hasql

hasql is a library for acquiring actual connections to masters and replicas in high available PostgreSQL clusters.

Features

• completely asynchronous api
• automatic detection of the host role in the cluster
• health-checks for each host and automatic traffic outage for unavailable hosts
• autodetection of hosts role changes, in case replica host will be promoted to master
• different policies for load balancing
• support for asyncpg, psycopg3, aiopg, sqlalchemy and asyncpgsa

Usage

Some useful examples

Creating connection pool

When acquiring a connection, the connection object of the used driver is returned (aiopg.connection.Connection for aiopg and asyncpg.pool.PoolConnectionProxy for asyncpg and asyncpgsa)

Database URL specirication rules

• Multiple hosts should be passed comma separated
  • multihost example:
    • postgresql://db1,db2,db3/
  • split result:
    • postgresql://db1:5432/
    • postgresql://db2:5432/
    • postgresql://db3:5432/
• The non-default port for each host might be passed after hostnames. e.g.
  • multihost example:
    • postgresql://db1:1234,db2:5678,db3/
  • split result:
    • postgresql://db1:1234/
    • postgresql://db2:5678/
    • postgresql://db3:5432/
• The special case for non-default port for all hosts
  • multihost example:
    • postgresql://db1,db2,db3:6432/
  • split result:
    • postgresql://db1:6432/
    • postgresql://db2:6432/
    • postgresql://db3:6432/

For aiopg or aiopg.sa

aiopg must be installed as a requirement.

Code example using aiopg:

from hasql.aiopg import PoolManager

hosts = ",".join([
    "master-host:5432",
    "replica-host-1:5432",
    "replica-host-2:5432",
])

multihost_dsn = f"postgresql://user:password@{hosts}/dbname"

async def create_pool(dsn) -> PoolManager:
    pool = PoolManager(multihost_dsn)

    # Waiting for 1 master and 1 replica will be available
    await pool.ready(masters_count=1, replicas_count=1)
    return pool

Code example using aiopg.sa:

from hasql.aiopg_sa import PoolManager

hosts = ",".join([
    "master-host:5432",
    "replica-host-1:5432",
    "replica-host-2:5432",
])

multihost_dsn = f"postgresql://user:password@{hosts}/dbname"

async def create_pool(dsn) -> PoolManager:
    pool = PoolManager(multihost_dsn)

    # Waiting for 1 master and 1 replica will be available
    await pool.ready(masters_count=1, replicas_count=1)
    return pool

For asyncpg

asyncpg must be installed as a requirement

from hasql.asyncpg import PoolManager

hosts = ",".join([
    "master-host:5432",
    "replica-host-1:5432",
    "replica-host-2:5432",
])

multihost_dsn = f"postgresql://user:password@{hosts}/dbname"

async def create_pool(dsn) -> PoolManager:
    pool = PoolManager(multihost_dsn)

    # Waiting for 1 master and 1 replica will be available
    await pool.ready(masters_count=1, replicas_count=1)
    return pool

For sqlalchemy

sqlalchemy[asyncio] & asyncpg must be installed as requirements

from hasql.asyncsqlalchemy import PoolManager

hosts = ",".join([
    "master-host:5432",
    "replica-host-1:5432",
    "replica-host-2:5432",
])

multihost_dsn = f"postgresql://user:password@{hosts}/dbname"


async def create_pool(dsn) -> PoolManager:
    pool = PoolManager(
        multihost_dsn,

        # Use master for acquire_replica, if no replicas available
        fallback_master=True,

        # You can pass pool-specific options
        pool_factory_kwargs=dict(
            pool_size=10,
            max_overflow=5
        )
    )

    # Waiting for 1 master and 1 replica will be available
    await pool.ready(masters_count=1, replicas_count=1)
    return pool

For asyncpgsa

asyncpgsa must be installed as a requirement

from hasql.asyncpgsa import PoolManager

hosts = ",".join([
    "master-host:5432",
    "replica-host-1:5432",
    "replica-host-2:5432",
])

multihost_dsn = f"postgresql://user:password@{hosts}/dbname"

async def create_pool(dsn) -> PoolManager:
    pool = PoolManager(multihost_dsn)

