The Future of Distributed Databases is Relational | QCon London 2018 | Sumedh Pathak

The Future of Distributed Databases is Relational | QCon London 2018 | Sumedh Pathak

Years ago when working at Amazon on shopping cart infrastructure and the precursor to DynamoDB, my co-founder and I realized that while distributed key value stores were useful for a few use-cases, we missed many of the benefits of relational databases: transactions, joins, and the power of the lingua franca of RDBMS’s: SQL. So we challenged ourselves to modernize the traditional relational database, to take a robust open source relational database and transform it into a distributed database.

This talk is about my team’s journey to create a more modern relational database. I’ll talk about the distributed systems problems we had to solve in order to scale out the Postgres open source database, in order to achieve parallelism and a concomitant increase in performance. I'll describe the architecture of the distributed query planner; how we extend traditional relational algebra operators to plan distributed queries and scale reads. I’ll also describe distributed deadlock detection, and how that enabled us to scale out transactions spanning multiple machines.

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Citus Data

March 06, 2018
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Transcript

  1. 1.

    Sumedh Pathak, Co-Founder & VP Engineering, Citus Data QCon London

    2018 The Future of Distributed Databases is Relational @moss_toss | @citusdata
  2. 2.

    About Me • Co-Founder & VP Engineering at Citus Data

    • Amazon Shopping Cart (former) • Amazon Supply Chain & Order Fulfillment (former) • Stanford Computer Science Citus Data co-founders, left-to-right Ozgun Erdogan, Sumedh Pathak, & Umur Cubukcu Photo cred: Willy Johnson, Monterey CA, Sep 2017 Sumedh Pathak | Citus Data | QCon London 2018
  3. 3.
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    Streaming Storage Map Reduce NoSQL SQL Database SELECT ... Application

    Application Sumedh Pathak | Citus Data | QCon London 2018 Because your architecture could be simpler
  5. 6.

    An RDBMS is a general-purpose data platform Sumedh Pathak |

    Citus Data | QCon London 2018 Fast writes Real-time & bulk High throughput High concurrency Data consistency Query optimizers
  6. 7.

    PostgreSQL MySQL MongoDB SQL Server + Oracle Source: Hacker News,

    https://news.ycombinator.com Startups Are Choosing Postgres % database job posts mentioning each database, across 20K+ job posts Sumedh Pathak | Citus Data | QCon London 2018
  7. 9.
  8. 10.

    What exactly needs to Scale? - Tables (Data) - Partitioning,

    Co-location, Reference Tables - SQL (Reads) - How do we express and optimize distributed SQL - Transactions (Writes) - Cross Shard updates/deletes, Global Atomic Transactions Sumedh Pathak | Citus Data | QCon London 2018 1 2 3
  9. 12.

    How to partition the data? - Pick a column -

    Date - Id (customer_id, cart_id) - Pick a method - Hash - Range
  10. 13.

    Partition data across nodes R 1 R 2 R 3

    R 4 R 5 R 6 R 7 Coordinator Node Worker Nodes Shards Sumedh Pathak | Citus Data | QCon London 2018
  11. 15.

    Reference Tables N 1 N 1 N 1 N 1

    Copies of same table Sumedh Pathak | Citus Data | QCon London 2018 Coordinator Node Worker Nodes
  12. 16.

    Co-Location R 1 R 2 R 3 R 4 S

    1 S 2 S 3 S 4 Explicit Co-Location API. E.g. Partition by Tenant Sumedh Pathak | Citus Data | QCon London 2018 Coordinator Node Worker Nodes
  13. 18.

    The key to scaling tables... - Use relational databases as

    a building block - Understand semantics of application—to be smart about partitioning - Multi-tenant applications
  14. 20.

    FROM table R SELECT x Project x (R) → R’

    WHERE f(x) Filter f(x) (R) → R’ … JOIN … R × S → R’ SQL ↔ Relational Algebra Sumedh Pathak | Citus Data | QCon London 2018
  15. 21.

