22. Merged Data Store View

The GeoMesa Merged Data Store View provides a unified way to query multiple data stores concurrently. For example, you may want to store more recent data in an HBase data store instance, and older data in a FileSystem data store instance to reduce storage costs, but still provide a single layer to query both.

Warning

The Merged Data Store View is an alpha-level feature, and may change without notice

In comparison to the Lambda Data Store, the Merged Data Store View does not provide any management for transitioning features between data stores. All writes must be done through the underlying stores directly, and if the same features exist in multiple stores, they may be returned multiple times in a single query response.

In order to use a layer through the Merged Data Store View, the SimpleFeatureType must exist (and match) in all of the underlying stores. If a schema exists in some stores but not all of them, it will not show up in the merged view.

22.1. Installation

The Merged Data Store View is available in the geomesa-index-api JAR, which is bundled by default with all GeoMesa data stores. The underlying stores being merged must be installed separately; see the relevant documentation for each store. Depending on the stores being merged, you may need to resolve classpath conflicts between the store dependencies.

22.2. Usage

The Merged Data Store View can be instantiated through the standard GeoTools DataStoreFinder or the GeoServer New Data Source page. It only requires a single parameter: geomesa.merged.stores.

geomesa.merged.stores should be a TypeSafe Config string the defines the parameters for each merged store. The config should have a top-level key of stores that is a list of objects, where each object is a set of key-value pairs corresponding to the parameters for a single data store.

For example, to merge a GeoMesa Accumulo data store with a PostGis data store, you could use the following config:

{
  "stores": [
    {
      "accumulo.zookeepers": "localhost",
      "accumulo.instance.id": "test",
      "accumulo.catalog": "test",
      "accumulo.user": "test",
      "accumulo.password": "test"
    },
    {
      "dbtype": "postgis",
      "host": "localhost",
      "port": "5432",
      "database": "test",
      "user": "test",
      "passwd": "test"
    }
  ]
}

22.3. Query Filtering

If the stores being merged have known characteristics, filters can be applied selectively to each store in order to speed up queries. The filter is specified along with the other store parameters, under the key geomesa.merged.store.filter, and should be an ECQL filter string.

The filter will be applied against any query, in addition to the query filter. This can be used to short-circuit queries that are not relevant for a particular store. For example, if one store contains features from the past 24 hours, and a second store contains features older than 24 hours, then you could configure them with time-based filters:

{
  "stores": [
    {
      // regular store parameters go here
      "geomesa.merged.store.filter": "dtg >= currentDate('-P1D')"
    },
    {
      // regular store parameters go here
      "geomesa.merged.store.filter": "dtg < currentDate('-P1D')"
    }
  ]
}

22.4. Config Provider

As an alternative to specifying geomesa.merged.stores, config loading can be delegated to a provider interface: org.locationtech.geomesa.index.view.MergedViewConfigLoader. Implementations of this class must be made available via Java SPI loading, with a special descriptor defined in META-INF/services. To use a config provider, use the parameter geomesa.merged.loader set to the full class name of the provider class. In GeoServer, available providers will be displayed in a selection list.

Note that you may combine the explicit configuration of geomesa.merged.stores with the delegated configuration of geomesa.merged.loader, in which case the two configurations will be merged.