mirror of https://github.com/citusdata/citus.git
1 Commits (8bba66f20768792f08d717cdd5cedb0bcfe00558)
| Author | SHA1 | Message | Date |
|---|---|---|---|
Muhammad Usama
|
f743b35fc2
|
Parallelize Shard Rebalancing & Unlock Concurrent Logical Shard Moves (#7983)
DESCRIPTION: Parallelizes shard rebalancing and removes the bottlenecks
that previously blocked concurrent logical-replication moves.
These improvements reduce rebalance windows—particularly for clusters
with large reference tables and enable multiple shard transfers to run in parallel.
Motivation:
Citus’ shard rebalancer has some key performance bottlenecks:
**Sequential Movement of Reference Tables:**
Reference tables are often assumed to be small, but in real-world
deployments, they can grow significantly large. Previously, reference
table shards were transferred as a single unit, making the process
monolithic and time-consuming.
**No Parallelism Within a Colocation Group:**
Although Citus distributes data using colocated shards, shard
movements within the same colocation group were serialized. In
environments with hundreds of distributed tables colocated
together, this serialization significantly slowed down rebalance
operations.
**Excessive Locking:**
Rebalancer used restrictive locks and redundant logical replication
guards, further limiting concurrency.
The goal of this commit is to eliminate these inefficiencies and enable
maximum parallelism during rebalance, without compromising correctness
or compatibility. Parallelize shard rebalancing to reduce rebalance
time.
Feature Summary:
**1. Parallel Reference Table Rebalancing**
Each reference-table shard is now copied in its own background task.
Foreign key and other constraints are deferred until all shards are
copied.
For single shard movement without considering colocation a new
internal-only UDF '`citus_internal_copy_single_shard_placement`' is
introduced to allow single-shard copy/move operations.
Since this function is internal, we do not allow users to call it
directly.
**Temporary Hack to Set Background Task Context** Background tasks
cannot currently set custom GUCs like application_name before executing
internal-only functions. 'citus_rebalancer ...' statement as a prefix in
the task command. This is a temporary hack to label internal tasks until
proper GUC injection support is added to the background task executor.
**2. Changes in Locking Strategy**
- Drop the leftover replication lock that previously serialized shard
moves performed via logical replication. This lock was only needed when
we used to drop and recreate the subscriptions/publications before each
move. Since Citus now removes those objects later as part of the “unused
distributed objects” cleanup, shard moves via logical replication can
safely run in parallel without additional locking.
- Introduced a per-shard advisory lock to prevent concurrent operations
on the same shard while allowing maximum parallelism elsewhere.
- Change the lock mode in AcquirePlacementColocationLock from
ExclusiveLock to RowExclusiveLock to allow concurrent updates within the
same colocation group, while still preventing concurrent DDL operations.
**3. citus_rebalance_start() enhancements**
The citus_rebalance_start() function now accepts two new optional
parameters:
```
- parallel_transfer_colocated_shards BOOLEAN DEFAULT false,
- parallel_transfer_reference_tables BOOLEAN DEFAULT false
```
This ensures backward compatibility by preserving the existing behavior
and avoiding any disruption to user expectations and when both are set
to true, the rebalancer operates with full parallelism.
**Previous Rebalancer Behavior:**
`SELECT citus_rebalance_start(shard_transfer_mode := 'force_logical');`
This would:
Start a single background task for replicating all reference tables
Then, move all shards serially, one at a time.
```
Task 1: replicate_reference_tables()
↓
Task 2: move_shard_1()
↓
Task 3: move_shard_2()
↓
Task 4: move_shard_3()
```
Slow and sequential. Reference table copy is a bottleneck. Colocated
shards must wait for each other.
**New Parallel Rebalancer:**
```
SELECT citus_rebalance_start(
shard_transfer_mode := 'force_logical',
parallel_transfer_colocated_shards := true,
parallel_transfer_reference_tables := true
);
```
This would:
- Schedule independent background tasks for each reference-table shard.
- Move colocated shards in parallel, while still maintaining dependency
order.
- Defer constraint application until all reference shards are in place.
-
```
Task 1: copy_ref_shard_1()
Task 2: copy_ref_shard_2()
Task 3: copy_ref_shard_3()
→ Task 4: apply_constraints()
↓
Task 5: copy_shard_1()
Task 6: copy_shard_2()
Task 7: copy_shard_3()
↓
Task 8-10: move_shard_1..3()
```
Each operation is scheduled independently and can run as soon as
dependencies are satisfied.
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