With this commit, rebalancer backends are identified by application_name = citus_rebalancer
and the regular internal backends are identified by application_name = citus_internal
To be able to report progress of the rebalancer, the rebalancer updates
the state of a shard move in a shared memory segment. To then fetch the
progress, `get_rebalance_progress` can be called which reads this shared
memory.
Without this change it did so without using any synchronization
primitives, allowing for data races. This fixes that by using atomic
operations to update and read from the parts of the shared memory that
can be changed after initialization.
Without this change the rebalancer progress monitor gets the shard sizes
from the `shardlength` column in `pg_dist_placement`. This column needs to
be updated manually by calling `citus_update_table_statistics`.
However, `citus_update_table_statistics` could lead to distributed
deadlocks while database traffic is on-going (see #4752).
To work around this we don't use `shardlength` column anymore. Instead
for every rebalance we now fetch all shard sizes on the fly.
Two additional things this does are:
1. It adds tests for the rebalance progress function.
2. If a shard move cannot be done because a source or target node is
unreachable, then we error in stop the rebalance, instead of showing
a warning and continuing. When using the by_disk_size rebalance
strategy it's not safe to continue with other moves if a specific
move failed. It's possible that the failed move made space for the
next move, and because the failed move never happened this space now
does not exist.
3. Adds two new columns to the result of `get_rebalancer_progress` which
shows the size of the shard on the source and target node.
Fixes#4930
Every move in the rebalancer algorithm results in an improvement in the
balance. However, even if the improvement in the balance was very small
the move was still chosen. This is especially problematic if the shard
itself is very big and the move will take a long time.
This changes the rebalancer algorithm to take the relative size of the
balance improvement into account when choosing moves. By default a move
will not be chosen if it improves the balance by less than half of the
size of the shard. An extra argument is added to the rebalancer
functions so that the user can decide to lower the default threshold if
the ignored move is wanted anyway.
Teja Mupparti
Jelte Fennema
Marco Slot