feat: BigQuery Storage v1beta1 API migration guide

Author: benhxy

bigquery_storage/quickstart/v1beta1-migration-guide.md

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+# Migrating BigQuery Storage API from v1beta1 to v1: Python
+
+This guide shows how to migrate Python code using the BigQuery Storage API from
+version `v1beta1` to `v1`.
+
+## Key Changes
+
+*   **Package Imports**: `google.cloud.bigquery_storage_v1beta1` ->
+    `google.cloud.bigquery_storage_v1`
+*   **Service Client**: `BigQueryStorageClient` is replaced by
+    `BigQueryReadClient`.
+*   **Table Reference**: `TableReference` type is replaced by a simple string
+    representation of the table path in `ReadSession.table`.
+*   **Session Configuration**: Configuration fields (table, format, read
+    options) have moved into `ReadSession` type, which is passed in
+    `CreateReadSessionRequest`.
+*   **Parallelism**: `requested_streams` parameter in `create_read_session` is
+    replaced by `max_stream_count`.
+*   **Sharding Strategy**: `sharding_strategy` is removed. The server now
+    automatically balances the streams.
+*   **Read Rows Request**: `StreamPosition` is flattened. You now pass the
+    stream name directly to `read_rows` as `name` (maps to `read_stream` in
+    proto) and the `offset`.
+
+## Code Comparison
+
+### 1. Client Initialization
+
+**v1beta1:**
+
+```python
+from google.cloud import bigquery_storage_v1beta1
+
+client = bigquery_storage_v1beta1.BigQueryStorageClient()
+```
+
+**v1:**
+
+```python
+from google.cloud import bigquery_storage_v1
+
+client = bigquery_storage_v1.BigQueryReadClient()
+```
+
+### 2. Creating a Read Session
+
+**v1beta1:**
+
+```python
+from google.cloud import bigquery_storage_v1beta1
+
+table_ref = bigquery_storage_v1beta1.types.TableReference()
+table_ref.project_id = "bigquery-public-data"
+table_ref.dataset_id = "usa_names"
+table_ref.table_id = "usa_1910_current"
+
+read_options = bigquery_storage_v1beta1.types.TableReadOptions()
+read_options.selected_fields.append("name")
+read_options.row_restriction = 'state = "WA"'
+
+session = client.create_read_session(
+    table_reference=table_ref,
+    parent='projects/read-session-project',
+    read_options=read_options,
+    requested_streams=1,
+    format=bigquery_storage_v1beta1.types.DataFormat.AVRO,
+    sharding_strategy=bigquery_storage_v1beta1.types.ShardingStrategy.LIQUID
+)
+```
+
+**v1:**
+
+```python
+from google.cloud import bigquery_storage_v1
+
+# Table path is now a string: projects/{project}/datasets/{dataset}/tables/{table}
+table_path = "projects/bigquery-public-data/datasets/usa_names/tables/usa_1910_current"
+
+read_options = bigquery_storage_v1.types.ReadSession.TableReadOptions()
+read_options.selected_fields.append("name")
+read_options.row_restriction = 'state = "WA"'
+
+# ReadSession holds the session configuration
+read_session = bigquery_storage_v1.types.ReadSession(
+    table=table_path,
+    data_format=bigquery_storage_v1.types.DataFormat.AVRO, # format renamed to data_format
+    read_options=read_options
+)
+
+session = client.create_read_session(
+    parent="projects/read-session-project",
+    read_session=read_session,
+    max_stream_count=1 # requested_streams renamed to max_stream_count
+)
+```
+
+### 3. Reading Rows
+
+**v1beta1:**
+
+```python
+from google.cloud import bigquery_storage_v1beta1
+
+position = bigquery_storage_v1beta1.types.StreamPosition(
+    stream=session.streams[0],
+    offset=0
+)
+
+reader = client.read_rows(read_position=position)
+
+for row in reader.rows(session):
+    print(row["name"])
+```
+
+**v1:**
+
+```python
+# read_rows accepts stream name and offset directly.
+# Note that the parameter name in the library helper is 'name', which maps to 'read_stream' in proto.
+reader = client.read_rows(
+    name=session.streams[0].name,
+    offset=0
+)
+
+# In v1, you don't need to pass session to rows() if schema info is already present,
+# but the client library handles it.
+for row in reader.rows():
+    print(row["name"])
+```