Improve landing page. (#28)

README.md

@@ -1,11 +1,101 @@
 [![Docker](https://img.shields.io/docker/v/linkedfactory/linkedfactory-pod?label=Docker&style=flat)](https://hub.docker.com/r/linkedfactory/linkedfactory-pod)
 
-# Welcome to the LinkedFactory POD repository
+# Welcome to the LinkedFactory Pod repository
 
 This is the reference implementation of the 
-[LinkedFactory specification](https://github.com/linkedfactory/specification).
+**[LinkedFactory architecture](https://github.com/linkedfactory/specification)**.
 
-## Run with docker
+LinkedFactory (LF) is a data architecture concept for structuring time-series data by using semantic descriptions of assets to create connected digital representations of technical systems.
+
+The core idea is to use an RDF knowledge graph for describing the structure of systems with their components and relationships. The components are identified by URIs and may have associated time-series data. To organize these measurements, the component's URI **S** is used as a key in combination with a predicate URI **P** and a timestamp **T** to uniquely qualify values. The resulting composite key is then used to store the associated data in a key-value store or it is mapped to the structure of a tailored time-series database.
+
+<img style="background-color: white; padding: 15px; width: 100%; max-width: 800px" alt="LinkedFactory semantic linking" src="docs/assets/lf-linking.svg">
+
+## Data representation
+Formally, the triple-based data model of RDF _(S, P, O)_ is extended to a quad-based data model _(S, P, T, O)_. If named graphs are used to manage multiple RDF datasets then an additional context **C** can be introduced to extend the data model to _(C, S, P, T, O)_. We call this the **Kvin** data model.
+
+### JSON format
+The primary data format is the __LF JSON format__ that uses a nested structure where the first level contains the items and the second level the associated properties with their values:
+
+```json
+{
+    "http://example.org/resource1": {
+       "http://example.org/properties/p1": [
+           { "value": 20.4, "time": 1619424246120 },
+           { "value": 20.3, "time": 1619424246100 }
+       ],
+       "http://example.org/properties/p2": [
+           { "value": { "msg" : "Error 1", "nr" : 1 }, "time": 1619424246100 }
+       ]
+    }
+}
+```
+
+A concrete example for modeling the captured force data of a strain gauge could be represented as:
+```json
+{
+  "https://example.org/Press/Frame/StrainGauge": {
+    "p:force" : [{ "value": 7.10096884, "time": 1541521440000 }]
+  }
+}
+```
+### RDF format
+For representing time-series data in plain RDF the following encoding is used:
+```
+S P [ <kvin:value> O ; <kvin:time> T ] .
+```
+
+The data of the strain gauge example above could then expressed as:
+```
+@base <https://example.org/Press/Frame/> .
+
+<StrainGauge> <p:force> [ <kvin:value> 7.10096884 ; <kvin:time> 1541521440000 ] .
+```
+
+## Data insertion and retrieval
+### HTTP APIs
+The data can be inserted and queried by using __[HTTP-based APIs](https://linkedfactory.github.io/specification/overview/1/timeseries/api)__.
+
+For inserting the data the LF JSON format can be used as follows:
+```sh
+curl -H "Content-Type:application/json" http://localhost:8080/linkedfactory/values -d '{
+  "https://example.org/Press/Frame/StrainGauge": {
+    "p:force" : [{ "value": 7.10096884, "time": 1541521440000 }]
+  }
+}'
+```
+If data with the same timestamps should be inserted then the CSV format is more concise:
+```sh
+curl -H "Content-Type:text/csv" http://localhost:8080/linkedfactory/values -d 'time,<https://example.org/Press/Frame/StrainGauge>@<p:force>
+1541521440000, 7.10096884'
+```
+
+Retrieving the data in JSON format is possible through a simple GET request:
+```sh
+curl -G -d "item=https://example.org/Press/Frame/StrainGauge" -d "property=p:force" http://localhost:8080/linkedfactory/values
+```
+
+Just specify an accept header to retrieve data in CSV format:
+```sh
+curl -G -H "Accept:text/csv" -d "item=https://example.org/Press/Frame/StrainGauge" -d "property=p:force" http://localhost:8080/linkedfactory/values
+```
+
+### SPARQL
+Additionaly, due to the compatiblity with RDF, it is possible to query the data with SPARQL.
+
+```
+curl -H "Accept:text/csv" http://localhost:8080/sparql --data-urlencode 'query=
+base <https://example.org/Press/Frame/>
+
+select ?time ?value {
+  service <kvin:> {
+    <StrainGauge> <p:force> [ <kvin:value> ?value ; <kvin:time> ?time ] .
+  }
+}' --data-urlencode 'model=http://linkedfactory.github.io/data/'
+```
+
+## Docker
+* We provide containers on [Docker Hub](https://hub.docker.com/r/linkedfactory/linkedfactory-pod)
 * `docker run -p 8080:8080 -v /tmp/workspace:/linkedfactory-pod/workspace linkedfactory/linkedfactory-pod`
 
 ## Building