diff --git a/business_model.md b/business_model.md
new file mode 100644
index 0000000..3ab2cb8
--- /dev/null
+++ b/business_model.md
@@ -0,0 +1,43 @@
+Yes. Most business questions map to a model family once you work out the unit of analysis (who or what is being counted) and the type of outcome. Here's the mapping using your Trakt and Spotify examples.
+
+**Who**
+- *Questions:* Who are my heavy skippers? Who is most like whom?
+- *Frameworks:* clustering (K-means) for segments, logistic models for propensity (e.g. who completes shows), similarity scores (Jaccard, cosine) for how alike two users are.
+- *Example:* Spotify user A vs. B overlap, or the viewing-day personas from K-means.
+
+**What**
+- *Questions:* What should we recommend? What drives completion?
+- *Frameworks:* recommenders (item-item, Word2Vec), association rules (FP-growth), and GLM coefficients that rank drivers.
+- *Example:* tracks B plays that A hasn't heard, or runtime's effect on finishing a show.
+
+**When**
+- *Questions:* When will demand peak? How long until someone abandons a show? When did habits change?
+- *Frameworks:* time series (ETS, Prophet) for forecasts, survival analysis for time-to-event, Poisson with seasonality terms, change-point detection.
+- *Example:* predicted days until a 50% chance of abandoning a series.
+
+**Where**
+- *Questions:* Which region, device or channel?
+- *Frameworks:* group-level GLMs with country or platform terms, hierarchical models to shrink small-sample regions toward the average.
+- *Example:* Spotify's country and platform fields. Trakt has almost no "where", so there it's mostly a time-zone question.
+
+**How many / how much**
+- *Frameworks:* Poisson or negative binomial for counts, gamma for skewed amounts, with an offset for exposure.
+- *Example:* plays per day on weekends vs. weekdays.
+
+**Why**
+- *Frameworks:* GLM coefficients show association. Causal designs (A/B tests, difference-in-differences) show effect.
+- *Example:* "Did B hearing it first cause A's plays?" needs more than a coefficient.
+
+**The shortcut:** ask what the outcome looks like.
+- A count → Poisson
+- Yes/no → logistic
+- Time until something → survival
+- A skewed amount → gamma
+- Groups of similar things → clustering
+- Next best item → recommender
+
+**Two cautions**
+- Plain SQL answers many who/what/when/where questions descriptively (top artists, busiest days). Reach for a model when you need prediction, comparison with uncertainty, or an effect size.
+- Models give probabilities and rates, not certainties, and sparse data (one or two users) limits which ones are valid.
+
+Want to take one real business question from your data and walk it through question → outcome type → model → features?
\ No newline at end of file
diff --git a/etl_spark.md b/etl_spark.md
new file mode 100644
index 0000000..34da1bd
--- /dev/null
+++ b/etl_spark.md
@@ -0,0 +1,490 @@
+# Building an ETL Pipeline with PySpark and Spark SQL
+
+**Setup:** `pip install pyspark`. Java 8, 11, or 17 must be installed.
+
+---
+
+## Extract Phase
+
+The extract phase is the first step in an ETL pipeline, where raw data is collected from files, databases, and APIs. In PySpark, every extract starts with a `SparkSession`, the entry point for DataFrames and Spark SQL.
+
+In this task you will learn how to:
+
+- Create a SparkSession
+- Read flat files into Spark DataFrames
+- Read data from a SQL database over JDBC
+- Read data from REST APIs into Spark DataFrames
+
+To get started open the `extract.py` file.
+
+### Task 1.0.1: Create a SparkSession
+
+```python
+from pyspark.sql import SparkSession
+
+spark = (
+    SparkSession.builder
+    .appName("etl_pipeline")
+    .config("spark.jars.packages", "com.mysql:mysql-connector-j:8.3.0")  # MySQL JDBC driver, used in Task 1.0.3
+    .getOrCreate()
+)
+```
+
+`.master("local[*]")` runs Spark on your machine. On a real cluster you would remove `.master()` and let the cluster manager supply it.
+
+0 / 1 checks: Create a SparkSession named `spark`.
+
+### Task 1.0.2: Extracting Data from Flat Files
+
+Spark reads these formats natively:
+
+- `.csv`
+- `.json`
+- `.parquet`
+
+Excel is not supported natively, so you would read it with Pandas and convert it (see Task 1.0.4).
