# Document Store#

Documents inside a DocumentArray can live in a document store instead of in memory, e.g. in SQLite, Redis. Comparing to the in-memory storage, the benefit of using an external store is often about longer persistence and faster retrieval.

The look-and-feel of a DocumentArray with external store is almost the same as a regular in-memory DocumentArray. This allows users to easily switch between backends under the same DocArray idiom.

Take SQLite as an example, using it as the store backend of a DocumentArray is as simple as follows:

from docarray import DocumentArray, Document

da = DocumentArray(storage='sqlite', config={'connection': 'example.db'})

da.append(Document())
da.summary()

        Documents Summary

Length                 1
Homogenous Documents   True
Common Attributes      ('id',)

Attributes Summary

Attribute   Data type   #Unique values   Has empty value
──────────────────────────────────────────────────────────
id          ('str',)    1                False



Creating, retrieving, updating, deleting Documents are identical to the regular DocumentArray. All DocumentArray methods such as .summary(), .embed(), .plot_embeddings() should work out of the box.

## Construct#

There are two ways for initializing a DocumentArray with a store backend.

from docarray import DocumentArray

da = DocumentArray(storage='sqlite')

<DocumentArray[SQLite] (length=0) at 4477814032>

from docarray.array.sqlite import DocumentArraySqlite as DocumentArray

da = DocumentArray()

<DocumentArray[SQLite] (length=0) at 4477814032>


Depending on the context, you can choose the style that fits better. For example, if one wants to use class method such as DocumentArray.empty(10), then explicit importing DocumentArraySqlite is the way to go. Of course, you can choose not to alias the imported class to make the code even more explicit.

### Construct with config#

The config of a store backend is either store-specific dataclass object or a dict that can be parsed into the former.

One can pass the config in the constructor via config:

from docarray import DocumentArray
from docarray.array.sqlite import SqliteConfig

cfg = SqliteConfig(connection='example.db', table_name='test')

da = DocumentArray(storage='sqlite', config=cfg)

from docarray import DocumentArray

da = DocumentArray(
storage='sqlite', config={'connection': 'example.db', 'table_name': 'test'}
)


Using dataclass gives you better type-checking in IDE but requires an extra import; using dict is more flexible but can be error-prone. You can choose the style that fits best to your context.

## Known limitations#

### Multiple references to the same storage backend#

Let’s see an example with ANNLite storage backend, other storage backends would also have the same problem. Let’s create two DocumentArrays da and db that point the same storage backend:

from docarray import DocumentArray, Document

da = DocumentArray(storage='annlite', config={'data_path': './temp3', 'n_dim': 2})
da.append(Document(text='hello'))
print(len(da))

db = DocumentArray(storage='annlite', config={'data_path': './temp3', 'n_dim': 2})
print(len(db))


The output is:

1
0


Looks like db is not really up-to-date with da. This is true and false. True in the sense that 1 is not 0, number speaks by itself. False in the sense that, the Document is already written to the storage backend. You just can’t see it.

To prove it does persist, run the following code snippet multiple times and you will see the length is increasing one at a time:

from docarray import DocumentArray, Document

da = DocumentArray(storage='annlite', config={'data_path': './temp3', 'n_dim': 2})
da.append(Document(text='hello'))
print(len(da))


Simply put, the reason of this behavior is that certain meta information not synced immediately to the backend on every operation; it would be very costly to do so. As a consequence, your multiple references to the same backend would look different if they are written in one code block as the example above.

To solve this problem, simply use with statement and use DocumentArray as a context manager. The last example can be refactored into the following:

from docarray import DocumentArray, Document

da = DocumentArray(storage='annlite', config={'data_path': './temp4', 'n_dim': 2})
with da:
da.append(Document(text='hello'))
print(len(da))

db = DocumentArray(storage='annlite', config={'data_path': './temp4', 'n_dim': 2})
print(len(db))


Now you get the correct output:

1
1


Take home message is, use the context manager and put your write operations into the with block, when you work with multiple references in a row.

### Out-of-array modification#

One can not take a Document out from a DocumentArray and modify it, then expect its modification to be committed back to the DocumentArray.

Specifically, the pattern below is not supported by any external store backend:

from docarray import DocumentArray

da = DocumentArray(storage='any_store_beyond_in_memory')
d = da[0]  # or any access-element method, e.g. by id, by slice
d.text = 'hello'

print(da[0])  # this will NOT change to hello


The solution is simple: use column-selector:

da[0, 'text'] = 'hello'


### Elements access is slower#

Obviously, a DocumentArray with on-disk storage is slower than in-memory DocumentArray. However, if you choose to use on-disk storage, then often your concern of persistence overwhelms the concern of efficiency.

Slowness can affect all functions of DocumentArray. On the bright side, they may not be that severe as you would expect. Modern database are highly optimized. Moreover, some database provides faster method for resolving certain queries, e.g. nearest-neighbour queries. We are actively and continuously improving DocArray to better leverage those features.