SQLite Tagging Service¶
SQLite tagging service provide APIs to tag both topic names(device points) and topic name prefixes (campus, building, unit/equipment, sub unit) and then query for relevant topics based on saved tag names and values. The SQLite tagging services stores the tags in a sqlite3 database and hence provides a way to use this feature in VOLTTRON out of the box.
Tags used by this agent are not user defined. They have to be pre-defined in a resource file at volttron_data/tagging_resources. The agent validates against this predefined list of tags every time user add tags to topics. Tags can be added to one topic at a time or multiple topics by using a topic name pattern(regular expression). This agent uses tags from project haystack. and adds a few custom tags for campus and VOLTTRON point name.
Each tag has an associated value and users can query for topic names based tags and its values using a simplified sql-like query string. Queries can specify tag names with values or tags without values for boolean tags(markers). Queries can combine multiple conditions with keyword AND and OR, and use the keyword NOT to negate a conditions.
Dependencies and Limitations¶
When adding tags to topics this agent calls the platform.historian's get_topic_list and hence requires the platform.historian to be running but it doesn't require the historian to use sqlite or any specific database. It does not require platform.historian to be running for using its query APIs.
Resource files that provides the list of valid tags is mandatory and should be in volttron_data/tagging_reosurces/tags.csv
Tagging service only provides APIs query for topic names based on tags. Once the list of topic names is retrieved, users should use the historian APIs to get the data corresponding to those topics.
Since RDMS is not a natural fit for tagname=value kind of data, performance of queries will not be high if you have several thousands of topics and several hundreds tags for each topic and perform complex queries. For intermediate level data and query complexity, performance can be improved by increasing the page limit of sqlite.
Current version of tagging service does not support versioning of tag/values. When tags values set using tagging service APIs update/overwrite any existing tag entries in the database
Configuration Options¶
The following JSON configuration file shows all the options currently supported by this agent.
{
# sqlite connection parameters
"connection": {
"type": "sqlite",
"params": {
"database": "~/.volttron/data/volttron.tags.sqlite"
}
},
# optional. Specify if collections created for tagging should have names
# starting with a specific prefix <given prefix>_<collection_name>
"table_prefix":"volttron",
# optional. Specify if you want tagging service to query the historian
# with this vip identity. defaults to platform.historian
"historian_vip_identity": "crate.historian"
}