Detect and deal with knowledge skew on AWS Glue

AWS Glue is a totally managed, serverless knowledge integration service supplied by Amazon Net Providers (AWS) that makes use of Apache Spark as one in all its backend processing engines (as of this writing, you should utilize Python Shell, Spark, or Ray).

Information skew happens when the information being processed shouldn’t be evenly distributed throughout the Spark cluster, inflicting some duties to take considerably longer to finish than others. This may result in inefficient useful resource utilization, longer processing instances, and finally, slower efficiency. Information skew can come up from numerous components, together with uneven knowledge distribution, skewed be part of keys, or uneven knowledge processing patterns. Although the most important concern is usually having nodes working out of disk throughout shuffling, which results in nodes falling like dominoes and job failures, it’s additionally essential to say that knowledge skew is hidden. The stealthy nature of information skew means it might usually go undetected as a result of monitoring instruments may not flag an uneven distribution as a crucial concern, and logs don’t at all times make it evident. Consequently, a developer might observe that their AWS Glue jobs are finishing with out obvious errors, but the system may very well be working removed from its optimum effectivity. This hidden inefficiency not solely will increase operational prices resulting from longer runtimes however may also result in unpredictable efficiency points which might be troublesome to diagnose with no deep dive into the information distribution and job run patterns.

For instance, in a dataset of buyer transactions, if one buyer has considerably extra transactions than the others, it might trigger a skew within the knowledge distribution.

Figuring out and dealing with knowledge skew points is vital to having good efficiency on Apache Spark and subsequently on AWS Glue jobs that use Spark as a backend. On this submit, we present how one can determine knowledge skew and talk about the completely different methods to mitigate knowledge skew.

Tips on how to detect knowledge skew

When an AWS Glue job has points with native disks (break up disk points), doesn’t scale with the variety of employees, or has low CPU utilization (you may enable Amazon CloudWatch metrics to your job to have the ability to see this), you’ll have a knowledge skew concern. You may detect knowledge skew with knowledge evaluation or by utilizing the Spark UI. On this part, we talk about the best way to use the Spark UI.

The Spark UI gives a complete view of Spark purposes, together with the variety of duties, levels, and their length. To make use of it you should allow Spark UI event logs for your job runs. It’s enabled by default on Glue console and as soon as enabled, Spark occasion log information might be created throughout the job run and saved in your S3 bucket. Then, these logs are parsed, and you should utilize the AWS Glue serverless Spark UI to visualise them. You may confer with this blogpost for extra particulars. In these jobs the place the AWS Glue serverless Spark UI doesn’t work because it has a restrict of 512 MB of logs, you may set up the Spark UI utilizing an EC2 occasion.

You need to use the Spark UI to determine which duties are taking longer to finish than others, and if the information distribution amongst partitions is balanced or not (keep in mind that in Spark, one partition is mapped to 1 job). If there’s knowledge skew, you will notice that some partitions have considerably extra knowledge than others. The next determine exhibits an instance of this. We are able to see that one job is taking much more time than the others, which might point out knowledge skew.

One other factor that you should utilize is the abstract metrics for every stage. The next screenshot exhibits one other instance of information skew.

These metrics characterize the task-related metrics beneath which a sure proportion of duties accomplished. For instance, the seventy fifth percentile job length signifies that 75% of duties accomplished in much less time than this worth. When the duties are evenly distributed, you will notice related numbers in all of the percentiles. When there’s knowledge skew, you will notice very biased values in every percentile. Within the previous instance, it didn’t write many shuffle information (lower than 50 MiB) in Min, twenty fifth percentile, Median, and seventy fifth percentile. Nonetheless, in Max, it wrote 460 MiB, 10 instances the seventy fifth percentile. It means there was at the very least one job (or as much as 25% of duties) that wrote a lot larger shuffle information than the remainder of the duties. You too can see that the length of the tax in Max is 46 seconds and the Median is 2 seconds. These are all indicators that your dataset might have knowledge skew.

