Migration guide

Migrate workloads to Delta Lake

When you migrate workloads to Delta Lake, you should be aware of the following simplifications and differences compared with the data sources provided by Apache Spark and Apache Hive.

Delta Lake handles the following operations automatically, which you should never perform manually:

Add and remove partitions: Delta Lake automatically tracks the set of partitions present in a table and updates the list as data is added or removed. As a result, there is no need to run ALTER TABLE [ADD|DROP] PARTITION or MSCK.
Load a single partition: As an optimization, you may sometimes directly load the partition of data you are interested in. For example, spark.read.format("parquet").load("/data/date=2017-01-01"). This is unnecessary with Delta Lake, since it can quickly read the list of files from the transaction log to find the relevant ones. If you are interested in a single partition, specify it using a WHERE clause. For example, spark.read.delta("/data").where("date = '2017-01-01'"). For large tables with many files in the partition, this can be much faster than loading a single partition (with direct partition path, or with WHERE) from a Parquet table because listing the files in the directory is often slower than reading the list of files from the transaction log.

When you port an existing application to Delta Lake, you should avoid the following operations, which bypass the transaction log:

Manually modify data: Delta Lake uses the transaction log to atomically commit changes to the table. Because the log is the source of truth, files that are written out but not added to the transaction log are not read by Spark. Similarly, even if you manually delete a file, a pointer to the file is still present in the transaction log. Instead of manually modifying files stored in a Delta table, always use the commands that are described in this guide.

External readers: Directly reading the data stored in Delta Lake. For information on how to read Delta tables, see Access Delta tables from external data processing engines.

Example

Suppose you have Parquet data stored in a directory named /data-pipeline, and you want to create a Delta table named events.

The first example shows how to:

Read the Parquet data from its original location, /data-pipeline, into a DataFrame.
Save the DataFrame’s contents in Delta format in a separate location, /tmp/delta/data-pipeline/.
Create the events table based on that separate location, /tmp/delta/data-pipeline/.

The second example shows how to use CONVERT TO TABLE to convert data from Parquet to Delta format without changing its original location, /data-pipeline/.

Save as Delta table

Read the Parquet data into a DataFrame and then save the DataFrame’s contents to a new directory in delta format:

data = spark.read.format("parquet").load("/data-pipeline")
data.write.format("delta").save("/tmp/delta/data-pipeline/")

Create a Delta table named events that refers to the files in the new directory:

spark.sql("CREATE TABLE events USING DELTA LOCATION '/tmp/delta/data-pipeline/'")

Convert to Delta table

You have two options for converting a Parquet table to a Delta table:

Convert files to Delta Lake format and then create a Delta table:

CONVERT TO DELTA parquet.`/data-pipeline/`
CREATE TABLE events USING DELTA LOCATION '/data-pipeline/'

Create a Parquet table and then convert it to a Delta table:

CREATE TABLE events USING PARQUET OPTIONS (path '/data-pipeline/')
CONVERT TO DELTA events

For details, see Convert a Parquet table to a Delta table.

Migrate Delta Lake workloads to newer versions

This section discusses any changes that may be required in the user code when migrating from older to newer versions of Delta Lake.

Delta Lake 2.0 or below to Delta Lake 2.1 or above

When calling CONVERT TO DELTA on a catalog table Delta Lake 2.1 infers the data schema from the catalog. In version 2.0 and below, Delta Lake infers the data schema from the data. This means in Delta 2.1 data columns that are not defined in the original catalog table will not be present in the converted Delta table. This behavior can be disabled by setting the Spark session configuration spark.databricks.delta.convert.useCatalogSchema=false.

Delta Lake 1.2 or below to Delta Lake 2.0 or above

Delta Lake 2.0.0 introduced a behavior change for DROP CONSTRAINT. In version 1.2 and below, no error was thrown when trying to drop a non-existent constraint. In version 2.0.0 and above, the behavior is changed to throw a constraint not exists error. To avoid the error, use IF EXISTS construct (for example, ALTER TABLE events DROP CONSTRAINT IF EXISTS constraint_name). There is no change in behavior in dropping an existing constraint.

