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Using FileStore for files on disk

The first step in any maggma pipeline is creating a Store so that data can be queried and transformed. Often times your data will originate as files on disk (e.g., calculation output files, files generated by instruments, etc.). FileStore provides a convenient way to access this type of data as if it were in a database, making it possible to query, add metadata, and run Builder on it.

Suppose you have some data files organized in the following directory structure:

Example directory structure

Creating the FileStore

To create a Filestore, simply pass the path to the top-level directory that contains the files.

>>> fs = FileStore('/path/to/file_store_test/')
>>> fs.connect()

On connect(), FileStore iterates through all files in the base directory and all subdirectories. For each file, it creates dict-like record based on the file's metadata such as name, size, last modification date, etc. These records are kept in memory using an internal MemoryStore. An example record is shown below.

{'_id': ObjectId('625e581113cef6275a992abe'),
 'name': '',
 'path': '/test_files/file_store_test/calculation1/',
 'parent': 'calculation1',
 'size': 90,
 'file_id': '2d12e9803fa0c6eaffb065c8dc3cf4fe',
 'last_updated': datetime.datetime(2022, 4, 19, 5, 23, 54, 109000),
 'hash': 'd42c9ff24dc2fde99ed831ec767bd3fb',
 'orphan': False,
 'contents': 'This is the file named\nIn directory calculation1\nin the FileStore test directory.'}

Choosing files to index

To restrict which files are indexed by the Store (which can improve performance), the optional keyword arguments max_depth and file_filters can be used. For example, to index only files ending in ".in", use

>>> fs = FileStore('/path/to/my/data', file_filters=["*.in"])

You can pass multiple file_filters and use regex-like fnmatch patterns as well. For example, to index all files ending in ".in" or named "test-X.txt" where X is any single letter between a and d, use

>>> fs = FileStore('/path/to/my/data', file_filters=["*.in","test-[abcd].txt"])

If you only want to index the root directory and exclude all subdirectories, use max_depth=0, e.g.

>>> fs = FileStore('/path/to/my/data', max_depth=0)

Write access

By default, the FileStore is read-only. However you can set read_only=False if you want to add additional metadata to the data (See "Adding Metadata" below). This metadata is stored in a .json file placed in the root directory of the FileStore (the name of the file can be customized with the json_name keyword argument.)

>>> fs = FileStore('/path/to/my/data', read_only=False, json_name='my_store.json')

Several methods that modify the contents of the FileStore such as add_metadata, update, and remove_docs will not work unless the store is writable (i.e., read_only=False).

File identifiers (file_id)

Each file is uniquely identified by a file_id key, which is computed from the hash of the file's path relative to the base FileStore directory. Unique identifiers for every file are necessary to enable Builder to work correctly and for associating custom metadata (See "Adding Metadata" below). By using the relative path instead of the absolute path makes it possible to move the entire FileStore to a new location on disk without changing file_id (as long as the relative paths don't change).

Connecting and querying

As with any Store, you have to connect() before you can query any data from a FileStore. After that, you can use query_one() to examine a single document or query() to return an interator of matching documents. For example, let's print the parent directory of each of the files named "" in our example FileStore:

>>> fs.connect()
>>> [d["parent"] for d in fs.query({"name":""})]
['calculation2', 'calculation1']


NOTE FileStore can take a long time to connect() when there are more than a few hundred files in the directory. This is due to limitations of the mongomock package that powers the internal MemoryStore. We hope to identify a more performant alternative in the near future. In the mean time, use file_filters and max_depth to limit the total number of files in the FileStore.

File Contents

When you query() data, FileStore attempts to read the contents of each matching file and include them in the contents key of the returned dictionary, as you can see in the example above. There is an optional keyword argument contents_size_limit which specifies the maximum size of file that FileStore will attempt to read.

At present, this only works with text files and the entire file contents are returned as a single string. If a file is too large to read, or if FileStore was unable to open the file (because it is a binary file, etc.), then you will see contents populated with a message that beings with "Unable to read:. This behavior may change in the future.

Adding metadata

As long as a store is not read-only (see #write-access), you can update() documents in it just like any other Store. This is a great way to associate additional information with raw data files. For example, if you have a store of files generated by an instrument, you can add metadata related to the environmental conditions, the sample that was tested, etc.

update method

You can use update() to add keys to the FileStore records. For example, to add some tags to the files named "", use:

docs = [d for d in fs.query({"name":""})]
for d in docs:
    d["tags"] = ["preliminary"]

The above steps will result in the following contents being added to the .json file. This metadata will be automatically read back in next time you connect to the Store.


Notice that only the items modified with extra keys are written to the JSON (i.e., if you have 10 items in the store but add metadata to just two, only the two items will be written to the JSON). The purpose of this behavior is to prevent any duplication of data. The file_id and path are retained in the JSON file to make each metadata record manually identifiable.

add_metadata convenience method

A more convenient way to add metadata is via the add_metadata method. To use it, just pass a query to identify the documents you want to update, and a dict to add to the document. Here is what the example above would look like using add_metadata

fs.add_metadata({"name":""}, {"tags":["preliminary"]})

Automatic metadata

You can even define a function to automatically crate metadata from file or directory names. For example, if you prefix all your files with datestamps (e.g., '2022-05-07_experiment.csv'), you can write a simple string parsing function to extract information from any key in a FileStore record and pass the function as an argument to add_metadata.

For example, to extract the date from files named like '2022-05-07_experiment.csv' and add it to the 'date' field:

>>> def get_date_from_filename(d):
        d: An item returned from the `FileStore`
    return {"date": d["name"].split("_")[0],
            "test_name": d["name"].split("_")[1]

>>> fs.add_metadata({}, auto_data=get_date_from_filename)

Protected Keys

Note that when using any of the above methods, you cannot modify any keys that are populated by default (e.g. name, parent, file_id), because they are derived directly from the files on disk.

Orphaned Metadata

In the course of working with FileStore you may encounter a situation where there are metadata records stored in the JSON file that no longer match files on disk. This can happen if, for example, you init a FileStore and later delete a file, or if you init the store with the default arguments but later restrict the file selection with max_depth or file_filters.

These orphaned metadata records will appear in the FileStore with the field {"orphan": True}. The goal with this behavior is to preserve all metadata the user may have added and prevent data loss.

By default, orphaned metadata is excluded from query results. There is an include_orphans keyword argument you can set on init if you want orphaned metadata to be returned in queries.

Deleting files

For consistency with the Store interface, FileStore provides the remove_docs method whenever read_only=False. This method will delete files on disk, because FileStore documents are simply representations of those files. It has an additional guard argument confirm which must be set to the non-default value True for the method to actually do anything.

>>> fs.remove_docs({"name":""})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../maggma/src/maggma/stores/", line 496, in remove_docs
    raise StoreError( (StoreError(...), 'Warning! This command is about '
    'to delete 2 items from disk! If this is what you want, reissue '
    'this command with confirm=True.')

Processing files with a Builder

Now that you can access your files on disk via a FileStore, it's time to write a Builder to read and process the data (see Writing a Builder). Keep in mind that get_items will return documents like the one shown in (#creating-the-filestore). You can then use process_items to

  • Create strucured data from the contents
  • Open the file for reading using a custom piece of code
  • etc.

Once you can process data on your disk with a Builder, you can send that data to any kind of Store you like - another FileStore, a database, etc.