+
+For this task:
+
+1. Read `data/example.csv` into `df1` with `spark.read.csv`. Set `header=True` and `inferSchema=True`, otherwise every column is read as a string.
+2. Read `data/example.json` into `df2` with `spark.read.json`.
+3. Read `data/example.parquet` into `df3` with `spark.read.parquet`.
+
+```python
+df1 = spark.read.csv("data/example.csv", header=True, inferSchema=True)
+df2 = spark.read.json("data/example.json")
+df3 = spark.read.parquet("data/example.parquet")
+
+df1.printSchema()
+df1.show(5)
+```
+
+0 / 1 checks: Read different file types into Spark DataFrames using `spark.read`.
+
+### Task 1.0.3: Loading Multiple Files with Wildcards
+
+Spark is built for parallel processing, so reading many files at once is a core use case. Spark will split the files into partitions and process them across your cores or cluster.
+
+1. Observe the `dask_data_*.csv` files in the `data` folder.
+2. Assign `df4` the result of:
+
+```python
+df4 = spark.read.csv("data/dask_data_*.csv", header=True, inferSchema=True)
+```
+
+3. Optionally inspect metadata about the DataFrame:
+
+```python
+print(df4.count())                  # row count (triggers a job)
+print(len(df4.columns))             # column count
+print(df4.rdd.getNumPartitions())   # how many partitions Spark created
+```
+
+0 / 1 checks: Read the `dask_data` files in `data` into the `df4` DataFrame.
+
+### Task 1.0.4: Extract Data from Database Tables (JDBC)
+
+Spark reads databases through JDBC. Instead of a SQLAlchemy engine, you pass connection options to `spark.read.format("jdbc")`.
+
+1. Assign the query to the `query` variable:
+
+```python
+query = "SELECT TABLE_NAME FROM information_schema.TABLES"
+```
+
+2. Read the results into `df5`:
+
+```python
+df5 = (
+    spark.read.format("jdbc")
+    .option("url", "jdbc:mysql://localhost:3306/mydb")
+    .option("query", query)
+    .option("user", "root")
+    .option("password", "your_password")
+    .option("driver", "com.mysql.cj.jdbc.Driver")
+    .load()
+)
+df5.show()
+```
+
+For large tables, add `partitionColumn`, `lowerBound`, `upperBound`, and `numPartitions` so Spark reads in parallel rather than through a single connection.
+
+0 / 1 checks: Read the provided query results into the `df5` Spark DataFrame.
+
+### Task 1.0.5: Extracting REST API Data with Requests
+
+Spark has no built-in REST reader. The common pattern is to fetch the JSON with `requests` on the driver, then convert it into a DataFrame.
+
+1. Open a new terminal tab and run `python3 scripts/app.py` to start the Flask app.
+2. Fetch the data and create a DataFrame:
+
+```python
+import requests
+
+url = "http://localhost:5000/random-user"
+r = requests.get(url)
+print(r.json())
+
+df6 = spark.createDataFrame([r.json()])   # a dict becomes a one-row DataFrame
+df6.show()
+```
+
+For a list of records, pass the list directly: `spark.createDataFrame(r.json())`.
+
+### Task 1.0.6: Authenticating with Requests
+
+Add a Bearer token to the request headers to authenticate to the `/protected` endpoint.
+
+1. In the `headers` dictionary, add the key `Authorization`.
+2. Set the value to `Bearer mysecrettoken123`.
+3. Assign the result to `response`:
+
+```python
+headers = {"Authorization": "Bearer mysecrettoken123"}
+response = requests.get("http://localhost:5000/protected", headers=headers)
+df7 = spark.createDataFrame([response.json()])
+```
+
+0 / 1 checks: Assign the header values and authenticate to the `/protected` endpoint, assigning the results to `response`.
+
+### (Optional) Reading JSON strings with a schema
+
+`createDataFrame` infers types, which can be fragile with nested or inconsistent API data. Define a schema explicitly for predictable results:
+
+```python
+from pyspark.sql.types import StructType, StructField, StringType, IntegerType
+
+schema = StructType([
+    StructField("name", StringType(), True),
+    StructField("age", IntegerType(), True),
+])
+df8 = spark.createDataFrame(response.json(), schema=schema)
+```
+
+---
+
+## Transform Phase
+
+The transform phase is where raw data is cleansed, standardized, aggregated, and joined. PySpark gives you two interchangeable APIs for this: the **DataFrame API** (`pyspark.sql.functions`) and **Spark SQL**. Both compile to the same optimized execution plan, so use whichever reads better.
