Advanced Topics


Transaction and Context

In normal cases when db is bound to a pool, you can start a transaction through db directly:

async with db.transaction() as (conn, tx):
    # play within a transaction

As you can see from the unpacked arguments, db.transaction() acquired a connection and started a transaction in one go. It is identical to do it separately:

async with db.acquire() as conn:
    async with conn.transaction() as tx:
        # play within a transaction

There is an alternative to do this without async with, but this may be changed in next version, as discussed in #59. Also, tx is always None for now.

Because GINO offers query APIs on not only connections but also model classes and objects and even query objects, it would be too much trouble passing connection object around when dealing with transactions. Therefore GINO offers an optional feature to automatically manage connection objects, by enabling a builtin task local hack before any tasks are created:

from gino import enable_task_local

This switch creates a local storage for each coroutine, where db.acquire() shall store the connection object. Hence executions within the acquire context will be able to make use of the same connection right in the local storage. Furthermore, nested db.acquire() will simply return the same connection. This allows db.transaction() to be nested in the same way that asyncpg conn.transaction() does it - to use database save points.

async with db.transaction() as (conn1, tx1):      # BEGIN
    async with db.transaction() as (conn2, tx2):  # SAVEPOINT ...
        assert conn1 == conn2

If nested transactions or reused connections are not expected, you can explicitly use db.acquire(reuse=False) or db.transaction(reuse=False) to borrow new connections from the pool. Non-reused connections are stacked, they will be returned to the pool in the reversed order as they were borrowed. Local storage covers between different tasks that are awaited in a chain, it is theoretically safe in most cases. However it is still some sort of a hack, but it would be like this before Python officially supports task local storage in PEP 550.

Sanic Support

To integrate with Sanic_, a few configurations needs to be set in app.config (with default value though):

  • DB_HOST: if not set, localhost
  • DB_PORT: if not set, 5432
  • DB_USER: if not set, postgres
  • DB_PASSWORD: if not set, empty string
  • DB_DATABASE: if not set, postgres
  • DB_POOL_MIN_SIZE: if not set, 5
  • DB_POOL_MAX_SIZE: if not set, 10

An example:

from sanic import Sanic
from gino.ext.sanic import Gino

app = Sanic()
app.config.DB_HOST = 'localhost'
app.config.DB_USER = 'postgres'

db = Gino()

After db.init_app, a connection pool with configured settings shall be created and bound to db when Sanic server is started, and closed on stop. Furthermore, a lazy connection context is created on each request, and released on response. That is to say, within Sanic request handlers, you can directly access db by e.g. User.get(1), everything else is settled: database pool is created on server start, connection is lazily borrowed from pool on the first database access and shared within the rest of the same request handler, and automatically returned to the pool on response.

Please be noted that, in the async world, await may block unpredictably for a long time. When this world is crossing RDBMS pools and transactions, it is a very dangerous bite for performance, even causing disasters sometimes. Therefore we recommend, during the time enjoying fast development, do pay special attention to the scope of transactions and borrowed connections, make sure that transactions are closed as soon as possible, and connections are not taken for unnecessarily long time. As for the Sanic support, if you want to release the concrete connection in the request context before response is reached, just do it like this:

await request['connection'].release()

Or if you prefer not to use the contextual lazy connection in certain handlers, prefer explicitly manage the connection lifetime, you can always borrow a new connection by setting reuse=False:

async with db.acquire(reuse=False):
    # new connection context is created

Or if you prefer not to use the builtin request-scoped lazy connection at all, you can simply turn it off:


JSON Property

PostgreSQL started to support native JSON type since 9.2, and became more feature complete in 9.4. JSON is ideal to store varying key-value data. GINO offers objective support for this scenario, requiring PostgreSQL 9.5 for now.

from gino import Gino

db = Gino()

class User(db.Model):
    __tablename__ = 'users'

    id = db.Column(db.Integer(), primary_key=True)
    profile = db.Column(db.JSONB())
    nickname = db.StringProperty(default='noname')
    age = db.IntegerProperty()

nickname and age look just like normal columns, but they are actually key-value pairs in the profile column. profile is the default column name for JSON properties, you can specify a different name by offering the argument column_name when defining a JSON property. Actually multiple JSON columns are allowed, storing different JSON properties as needed. Also, both JSON and JSONB can be used, depending on your choice. For example:

from gino import Gino

db = Gino()

class Article(db.Model):
    __tablename__ = 'articles'

    id = db.Column(db.Integer(), primary_key=True)

    profile = db.Column(db.JSONB())
    author = db.StringProperty(default='noname')
    pub_index = db.IntegerProperty()

    values = db.Column(db.JSON())
    read_count = db.IntegerProperty(default=0, column_name='values')
    last_update = db.DateTimeProperty(column_name='values')

JSON properties work like normal columns too:

# Create with JSON property values
u = await User.create(age=18)

# Default value is immediately available
u.nickname = 'Name: ' + u.nickname
# identical to: u.update(nickname='Name' + u.nickname)

# Updating only age, accept clause:
await u.update(age=User.age + 2).apply()