Astronomical Data Processing on the LSST Scale with Apache Spark

The next decade promises to be exciting for both astronomy and computer science with a number of large-scale astronomical surveys in preparation. One of the most important ones is Large Scale Survey Telescope, or LSST. LSST will produce the first ‘video’ of the deep sky in history by continually scanning the visible sky and taking one 3.2 giga-pixel image every 20 seconds. In this talk we will describe LSST’s unique design and how its image processing pipeline produces catalogs of astronomical objects. To process and quickly cross-match catalog data we built AXS (Astronomy Extensions for Spark), a system based on Apache Spark. We will explain its design and what is behind its great cross-matching performance.

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1.WIFI SSID:Spark+AISummit | Password: UnifiedDataAnalytics

2.AXS - Astronomical Data Processing on the LSST Scale with Apache Spark Petar Zečević, SV Group, University of Zagreb Mario Jurić, DIRAC Institute, University of Washington #UnifiedDataAnalytics #SparkAISummit

3.About us Mario Jurić • Prof. of Astronomy at the University of Washington • Founding faculty of DIRAC & eScience Institute Fellow • Fmr. lead of LSST Data Management Petar Zečević • CTO at SV Group, Croatia • CS PhD student at University of Zagreb • Visiting Fellow at DiRAC institute @ UW • Author of “Spark in Action” #UnifiedDataAnalytics #SparkAISummit 3

4.About us #UnifiedDataAnalytics #SparkAISummit 4

5.Context: The Large Survey Revolution in Astronomy

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7.Hipparchus of Rhodes (180-125 BC) In 129 BC, constructed one of the first star catalogs, containing about 850 stars.

8.Galileo Galilei (1564-1642) Researched a variety of topics in physics, but called out here for the introduction of the Galilean telescope. Galileo’s telescope allowed us for the first time to zoom in on the cosmos, and study the individual objects in great detail.

9.The Astrophysics Two-Step • Surveys – Construct catalogs and maps of objects in the sky. Focus on coarse classification and discovering targets for further follow-up. • Large telescopes – Acquire detailed observations of a few representative objects. Understand the details of astrophysical processes that govern them, and extrapolate that understanding to the entire class.

10. The Story of Astronomy: 2000 Years of being Data Poor 10

11. Sloan Digital Sky Survey 2.5m telescope >14,500 deg2 0.1” astrometry r<22.5 flux limit 5 band, 1%, photometry for over 900M stars Over 3M R=2000 spectra 10 years of ops: ~10 TB of imaging

12. Facilitated the development of large databases, data- driven discovery, motion towards what we recognize as Data Science today. 1,231,051,050 rows (SDSS DR10, PhotoObjAll table) ~500 columns

13.Panoramic Survey Telescope and Rapid Response System 1.8m telescope 30,000 deg2 50mas astrometry r<23 flux limit 5 band, better than 1% photometry (goal) ~700 GB/night

14.Gaia DR2: 1.7 billion stars https://sci.esa.int/s/wV6oG5w 14

15.The Large Synoptic Survey Telescope A Public, Deep, Wide and Fast, Optical Sky Survey First Light: 2020 Operations: 2022 Deep (24th mag), Wide (60% of the sky), Fast (every 15 seconds) Largest astronomical camera in the world Will repeatedly observe the night sky over 10 years 10 million alerts each night (60 seconds) 37 billion astronomical sources, with time series 30 trillion measurements

16.LSST’s mission is to build a well-understood system that Overview provides a vast astronomical dataset for unprecedented discovery of the deep and dynamic universe.

17.The Scale of Things to Come Metric Amount Number of detections 7 trillion rows Number of objects 37 billion rows Nightly alert rate 10 million Nightly data rate >15 TB Alert latency 60 seconds Total images after 10 yrs 50 PB Total data after 10 yrs 83 PB Objects detected, measured, and stored in queryable catalogs (tables) 17

18.Catalog-driven Science • Once a catalog is available, astronomers “ask” all kinds of questions • The traditional paradigm: – Subset (filter data using a catalog SQL interface online) – Download data locally – Analyze (usually Python) #UnifiedDataAnalytics #SparkAISummit 18

19.Challenges (part 0) Dataset Size (keeping ~PBs of data in RBDMSes is not easy, or cheap) What do you do when the dataset subset is a few ~TBs?

20.Challenges (part 1) Dataset Size (keeping ~TBs of data in RBDMs-es is not easy) Better Together I Want it All (joining datasets is powerful) (interesting science w. whole dataset operations)

21.Challenges (part 2) Scalability Resources (how do I write an analysis code that will (where are the resources to run this code?) scale to petabytes of data?) How do you scale exploratory data analysis to ~PB-sized datasets and thousands of simultaneous users?

22.Enter Spark, AXS • AXS: Astronomy eXtensions for Spark • The main idea: – Spark is a proven, scalable, cloud-ready and widely-supported analytics framework with full SQL support (legacy support). – Extend it to exploratory data analysis. – Add a scalable positional cross-match operator – Add a domain-specific Python API layer to PySpark – Couple to S3 API for storage, Kubernetes for orchestration… • … A scalable platform supporting an arbitrarily sized dataset and a large number of users, deployable on either public or private cloud. 22

23.Key Issue: Scalable Cross-matching A match Search perimeter (can also use similarity) DEC and RA coordinates #UnifiedDataAnalytics #SparkAISummit 23

24.AXS data partitioning • Data partitioning is at the root of AXS' efficient cross- matching • Based on (late) Jim Gray's “zones algorithm” (MS Rsch) • Sky divided into horizontal “zones” of a certain height • Adapted for distributed architectures • Data stored in Parquet files – bucketed by zone – sorted by zone and ra columns – data from zone borders duplicated to the zone below 24

25.AXS data partitioning 25

26.AXS - optimal joins 26

27.AXS - optimal joins 27

28.Epsilon join SELECT ... FROM TA, TB WHERE TA.zone = TB.zone AND TA.ra BETWEEN TB.ra - e AND TB.ra + e SPARK-24020: Sort-merge join “inner range optimization” 28

29.Other approaches Other systems use HEALPix or Hierarchical Triangular Mesh (HTM) 29