Guide

Introduction

The itemdb library allows you to store and retrieve Python dicts in a database on the local filesystem, in an easy, fast, and reliable way.

To be more precise: it is an ACID compliant transactional database for storage and retrieval of JSON-compatible dict items. That sounds very technical; let’s break it down:

  • ACID means it has desirable database features of atomicity, consistency, isolation and durability. We’ll get back to these when we talk about transactions.
  • The itemdb API focuses on making transactions easy and explicit.
  • JSON is used to serialize the dict items, so the values in the dicts are limited to: None, bool, int, float, str, list, dict.

In practice, itemdb uses the rock-solid SQLite, and provides an object-based API that requires no knowledge of SQL.

You can use itemdb in a wide variety of applications. This includes web-servers, though once your traffic scales up, you may want to consider something like PostgreSQL or perhasps a hosted db.

Opening a database

In itemdb (like SQLite) each database is represented as a file. One can also use ":memory:" to create an in-memory database for testing/demo purposes.

db = ItemDB(filename)
db = ItemDB(":memory:")

Creating tables and indices

Each database consists of tables, and the tables contain the items. A “table” is what is also called “table” in SQL databases, a “collection” in MongoDB, and an “object store” in IndexedDB.

You can create a table using ensure_table(). It is safe to call this before every time that you use the database, because it returns fast if the table already exist:

db.ensure_table("some_table_name")

In the same call we can also specify indices. Indicices represent fields in the items that are indexed, so that they can be used to retrieve items fast, using select(), count() and delete().

Indices can also be prefixed with a “!”, marking the field as mandatory and unique, making it possible to identify items.

db.ensure_table("persons", "!name", "age")

We can now select() items based on the name and age fields, and no two items can have the same value for name.

Note

No new fields can be marked unique once the table has been created.

Note

In the examples below we mark the “name” field as unique, but strictly speaking this is wrong, because different persons can have the same name. Another form of ID would be more appropriate in real use-cases.

Add some items

An “item” is what is called a “row” in SQL databases, a “document” in MongoDB, and an “object” in IndexedDB. Let’s add some to our table!

with db:
    db.put_one("persons", name="Jane", age=22)
    db.put_one("persons", name="John", age=18, fav_number=7)
    db.put("persons", {"name": "Guido"}, {"name": "Anne", "age": 42})

You can see how we use with db here. This is because itemdb requires using a transaction when making changes to the database. Everything inside the with statement is a single transaction. More on that later.

You can also see that with put_one() we can use keyword arguments to specify fields, while with put() we can specify multiple items, each items a dict.

The dictionary can contain as many fields as you want, including sub-dicts and lists. Although the age field is indexed, it is not mandatory (you can select items with missing age using db.select("persons", "age is NULL")).

Since the name field is unique, if we put an item with an existing name, it will simply update it:

# John had his birthday and changed his favourite number
with db:
    db.put_one("persons", name="John", age=19, fav_number=8)

Make some queries

Use e.g. count(), select() to query the database:

>>> db.count_all("persons")
4

>>> db.select("persons", "age > 20")
[{'name': 'Jane', 'age': 22}, {'name': 'Anne', 'age': 42}]

>>> select_name = "John"
>>> db.select_one("persons", "name = ?", select_name)
{'name': 'John', 'age': 19, 'fav_number': 8}

Transactions

Transactions are an important concept in databases. In ACID databases (like itemdb) it has a number of features:

  • A transaction is atomic (either the whole transaction is applied, or the whole transaction is not applied)
  • A transaction is applied in isolation, even when multiple processes are interacting with the database at the same time. This means that when a transaction is in progress, another process/thread that wants to apply a transaction that “intersects” with the ongoing operation, it will wait. (This even works for multiple Docker containers operating on the same SQLite database.)
  • The remaining elements of ACID (consistency and durability) mean that the database always remains in a healthy state. Even on a power outage or if the system crashes halfway a transaction.

In itemdb, transactions are easy, using a context manager. Let’s have a look at some examples:

# Increasing a value is recommended to do in a transaction.
with db:
    player = db.select("players", "name == ?", player_name)
    player["position"] += 2
    db.put("players", player)

# The below has no effect: the transaction fails and is rolled back
with db:
    db.put_one("persons", name="John", age=21, fav_number=8)
    raise RuntimeError()

Database maintenance

Sometimes, you may want to add unique keys to a table or remove existing indices. This is possible by copying the items to a new table and then replacing the new table with the old. By doing this inside a transaction, it can be done safely:

with db:
    db.ensure_table("persons2", "!id", "name", "age")
    for i, person in enumerate(db.select_all("persons")):
        # Make sure each person has an id, e.g.:
        person["id"] = i
        db.put("persons2", person)
    db.delete_table("persons")
    db.rename_table("persons2", "persons")

At the time of writing, itemdb does not provide an API for backups or vacuuming, but it’s just SQLite under the hood, so you can use the common methods.

Going Async

The API of ItemDB is synchronous. It operates with the filesystem, so it can benefit from async use a lot.

There are two ways to make your code async. The first is by using the AsyncItemDB class. It has the exact same API as ItemDB, but all its methods are async. Note that you must also use async with.

The second approach is to asyncify a synchronous function. The idea of this approach is to do all itemdb operations inside a function and then wrap that function if you want to use it in an async environment. Consider the following example of a web server:

@itemdb.asycify
def push_items(filename, items):
    db = ItemDB(filename)
    db.ensure_table("my_table", "!id", "mtime")

    with db:
        ...
        db.put("my_table", items)

async def my_request_handler(request):
    ...
    # Because we decorated the function with asyncify,
    # we can now await it, while the db interaction
    # occurs in a separate thread.
    await push_items(filename, items)
    ...

Of the two mentioned approaches, the asyncify-approach is slightly more efficient, because it makes use of a thread pool, and only switches to a thread for the duration of the function you’ve asyncified. However, using AsyncItemDB probably makes your code easier to read and maintain, which is probably worth more.