chrontext 0.9.4

Hybrid SPARQL query engine for timeseries data

chrontext: High-performance hybrid query engine for knowledge graphs and analytical data (e.g. time-series)

Chrontext allows you to use your knowledge graph to access large amounts of time-series or other analytical data. It uses a commodity SPARQL Triplestore and your existing data storage infrastructure. It currently supports time-series stored in a PostgreSQL-compatible Database such as DuckDB, Google Cloud BigQuery (SQL) and OPC UA HA, but can easily be extended to other APIs and databases.

Chrontext forms a semantic layer that allows self-service data access, abstracting away technical infrastructure. Users can create query-based inputs for data products, that maintains these data products as the knowledge graph is maintained, and that can be deployed across heterogeneous on-premise and cloud infrastructures with the same API.

Chrontext is a high-performance Python library built in Rust using Polars, and relies heavily on packages from the Oxigraph project. Chrontext works with Apache Arrow, prefers time-series transport using Apache Arrow Flight and delivers results as Polars DataFrames.

Please reach out to Data Treehouse if you would like help trying Chrontext, or require support for a different database backend.

Installing

Chrontext is in pip, just use:

pip install chrontext

The API is documented HERE.

Example query in Python

The code assumes that we have a SPARQL-endpoint and BigQuery set up with time-series. The query uses a bit of syntactic sugar, but is converted to pure SPARQL before execution.

... 

df = engine.query("""
    PREFIX xsd:<http://www.w3.org/2001/XMLSchema#>
    PREFIX ct:<https://github.com/DataTreehouse/chrontext#>
    PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> 
    PREFIX rds: <https://github.com/DataTreehouse/solar_demo/rds_power#> 
    SELECT ?inv_path WHERE {
        # We are navigating th Solar PV site "Metropolis", identifying every inverter. 
        ?site a rds:Site .
        ?site rdfs:label "Metropolis" .
        ?site rds:functionalAspect+ ?inv .    
        ?inv a rds:TBB .                    # RDS code TBB: Inverter
        ?inv rds:path ?inv_path .
        
        # Find the timeseries associated with the inverter
        ?inv ct:hasTimeseries ?ts_pow .
        ?ts_pow rdfs:label "InvPDC_kW" .    
        DT {
            timestamp = ?t,
            timeseries = ?ts_pow, 
            interval = "10m",
            from = "2018-12-25T00:00:00Z",
            aggregation = "avg" }
        }
    ORDER BY ?inv_path ?t
""")

This produces the following DataFrame:

inv_path	t	ts_pow_value_avg
str	datetime[ns]	f64
=<Metropolis>.A1.RG1.TBB1	2018-12-25 00:00:00	0.0
…	…	…
=<Metropolis>.A5.RG9.TBB1	2019-01-01 04:50:00	0.0

Not much power being produced at night in the middle of winter :-)

API

The API is documented HERE.

Tutorial using DuckDB

In the following tutorial, we assume that you have a couple of CSV-files on disk that you want to query. We assume that you have DuckDB and chrontext installed, if not, do pip install chrontext duckdb. Installing chrontext will also install sqlalchemy, which we rely on to define the virtualized DuckDB tables.

CSV files

Our csv files look like this.

ts1.csv :

timestamp,value
2022-06-01T08:46:52,1
2022-06-01T08:46:53,10
..
2022-06-01T08:46:59,105

ts2.csv:

timestamp,value
2022-06-01T08:46:52,2
2022-06-01T08:46:53,20
...
2022-06-01T08:46:59,206

DuckDB setup:

We need to create a class with a method query that takes a SQL string its argument, returning a Polars DataFrame. In this class, we just hard code the DuckDB setup in the constructor.

import duckdb
import polars as pl

class MyDuckDB():
    def __init__(self):
        con = duckdb.connect()
        con.execute("SET TIME ZONE 'UTC';")
        con.execute("""CREATE TABLE ts1 ("timestamp" TIMESTAMPTZ, "value" INTEGER)""")
        ts_1 = pl.read_csv("ts1.csv", try_parse_dates=True).with_columns(pl.col("timestamp").dt.replace_time_zone("UTC"))
        con.append("ts1", df=ts_1.to_pandas())
        con.execute("""CREATE TABLE ts2 ("timestamp" TIMESTAMPTZ, "value" INTEGER)""")
        ts_2 = pl.read_csv("ts2.csv", try_parse_dates=True).with_columns(pl.col("timestamp").dt.replace_time_zone("UTC"))
        con.append("ts2", df=ts_2.to_pandas())
        self.con = con


    def query(self, sql:str) -> pl.DataFrame:
        # We execute the query and return it as a Polars DataFrame.
        # Chrontext expects this method to exist in the provided class.
        df = self.con.execute(sql).pl()
        return df

my_db = MyDuckDB()

Defining a virtualized SQL

We first define a sqlalchemy select query involving the two tables. Chrontext will modify this query when executing hybrid queries.

from sqlalchemy import MetaData, Table, Column, bindparam
metadata = MetaData()
ts1_table = Table(
    "ts1",
    metadata,
    Column("timestamp"),
    Column("value")
)
ts2_table = Table(
    "ts2",
    metadata,
    Column("timestamp"),
    Column("value")
)
ts1 = ts1_table.select().add_columns(
    bindparam("id1", "ts1").label("id"),
)
ts2 = ts2_table.select().add_columns(
    bindparam("id2", "ts2").label("id"),
)
sql = ts1.union(ts2)

Now, we are ready to define the virtualized backend. We will annotate nodes of the graph with a resource data property. These data properties will be linked to virtualized RDF triples in the DuckDB backend. The resource_sql_map decides which SQL is used for each resource property.

from chrontext import VirtualizedPythonDatabase

vdb = VirtualizedPythonDatabase(
    database=my_db,
    resource_sql_map={"my_resource": sql},
    sql_dialect="postgres"
)

The triple below will link the ex:myWidget1 to triples defined by the above sql.

ex:myWidget1 ct:hasResource "my_resource" . 

