[FLINK-15999][doc] Remove programming-model section and incorporate i…

…nto overview

This also adds explanation about the concepts section

docs/concepts/index.md

@@ -4,8 +4,7 @@ nav-id: concepts
 nav-pos: 2
 nav-title: '<i class="fa fa-map-o title appetizer" aria-hidden="true"></i> Concepts'
 nav-parent_id: root
-layout: redirect
-redirect: /concepts/programming-model.html
+nav-show_overview: true
 permalink: /concepts/index.html
 always-expand: true
 ---
@@ -27,3 +26,58 @@ KIND, either express or implied. See the License for the
 specific language governing permissions and limitations
 under the License.
 -->
+
+Flink offers different levels of abstraction for developing streaming/batch applications.
+
+<img src="{{ site.baseurl }}/fig/levels_of_abstraction.svg" alt="Programming levels of abstraction" class="offset" width="80%" />
+
+ - The lowest level abstraction simply offers **stateful streaming**. It is
+ embedded into the [DataStream API]({{ site.baseurl}}{% link
+ dev/datastream_api.md %}) via the [Process Function]({{ site.baseurl }}{%
+ link dev/stream/operators/process_function.md %}). It allows users freely
+ process events from one or more streams, and use consistent fault tolerant
+ *state*. In addition, users can register event time and processing time
+ callbacks, allowing programs to realize sophisticated computations.
+
+ - In practice, most applications would not need the above described low level
+ abstraction, but would instead program against the **Core APIs** like the
+ [DataStream API]({{ site.baseurl }}{% link dev/datastream_api.md %})
+ (bounded/unbounded streams) and the [DataSet API]({{ site.baseurl }}{% link
+ dev/batch/index.md %}) (bounded data sets). These fluent APIs offer the
+ common building blocks for data processing, like various forms of
+ user-specified transformations, joins, aggregations, windows, state, etc.
+ Data types processed in these APIs are represented as classes in the
+ respective programming languages.
+
+ The low level *Process Function* integrates with the *DataStream API*,
+ making it possible to go the lower level abstraction for certain operations
+ only. The *DataSet API* offers additional primitives on bounded data sets,
+ like loops/iterations.
+
+ - The **Table API** is a declarative DSL centered around *tables*, which may
+ be dynamically changing tables (when representing streams). The [Table
+ API]({{ site.baseurl }}{% link dev/table/index.md %}) follows the
+ (extended) relational model: Tables have a schema attached (similar to
+ tables in relational databases) and the API offers comparable operations,
+ such as select, project, join, group-by, aggregate, etc. Table API
+ programs declaratively define *what logical operation should be done*
+ rather than specifying exactly *how the code for the operation looks*.
+ Though the Table API is extensible by various types of user-defined
+ functions, it is less expressive than the *Core APIs*, but more concise to
+ use (less code to write). In addition, Table API programs also go through
+ an optimizer that applies optimization rules before execution.
+
+ One can seamlessly convert between tables and *DataStream*/*DataSet*,
+ allowing programs to mix *Table API* and with the *DataStream* and
+ *DataSet* APIs.
+
+ - The highest level abstraction offered by Flink is **SQL**. This abstraction
+ is similar to the *Table API* both in semantics and expressiveness, but
+ represents programs as SQL query expressions. The [SQL]({{ site.baseurl
+ }}{% link dev/table/index.md %}#sql) abstraction closely interacts with the
+ Table API, and SQL queries can be executed over tables defined in the
+ *Table API*.
+
+This _concepts_ section explains the basic concepts behind the different APIs,
+that is the concepts behind Flink as a stateful and timely stream processing
+system.

docs/concepts/index.zh.md

@@ -4,8 +4,7 @@ nav-id: concepts
 nav-pos: 2
 nav-title: '<i class="fa fa-map-o title appetizer" aria-hidden="true"></i> 概念'
 nav-parent_id: root
-layout: redirect
-redirect: /concepts/programming-model.html
+nav-show_overview: true
 permalink: /concepts/index.html
 always-expand: true
 ---
@@ -27,3 +26,58 @@ KIND, either express or implied. See the License for the
 specific language governing permissions and limitations
 under the License.
 -->
+
+Flink offers different levels of abstraction for developing streaming/batch applications.
+
+<img src="{{ site.baseurl }}/fig/levels_of_abstraction.svg" alt="Programming levels of abstraction" class="offset" width="80%" />
+
+ - The lowest level abstraction simply offers **stateful streaming**. It is
+ embedded into the [DataStream API]({{ site.baseurl}}{% link
+ dev/datastream_api.md %}) via the [Process Function]({{ site.baseurl }}{%
+ link dev/stream/operators/process_function.md %}). It allows users freely
+ process events from one or more streams, and use consistent fault tolerant
+ *state*. In addition, users can register event time and processing time
+ callbacks, allowing programs to realize sophisticated computations.
+
+ - In practice, most applications would not need the above described low level
+ abstraction, but would instead program against the **Core APIs** like the
+ [DataStream API]({{ site.baseurl }}{% link dev/datastream_api.md %})
+ (bounded/unbounded streams) and the [DataSet API]({{ site.baseurl }}{% link
+ dev/batch/index.md %}) (bounded data sets). These fluent APIs offer the
+ common building blocks for data processing, like various forms of
+ user-specified transformations, joins, aggregations, windows, state, etc.
+ Data types processed in these APIs are represented as classes in the
+ respective programming languages.
+
+ The low level *Process Function* integrates with the *DataStream API*,
+ making it possible to go the lower level abstraction for certain operations
+ only. The *DataSet API* offers additional primitives on bounded data sets,
+ like loops/iterations.
+
+ - The **Table API** is a declarative DSL centered around *tables*, which may
+ be dynamically changing tables (when representing streams). The [Table
+ API]({{ site.baseurl }}{% link dev/table/index.md %}) follows the
+ (extended) relational model: Tables have a schema attached (similar to
+ tables in relational databases) and the API offers comparable operations,
+ such as select, project, join, group-by, aggregate, etc. Table API
+ programs declaratively define *what logical operation should be done*
+ rather than specifying exactly *how the code for the operation looks*.
+ Though the Table API is extensible by various types of user-defined
+ functions, it is less expressive than the *Core APIs*, but more concise to
+ use (less code to write). In addition, Table API programs also go through
+ an optimizer that applies optimization rules before execution.
+
+ One can seamlessly convert between tables and *DataStream*/*DataSet*,
+ allowing programs to mix *Table API* and with the *DataStream* and
+ *DataSet* APIs.
+
+ - The highest level abstraction offered by Flink is **SQL**. This abstraction
+ is similar to the *Table API* both in semantics and expressiveness, but
+ represents programs as SQL query expressions. The [SQL]({{ site.baseurl
+ }}{% link dev/table/index.md %}#sql) abstraction closely interacts with the
+ Table API, and SQL queries can be executed over tables defined in the
+ *Table API*.
+
+This _concepts_ section explains the basic concepts behind the different APIs,
+that is the concepts behind Flink as a stateful and timely stream processing
+system.