[YAML] Add documentation to Beam site.

website/www/site/content/en/documentation/sdks/yaml-combine.md

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+<!--
+    Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+-->
+
+# Beam YAML Aggregations
+
+Beam YAML has EXPERIMENTAL ability to do aggregations to group and combine
+values across records. The is accomplished via the `Combine` transform type.
+Currently `Combine` needs to be in the `yaml_experimental_features`
+option to use this transform.
+
+For example, one can write
+
+```
+- type: Combine
+  config:
+    group_by: col1
+    combine:
+      total:
+        value: col2
+        fn:
+          type: sum
+```
+
+If the function has no configuration requirements, it can be provided directly
+as a string
+
+```
+- type: Combine
+  config:
+    group_by: col1
+    combine:
+      total:
+        value: col2
+        fn: sum
+```
+
+This can be simplified further if the output field name is the same as the input
+field name
+
+```
+- type: Combine
+  config:
+    group_by: col1
+    combine:
+      col2: sum
+```
+
+One can aggregate over may fields at once
+
+```
+- type: Combine
+  config:
+    group_by: col1
+    combine:
+      col2: sum
+      col3: max
+```
+
+and/or group by more than one field
+
+```
+- type: Combine
+  config:
+    group_by: [col1, col2]
+    combine:
+      col3: sum
+```
+
+or none at all (which will result in a global combine with a single output)
+
+```
+- type: Combine
+  config:
+    group_by: []
+    combine:
+      col2: sum
+      col3: max
+```
+
+## Windowed aggregation
+
+As with all transforms, `Combine` can take a windowing parameter
+
+```
+- type: Combine
+  windowing:
+    type: fixed
+    size: 60
+  config:
+    group_by: col1
+    combine:
+      col2: sum
+      col3: max
+```
+
+If no windowing specification is provided, it inherits the windowing
+parameters from upstream, e.g.
+
+```
+- type: WindowInto
+  windowing:
+    type: fixed
+    size: 60
+- type: Combine
+  config:
+    group_by: col1
+    combine:
+      col2: sum
+      col3: max
+```
+
+is equivalent to the previous example.
+
+
+## Custom aggregation functions
+
+One can use aggregation functions defined in Python by setting the language
+parameter.
+
+```
+- type: Combine
+  config:
+    language: python
+    group_by: col1
+    combine:
+      biggest:
+        value: "col2 + col2"
+        fn:
+          type: 'apache_beam.transforms.combiners.TopCombineFn'
+          config:
+            n: 10
+```
+
+## SQL-style aggregations
+
+By setting the language to SQL, one can provide full SQL snippets as the
+combine fn.
+
+```
+- type: Combine
+  config:
+    language: sql
+    group_by: col1
+    combine:
+      num_values: "count(*)"
+      total: "sum(col2)"
+```
+
+One can of course also use the `Sql` transform type and provide a query
+directly.