AWS Glue interactive periods

You need to use interactive sessions to load your knowledge from the AWS Glue Information Catalog or simply use Spark strategies to load the information resembling Parquet or CSV that you simply need to analyze. You need to use an analogous script to the next to detect knowledge skew from the partition dimension perspective; the extra essential concern is expounded to knowledge skew whereas shuffling, and this script doesn’t detect that form of skew:

from pyspark.sql.features import spark_partition_id, asc, desc
#input_dataframe being the dataframe the place you need to test for knowledge skew
    .withColumn("partitionId", spark_partition_id())
#calculate common and standar deviation for the partition sizes
avg_size = partition_sizes_df.agg({"partition_size": "avg"}).gather()[0][0]
std_dev_size = partition_sizes_df.agg({"partition_size": "stddev"}).gather()[0][0]

 the code calculates absolutely the distinction between every worth within the "partition_size" column and the calculated common (avg_size).
 then, calculates twice the usual deviation (std_dev_size) and use 
 that as a boolean masks the place the situation checks if absolutely the distinction is bigger than twice the usual deviation
 as a way to mark a partition 'skewed'
skewed_partitions_df = partition_sizes_df.filter(abs(partition_sizes_df["partition_size"] - avg_size) > 2 * std_dev_size)
if skewed_partitions_df.rely() > 0:
    skewed_partitions = [row["partition_id"] for row in skewed_partitions_df.gather()]
    print(f"The next partitions have considerably completely different sizes: {skewed_partitions}")
    print("No knowledge skew detected.")

You may calculate the common and customary deviation of partition sizes utilizing the agg() perform and determine partitions with considerably completely different sizes utilizing the filter() perform, and you may print their indexes if any skewed partitions are detected. In any other case, the output prints that no knowledge skew is detected.

This code assumes that your knowledge is structured, and it’s possible you’ll want to switch it in case your knowledge is of a special kind.

Tips on how to deal with knowledge skew

You need to use completely different methods in AWS Glue to deal with knowledge skew; there is no such thing as a single common resolution. The very first thing to do is verify that you simply’re utilizing newest AWS Glue model, for instance AWS Glue 4.0 based mostly on Spark 3.3 has enabled by default some configs like Adaptative Question Execution (AQE) that may assist enhance efficiency when knowledge skew is current.

The next are among the methods which you can make use of to deal with knowledge skew:

  • Filter and carry out – If you realize which keys are inflicting the skew, you may filter them out, carry out your operations on the non-skewed knowledge, after which deal with the skewed keys individually.
  • Implementing incremental aggregation – If you’re performing a big aggregation operation, you may break it up into smaller levels as a result of in giant datasets, a single aggregation operation (like sum, common, or rely) will be resource-intensive. In these circumstances, you may carry out intermediate actions. This might contain filtering, grouping, or extra aggregations. This can assist distribute the workload throughout the nodes and scale back the scale of intermediate knowledge.
  • Utilizing a customized partitioner – In case your knowledge has a particular construction or distribution, you may create a customized partitioner that partitions your knowledge based mostly on its traits. This can assist guarantee that knowledge with related traits is in the identical partition and scale back the scale of the most important partition.
  • Utilizing broadcast be part of – In case your dataset is small however exceeds the spark.sql.autoBroadcastJoinThreshold worth (default is 10 MB), you’ve gotten the choice to both present a touch to make use of broadcast be part of or alter the edge worth to accommodate your dataset. This may be an efficient technique to optimize be part of operations and mitigate knowledge skew points ensuing from shuffling giant quantities of information throughout nodes.
  • Salting – This includes including a random prefix to the important thing of skewed knowledge. By doing this, you distribute the information extra evenly throughout the partitions. After processing, you may take away the prefix to get the unique key values.

These are just some methods to deal with knowledge skew in PySpark; the perfect strategy will rely on the traits of your knowledge and the operations you might be performing.

