This service writes and reads item states to and from a number of relational database systems that support [Java Database Connectivity (JDBC)](https://en.wikipedia.org/wiki/Java_Database_Connectivity).
This service allows you to persist state updates using one of several different underlying database services.
It is designed for a maximum of scalability, to store very large amounts of data and still over the years not lose its speed.
The generic design makes it relatively easy for developers to integrate other databases that have JDBC drivers.
The following databases are currently supported and tested:
| url | | Yes | JDBC URL to establish a connection to your database. Examples:<br/><br/>`jdbc:derby:./testDerby;create=true`<br/>`jdbc:h2:./testH2`<br/>`jdbc:hsqldb:./testHsqlDb`<br/>`jdbc:mariadb://192.168.0.1:3306/testMariadb`<br/>`jdbc:mysql://192.168.0.1:3306/testMysql?serverTimezone=UTC`<br/>`jdbc:postgresql://192.168.0.1:5432/testPostgresql`<br/>`jdbc:sqlite:./testSqlite.db`.<br/><br/>If no database is available it will be created; for example the url `jdbc:h2:./testH2` creates a new H2 database in openHAB folder. Example to create your own MySQL database directly:<br/><br/>`CREATE DATABASE 'yourDB' CHARACTER SET utf8 COLLATE utf8_general_ci;` |
| user | | if needed | database user name |
| password | | if needed | database user password |
| errReconnectThreshold | 0 | No | when the service is deactivated (0 means ignore) |
| sqltype.CALL | `VARCHAR(200)` | No | All `sqlType` options allow you to change the SQL data type used to store values for different openHAB item states. See the following links for further information: [mybatis](https://mybatis.github.io/mybatis-3/apidocs/reference/org/apache/ibatis/type/JdbcType.html) [H2](https://www.h2database.com/html/datatypes.html) [PostgresSQL](https://www.postgresql.org/docs/9.3/static/datatype.html) |
| sqltype.ROLLERSHUTTER | `TINYINT` | No | see above |
| sqltype.STRING | `VARCHAR(65500)` | No | see above |
| sqltype.SWITCH | `VARCHAR(6)` | No | see above |
| sqltype.tablePrimaryKey | `TIMESTAMP` | No | type of `time` column for newly created item tables |
| sqltype.tablePrimaryValue | `NOW()` | No | value of `time` column for newly inserted rows |
| numberDecimalcount | 3 | No | for Itemtype "Number" default decimal digit count |
| tableNamePrefix | `item` | No | table name prefix. For Migration from MySQL Persistence, set to `Item`. |
| tableUseRealItemNames | `false` | No | table name prefix generation. When set to `true`, real item names are used for table names and `tableNamePrefix` is ignored. When set to `false`, the `tableNamePrefix` is used to generate table names with sequential numbers. |
| tableIdDigitCount | 4 | No | when `tableUseRealItemNames` is `false` and thus table names are generated sequentially, this controls how many zero-padded digits are used in the table name. With the default of 4, the first table name will end with `0001`. For migration from the MySQL persistence service, set this to 0. |
| rebuildTableNames | false | No | rename existing tables using `tableUseRealItemNames` and `tableIdDigitCount`. USE WITH CARE! Deactivate after Renaming is done! |
| jdbc.maximumPoolSize | configured per database in package `org.openhab.persistence.jdbc.db.*` | No | Some embedded databases can handle only one connection. See [this link](https://github.com/brettwooldridge/HikariCP/issues/256) for more information |
### Migration from MySQL to JDBC Persistence Services
The JDBC Persistence service can act as a replacement for the MySQL Persistence service.
Here is an example of a configuration for a MySQL database named `testMysql` with user `test` and password `test`:
services/jdbc.cfg
```
url=jdbc:mysql://192.168.0.1:3306/testMysql
user=test
password=test
tableNamePrefix=Item
tableUseRealItemNames=false
tableIdDigitCount=0
```
Remember to install and uninstall the services you want, and rename `persistence/mysql.persist` to `persistence/jdbc.persist`.
## Technical Notes
### Database Table Schema
The table name schema can be reconfigured after creation, if needed.
The service will create a mapping table to link each item to a table, and a separate table is generated for each item.
The item data tables include time and data values.
The SQL data type used depends on the openHAB item type, and allows the item state to be recovered back into openHAB in the same way it was stored.
With this *per-item* layout, the scalability and easy maintenance of the database is ensured, even if large amounts of data must be managed.
To rename existing tables, use the parameters `tableUseRealItemNames` and `tableIdDigitCount` in the configuration.
### Number Precision
Default openHAB number items are persisted with SQL datatype `double`.
Internally openHAB uses `BigDecimal`.
If better numerical precision is needed, for example set `sqltype.NUMBER = DECIMAL(max digits, max decimals)`, then on the Java side, the service works with `BigDecimal` without type conversion.
If more come decimals as `max decimals` provides, this persisted value is rounded mathematically correctly.
The SQL types `DECIMAL` or `NUMERIC` are precise, but to work with `DOUBLE` is faster.
### Rounding results
The results of database queries of number items are rounded to three decimal places by default.
With `numberDecimalcount` decimals can be changed.
