mirror of https://github.com/citusdata/citus.git
be6668e440
**DESCRIPTION:**
This pull request introduces the foundation and core logic for the
snapshot-based node split feature in Citus. This feature enables
promoting a streaming replica (referred to as a clone in this feature
and UI) to a primary node and rebalancing shards between the original
and the newly promoted node without requiring a full data copy.
This significantly reduces rebalance times for scale-out operations
where the new node already contains a full copy of the data via
streaming replication.
Key Highlights:
**1. Replica (Clone) Registration & Management Infrastructure**
Introduces a new set of UDFs to register and manage clone nodes:
- citus_add_clone_node()
- citus_add_clone_node_with_nodeid()
- citus_remove_clone_node()
- citus_remove_clone_node_with_nodeid()
These functions allow administrators to register a streaming replica of
an existing worker node as a clone, making it eligible for later
promotion via snapshot-based split.
**2. Snapshot-Based Node Split (Core Implementation)**
New core UDF:
- citus_promote_clone_and_rebalance()
This function implements the full workflow to promote a clone and
rebalance shards between the old and new primaries. Steps include:
1. Ensuring Exclusivity – Blocks any concurrent placement-changing
operations.
2. Blocking Writes – Temporarily blocks writes on the primary to ensure
consistency.
3. Replica Catch-up – Waits for the replica to be fully in sync.
4. Promotion – Promotes the replica to a primary using pg_promote.
5. Metadata Update – Updates metadata to reflect the newly promoted
primary node.
6. Shard Rebalancing – Redistributes shards between the old and new
primary nodes.
**3. Split Plan Preview**
A new helper UDF get_snapshot_based_node_split_plan() provides a preview
of the shard distribution post-split, without executing the promotion.
**Example:**
```
reb 63796> select * from pg_catalog.get_snapshot_based_node_split_plan('127.0.0.1',5433,'127.0.0.1',5453);
table_name | shardid | shard_size | placement_node
--------------+---------+------------+----------------
companies | 102008 | 0 | Primary Node
campaigns | 102010 | 0 | Primary Node
ads | 102012 | 0 | Primary Node
mscompanies | 102014 | 0 | Primary Node
mscampaigns | 102016 | 0 | Primary Node
msads | 102018 | 0 | Primary Node
mscompanies2 | 102020 | 0 | Primary Node
mscampaigns2 | 102022 | 0 | Primary Node
msads2 | 102024 | 0 | Primary Node
companies | 102009 | 0 | Clone Node
campaigns | 102011 | 0 | Clone Node
ads | 102013 | 0 | Clone Node
mscompanies | 102015 | 0 | Clone Node
mscampaigns | 102017 | 0 | Clone Node
msads | 102019 | 0 | Clone Node
mscompanies2 | 102021 | 0 | Clone Node
mscampaigns2 | 102023 | 0 | Clone Node
msads2 | 102025 | 0 | Clone Node
(18 rows)
```
**4 Test Infrastructure Enhancements**
- Added a new test case scheduler for snapshot-based split scenarios.
- Enhanced pg_regress_multi.pl to support creating node backups with
slightly modified options to simulate real-world backup-based clone
creation.
### 5. Usage Guide
The snapshot-based node split can be performed using the following
workflow:
**- Take a Backup of the Worker Node**
Run pg_basebackup (or an equivalent tool) against the existing worker
node to create a physical backup.
`pg_basebackup -h <primary_worker_host> -p <port> -D
/path/to/replica/data --write-recovery-conf
`
**- Start the Replica Node**
Start PostgreSQL on the replica using the backup data directory,
ensuring it is configured as a streaming replica of the original worker
node.
**- Register the Backup Node as a Clone**
Mark the registered replica as a clone of its original worker node:
`SELECT * FROM citus_add_clone_node('<clone_host>', <clone_port>,
'<primary_host>', <primary_port>);
`
**- Promote and Rebalance the Clone**
Promote the clone to a primary and rebalance shards between it and the
original worker:
`SELECT * FROM citus_promote_clone_and_rebalance('clone_node_id');
`
**- Drop Any Replication Slots from the Original Worker**
After promotion, clean up any unused replication slots from the original
worker:
`SELECT pg_drop_replication_slot('<slot_name>');
`
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| hammerdb | ||
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