Using multiple bindings

It's easy to use more than one database in your reports. Here's an example. In my office I have two consoles: a VantagePro2 connected to a Dell Optiplex, and a WMR100N, connected to a Raspberry Pi. Each is running WeeWX. The Dell is using SQLite, the RPi, MySQL.

Suppose I wish to compare the inside temperatures of the two consoles. How would I do that?

It's easier to access MySQL across a network than SQLite, so let's run the reports on the Dell, but access the RPi's MySQL database remotely. Here's how the bindings and database sections of weewx.conf would look on the Dell:

[DataBindings]
    # This section binds a data store to an actual database

    [[wx_binding]]
        # The database to be used - it should match one of the sections in [Databases]
        database = archive_sqlite
        # The name of the table within the database
        table_name = archive
        # The class to manage the database
        manager = weewx.manager.DaySummaryManager
        # The schema defines to structure of the database contents
        schema = schemas.wview_extended.schema

    [[wmr100_binding]]
        # Binding for my WMR100 on the RPi
        database = rpi_mysql
        # The name of the table within the database
        table_name = archive
        # The class to manage the database
        manager = weewx.manager.DaySummaryManager
        # The schema defines to structure of the database contents
        schema = schemas.wview_extended.schema

[Databases]
    # This section binds to the actual database to be used

    [[archive_sqlite]]
        database_type = SQLite
        database_name = weewx.sdb

    [[rpi_mysql]]
        database_type = MySQL
        database_name = weewx
        host = rpi-bug

[DatabaseTypes]
    #   This section defines defaults for the different types of databases.

    [[SQLite]]
        driver = weedb.sqlite
        # Directory in which the database files are located
        SQLITE_ROOT = archive

    [[MySQL]]
        driver = weedb.mysql
        # The host where the database is located
        host = localhost
        # The user name for logging in to the host
        user = weewx
        # The password for the user name
        password = weewx

The two additions have been highlighted. The first, [[wmr100_binding]], adds a new binding called wmr100_binding. It links ("binds") to the new database, called rpi_mysql, through the option database. It also defines some characteristics of the binding, such as which manager is to be used and what its schema looks like.

The second addition, [[rpi-mysql]], defines the new database. Option database_type is set to MySQL, indicating that it is a MySQL database. Defaults for MySQL databases are defined in the section [[MySQL]]. The new database accepts all of them, except for host, which as been set to the remote host rpi-bug, the name of my Raspberry Pi.

Explicit binding in tags

How do we use this new binding? First, let's do a text comparison, using tags. Here's what our template looks like:

<table>
  <tr>
    <td class="stats_label">Inside Temperature, Vantage</td>
    <td class="stats_data">$current.inTemp</td>
  </tr>
  <tr>
    <td class="stats_label">Inside Temperature, WMR100</td>
    <td class="stats_data">$latest($data_binding='wmr100_binding').inTemp</td>
  </tr>
</table>

The explicit binding to wmr100_binding is highlighted. This tells the reporting engine to override the default binding specifed in [StdReport], generally wx_binding, and use wmr100_binding instead.

Inside Temperature, Vantage 68.7°F
Inside Temperature, WMR100 68.9°F

Explicit binding in images

How would we produce a graph of the two different temperatures? Here's what the relevant section of the skin.conf file would look like.

[[[daycompare]]]
   [[[[inTemp]]]]
       label = Vantage inTemp
   [[[[WMR100Temp]]]]
       data_type = inTemp
       data_binding = wmr100_binding
       label = WMR100 inTemp

This will produce an image with name daycompare.png, with two plot lines. The first will be of the temperature from the Vantage. It uses the default binding, wx_binding, and will be labeled Vantage inTemp. The second line explicitly uses the wmr100_binding. Because it uses the same variable name (inTemp) as the first line, we had to explicitly specify it using option data_type, in order to avoid using the same subsection name twice (see the section Including a type more than once in a plot for details). It will be labeled WMR100 inTemp. The results look like this:

Stupid detail

At first, I could not get this example to work. The problem turned out to be that the RPi was processing things just a beat behind the Dell, so the temperature for the "current" time wasn't ready when the Dell needed it. I kept getting N/A. To avoid this, I introduced the tag $latest, which uses the last available timestamp in the binding, which may or may not be the same as what $current uses. That's why the example above uses $latest instead of $current.