit size stored in the block header (except for, optionally, the last block).

Note also that, by design, an ASDF file containing no binary blocks is also a completely standard and valid YAML file.

Additionally, the spec allows for extra unallocated space after the tree and between blocks. This allows libraries to more easily update the files in place, since it allows expansion of certain areas without rewriting of the entire file.


Additional comment lines may appear between the Header and the Tree.

The use of comments here is intended for information for the ASDF parser, and not information of general interest to the end user. All data of interest to the end user should be in the Tree.

Each line must begin with a # character.


The tree stores structured information using a subset of YAML Ain’t Markup Language (YAML™) 1.1 syntax (see YAML subset for details on YAML features that are excluded from ASDF). While it is the main part of most ASDF files, it is entirely optional, and a ASDF file may skip it completely. This is useful for creating files in Exploded form. Interpreting the contents of this section is described in greater detail in The tree in-depth. This section only deals with the serialized representation of the tree, not its logical contents.

The tree is always encoded in UTF-8, without an explicit byteorder marker (BOM). Newlines in the tree may be either DOS ("\r\n") or UNIX ("\n") format.

In ASDF 1.6.0 , the tree must be encoded in YAML version 1.1. At the time of this writing, the latest version of the YAML specification is 1.2, however most YAML parsers only support YAML 1.1, and the benefits of YAML 1.2 are minor. Therefore, for maximum portability, ASDF requires that the YAML is encoded in YAML 1.1. To declare that YAML 1.1 is being used, the tree must begin with the following line:

%YAML 1.1

The tree must contain exactly one YAML document, starting with --- (YAML document start marker) and ending with ... (YAML document end marker), each on their own line. Between these two markers is the YAML content. For example:

%YAML 1.1
%TAG ! tag:stsci.edu:asdf/
--- !core/asdf-1.0.0
data: !core/ndarray-1.0.0
  source: 0
  datatype: float64
  shape: [1024, 1024]

The size of the tree is not explicitly specified in the file, so that it can easily be edited by hand. Therefore, ASDF parsers must search for the end of the tree by looking for the end-of-document marker (...) on its own line. For example, the following regular expression may be used to find the end of the tree:


Though not required, the tree should be followed by some unused space to allow for the tree to be updated and increased in size without performing an insertion operation in the file. It also may be desirable to align the start of the first block to a filesystem block boundary. This empty space may be filled with any content (as long as it doesn’t contain the block_magic_token described in Blocks). It is recommended that the content is made up of space characters (0x20) so it appears as empty space when viewing the file.


Following the tree and some empty space, or immediately following the header, there are zero or more binary blocks.

Blocks represent a contiguous chunk of binary data and nothing more. Information about how to interpret the block, such as the data type or array shape, is stored entirely in ndarray structures in the tree, as described in ndarray. This allows for a very flexible type system on top of a very simple approach to memory management within the file. It also allows for new extensions to ASDF that might interpret the raw binary data in ways that are yet to be defined.

There may be an arbitrary amount of unused space between the end of the tree and the first block. To find the beginning of the first block, ASDF parsers should search from the end of the tree for the first occurrence of the block_magic_token. If the file contains no tree, the first block must begin immediately after the header with no padding.

Block header

Each block begins with the following header:

  • block_magic_token (4 bytes): Indicates the start of the block. This allows the file to contain some unused space in which to grow the tree, and to perform consistency checks when jumping from one block to the next. It is made up of the following 4 8-bit characters:

    • in hexadecimal: d3, 42, 4c, 4b

    • in ascii: "\323BLK"

  • header_size (16-bit unsigned integer, big-endian): Indicates the size of the remainder of the header (not including the length of the header_size entry itself or the block_magic_token), in bytes. It is stored explicitly in the header itself so that the header may be enlarged in a future version of the ASDF standard while retaining backward compatibility. Importantly, ASDF parsers should not assume a fixed size of the header, but should obey the header_size defined in the file. In ASDF version 0.1, this should be at least 48, but may be larger, for example to align the beginning of the block content with a file system block boundary.

  • flags (32-bit unsigned integer, big-endian): A bit field containing flags (described below).

  • compression (4-byte byte string): The name of the compression algorithm, if any. Should be \0\0\0\0 to indicate no compression. See Compression for valid values.

  • allocated_size (64-bit unsigned integer, big-endian): The amount of space allocated for the block (not including the header), in bytes.

  • used_size (64-bit unsigned integer, big-endian): The amount of used space for the block on disk (not including the header), in bytes.