Delta Lake 2.0.0 introduced support for Dynamic Partition Overwrites. In version 1.2 and below, enabling dynamic partition overwrite mode in either the Spark session configuration or a DataFrameWriter option was a no-op, and writes in overwrite mode replaced all existing data in every partition of the table. In version 2.0.0 and above, when dynamic partition overwrite mode is enabled, Delta Lake replaces all existing data in each logical partition for which the write will commit new data.

Delta Lake 1.1 or below to Delta Lake 1.2 or above

The LogStore related code is extracted out from the delta-core Maven module into a new module delta-storage as part of the issue #951 for better code manageability. This results in an additional JAR delta-storage-<version>.jar dependency for delta-core. By default, the additional JAR is downloaded as part of the delta-core-<version>_<scala-version>.jar dependency. In clusters where there is no internet connectivity, delta-storage-<version>.jar cannot be downloaded. It is advised to download the delta-storage-<version>.jar manually and place it in the Java classpath.

Delta Lake 1.0 or below to Delta Lake 1.1 or above

If the name of a partition column in a Delta table contains invalid characters ( ,;{}()\n\t=), you cannot read it in Delta Lake 1.1 and above, due to SPARK-36271. However, this should be rare as you cannot create such tables by using Delta Lake 0.6 and above. If you still have such legacy tables, you can overwrite your tables with new valid column names by using Delta Lake 1.0 and below before upgrading Delta Lake to 1.1 and above, such as the following:

spark.read \
  .format("delta") \
  .load("/the/delta/table/path") \
  .withColumnRenamed("column name", "column-name") \
  .write \
  .format("delta") \
  .mode("overwrite") \
  .option("overwriteSchema", "true") \
  .save("/the/delta/table/path")

spark.read
  .format("delta")
  .load("/the/delta/table/path")
  .withColumnRenamed("column name", "column-name")
  .write
  .format("delta")
  .mode("overwrite")
  .option("overwriteSchema", "true")
  .save("/the/delta/table/path")

Delta Lake 0.6 or below to Delta Lake 0.7 or above

If you are using DeltaTable APIs in Scala, Java, or Python to update or run utility operations on them, then you may have to add the following configurations when creating the SparkSession used to perform those operations.

from pyspark.sql import SparkSession

spark = SparkSession \
  .builder \
  .appName("...") \
  .master("...") \
  .config("spark.sql.extensions", "io.delta.sql.DeltaSparkSessionExtension") \
  .config("spark.sql.catalog.spark_catalog", "org.apache.spark.sql.delta.catalog.DeltaCatalog") \
  .getOrCreate()

import org.apache.spark.sql.SparkSession

val spark = SparkSession
  .builder()
  .appName("...")
  .master("...")
  .config("spark.sql.extensions", "io.delta.sql.DeltaSparkSessionExtension")
  .config("spark.sql.catalog.spark_catalog", "org.apache.spark.sql.delta.catalog.DeltaCatalog")
  .getOrCreate()

import org.apache.spark.sql.SparkSession;

SparkSession spark = SparkSession
  .builder()
  .appName("...")
  .master("...")
  .config("spark.sql.extensions", "io.delta.sql.DeltaSparkSessionExtension")
  .config("spark.sql.catalog.spark_catalog", "org.apache.spark.sql.delta.catalog.DeltaCatalog")
  .getOrCreate();

Alternatively, you can add additional configurations when submitting you Spark application using spark-submit or when starting spark-shell/pyspark by specifying them as command line parameters.

spark-submit --conf "spark.sql.extensions=io.delta.sql.DeltaSparkSessionExtension" --conf "spark.sql.catalog.spark_catalog=org.apache.spark.sql.delta.catalog.DeltaCatalog"  ...

pyspark --conf "spark.sql.extensions=io.delta.sql.DeltaSparkSessionExtension" --conf "spark.sql.catalog.spark_catalog=org.apache.spark.sql.delta.catalog.DeltaCatalog"  ...