|`druid.query.extraction.namespace.cache.type`|Specifies the type of caching to be used by the namespaces. May be one of [`offHeap`, `onHeap`]. `offHeap` uses a temporary file for off-heap storage of the namespace (memory mapped files). `onHeap` stores all cache on the heap in standard java map types.|`onHeap`|
|`pollPeriod`|Period between polling for updates|No|0 (only once)|
|`versionRegex`|Regex to help find newer versions of the namespace data|Yes||
|`namespaceParseSpec`|How to interpret the data at the URI|Yes||
The `pollPeriod` value specifies the period in ISO 8601 format between checks for updates. If the source of the lookup is capable of providing a timestamp, the lookup will only be updated if it has changed since the prior tick of `pollPeriod`. A value of 0, an absent parameter, or `null` all mean populate once and do not attempt to update. Whenever an update occurs, the updating system will look for a file with the most recent timestamp and assume that one with the most recent data.
The `versionRegex` value specifies a regex to use to determine if a filename in the parent path of the uri should be considered when trying to find the latest version. Omitting this setting or setting it equal to `null` will match to all files it can find (equivalent to using `".*"`). The search occurs in the most significant "directory" of the uri.
The `namespaceParseSpec` can be one of a number of values. Each of the examples below would rename foo to bar, baz to bat, and buck to truck. All parseSpec types assumes each input is delimited by a new line. See below for the types of parseSpec supported.
### csv lookupParseSpec
|Parameter|Description|Required|Default|
|---------|-----------|--------|-------|
|`columns`|The list of columns in the csv file|yes|`null`|
|`keyColumn`|The name of the column containing the key|no|The first column|
|`valueColumn`|The name of the column containing the value|no|The second column|
The `simpleJson` lookupParseSpec does not take any parameters. It is simply a line delimited json file where the field is the key, and the field's value is the value.
The JDBC lookups will poll a database to populate its local cache. If the `tsColumn` is set it must be able to accept comparisons in the format `'2015-01-01 00:00:00'`. For example, the following must be valid sql for the table `SELECT * FROM some_lookup_table WHERE timestamp_column > '2015-01-01 00:00:00'`. If `tsColumn` is set, the caching service will attempt to only poll values that were written *after* the last sync. If `tsColumn` is not set, the entire table is pulled every time.
|Parameter|Description|Required|Default|
|---------|-----------|--------|-------|
|`namespace`|The namespace to define|Yes||
|`connectorConfig`|The connector config to use|Yes||
|`table`|The table which contains the key value pairs|Yes||
|`keyColumn`|The column in `table` which contains the keys|Yes||
|`valueColumn`|The column in `table` which contains the values|Yes||
|`tsColumn`| The column in `table` which contains when the key was updated|No|Not used|
|`pollPeriod`|How often to poll the DB|No|0 (only once)|
If you need updates to populate as promptly as possible, it is possible to plug into a kafka topic whose key is the old value and message is the desired new value (both in UTF-8). This requires the following extension: "io.druid.extensions:kafka-extraction-namespace"
The extension `kafka-extraction-namespace` enables reading from a kafka feed which has name/key pairs to allow renaming of dimension values. An example use case would be to rename an ID to a human readable format.
Currently the historical node caches the key/value pairs from the kafka feed in an ephemeral memory mapped DB via MapDB.
