format.rst

File Format

Dataset Directory

A Lance Dataset is organized in a directory.

/path/to/dataset:
    data/*.lance  -- Data directory
    latest.manifest -- The manifest file for the latest version.
    _versions/*.manifest -- Manifest file for each dataset version.
    _indices/{UUID-*}/index.idx -- Secondary index, each index per directory.
    _deletions/*.{arrow,bin} -- Deletion files, which contain ids of rows
      that have been deleted.

A Manifest file includes the metadata to describe a version of the dataset.

.. literalinclude:: ../protos/format.proto
   :language: protobuf
   :linenos:
   :start-at: // Manifest is
   :end-at: } // Manifest

Fragments

DataFragment represents a chunk of data in the dataset. Itself includes one or more DataFile, where each DataFile can contain several columns in the chunk of data. It also may include a DeletionFile, which is explained in a later section.

.. literalinclude:: ../protos/format.proto
   :language: protobuf
   :linenos:
   :start-at: // Data fragment
   :end-at: } // DataFile

The overall structure of a fragment is shown below. One or more data files store the columns of a fragment. New columns can be added to a fragment by adding new data files. The deletion file (if present), stores the rows that have been deleted from the fragment.

Every row has a unique id, which is an u64 that is composed of two u32s: the fragment id and the local row id. The local row id is just the index of the row in the data files.

File Structure

Each .lance file is the container for the actual data.

At the tail of the file, a Metadata protobuf block is used to describe the structure of the data file.

.. literalinclude:: ../protos/format.proto
   :language: protobuf
   :linenos:
   :start-at: message Metadata {
   :end-at: } // Metadata

Optionally, a Manifest block can be stored after the Metadata block, to make the lance file self-describable.

In the end of the file, a Footer is written to indicate the closure of a file:

+---------------+----------------+
| 0 - 3 byte    | 4 - 7 byte     |
+===============+================+
| metadata position (uint64)     |
+---------------+----------------+
| major version | minor version  |
+---------------+----------------+
|   Magic number "LANC"          |
+--------------------------------+

Feature Flags

As the file format and dataset evolve, new feature flags are added to the format. There are two separate fields for checking for feature flags, depending on whether you are trying to read or write the table. Readers should check the reader_feature_flags to see if there are any flag it is not aware of. Writers should check writer_feature_flags. If either sees a flag they don't know, they should return an "unsupported" error on any read or write operation.

Fields

Fields represent the metadata for a column. This includes the name, data type, id, nullability, and encoding.

Fields are listed in depth first order, and can be one of (1) parent (struct), (2) repeated (list/array), or (3) leaf (primitive). For example, the schema:

a: i32
b: struct {
    c: list<i32>
    d: i32
}

Would be represented as the following field list:

name	id	type	parent_id	logical_type
a	1	LEAF	0	"int32"
b	2	PARENT	0	"struct"
b.c	3	REPEATED	2	"list"
b.c	4	LEAF	3	"int32"
b.d	5	LEAF	2	"int32"

Encodings

Lance uses encodings that can render good both point query and scan performance. Generally, it requires:

1. It takes no more than 2 disk reads to access any data points.
2. It takes sub-linear computation (O(n)) to locate one piece of data.

Plain Encoding

Plain encoding stores Arrow array with fixed size values, such as primitive values, in contiguous space on disk. Because the size of each value is fixed, the offset of a particular value can be computed directly.

Null: TBD

Variable-Length Binary Encoding

For variable-length data types, i.e., (Large)Binary / (Large)String / (Large)List in Arrow, Lance uses variable-length encoding. Similar to Arrow in-memory layout, the on-disk layout include an offset array, and the actual data array. The offset array contains the absolute offset of each value appears in the file.

+---------------+----------------+
| offset array  | data array     |
+---------------+----------------+

If offsets[i] == offsets[i + 1], we treat the i-th value as Null.

Dictionary Encoding

Directory encoding is a composite encoding for a Arrow Dictionary Type, where Lance encodes the key and value separately using primitive encoding types, i.e., key are usually encoded with Plain Encoding.

Dataset Update and Schema Evolution

Lance supports fast dataset update and schema evolution via manipulating the Manifest metadata.

Appending is done by appending new Fragment to the dataset. While adding columns is done by adding new DataFile of the new columns to each Fragment. Finally, Overwrite a dataset can be done by resetting the Fragment list of the Manifest.

Deletion

Rows can be marked deleted by adding a deletion file next to the data in the _deletions folder. These files contain the indices of rows that have between deleted for some fragment. For a given version of the dataset, each fragment can have up to one deletion file. Fragments that have no deleted rows have no deletion file.

Readers should filter out row ids contained in these deletion files during a scan or ANN search.

Deletion files come in two flavors:

1. Arrow files: which store a column with a flat vector of indices
2. Roaring bitmaps: which store the indices as compressed bitmaps.

Roaring Bitmaps are used for larger deletion sets, while Arrow files are used for small ones. This is because Roaring Bitmaps are known to be inefficient for small sets.

The filenames of deletion files are structured like:

_deletions/{fragment_id}-{read_version}-{random_id}.{arrow|bin}

Where fragment_id is the fragment the file corresponds to, read_version is the version of the dataset that it was created off of (usually one less than the version it was committed to), and random_id is a random i64 used to avoid collisions. The suffix is determined by the file type (.arrow for Arrow file, .bin for roaring bitmap).

.. literalinclude:: ../protos/format.proto
   :language: protobuf
   :linenos:
   :start-at: // Deletion File
   :end-at: } // DeletionFile

Deletes can be materialized by re-writing data files with the deleted rows removed. However, this invalidates row indices and thus the ANN indices, which can be expensive to recompute.