+
+In this task you will learn how to:
+
+- Convert data types and date formats
+- Replace values and clean dirty data with regex
+- Filter, aggregate, and create calculated columns
+- Join DataFrames
+- Run the same logic with Spark SQL using temp views
+
+**Example Data*
+
+```python
+data = {
+    "Name": ["Adam", "Bob", "Charlie"],
+    "Children": [1, 2, 0],
+    "ZipCode":[10001,90001,60601],
+    "City": ["New York", "Los Angeles", "Chicago"],
+    "Birthdate": [pd.Timestamp('20000101'),pd.Timestamp('19900418'),pd.Timestamp('20180310')],
+    "CharacterSeq": ['****','____','****'],
+    "UserId":['A111','B222','C333'],
+    "TShirtSize":['M','XL','XL'],
+    "Salary":[50000,60000,70000]
+}
+```
+
+```python
+from pyspark.sql import functions as F
+from pyspark.sql.types import StringType, DoubleType
+```
+
+### Task 2.0.1: Convert Column Data Types
+
+Observe the data dictionary. Notice that `ZipCode` is an integer. Convert it to a string with `cast`:
+
+```python
+df = df.withColumn("ZipCode", F.col("ZipCode").cast(StringType()))
+df.printSchema()
+```
+
+DataFrames are immutable in Spark. `withColumn` returns a new DataFrame, so you must assign the result back.
+
+0 / 1 checks: Convert the `ZipCode` column to a string data type.
+
+### Task 2.0.2: Convert Date Formats
+
+`Birthdate` is in `yyyy-MM-dd` format. We want `MM-dd-yyyy`. Parse the string into a date, then format it back out:
+
+```python
+df = df.withColumn(
+    "Birthdate",
+    F.date_format(F.to_date("Birthdate", "yyyy-MM-dd"), "MM-dd-yyyy")
+)
+df.select("Birthdate").show(5)
+```
+
+Spark date patterns are case-sensitive. `MM` is month and `mm` is minutes.
+
+0 / 1 checks: Convert the `Birthdate` column to the `MM-dd-yyyy` format.
+
+### Task 2.0.3: Replace Values with a Mapping
+
+1. Observe the `replace_map` dictionary. The value being replaced comes first, followed by the replacement.
+2. Apply it to the `City` column:
+
+```python
+replace_map = {"NYC": "New York", "LA": "Los Angeles"}
+df = df.replace(replace_map, subset=["City"])
+```
+
+0 / 1 checks: Convert the existing `City` values using `df.replace()`.
+
+### Task 2.0.4: Using Regex to Replace Dirty Data
+
+Use `regexp_replace` to find and replace patterns. `*` is a special regex character, so it must be escaped.
+
+1. Observe the `CharacterSeq` column.
+2. Write a regex pattern that matches four asterisks: `\*{4}` (or `\*\*\*\*`).
+3. Replace them with underscores:
+
+```python
+df = df.withColumn("CharacterSeq", F.regexp_replace("CharacterSeq", r"\*{4}", "____"))
+```
+
+0 / 1 checks: Use a regex on `CharacterSeq` to replace `****` with `____`.
+
+### (Optional) DataFrame Filtering
+
+```python
+# Column expression syntax
+df.filter(F.col("Name") == "Adam")
+
+# SQL string syntax
+df.filter("Name = 'Adam'")
+
+# Assign to a new DataFrame for a sub-dataset
+adams = df.filter(F.col("Name") == "Adam")
+adams.show()
+```
+
+`where()` is an alias for `filter()`.
+
+### Task 2.0.5: Creating Custom Calculated Columns
+
+1. Observe the `cost_of_living_adjust` function, which takes an original value and an `inc_percent`.
+2. Wrap it in a UDF (user-defined function) and create `Salary_COL`:
+
+```python
+def cost_of_living_adjust(value, inc_percent):
+    return value * (1 + inc_percent / 100) if value is not None else None
+
+col_udf = F.udf(lambda s: cost_of_living_adjust(s, 5), DoubleType())
+df = df.withColumn("Salary_COL", col_udf(F.col("Salary")))
+```
+
+Python UDFs are slow because every row is serialized between the JVM and Python. When you can, use native column expressions, which Spark can optimize:
+
+```python
+df = df.withColumn("Salary_COL", F.col("Salary") * (1 + 5 / 100))
+```
+
+0 / 1 checks: Create a new `Salary_COL` column by applying `cost_of_living_adjust` to `Salary` with `inc_percent=5`.
