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72bb3113ac
Add a new next_replication column to system_schema.keyspaces table. While there is an ongoing RF change: - next_replication keeps the target RF values; - existing replication_v2 column keeps initial RF values - the ones we started the RF change with. DESCRIBE KEYSPACE statement shows replication_v2. When there is no ongoing RF change for this keyspace, its next_replication is empty. In this commit no data is kept in the new column.
106 lines
3.8 KiB
Markdown
106 lines
3.8 KiB
Markdown
# System schema keyspace layout
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This section describes layouts and usage of system\_schema.* tables.
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## system\_schema.keyspaces
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This table contains one row per keyspaces.
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Schema:
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```
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CREATE TABLE system_schema.keyspaces (
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keyspace_name text PRIMARY KEY,
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durable_writes boolean,
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replication frozen<map<text, text>>,
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replication_v2 frozen<map<text, text>>,
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next_replication frozen<map<text, text>>
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)
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```
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Columns:
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* `keyspace_name` - name of the keyspace
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* `durable_writes` - whether writes to the keyspace are using commitlog.
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* `replication` - Deprecated replication settings for the keyspace. Contains the same options as
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the `replication_v2` column, but rack lists are replaced with replica count (numeric RF)
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for backwards compatibility with drivers which don't recognize rack lists here.
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* `replication_v2` - replication settings for the keyspace. The value for the `"class"` key determines
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replication strategy name. The structure of other options depends
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on the replication strategy.
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For `NetworkTopologyStrategy` the other options specify replication factors for datacenters,
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stored as a flattened map of the extended options map (see below).
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For `SimpleStrategy` there is a single option `"replication_factor"` specifying the replication factor.
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* `next_replication` - the target replication factor for the keyspace during rf change.
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If there is no ongoing rf change, `next_replication` value is not set.
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Extended options map used by NetworkTopologyStrategy is a map where values can be either strings or lists of strings.
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For example:
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```
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{
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'dc1': '3',
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'dc2': ['rack1', 'rack2'],
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'dc3': []
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}
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```
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The options above mean that the replication factor for datacenter `dc1` is 3, for datacenter `dc2` it is 2,
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with replicas placed on racks `rack1` and `rack2`. For 'dc3' the replication factor is 0, expressed as an empty list of racks.
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The extended map is stored in the "replication" column in a flattened form, where values which are lists
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are represented as multiple entries in the map with the list index appended to the key, with `:` as the separator.
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The index can be negative, which is used to indicate that the list is empty.
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The example extended options map from above has a flattened representation of:
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```
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{
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'dc1': '3',
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'dc2:0': 'rack1',
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'dc2:1': 'rack2',
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'dc3:-1': ''
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}
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```
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## system\_schema.computed\_columns
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Computed columns are a special kind of columns. Rather than having their value provided directly
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by the user, they are computed - possibly from other column values. Examples of such computed
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columns could be:
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* token column generated from the base partition key for secondary indexes
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* map value column, generated as the extraction of a single value from a map stored in a different column
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Computed columns in many ways act as regular columns, so they are also present in the `system_schema.columns` table -
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`system_schema.computed_columns` is an additional mapping that marks the column as computed and provides its computation.
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Schema:
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~~~
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CREATE TABLE system_schema.computed_columns (
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keyspace_name text,
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table_name text,
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column_name text,
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computation blob,
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PRIMARY KEY (keyspace_name, table_name, column_name)
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) WITH CLUSTERING ORDER BY (table_name ASC, column_name ASC);
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~~~
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`computation` is stored as a blob and its contents are assumed to be a JSON representation of computation's type
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and any custom fields needed.
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Example representations:
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~~~
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{'type':'token'}
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{'type':'map_value','map':'my_map_column_name','key':'AF$^GESHHgge6yhf'}
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~~~
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The token computation does not need additional arguments, as it returns the token of base's partition key.
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In order to compute a map value, what's additionally needed is the column name that stores the map and the key
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at which the value is expected.
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