Committing Datasets

A new version of a dataset is committed by writing a new manifest file to the _versions directory. Only after successfully committing this file should the _latest.manifest file be updated.

To prevent concurrent writers from overwriting each other, the commit process must be atomic and consistent for all writers. If two writers try to commit using different mechanisms, they may overwrite each other's changes. For any storage system that natively supports atomic rename-if-not-exists or put-if-not-exists, these operations should be used. This is true of local file systems and cloud object stores, with the notable except of AWS S3. For ones that lack this functionality, an external locking mechanism can be configured by the user.

Conflict resolution

If two writers try to commit at the same time, one will succeed and the other will fail. The failed writer should attempt to retry the commit, but only if it's changes are compatible with the changes made by the successful writer.

The changes for a given commit are recorded as a transaction file, under the _transactions prefix in the dataset directory. The transaction file is a serialized Transaction protobuf message. See the transaction.proto file for its definition.

The commit process is as follows:

1. The writer finishes writing all data files.
2. The writer creates a transaction file in the _transactions directory. This files describes the operations that were performed, which is used for two purposes: (1) to detect conflicts, and (2) to re-build the manifest during retries.
3. Look for any new commits since the writer started writing. If there are any, read their transaction files and check for conflicts. If there are any conflicts, abort the commit. Otherwise, continue.
4. Build a manifest and attempt to commit it to the next version. If the commit fails because another writer has already committed, go back to step 3.
5. If the commit succeeds, update the _latest.manifest file.

When checking whether two transactions conflict, be conservative. If the transaction file is missing, assume it conflicts. If the transaction file has an unknown operation, assume it conflicts.

External Manifest Store

If the backing object store does not support *-if-not-exists operations, an external manifest store can be used to allow concurrent writers. An external manifest store is a KV store that supports put-if-not-exists operation. The external manifest store supplements but does not replace the manifests in object storage. A reader unaware of the external manifest store could read a table that uses it, but it might be up to one version behind the true latest version of the table.

The commit process is as follows:

1. PUT_OBJECT_STORE mydataset.lance/_versions/{version}.manifest-{uuid} stage a new manifest in object store under a unique path determined by new uuid
2. PUT_EXTERNAL_STORE base_uri, version, mydataset.lance/_versions/{version}.manifest-{uuid} commit the path of the staged manifest to the external store.
3. COPY_OBJECT_STORE mydataset.lance/_versions/{version}.manifest-{uuid} mydataset.lance/_versions/{version}.manifest copy the staged manifest to the final path
4. PUT_EXTERNAL_STORE base_uri, version, mydataset.lance/_versions/{version}.manifest update the external store to point to the final manifest

Note that the commit is effectively complete after step 2. If the writer fails after step 2, a reader will be able to detect the external store and object store are out-of-sync, and will try to synchronize the two stores. If the reattempt at synchronization fails, the reader will refuse to load. This is to ensure the that the dataset is always portable by copying the dataset directory without special tool.

The reader load process is as follows:

1. GET_EXTERNAL_STORE base_uri, version, path then, if path does not end in a UUID return the path
2. COPY_OBJECT_STORE mydataset.lance/_versions/{version}.manifest-{uuid} mydataset.lance/_versions/{version}.manifest reattempt synchronization
3. PUT_EXTERNAL_STORE base_uri, version, mydataset.lance/_versions/{version}.manifest update the external store to point to the final manifest
4. RETURN mydataset.lance/_versions/{version}.manifest always return the finalized path, return error if synchronization fails

Statistics

Statistics are stored within Lance files. The statistics can be used to determine which pages can be skipped within a query. The null count, lower bound (min), and upper bound (max) are stored.

Statistics themselves are stored in Lance's columnar format, which allows for selectively reading only relevant stats columns.

Statistic values

Three types of statistics are stored per column: null count, min value, max value. The min and max values are stored as their native data types in arrays.

There are special behavior for different data types to account for nulls:

For integer-based data types (including signed and unsigned integers, dates, and timestamps), if the min and max are unknown (all values are null), then the minimum/maximum representable values should be used instead.

For float data types, if the min and max are unknown, then use -Inf and +Inf, respectively. (-Inf and +Inf may also be used for min and max if those values are present in the arrays.) NaN values should be ignored for the purpose of min and max statistics. If the max value is zero (negative or positive), the max value should be recorded as +0.0. Likewise, if the min value is zero (positive or negative), it should be recorded as -0.0.

For binary data types, if the min or max are unknown or unrepresentable, then use null value. Binary data type bounds can also be truncated. For example, an array containing just the value "abcd" could have a truncated min of "abc" and max of "abd". If there is no truncated value greater than the maximum value, then instead use null for the maximum.

Warning

The min and max values are not guaranteed to be within the array; they are simply upper and lower bounds. Two common cases where they are not contained in the array is if the min or max original value was deleted and when binary data is truncated. Therefore, statistic should not be used to compute queries such as SELECT max(col) FROM table.

Page-level statistics format

Page-level statistics are stored as arrays within the Lance file. Each array contains one page long and is num_pages long. The page offsets are stored in an array just like the data page table. The offset to the statistics page table is stored in the metadata.

The schema for the statistics is:

<field_id_1>: struct
    null_count: i64
    min_value: <field_1_data_type>
    max_value: <field_1_data_type>
...
<field_id_N>: struct
    null_count: i64
    min_value: <field_N_data_type>
    max_value: <field_N_data_type>

Any number of fields may be missing, as statistics for some fields or of some kind may be skipped. In addition, readers should expect there may be extra fields that are not in this schema. These should be ignored. Future changes to the format may add additional fields, but these changes will be backwards compatible.

However, writers should not write extra fields that aren't described in this document. Until they are defined in the specification, there is no guarantee that readers will be able to safely interpret new forms of statistics.