However, it will only be linked to those triples corresponding to rows where the identifier column equals the identifier associated with ex:myWidget1. Below, we define that ex:instanceA is only linked to those rows where the id column is ts1.

ex:myWidget1 ct:hasIdentifier "ts1" . 

In any such resource sql, the id column is mandatory.

Relating the Database to RDF Triples

Next, we want to relate the rows in this sql, each containing id, timestamp, value to RDF triples, using a template.

from chrontext import Prefix, Variable, Template, Parameter, RDFType, Triple, XSD
ct = Prefix("ct", "https://github.com/DataTreehouse/chrontext#")
xsd = XSD()
id = Variable("id")
timestamp = Variable("timestamp")
value = Variable("value")
dp = Variable("dp")
resources = {
    "my_resource": Template(
        iri=ct.suf("my_resource"),
        parameters=[
            Parameter(id, rdf_type=RDFType.Literal(xsd.string)),
            Parameter(timestamp, rdf_type=RDFType.Literal(xsd.dateTime)),
            Parameter(value, rdf_type=RDFType.Literal(xsd.double)),
        ],
        instances=[
            Triple(id, ct.suf("hasDataPoint"), dp),
            Triple(dp, ct.suf("hasValue"), value),
            Triple(dp, ct.suf("hasTimestamp"), timestamp)
        ]
)}

This means that our instance ex:myWidget1, will be associated with a value and a timestamp (and a blank data point) for each row in ts1.csv. For instance, the first row means we have:

ex:widget1 ct:hasDataPoint _:b1 .
_:b1 ct:hasTimestamp "2022-06-01T08:46:52Z"^^xsd:dateTime .
_:b1 ct:hasValue 1 .

Chrontext is created for those cases when this is infeasibly many triples, so we do not want to materialize them, but query them.

Creating the engine and querying:

The context for our analytical data (e.g. a model of an industrial asset) has to be stored in a SPARQL endpoint. In this case, we use an embedded Oxigraph engine that comes with chrontext. Now we assemble the pieces and create the engine.

from chrontext import Engine, SparqlEmbeddedOxigraph
oxigraph_store = SparqlEmbeddedOxigraph(rdf_file="my_graph.ttl", path="oxigraph_db_tutorial")
engine = Engine(
    resources,
    virtualized_python_database=vdb,
    sparql_embedded_oxigraph=oxigraph_store)
engine.init()

Now we can use our context to query the dataset. The aggregation below are pushed into DuckDB. The example below is a bit simple, but complex conditions can identify the ?w and ?s.

q = """
    PREFIX xsd:<http://www.w3.org/2001/XMLSchema#>
    PREFIX chrontext:<https://github.com/DataTreehouse/chrontext#>
    PREFIX types:<http://example.org/types#>
    SELECT ?w (SUM(?v) as ?sum_v) WHERE {
        ?w types:hasSensor ?s .
        ?s a types:ThingCounter .
        ?s chrontext:hasTimeseries ?ts .
        ?ts chrontext:hasDataPoint ?dp .
        ?dp chrontext:hasTimestamp ?t .
        ?dp chrontext:hasValue ?v .
        FILTER(?t > "2022-06-01T08:46:53Z"^^xsd:dateTime) .
    } GROUP BY ?w
    """
df = engine.query(q)
print(df)

This produces the following result:

w	sum_v
str	decimal[38,0]
http://example.org/case#myWidget1	1215
http://example.org/case#myWidget2	1216

Roadmap in brief

Let us know if you have suggestions!

Stabilization

Chrontext will be put into use in the energy industry during the period, and will be stabilized as part of this process. We are very interested in your bug reports!

Support for Azure Data Explorer / KustoQL

We are likely adding support for ADX/KustoQL. Let us know if this is something that would be useful for you.

Support for Databricks SQL

We are likely adding support for Databricks SQL as the virtualization backend.

Generalization to analytical data (not just time series!)

While chrontext is currently focused on time series data, we are incrementally adding support for contextualization of arbitrary analytical data.

Support for multiple databases

Currently, we only support one database backend at a given time. We plan to support hybrid queries across multiple virtualized databases.

References

Chrontext is joint work by Magnus Bakken and Professor Ahmet Soylu at OsloMet. To read more about Chrontext, read the article Chrontext: Portable Sparql Queries Over Contextualised Time Series Data in Industrial Settings.

License

All code produced since August 1st. 2023 is copyrighted to Data Treehouse AS with an Apache 2.0 license unless otherwise noted.

All code which was produced before August 1st. 2023 copyrighted to Prediktor AS with an Apache 2.0 license unless otherwise noted, and has been financed by The Research Council of Norway (grant no. 316656) and Prediktor AS as part of a PhD Degree. The code at this state is archived in the repository at https://github.com/DataTreehouse/chrontext.