    # Waiting for 1 master and 1 replica will be available
    await asyncio.gather(
        pool.wait_masters_ready(1),
        pool.wait_replicas_ready(1)
    )
    return pool

For psycopg3

psycopg3 must be installed as a requirement (package name is psycopg)

from hasql.psycopg3 import PoolManager


hosts = ",".join([
    "master-host:5432",
    "replica-host-1:5432",
    "replica-host-2:5432",
])
multihost_dsn = f"postgresql://user:password@{hosts}/dbname"

async def create_pool(dsn) -> PoolManager:
    pool = PoolManager(multihost_dsn)

    # Waiting for 1 master and 1 replica will be available
    await pool.ready(masters_count=1, replicas_count=1)
    return pool

Acquiring connections

Connections should be acquired with async context manager:

Acquiring master connection

async def do_something():
    pool = await create_pool(multihost_dsn)
    async with pool.acquire(read_only=False) as connection:
        ...

or

async def do_something():
    pool = await create_pool(multihost_dsn)
    async with pool.acquire_master() as connection:
        ...

Acquiring replica connection

async def do_something():
    pool = await create_pool(multihost_dsn)
    async with pool.acquire(read_only=True) as connection:
        ...

or

async def do_something():
    pool = await create_pool(multihost_dsn)
    async with pool.acquire_replica() as connection:
        ...

Without context manager (really not recommended)

async def do_something():
    pool = await create_pool(multihost_dsn)
    connection = await pool.acquire(read_only=False)
    await pool.release(connection)

or more useful

async def do_something():
    pool = await create_pool(multihost_dsn)
    try:
        connection = await pool.acquire(read_only=False)
    finally:
        await pool.release(connection)

How it works?

For each host from dsn string, a connection pool is created. From each pool one connection is reserved, which is used to check the availability of the host and its role. The minimum and maximum number of connections in the pool increases by 1 (to reserve a system connection).

For each pool a background task is created, in which the host availability and its role (master or replica) is checked once every refresh_delay second.

When switching hosts roles, hasql detects this with a slight delay.

For PostgreSQL, when switching the master, all connections to all hosts are broken (the details of implementing PostgreSQL).

If there are no available hosts, the methods acquire(), acquire_master(), and acquire_replica() wait until the host with the desired role startup.

Overview

• hasql.base.BasePoolManager

  • __init__(dsn, acquire_timeout, refresh_delay, refresh_timeout, fallback_master, master_as_replica_weight, balancer_policy, pool_factory_kwargs):

    • dsn: str - Connection string used by the connection.
    • acquire_timeout: Union[int, float] - Default timeout (in seconds) for connection operations. 1 sec by default.
    • refresh_delay: Union[int, float] - Delay time (in seconds) between host polls. 1 sec by default.
    • refresh_timeout: Union[int, float] - Timeout (in seconds) for trying to connect and get the host role. 1 sec by default.
    • fallback_master: bool - Use connections from master if replicas are missing. False by default.
    • master_as_replica_weight: float - Probability of using the master as a replica (from 0. to 1.; 0. - master is not used as a replica; 1. - master can be used as a replica).
    • balancer_policy: type - Connection pool balancing policy (hasql.balancer_policy.GreedyBalancerPolicy, hasql.balancer_policy.RandomWeightedBalancerPolicy or hasql.balancer_policy.RoundRobinBalancerPolicy).
    • stopwatch_window_size: int - Window size for calculating the median response time of each pool.
    • pool_factory_kwargs: Optional[dict] - Connection pool creation parameters that are passed to pool factory.

  • get_pool_freesize(pool) Getting the number of free connections in the connection pool. Returns number of free connections in the connection pool.

    • pool - Pool for which you to be getting the number of free connections.

  • coroutine async-with acquire_from_pool(pool, **kwargs) Acquire a connection from pool. Returns connection to the database.

    • pool - Pool from which you to be acquiring the connection.
    • kwargs - Arguments to be passed to the pool acquire() method.

  • coroutine release_to_pool(connection, pool, **kwargs) A coroutine that reverts connection conn to pool for future recycling.

    • connection - Connection to be released.
    • pool - Pool to which you are returning the connection.
    • kwargs - Arguments to be passed to the pool release() method.

