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KeeperMap

This engine allows you to use Keeper/ZooKeeper cluster as consistent key-value store with linearizable writes and sequentially consistent reads.

To enable KeeperMap storage engine, you need to define a ZooKeeper path where the tables will be stored using <keeper_map_path_prefix> config.

For example:

<clickhouse>
    <keeper_map_path_prefix>/keeper_map_tables</keeper_map_path_prefix>
</clickhouse>

where path can be any other valid ZooKeeper path.

Creating a Table​

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
    ...
) ENGINE = KeeperMap(root_path, [keys_limit]) PRIMARY KEY(primary_key_name)

Engine parameters:

  • root_path - ZooKeeper path where the table_name will be stored.
    This path should not contain the prefix defined by <keeper_map_path_prefix> config because the prefix will be automatically appended to the root_path.
    Additionally, format of auxiliary_zookeper_cluster_name:/some/path is also supported where auxiliary_zookeper_cluster is a ZooKeeper cluster defined inside <auxiliary_zookeepers> config.
    By default, ZooKeeper cluster defined inside <zookeeper> config is used.
  • keys_limit - number of keys allowed inside the table.
    This limit is a soft limit and it can be possible that more keys will end up in the table for some edge cases.
  • primary_key_name – any column name in the column list.
  • primary key must be specified, it supports only one column in the primary key. The primary key will be serialized in binary as a node name inside ZooKeeper.
  • columns other than the primary key will be serialized to binary in corresponding order and stored as a value of the resulting node defined by the serialized key.
  • queries with key equals or in filtering will be optimized to multi keys lookup from Keeper, otherwise all values will be fetched.

Example:

CREATE TABLE keeper_map_table
(
    `key` String,
    `v1` UInt32,
    `v2` String,
    `v3` Float32
)
ENGINE = KeeperMap('/keeper_map_table', 4)
PRIMARY KEY key

with

<clickhouse>
    <keeper_map_path_prefix>/keeper_map_tables</keeper_map_path_prefix>
</clickhouse>

Each value, which is binary serialization of (v1, v2, v3), will be stored inside /keeper_map_tables/keeper_map_table/data/serialized_key in Keeper. Additionally, number of keys will have a soft limit of 4 for the number of keys.

If multiple tables are created on the same ZooKeeper path, the values are persisted until there exists at least 1 table using it.
As a result, it is possible to use ON CLUSTER clause when creating the table and sharing the data from multiple ClickHouse instances.
Of course, it's possible to manually run CREATE TABLE with same path on unrelated ClickHouse instances to have same data sharing effect.

Supported operations​

Inserts​

When new rows are inserted into KeeperMap, if the key does not exist, a new entry for the key is created. If the key exists, and setting keeper_map_strict_mode is set to true, an exception is thrown, otherwise, the value for the key is overwritten.

Example:

INSERT INTO keeper_map_table VALUES ('some key', 1, 'value', 3.2);

Deletes​

Rows can be deleted using DELETE query or TRUNCATE. If the key exists, and setting keeper_map_strict_mode is set to true, fetching and deleting data will succeed only if it can be executed atomically.

DELETE FROM keeper_map_table WHERE key LIKE 'some%' AND v1 > 1;
ALTER TABLE keeper_map_table DELETE WHERE key LIKE 'some%' AND v1 > 1;
TRUNCATE TABLE keeper_map_table;

Updates​

Values can be updated using ALTER TABLE query. Primary key cannot be updated. If setting keeper_map_strict_mode is set to true, fetching and updating data will succeed only if it's executed atomically.

ALTER TABLE keeper_map_table UPDATE v1 = v1 * 10 + 2 WHERE key LIKE 'some%' AND v3 > 3.1;

Related content​