The next is an instance of becoming a member of skewed knowledge with the salting approach:

from pyspark.sql import SparkSession
from pyspark.sql.features import lit, ceil, rand, concat, col

# Outline the variety of salt values
num_salts = 3

# Perform to determine skewed keys
def identify_skewed_keys(df, key_column, threshold):
    key_counts = df.groupBy(key_column).rely()
    return key_counts.filter(key_counts['count'] > threshold).choose(key_column)

# Establish skewed keys
skewed_keys = identify_skewed_keys(skewed_data, "key", skew_threshold)

# Splitting the dataset
skewed_data_subset = part of(skewed_keys, ["key"], "inside")
non_skewed_data_subset = part of(skewed_keys, ["key"], "left_anti")

# Apply salting to skewed knowledge
skewed_data_subset = skewed_data_subset.withColumn("salt", ceil((rand() * 10) % num_salts))
skewed_data_subset = skewed_data_subset.withColumn("salted_key", concat(col("key"), lit("_"), col("salt")))

# Replicate skewed rows in non-skewed dataset
def replicate_skewed_rows(df, keys, multiplier):
    replicated_df = part of(keys, ["key"]).crossJoin(spark.vary(multiplier).withColumnRenamed("id", "salt"))
    replicated_df = replicated_df.withColumn("salted_key", concat(col("key"), lit("_"), col("salt")))
    return replicated_df.drop("salt")

replicated_non_skewed_data = replicate_skewed_rows(non_skewed_data, skewed_keys, num_salts)

# Carry out the JOIN operation on the salted keys for skewed knowledge
result_skewed = part of(replicated_non_skewed_data, "salted_key")

# Carry out common be part of on non-skewed knowledge
result_non_skewed = part of(non_skewed_data, "key")

# Mix outcomes
final_result = result_skewed.union(result_non_skewed)

On this code, we first outline a salt worth, which could be a random integer or some other worth. We then add a salt column to our DataFrame utilizing the withColumn() perform, the place we set the worth of the salt column to a random quantity utilizing the rand() perform with a set seed. The perform replicate_salt_rows is outlined to duplicate every row within the non-skewed dataset (non_skewed_data) num_salts instances. This ensures that every key within the non-skewed knowledge has matching salted keys. Lastly, a be part of operation is carried out on the salted_key column between the skewed and non-skewed datasets. This be part of is extra balanced in comparison with a direct be part of on the unique key, as a result of salting and replication have mitigated the information skew.

The rand() perform used on this instance generates a random quantity between 0–1 for every row, so it’s essential to make use of a set seed to realize constant outcomes throughout completely different runs of the code. You may select any fastened integer worth for the seed.

The next figures illustrate the information distribution earlier than (left) and after (proper) salting. Closely skewed key2 recognized and salted into key2_0, key2_1, and key2_2, balancing the information distribution and stopping any single node from being overloaded. After processing, the outcomes will be aggregated again, in order that that the ultimate output is in step with the unsalted key values.

Different methods to make use of on skewed knowledge throughout the be part of operation

While you’re performing skewed joins, you should utilize salting or broadcasting methods, or divide your knowledge into skewed and common elements earlier than becoming a member of the common knowledge and broadcasting the skewed knowledge.

If you’re utilizing Spark 3, there are automated optimizations for attempting to optimize Information Skew points on joins. These will be tuned as a result of they’ve dedicated configs on Apache Spark.


This submit supplied particulars on the best way to detect knowledge skew in your knowledge integration jobs utilizing AWS Glue and completely different methods for dealing with it. Having knowledge distribution is vital to attaining the perfect efficiency on distributed processing methods like Apache Spark.

Though this submit centered on AWS Glue, the identical ideas apply to jobs it’s possible you’ll be working on Amazon EMR utilizing Apache Spark or Amazon Athena for Apache Spark.

As at all times, AWS welcomes your suggestions. Please depart your feedback and questions within the feedback part.

Concerning the Authors

Salim Tutuncu is a Sr. PSA Specialist on Information & AI, based mostly from Amsterdam with a concentrate on the EMEA North and EMEA Central areas. With a wealthy background within the know-how sector that spans roles as a Information Engineer, Information Scientist, and Machine Studying Engineer, Salim has constructed a formidable experience in navigating the complicated panorama of information and synthetic intelligence. His present function includes working intently with companions to develop long-term, worthwhile companies leveraging the AWS Platform, notably in Information and AI use circumstances.

Angel Conde Manjon is a Sr. PSA Specialist on Information & AI, based mostly in Madrid, and focuses on EMEA South and Israel. He has beforehand labored on analysis associated to Information Analytics and Synthetic Intelligence in various European analysis tasks. In his present function, Angel helps companions develop companies centered on Information and AI.

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