ALTER PARTITION is used to add, modify, reset, or remove replication zones for partitioning. It is combined with the CONFIGURE ZONE subcommand.
To view details about existing replication zones, use SHOW ZONE CONFIGURATIONS. For more information about replication zones, see Replication Controls.
You can use replication zones to control the number and location of replicas for specific sets of data, both when replicas are first added and when they are rebalanced to maintain cluster equilibrium.
Synopsis
Required privileges
The user must have the CREATE privilege on the table.
Parameters
| Parameter | Description |
|---|---|
table_name |
The name of the table with the replication zone configurations to modify. |
partition_name |
The name of the partition with the replication zone configurations to modify. |
index_name |
The name of the index with the replication zone configurations to modify. |
variable |
The name of the variable to change. |
value |
The value of the variable to change. |
Variables
| Variable | Description |
|---|---|
range_min_bytes |
The minimum size, in bytes, for a range of data in the zone. When a range is less than this size, CockroachDB will merge it with an adjacent range. Default: 134217728 (128 MiB) |
range_max_bytes |
The maximum size, in bytes, for a range of data in the zone. When a range reaches this size, CockroachDB will split it into two ranges. Default: 536870912 (512 MiB) |
gc.ttlseconds |
The number of seconds overwritten MVCC values will be retained before garbage collection. Default: 14400 (4 hours) Smaller values can save disk space and improve performance if values are frequently overwritten or for queue-like workloads. The smallest value we regularly test is 600 (10 minutes); smaller values are unlikely to be beneficial because of the frequency with which GC runs. If you use non-scheduled incremental backups, the GC TTL must be greater than the interval between incremental backups. Otherwise, your incremental backups will fail with the error message protected ts verification error. To avoid this problem, we recommend using scheduled backups instead, which automatically use protected timestamps to ensure they succeed. Larger values increase the interval allowed for AS OF SYSTEM TIME queries and allow for less frequent incremental backups. The largest value we regularly test is 90000 (25 hours). Increasing the GC TTL is not meant to be a solution for long-term retention of history; for that you should handle versioning in the schema design at the application layer. Setting the GC TTL too high can cause problems if the retained versions of a single row approach the maximum range size. This is important because all versions of a row are stored in a single range that never splits. |
num_replicas |
The number of replicas in the zone, also called the "replication factor". Default: 3For the system database and .meta, .liveness, and .system ranges, the default value is 5.For multi-region databases configured to survive region failures, the default value is 5; this will include both voting and non-voting replicas. |
constraints |
An array of required (+) and/or prohibited (-) constraints influencing the location of replicas. See Types of Constraints and Scope of Constraints for more details.To prevent hard-to-detect typos, constraints placed on store attributes and node localities must match the values passed to at least one node in the cluster. If not, an error is signalled. To prevent this error, make sure at least one active node is configured to match the constraint. For example, apply constraints = '[+region=west]' only if you had set --locality=region=west for at least one node while starting the cluster.Default: No constraints, with CockroachDB locating each replica on a unique node and attempting to spread replicas evenly across localities. |
lease_preferences |
An ordered list of required and/or prohibited constraints influencing the location of leaseholders. Whether each constraint is required or prohibited is expressed with a leading + or -, respectively. Note that lease preference constraints do not have to be shared with the constraints field. For example, it's valid for your configuration to define a lease_preferences field that does not reference any values from the constraints field. It's also valid to define a lease_preferences field with no constraints field at all. If the first preference cannot be satisfied, CockroachDB will attempt to satisfy the second preference, and so on. If none of the preferences can be met, the lease will be placed using the default lease placement algorithm, which is to base lease placement decisions on how many leases each node already has, trying to make all the nodes have around the same amount. Each value in the list can include multiple constraints. For example, the list [[+zone=us-east-1b, +ssd], [+zone=us-east-1a], [+zone=us-east-1c, +ssd]] means "prefer nodes with an SSD in us-east-1b, then any nodes in us-east-1a, then nodes in us-east-1c with an SSD."For a usage example, see Constrain leaseholders to specific availability zones. Default: No lease location preferences are applied if this field is not specified. |
global_reads |
If true, transactions operating on the range(s) affected by this zone config should be non-blocking, which slows down writes but allows reads from any replica in the range. Most users will not need to modify this setting; it is applied automatically when you use the GLOBAL table locality in a multi-region cluster. |
num_voters |
Specifies the number of voting replicas. When set, num_replicas will be the sum of voting and non-voting replicas. Most users will not need to modify this setting; it is part of the underlying machinery that enables improved multi-region capabilities in v21.1 and above. |
voter_constraints |
Specifies the constraints that govern the placement of voting replicas. This differs from the constraints field, which will govern the placement of all voting and non-voting replicas. Most users will not need to modify this setting; it is part of the underlying machinery that enables improved multi-region capabilities in v21.1 and above. |
If a value is not set, new zone configurations will inherit their values from their parent zone (e.g., a partition zone inherits from the table zone), which is not necessarily default.
If a variable is set to COPY FROM PARENT (e.g., range_max_bytes = COPY FROM PARENT), the variable will copy its value from its parent replication zone. The COPY FROM PARENT value is a convenient shortcut to use so you do not have to look up the parent's current value. For example, the range_max_bytes and range_min_bytes variables must be set together, so when editing one value, you can use COPY FROM PARENT for the other. Note that if the variable in the parent replication zone is changed after the child replication zone is copied, the change will not be reflected in the child zone.
Examples
Setup
The following examples use MovR, a fictional vehicle-sharing application, to demonstrate CockroachDB SQL statements. For more information about the MovR example application and dataset, see MovR: A Global Vehicle-sharing App.
To follow along, run cockroach demo with the --geo-partitioned-replicas flag. This command opens an interactive SQL shell to a temporary, 9-node in-memory cluster with the movr database.
$ cockroach demo --geo-partitioned-replicas
Create a replication zone for a partition
Once partitions have been defined for a table or a secondary index, to control replication for a partition, use ALTER PARTITION <partition> OF INDEX <table@index> CONFIGURE ZONE:
> ALTER PARTITION us_west OF INDEX vehicles@primary
CONFIGURE ZONE USING
num_replicas = 5,
constraints = '[+region=us-west1]';
CONFIGURE ZONE 1
> ALTER PARTITION us_west OF INDEX vehicles@vehicles_auto_index_fk_city_ref_users
CONFIGURE ZONE USING
num_replicas = 5,
constraints = '[+region=us-west1]';
CONFIGURE ZONE 1
To define replication zones for identically named partitions of a table and its secondary indexes, you can use the <table>@* syntax to save several steps:
> ALTER PARTITION us_west OF INDEX vehicles@*
CONFIGURE ZONE USING
num_replicas = 5,
constraints = '[+region=us-west1]';
To view the zone configuration for a partition, use SHOW ZONE CONFIGURATION FROM PARTITION <partition> OF INDEX <table@index>:
> SHOW ZONE CONFIGURATION FROM PARTITION us_west OF INDEX vehicles@primary;
target | raw_config_sql
----------------------------------------------+-------------------------------------------------------------------------
PARTITION us_west OF INDEX vehicles@primary | ALTER PARTITION us_west OF INDEX vehicles@primary CONFIGURE ZONE USING
| range_min_bytes = 134217728,
| range_max_bytes = 536870912,
| gc.ttlseconds = 90000,
| num_replicas = 5,
| constraints = '[+region=us-west1]',
| lease_preferences = '[]'
(1 row)
You can also use the SHOW CREATE TABLE statement or SHOW PARTITIONS statements to view details about all of the replication zones defined for the partitions of a table and its secondary indexes.
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