+
+### (Optional) DataFrame Aggregating
+
+```python
+# Whole-DataFrame aggregate
+df.agg(
+    F.count("Salary").alias("count"),
+    F.min("Salary").alias("min"),
+    F.max("Salary").alias("max"),
+    F.mean("Salary").alias("mean"),
+    F.sum("Salary").alias("sum"),
+).show()
+
+# Grouped aggregate
+df.groupBy("TShirtSize").agg(F.mean("Salary").alias("avg_salary")).show()
+```
+
+### Task 2.0.6: Joining Data
+
+Spark's `.join()` mirrors SQL joins. The join types are `inner`, `left`, `right`, `outer`, `left_semi`, and `left_anti`.
+
+1. Observe both DataFrames (`df` and `df2`).
+2. Use `UserId` as the `on` parameter.
+3. Use `left` as the `how` parameter.
+4. Assign the result to `df_merged`:
+
+```python
+df_merged = df.join(df2, on="UserId", how="left")
+df_merged.show()
+```
+
+Passing `on="UserId"` (a string) keeps a single `UserId` column. Joining on `df.UserId == df2.UserId` keeps both, which causes ambiguous-column errors later.
+
+0 / 1 checks: Use `.join` to combine data in `df` and `df2` into `df_merged`.
+
+### Task 2.0.7: Transforming with Spark SQL
+
+Spark SQL lets you query DataFrames with plain SQL. You first register a DataFrame as a temporary view:
+
+1. Register the view:
+
+```python
+df_merged.createOrReplaceTempView("users")
+```
+
+2. Query it with `spark.sql()`, which returns a DataFrame:
+
+```python
+result = spark.sql("""
+    SELECT
+        City,
+        COUNT(*)                       AS user_count,
+        ROUND(AVG(Salary_COL), 2)      AS avg_salary
+    FROM users
+    WHERE Salary_COL IS NOT NULL
+    GROUP BY City
+    HAVING COUNT(*) > 1
+    ORDER BY avg_salary DESC
+""")
+result.show()
+```
+
+3. Because the result is a DataFrame, you can switch back to the DataFrame API at any point and mix the two styles freely.
+
+0 / 1 checks: Register `df_merged` as a temp view named `users` and aggregate it with `spark.sql()`.
+
+---
+
+## Loading Data
+
+Data writing in PySpark refers to saving a Spark DataFrame to files or databases through the `df.write` interface. Spark supports `csv()`, `json()`, `parquet()`, `orc()`, and `jdbc()`, allowing you to export data to files, data lakes, or relational databases.
+
+There are three key differences from Pandas. Spark writes a **directory** of part files (one per partition) rather than a single file. You control overwrite behavior with **save modes** (`overwrite`, `append`, `ignore`, `errorifexists`). And **Parquet** is the preferred format for analytics because it is columnar, compressed, and keeps the schema.
+
+To get started open the `load.py` file.
+
+### Task 3.0.1: Loading Data to a CSV File
+
+1. Create a dictionary with the following python code:
+
+```python
+data = {
+    'Transaction_ID': ['T1001', 'T1002', 'T1003', 'T1004'],
+    'Customer_Name': ['Alice Johnson', 'Bob Lee', 'Charlie Kim', 'David Clark'],
+    'Product': ['Laptop', 'Smartphone', 'Tablet', 'Smartwatch'],
+    'Quantity': [1, 2, 3, 1],
+    'Price_Per_Unit': [1200, 800, 400, 200],
+    'Total_Sale': [1200, 1600, 1200, 200],
+    'Sale_Date': ['2025-03-20', '2025-03-21', '2025-03-21', '2025-03-22']
+}
+```
+
+2. Convert the dictionary into a Spark DataFrame variable `df`. Spark expects a list of rows, so reshape the dictionary first:
+
+```python
+import pandas as pd
+
+df = spark.createDataFrame(pd.DataFrame(data))
+```
+
+3. Use `df.write.csv` to write the contents to `data/Transactions`. Include the following options:
+   - `header=True`
+   - `mode="overwrite"`
+
+```python
+(
+    df.coalesce(1)                 # collapse to one partition, so one part file
+      .write
+      .mode("overwrite")
+      .option("header", True)
+      .csv("data/Transactions")
+)
+```
+
+Spark creates a folder named `Transactions` containing `part-00000-....csv` and a `_SUCCESS` marker. `coalesce(1)` is fine for small data, but avoid it on large datasets because it funnels everything through one task.
