[Sqlalchemy-tickets] Issue #3379: allow external APIs control over invalidation (zzzeek/sqlalchemy)

[Mike B. <iss...@bi...>]

New issue 3379: allow external APIs control over invalidation
https://bitbucket.org/zzzeek/sqlalchemy/issue/3379/allow-external-apis-control-over

Mike Bayer:

so that pool events can be used for clustering and other special cases where we want to invalidate only a subset of connections and have control over whether connections are bumped immediately, or only on next checkout, e.g.:

```
#!python




from sqlalchemy import create_engine
from sqlalchemy import event
import collections
import random

e = create_engine("mysql://scott:tiger@localhost/test")

hosts = ["host1", "host2", "host3"]

conn_recs = collections.defaultdict(set)


@event.listens_for(e, "do_connect")
def do_connect(dialect, conn_rec, carg, cparam):
    host = random.choice(hosts)

    cparam['host'] = host

    conn_rec.info['host'] = host
    conn_recs['host'].add(conn_rec)


@event.listens_for(e, "handle_error")
def handle_error(ctx):
    if ctx.is_disconnect:
        host = ctx.connection.info['host']
        ctx.invalidate_pool_on_disconnect = False
        ctx.connection.invalidate(soft=True)
        for conn_rec in conn_recs[host]:
            conn_rec.invalidate(soft=True)

```