With https://github.com/citusdata/citus/pull/5657, Citus uses
a fixed application_name while connecting to remote nodes
for internal purposes.
It means that we cannot allow users to override it via
citus.node_conninfo.
With this commit, we're introducing a new infrastructure to throttle
connections to the worker nodes. This infrastructure is useful for
multi-shard queries, router queries are have not been affected by this.
The goal is to prevent establishing more than citus.max_shared_pool_size
number of connections per worker node in total, across sessions.
To do that, we've introduced a new connection flag OPTIONAL_CONNECTION.
The idea is that some connections are optional such as the second
(and further connections) for the adaptive executor. A single connection
is enough to finish the distributed execution, the others are useful to
execute the query faster. Thus, they can be consider as optional connections.
When an optional connection is not allowed to the adaptive executor, it
simply skips it and continues the execution with the already established
connections. However, it'll keep retrying to establish optional
connections, in case some slots are open again.
We cache connections between nodes in our connection management code.
This is good for speed. For security this can be a problem though. If
the user changes settings related to TLS encryption they want those to
be applied to future queries. This is especially important when they did
not have TLS enabled before and now they want to enable it. This can
normally be achieved by changing citus.node_conninfo. However, because
connections are not reopened there will still be old connections that
might not be encrypted at all.
This commit changes that by marking all connections to be shutdown at
the end of their current transaction. This way running transactions will
succeed, even if placement requires connections to be reused for this
transaction. But after this transaction completes any future statements
will use a connection created with the new connection options.
If a connection is requested and a connection is found that is marked
for shutdown, then we don't return this connection. Instead a new one is
created. This is needed to make sure that if there are no running
transactions, then the next statement will not use an old cached
connection, since connections are only actually shutdown at the end of a
transaction.
Onder Kalaci
Halil Ozan Akgul
Jelte Fennema