+
+0 / 1 checks: Convert the dictionary values to a DataFrame and load it to `Transactions` in the `data` directory.
+
+### Task 3.0.2: Loading Data to Parquet
+
+```python
+df.write.mode("overwrite").parquet("data/Transactions_parquet")
+```
+
+Partitioning by a column splits the output into subfolders, which speeds up later filtered reads:
+
+```python
+df.write.mode("overwrite").partitionBy("Sale_Date").parquet("data/Transactions_by_date")
+```
+
+0 / 1 checks: Write the DataFrame to Parquet.
+
+### Task 3.0.3: Loading Data to a Database with JDBC
+
+```python
+(
+    df.write.format("jdbc")
+    .option("url", "jdbc:mysql://localhost:3306/mydb")
+    .option("dbtable", "transactions")
+    .option("user", "root")
+    .option("password", "your_password")
+    .option("driver", "com.mysql.cj.jdbc.Driver")
+    .mode("append")
+    .save()
+)
+```
+
+Use `append` to add rows. `overwrite` drops and recreates the table unless you also set `.option("truncate", True)`.
+
+0 / 1 checks: Load the DataFrame into a `transactions` table.
+
+### (Optional) Validating Data Operations
+
+Use `assert` statements to validate that the pipeline finished successfully. Check that row counts, column types, and totals match the expected results.
+
+```python
+csv_df = spark.read.csv("data/Transactions", header=True, inferSchema=True)
+
+assert csv_df.count() == 4, "Row counts do not match expected values!"
+print(f"Row counts match: {csv_df.count()} rows")
+
+assert csv_df.agg(F.sum("Total_Sale")).first()[0] == 4200, "Total sales mismatch!"
+
+expected_cols = {"Transaction_ID", "Customer_Name", "Product", "Quantity",
+                 "Price_Per_Unit", "Total_Sale", "Sale_Date"}
+assert set(csv_df.columns) == expected_cols, "Schema mismatch!"
+```
+
+To compare against the database table, read it back with `spark.read.format("jdbc")` and compare `.count()` to the source DataFrame.
+
+---
+
+## Putting It All Together
+
+Wrap the three phases in functions so the pipeline is testable and reusable:
+
+```python
+def extract(spark):
+    return spark.read.csv("data/example.csv", header=True, inferSchema=True)
+
+def transform(df):
+    return (
+        df.withColumn("ZipCode", F.col("ZipCode").cast("string"))
+          .withColumn("Birthdate", F.date_format(F.to_date("Birthdate"), "MM-dd-yyyy"))
+          .withColumn("Salary_COL", F.col("Salary") * 1.05)
+          .dropDuplicates()
+    )
+
+def load(df, path):
+    df.write.mode("overwrite").parquet(path)
+
+if __name__ == "__main__":
+    spark = SparkSession.builder.appName("etl").master("local[*]").getOrCreate()
+    raw = extract(spark)
+    clean = transform(raw)
+    load(clean, "data/output")
+    assert spark.read.parquet("data/output").count() == clean.count()
+    spark.stop()
+```
+
+I can also put this into a downloadable file or turn it into a Jupyter-style notebook.
