add schema validation to concepts (#284)

sdf/_embeds/schema/config-json.yaml

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+config:
+  converter: json

sdf/_embeds/schema/dataflow.yaml

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+apiVersion: 0.5.0
+meta:
+  name: person-age-validation
+  version: 0.1.0
+  namespace: examples
+config:
+  converter: json
+  consumer:
+    default_starting_offset:
+      value: 0
+      position: End
+
+types:
+  user:
+    type: object
+    properties:
+      name:
+        type: string
+      age:
+        type: u8
+
+topics:
+  user-topic:
+    name: user
+    schema:
+      value:
+        type: user
+
+  message-topic:
+    name: message
+    schema:
+      value:
+        type: string
+
+
+services:
+  check-adult:
+    sources:
+      - type: topic
+        id: user-topic
+    transforms:
+      - operator: map
+        run: |
+          fn age_check(user: User) -> Result<String> {
+            if user.age < 18 {
+              Ok("minor".to_string())
+            } else {
+              Ok("adult".to_string())
+            }
+           }
+    sinks:
+      - type: topic
+        id: message-topic

sdf/_embeds/schema/schema-error.log

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+$ sdf log -f
+Error deserializing input value ExpectedUnsigned at character 0

sdf/_embeds/schema/show-state.txt

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+>> show state
+ Namespace                                Keys  Type   
+ check-adult/user-topic/topic.offset      1     offset 
+ check-adult/age-check/metrics            1     table  
+ request-processing/request/metrics       1     table  
+ request-processing/request/topic.offset  1     offset 
+ check-adult/user-topic/metrics           1     table  
+>> show state check-adult/user-topic/metrics
+ Key    Window  succeeded  failed  last_error_offset 
+ stats  *       4          2       5                 
+>> 

sdf/_embeds/schema/topic-schema.yaml

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+topics:
+  persons:
+    schema:
+      value:
+        type: person

sdf/_embeds/schema/user-bad.json

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+{
+    "name": "joe",
+    "age": "30"
+}

sdf/_embeds/schema/user-good.json

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+{
+    "name": "joe",
+    "age": 30
+}

sdf/_embeds/schema/user-good.yaml

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+types:
+  person:
+    type: object
+    properties:
+      name:
+        type: string
+      weight:
+        type: u8

sdf/_embeds/schema/version.yaml

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+apiVersion: 0.5.0
+meta:
+  name: person-age-validation
+  version: 0.1.0
+  namespace: examples

sdf/concepts/schema_validation.mdx

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+---
+title: Schema Validation
+description: Schema Validation
+sidebar_position: 60
+---
+
+import CodeBlock from '@theme/CodeBlock';
+import UserGood from '!!raw-loader!../_embeds/schema/user-good.yaml';
+import TypeSchema from '!!raw-loader!../_embeds/schema/topic-schema.yaml';
+import JsonConfig from '!!raw-loader!../_embeds/schema/config-json.yaml';
+import UserGoodData from '!!raw-loader!../_embeds/schema/user-good.json';
+import UserBadData from '!!raw-loader!../_embeds/schema/user-bad.json';
+import ShowState from '!!raw-loader!../_embeds/schema/show-state.txt';
+import SchemaErrorLog from '!!raw-loader!../_embeds/schema/schema-error.log';
+import Version from '!!raw-loader!../_embeds/schema/version.yaml';
+import DataFlow from '!!raw-loader!../_embeds/schema/dataflow.yaml';
+
+SDF provides a schema validation feature to ensure that the data flowing through the dataflow is in the correct format. 
+
+# Schema
+
+First step is to define data schema.  Schema is defined in the [types] section of the dataflow.  The types can be define as inline or in the package which can be shared across multiple dataflows.
+
+For example, the following is a simple object type representing a person.
+
+<CodeBlock language="yaml">{UserGood}</CodeBlock>
+
+To enforce schema, all you have to is to specify the schema in the `topic` section.  For example, the following is a topic definition with schema:
+
+<CodeBlock language="yaml">{TypeSchema}</CodeBlock>
+
+Schema can be enforced for both key and value part of the record.
+
+Once defined, it can used to enforce the schema on the data from the source.  The enforcement is specific to serialization format.  Currently, SDF supports JSON serialization format but it can be extended to other formats in the future.
+
+The serialization format is defined int the configuration section: 
+
+<CodeBlock language="yaml">{JsonConfig}</CodeBlock>
+
+This will use `json` for all topics.  But you can override per topic. 
+
+
+Given the schema above, the following JSON object will pass the schema validation:
+
+<CodeBlock language="json">{UserGoodData}</CodeBlock>
+
+However, the following JSON object will fail the schema validation:
+
+<CodeBlock language="json">{UserBadData}</CodeBlock>
+
+The schema validation error will be reported in the [operator log].  The error message will indicate the field that failed the validation.  The failed record will be skipped and the dataflow will continue to process the next record.
+
+For example, with bad user data above, the error message will be:
+
+<CodeBlock language="bash">{SchemaErrorLog}</CodeBlock>
+
+Number of failed records will be also reflected in the internal metrics. The metrics can be accessed via the `sdf show state <operator>/metrics` command.
+
+<CodeBlock>{ShowState}</CodeBlock>
+
+The SDF type supports following concepts in the schema:
+- primitive types such as string, integer, float, boolean.
+- enum types
+- composite objects with nested properties
+- array or list of objects
+
+# Versioning
+
+Inline schema's version is inherited from dataflow version. If you want to version the schema, you can define the schema in the package and then version the package.  The versioned package can be then used in the dataflow.
+
+The schema package then can be published to [Hub] and imported into the dataflow.
+
+Version follows semver syntax.  For example, the following is a versioned schema package:
+
+<CodeBlock language="yaml">{Version}</CodeBlock>
+
+The `apiVersion` is the pkg syntax version and `version` in the `meta` section is the schema version.
+
+
+# Dataflow
+
+Full dataflow is defined as follows:
+<CodeBlock language="yaml">{DataFlow}</CodeBlock>
+
+
+
+[types]: /sdf/concepts/types.mdx
+[operator log]: /sdf/cli/log.mdx
+[Hub]: /sdf/composition/hub.mdx