website/www/site/content/en/documentation/sdks/yaml-errors.md

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+<!--
+    Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+-->
+
+# Beam YAML Error Handling
+
+The larger one's pipeline gets, the more common it is to encounter "exceptional"
+data that is malformatted, doesn't handle the proper preconditions, or otherwise
+breaks during processing.  Generally any such record will cause the pipeline to
+permanently fail, but often it is desirable to allow the pipeline to continue,
+re-directing bad records to another path for special handling or simply
+recording them for later off-line analysis.  This is often called the
+"dead letter queue" pattern.
+
+Beam YAML has special support for this pattern if the transform supports a
+`error_handling` config parameter with an `output` field.  For example,
+the following code will write all "good" processed records to one file and
+any "bad" records to a separate file.
+
+```
+pipeline:
+  transforms:
+    - type: ReadFromCsv
+      config:
+        path: /path/to/input*.csv
+
+    - type: MapToFields
+      input: ReadFromCsv
+      config:
+        language: python
+        fields:
+          col1: col1
+          # This could raise a divide-by-zero error.
+          ratio: col2 / col3
+        error_handling:
+          output: my_error_output
+
+    - type: WriteToJson
+      input: MapToFields
+      config:
+        path: /path/to/output.json
+
+    - type: WriteToJson
+      name: WriteErrorsToJson
+      input: MapToFields.my_error_output
+      config:
+        path: /path/to/errors.json
+```
+
+Note that with `error_handling` declared, `MapToFields.my_error_output`
+**must** be consumed; to ignore it will be an error.  Any use is fine, e.g.
+logging the bad records to stdout would be sufficient (though not recommended
+for a robust pipeline).
+
+Note also that the exact format of the error outputs is still being finalized.
+They can be safely printed and written to outputs, but their precise schema
+may change in a future version of Beam and should not yet be depended on.
+
+Some transforms allow for extra arguments in their error_handling config, e.g.
+for Python functions one can give a `threshold` which limits the relative number
+of records that can be bad before considering the entire pipeline a failure
+
+```
+pipeline:
+  transforms:
+    - type: ReadFromCsv
+      config:
+        path: /path/to/input*.csv
+
+    - type: MapToFields
+      input: ReadFromCsv
+      config:
+        language: python
+        fields:
+          col1: col1
+          # This could raise a divide-by-zero error.
+          ratio: col2 / col3
+        error_handling:
+          output: my_error_output
+          # If more than 10% of records throw an error, stop the pipeline.
+          threshold: 0.1
+
+    - type: WriteToJson
+      input: MapToFields
+      config:
+        path: /path/to/output.json
+
+    - type: WriteToJson
+      name: WriteErrorsToJson
+      input: MapToFields.my_error_output
+      config:
+        path: /path/to/errors.json
+```
+
+One can do arbitrary further processing on these failed records if desired,
+e.g.
+
+```
+pipeline:
+  transforms:
+    - type: ReadFromCsv
+      config:
+        path: /path/to/input*.csv
+
+    - type: MapToFields
+      name: ComputeRatio
+      input: ReadFromCsv
+      config:
+        language: python
+        fields:
+          col1: col1
+          # This could raise a divide-by-zero error.
+          ratio: col2 / col3
+        error_handling:
+          output: my_error_output
+
+    - type: MapToFields
+      name: ComputeRatioForBadRecords
+      input: ComputeRatio.my_error_output
+      config:
+        language: python
+        fields:
+          col1: col1
+          ratio: col2 / (col3 + 1)
+        error_handling:
+          output: still_bad
+
+    - type: WriteToJson
+      # Takes as input everything from the "success" path of both transforms.
+      input: [ComputeRatio, ComputeRatioForBadRecords]
+      config:
+        path: /path/to/output.json
+
+    - type: WriteToJson
+      name: WriteErrorsToJson
+      # These failed the first and the second transform.
+      input: ComputeRatioForBadRecords.still_bad
+      config:
+        path: /path/to/errors.json
+```
+
+When using the `chain` syntax, the required error consumption can happen
+in an `extra_transforms` block.
+
+```
+pipeline:
+  type: chain
+  transforms:
+    - type: ReadFromCsv
+      config:
+        path: /path/to/input*.csv
+
+    - type: MapToFields
+      name: SomeStep
+      config:
+        language: python
+        fields:
+          col1: col1
+          # This could raise a divide-by-zero error.
+          ratio: col2 / col3
+        error_handling:
+          output: errors
+
+    - type: MapToFields
+      name: AnotherStep
+      config:
+        language: python
+        fields:
+          col1: col1
+          # This could raise a divide-by-zero error.
+          inverse_ratio: 1 / ratio
+        error_handling:
+          output: errors
+
+    - type: WriteToJson
+      config:
+        path: /path/to/output.json
+
+  extra_transforms:
+    - type: WriteToJson
+      name: WriteErrors
+      input: [SomeStep.errors, AnotherStep.errors]
+      config:
+        path: /path/to/errors.json
+```