For MaterializeMySQL databases, attempt to reconnect if the connection
to MySQL is lost. The existing setting
`max_wait_time_when_mysql_unavailable` is used to control how often we
attempt to reconnect. This setting can now be set to a negative value
to disable reconnects.
If MySQL was unavailable when loading an existing database on startup,
we would previously load the database, fail to start synchronization,
but allow queries to the database. Change this to only allow queries
if the `allows_queries_when_mysql_lost` setting is on, so that the
behavior is consistent with what happens if the connection to MySQL is
lost while ClickHouse is running.
Also retry connection to MySQL if MySQL is unavailable when ClickHouse
is started (we would previously reconnect only if the connection was
lost during the initial dump of existing data).
Before it was silently try-catched for messages with additional {}, and
it is very easy to trigger, i.e.:
SELECT toDateTime(format('{}-{}-01 00:00:00', '2021', '1'))
Will print:
Code: 41. DB::Exception: Received from localhost:9000. DB::Exception: Cannot parse datetime 2021-1-01 00:00:00{}: Cannot parse DateTime from String: while executing 'FUNCTION toDateTime(format('{}-{}-01 00:00:00', '2021', '1') :: 3) -> toDateTime(format('{}-{}-01 00:00:00', '2021', '1')) DateTime : 2'.
Add two new settings for the Distributed engine:
- bytes_to_delay_insert
- max_delay_to_insert
If at the beginning of INSERT there will be too much pending data, more
then bytes_to_delay_insert, then the INSERT will wait until it will be
shrinked, and not more then max_delay_to_insert seconds.
If after this there will be still too much pending, it will throw an
exception.
Also new profile events were added (by analogy to the MergeTree):
- DistributedDelayedInserts (although you can use system.errors instead
of this, but still)
- DistributedRejectedInserts
- DistributedDelayedInsertsMilliseconds
Previous patch fixes the inaccuracy, but it's done using iterating over
directory on each request (to system.distribution_queue or to check
bytes_to_throw_insert), and like previous patch alredy stated, it may
have pretty huge overhead (especially when you have lots of distributed
files pending).
This patch remove that recalculation (but it will still be done, and
if there is different, there will be a log message), and replace it with
proper account at INSERT time (and after file has been sent, or marked
as broken).
So now system.distribution_queue will show accurate statistics, so tests
does not requires sleep anymore.
But note that with too much distributed pending this will iterate over
all directories.
Right now with distributed_directory_monitor_batch_inserts=1 and
insert_distributed_sync=0 INSERT into Distributed table will store
blocks that should be sent to remote (and in case of
prefer_localhost_replica=0 to the localhost too) on the local
filesystem, and sent it in background.
However there is no limit for this storage, and if the remote is
unavailable (or some other error), these pending blocks may take
significant space, and this is not always desired behaviour.
Add new Distributed setting - bytes_to_throw_insert, that will set the
limit for how much pending bytes is allowed, if the limit will be
reached an exception will be throw.
By default was set to 0, to avoid surprises.