  • data_size (64-bit unsigned integer, big-endian): The size of the block when decoded, in bytes. If compression is all zeros (indicating no compression), it must be equal to used_size. If compression is being used, this is the size of the decoded block data.

  • checksum (16-byte string): An optional MD5 checksum of the used data in the block. The special value of all zeros indicates that no checksum verification should be performed.


The following bit flags are understood in the flags field:

  • STREAMED (0x1): When set, the block is in streaming mode, and it extends to the end of the file. When set, the allocated_size, used_size and data_size fields are ignored. By necessity, any block with the STREAMED bit set must be the last block in the file.


Currently, two block compression types are supported:

  • zlib: The zlib lossless compression algorithm. It is widely used, patent-unencumbered, and has an implementation released under a permissive license in zlib.

  • bzp2: The bzip2 lossless compression algorithm. It is widely used, assumed to be patent-unencumbered, and has an implementation released under a permissive license in the bzip2 library.

Block content

Immediately following the block header, there are exactly used_space bytes of meaningful data, followed by allocated_space - used_space bytes of unused data. The exact content of the unused data is not enforced. The ability to have gaps of unused space allows an ASDF writer to reduce the number of disk operations when updating the file.

Block index

The block index allows for fast random access to each of the blocks in the file. It is completely optional: if not present, libraries may “skip along” the block headers to find the location of each block in the file. Libraries should detect invalid or obsolete block indices and ignore them and regenerate the index by skipping along the block headers.

The block index appears at the end of the file to make streaming an ASDF file possible without needing to determine the size of all blocks up front, which is non-trivial in the case of compression. It also allows for updating the index without an expensive insertion operation earlier in the file.

The block index must appear immediately after the allocated space for the last block in the file. If the last block is a streaming block, no block index may be present – the streaming block feature and block index are incompatible.

If no blocks are present in the file, the block index must also be absent.

The block index consists of a header, followed by a YAML document containing the indices of each block in the file.

The header must be exactly:


followed by a DOS or UNIX newline.

Following the header is a YAML document (in YAML version 1.1, like the Tree), containing a list of integers indicating the byte offset of each block in the file.

The following is an example block index:

%YAML 1.1
--- [2043, 16340]

The offsets in the block index must be monotonically increasing, and must, by definition, be at least “block header size” apart. If they were allowed to appear in any order, it would be impossible to rebuild the index by skipping blocks were the index to become damaged or out-of-sync.

Additional zero-valued bytes may appear after the block index. This is mainly to support operating systems, such as Microsoft Windows, where truncating the file may not be easily possible.

Implementation recommendations

Libraries should look for the block index by reading backward from the end of the file.

Libraries should be conservative about what is an acceptable index, since addressing incorrect parts of the file could result in undefined behavior.

The following checks are recommended:

  • Always ensure that the first offset entry matches the location of the first block in the file. This will catch the common use case where the YAML tree was edited by hand without updating the index. If they do not match, do not use the entire block index.

  • Ensure that the last entry in the index refers to a block magic token, and that the end of the allocated space in the last block is immediately followed by the block index. If they do not match, do not use the entire block index.

  • When using an offset in the block index, always ensure that the block magic token exists at that offset before reading data.

Exploded form

Exploded form expands a self-contained ASDF file into multiple files:

  • An ASDF file containing only the header and tree, which by design is also a valid YAML file.

  • n ASDF files, each containing a single block.

Exploded form is useful in the following scenarios:

  • Not all text editors may handle the hybrid text and binary nature of the ASDF file, and therefore either can’t open an ASDF file or would break an ASDF file upon saving. In this scenario, a user may explode the ASDF file, edit the YAML portion as a pure YAML file, and implode the parts back together.

  • Over a network protocol, such as HTTP, a client may only need to access some of the blocks. While reading a subset of the file can be done using HTTP Range headers, not all web servers support this HTTP feature. Exploded form allows each block to be requested directly by a specific URI.

  • An ASDF writer may stream a table to disk, when the size of the table is not known at the outset. Using exploded form simplifies this, since a standalone file containing a single table can be iteratively appended to without worrying about any blocks that may follow it.

Exploded form describes a convention for storing ASDF file content in multiple files, but it does not require any additions to the file format itself. There is nothing indicating that an ASDF file is in exploded form, other than the fact that some or all of its blocks come from external files. The exact way in which a file is exploded is up to the library and tools implementing the standard. In the simplest scenario, to explode a file, each ndarray source property in the tree is converted from a local block reference into a relative URI.