\ No newline at end of file
diff --git a/spotify_bronze_silver.ipynb b/spotify_bronze_silver.ipynb
index 684dec5..6654fc2 100644
--- a/spotify_bronze_silver.ipynb
+++ b/spotify_bronze_silver.ipynb
@@ -59,13 +59,14 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 16,
+   "execution_count": null,
    "id": "a898326e",
    "metadata": {},
    "outputs": [],
    "source": [
     "                                ## Transform ##\n",
     "def null_counts_by_column(df):\n",
+    "    \"\"\"Count nulls in each column of the DataFrame.\"\"\"\n",
     "    null_counts = df.select([\n",
     "        F.count(F.when(F.col(c).isNull(), c)).alias(c)\n",
     "        for c in df.columns\n",
@@ -73,9 +74,11 @@
     "    return null_counts\n",
     "\n",
     "def metadata_df(df):\n",
+    "    \"\"\"Rename verbose album/artist/track metadata columns to short names.\"\"\"\n",
     "    return df.withColumnRenamed(\"master_metadata_album_album_name\", \"album_name\").withColumnRenamed(\"master_metadata_album_artist_name\", \"artist_name\").withColumnRenamed(\"master_metadata_track_name\", \"track_name\")\n",
     "\n",
     "def rename_columns(df, rename_map, drop_columns):\n",
+    "    \"\"\"Drop specified columns and rename remaining ones per the rename map.\"\"\"\n",
     "    missing_rename = set(rename_map) - set(df.columns)\n",
     "    missing_dropped = set(drop_columns) - set(df.columns)\n",
     "\n",
@@ -89,21 +92,26 @@
     "    return df.toDF(*[rename_map.get(c, c) for c in df.columns])\n",
     "\n",
     "def track_df(df):\n",
+    "    \"\"\"Filter to rows where track_name is not null.\"\"\"\n",
     "    return df.where(df[\"track_name\"].isNotNull())\n",
     "\n",
     "def parse_timestamps(df: DataFrame, columns) -> DataFrame:\n",
+    "    \"\"\"Cast each column in the list to timestamp type.\"\"\"\n",
     "    for column in columns:\n",
     "        df = df.withColumn(column, F.to_timestamp(column))\n",
     "    return df\n",
     "\n",
     "def parse_booleans(df: DataFrame, columns) -> DataFrame:\n",
+    "    \"\"\"Cast each column in the list to string type.\"\"\"\n",
     "    df = df.select([F.col(c).cast(\"string\") for c in columns])\n",
     "    return df\n",
     "\n",
     "def add_minutes_played(df: DataFrame, src, dest) -> DataFrame:\n",
+    "    \"\"\"Convert milliseconds played to minutes and round to 2 decimals.\"\"\"\n",
     "    return (df.withColumn(dest, F.round(F.col(src) / 60000, 2)))\n",
     "\n",
     "def add_time_parts(df: DataFrame, ts_col) -> DataFrame:\n",
+    "    \"\"\"Add time/date/day-of-week/hour parts derived from the timestamp columns.\"\"\"\n",
     "    return (df\n",
     "            .withColumn(\"ts\", F.to_timestamp(\"ts\"))\n",
     "            .withColumn(\"offline_timestamped\", F.to_timestamp(\"offline_timestamp\"))\n",
@@ -116,6 +124,7 @@
     "            )\n",
     "\n",
     "def clean_track_df(df: DataFrame) -> DataFrame:\n",
+    "    \"\"\"Apply the full transform pipeline: rename, filter tracks, add minutes and time parts.\"\"\"\n",
     "    return(df\n",
     "            .transform(rename_columns, SPOTIFY_RENAMES, MUSIC_DROP_COLUMNS)\n",
     "            .transform(track_df)\n",
diff --git a/spotify_silver_gold.ipynb b/spotify_silver_gold.ipynb
index 2c7e1c3..96275e9 100644
--- a/spotify_silver_gold.ipynb
+++ b/spotify_silver_gold.ipynb
@@ -24,9 +24,9 @@
    "source": [
     "                            ## Extract (Load in) ##\n",
     "def start_spark(app_name):\n",
+    "    \"\"\"Initialize and return a SparkSession with the PostgreSQL JDBC driver.\"\"\"\n",
     "    jar_path = str(Path.cwd() / \"postgresql.jar\")\n",
     "    spark = SparkSession.builder.appName(app_name).config(\"spark.jars\", jar_path).config(\"spark.driver.extraClassPath\", jar_path).getOrCreate()\n",
-    "    spark.log.level(\"ERROR\")\n",
     "    return spark\n",
     "\n",
     "def get_jdbc_config():\n",
@@ -56,6 +56,7 @@
     "    return url, properties\n",
     "\n",
     "def read_jdbc(spark, table_name):\n",
+    "    \"\"\"Read a table from Postgres via JDBC into a Spark DataFrame.\"\"\"\n",
     "    url, properties = get_jdbc_config()\n",
     "    return spark.read.jdbc(url=url, table=table_name, properties=properties)\n",
     "\n",