A global read-only node gives you a single access point to query data stored across all primary nodes in a Multi-master Cluster (Limitless) cluster. Instead of building a separate synchronization pipeline for cross-database queries, you add one node to your existing cluster and connect through the global read-only endpoint.
How it works
All data files in a Multi-master Cluster (Limitless) cluster are stored in PolarStore and exposed to the global read-only node through PolarFileSystem, the distributed file system layer. When you send a query to the global read-only endpoint, PolarProxy automatically identifies which primary nodes the SQL statement involves and fetches data from each of them using read/write splitting.
Use cases
A global read-only node is most useful when:
Cross-database queries are frequent — your application joins or aggregates data from tables spread across multiple primary nodes (for example,
SELECT * FROM db1.tbl1 t1, db2.tbl2 t2 WHERE t1.id = t2.id)You want to avoid a separate sync pipeline — the global read-only node reads directly from PolarStore, so no external replication is needed
Why use a global read-only node
No separate cluster needed for cross-database queries
Without this feature, handling cross-database queries requires purchasing a separate PolarDB for MySQL cluster to consolidate data from all primary nodes, plus multiple Data Transmission Service (DTS) synchronization links to keep that cluster current.
A global read-only node eliminates both requirements. It reads directly from PolarStore — the same storage layer your primary nodes use — so no additional cluster or DTS links are needed.
Tens-of-milliseconds replication lag
DTS synchronization typically adds seconds of latency. Because a global read-only node accesses data directly through PolarFileSystem rather than replicating it, the lag is reduced to tens of milliseconds. This matters for cross-database queries that need timely results.
Supported versions
Global read-only nodes are available only on Multi-master Cluster (Limitless) clusters running PolarDB for MySQL 8.0.
Billing
You are charged only for the compute node. For pricing details, see Pay-as-you-go prices of compute nodes and Subscription prices of compute nodes.
Add a global read-only node
Adding a global read-only node follows the same process as adding a standard read-only node. See Add a read-only node.
After the node is added, connect through the global read-only endpoint to run cross-database queries. To adjust node specifications as your workload changes, see Manually change the specifications of a cluster.
Add a global read-only column store node
To add a global read-only column store node, first set the loose_polar_enable_imci_with_mm parameter to ON. For parameter details, see Parameter configuration. For instructions on adding the node, see Add a read-only IMCI node.
Remove a global read-only node
To remove a global read-only node, see Remove a read-only node.
Removing the global read-only node also deletes the global read-only endpoint. Cross-database queries through that endpoint will stop working. Reconnect through the cluster endpoint to the primary nodes before removing the node to avoid service interruption.