  • is_connection_closed(connection) Returns True if connection is closed.

  • get_last_response_time(pool) Returns database host last response time (in seconds).

  • coroutine async-with acquire(read_only, fallback_master, timeout, **kwargs) Acquire a connection from free pool.

    • readonly: bool - True if need return connection to replica, False - to master. False by default.
    • fallback_master: Optional[bool] - Use connections from master if replicas are missing. If None, then the default value is used.
    • master_as_replica_weight: float - Probability of using the master as a replica (from 0. to 1.; 0. - master is not used as a replica; 1. - master can be used as a replica).
    • timeout: Union[int, float] - Timeout (in seconds) for connection operations.
    • kwargs - Arguments to be passed to the pool acquire() method.

  • coroutine async-with acquire_master(timeout, **kwargs) Acquire a connection from free master pool. Equivalent acquire(read_only=False)

    • timeout: Union[int, float] - Timeout (in seconds) for connection operations.
    • kwargs - Arguments to be passed to the pool acquire() method.

  • coroutine async-with acquire_replica(fallback_master, timeout, **kwargs) Acquire a connection from free master pool. Equivalent acquire(read_only=True)

    • fallback_master: Optional[bool] - Use connections from master if replicas are missing. If None, then the default value is used.
    • master_as_replica_weight: float - Probability of using the master as a replica (from 0. to 1.; 0. - master is not used as a replica; 1. - master can be used as a replica).
    • timeout: Union[int, float] - Timeout (in seconds) for connection operations.
    • kwargs - Arguments to be passed to the pool acquire() method.

  • coroutine release(connection, **kwargs) A coroutine that reverts connection conn to pool for future recycling.

    • connection - Connection to be released.
    • kwargs - Arguments to be passed to the pool release() method.

  • coroutine close() Close pool. Mark all pool connections to be closed on getting back to pool. Closed pool doesn’t allow to acquire new connections.

  • coroutine terminate() Terminate pool. Close pool with instantly closing all acquired connections also.

  • coroutine wait_next_pool_check(timeout) Waiting for the next step to update host roles.

  • coroutine ready(masters_count, replicas_count, timeout) Waiting for a connection to the database hosts. If masters_count is None and replicas_count is None, then connection to all hosts is expected.

    • masters_count: Optional[int] - Minimum number of master hosts. None by default.
    • replicas_count: Optional[int] - Minimum number of replica hosts. None by default.
    • timeout: Union[int, float] - Timeout for database connections. 10 seconds by default.

  • coroutine wait_all_ready()` Waiting to connect to all database hosts.

  • coroutine wait_masters_ready(masters_count) Waiting for connection to the specified number of database master servers.

    • masters_count: int - Minimum number of master hosts.

  • coroutine wait_replicas_ready(replicas_count) Waiting for connection to the specified number of database replica servers.

    • replicas_count: int - Minimum number of replica hosts.

  • coroutine get_pool(read_only, fallback_master) Returns connection pool with the maximum number of free connections.

    • readonly: bool - True if need return replica pool, False - master pool.
    • fallback_master: Optional[bool] - Returns master pool if replicas are missing. False by default.

  • coroutine get_master_pools() Returns a list of all master pools.

  • coroutine get_replica_pools(fallback_master) Returns a list of all replica pools.

    • fallback_master: Optional[bool] - Returns a list of all master pools if replicas are missing. False by default.

  • pool_is_master(pool) Returns True if connection is master.

  • pool_is_replica(pool) Returns True if connection is replica.

  • register_connection(connection, pool) Match connection with the pool from which it was taken. It is necessary for the release() method to work correctly.

• hasql.aiopg.PoolManager

• hasql.aiopg_sa.PoolManager

• hasql.asyncpg.PoolManager

• hasql.asyncpgsa.PoolManager

• hasql.psycopg3.PoolManager

Balancer policies

• hasql.balancer_policy.GreedyBalancerPolicy Chooses pool with the most free connections. If there are several such pools, a random one is taken.
• hasql.balancer_policy.RandomWeightedBalancerPolicy Chooses random pool according to their weights. The weight is inversely proportional to the response time of the database of the respective pool (faster response - higher weight).
• hasql.balancer_policy.RoundRobinBalancerPolicy