    FROM sharded_table Collect(R 1 ,R 2 ,...) → R Distributed

    Relational Algebra R 1 R 2 C Sumedh Pathak | Citus Data | QCon London 2018
  16. 22.

    Project x (Collect(R 1 ,R 2 ,...)) = Collect(Project x

    (R 1 ), Project x (R 2 )...) Commutative property R 1 R 2 C R 1 R 2 C P x P x P x Sumedh Pathak | Citus Data | QCon London 2018
  17. 23.

    Collect(R 1 ,R 2 ,...) x Collect(S 1 ,S 2

    ,...) = Collect(R 1 × S 1 ,R 2 × S 2, ...) Distributive property R 1 R 2 C × C S 1 S 2 R 1 R 2 C × S 1 S 2 × X = Join Operator
  18. 24.

    SUM(x)(Collect(R 1 ,R 2 ,...)) = SUM(Collect(SUM(R 1 ), SUM(R

    2 )...)) Associative property R 1 R 2 C R 1 R 2 C Sum x Sum x Sum x Sumedh Pathak | Citus Data | QCon London 2018 Sum x
  19. 25.

    SELECT sum(price) FROM orders, nation WHERE orders.nation = nation.name AND

    orders.date >= '2012-01-01' AND nation.region = 'Asia'; Sumedh Pathak | Citus Data | QCon London 2018
  20. 27.

    Sumedh Pathak | Citus Data | QCon London 2018 Volcano

    style processing Data flows from bottom to top
  21. 30.

    Sumedh Pathak | Citus Data | QCon London 2018 Parallelize

    Aggregate Push Joins & Filters below collect. Run in parallel across all nodes Filters & Projections done before Join
  22. 31.

    SELECT sum(intermediate_col) FROM <concatenated results>; SELECT sum(price) FROM orders_2 JOIN

    nation_2 ON (orders_2.name = nation_2.name) WHERE orders_2.date >= '2017-01-01' AND nation_2.region = 'Asia'; SELECT sum(price) FROM orders_2 JOIN nation_1 ON (orders_2.name = nation_1.name) WHERE orders_2.date >= '2017-01-01' AND nation_2.region = 'Asia'; Sumedh Pathak | Citus Data | QCon London 2018
  23. 32.

    Executing Distributed SQL SQL database orders_1 nation_1 orders nation SELECT

    sum(price) FROM orders_2 o JOIN nation_1 ON (o.name = n.name) WHERE o.date >= '2017-01-01' AND n.region = 'Asia'; SELECT sum(price) FROM orders_1 o JOIN nation_1 ON (o.name = n.name) WHERE o.date >= '2017-01-01' AND n.region = 'Asia'; SELECT sum(price) FROM <results>; SQL database orders_2 nation_1
  24. 33.

    The key to scaling SQL... - New relational algebra operators

    for distributed processing - Relational Algebra Properties to optimize tree: Commutativity, Associativity, & Distributivity - Map / Reduce operators
  25. 35.

    Money Transfer, as an example BEGIN; UPDATE accounts SET balance

    = balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; Sumedh Pathak | Citus Data | QCon London 2018
  26. 36.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 Coordinator Sumedh Pathak | Citus Data | QCon London 2018
  27. 37.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 BEGIN; UPDATE accounts SET balance = balance - 100 WHERE id = ‘ALICE’; Sumedh Pathak | Citus Data | QCon London 2018 Coordinator
  28. 38.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 BEGIN; UPDATE accounts SET balance = balance - 100 WHERE id = ‘ALICE’; COMMIT; Sumedh Pathak | Citus Data | QCon London 2018 Coordinator
  29. 39.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 Coordinator BEGIN; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; Sumedh Pathak | Citus Data | QCon London 2018 BEGIN; UPDATE accounts SET balance = balance - 100 WHERE id = ‘ALICE’;
  30. 40.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 BEGIN; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT Sumedh Pathak | Citus Data | QCon London 2018 BEGIN; UPDATE accounts SET balance = balance - 100 WHERE id = ‘ALICE’; Coordinator
  31. 41.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 PREPARE TRANSACTION ‘citus_...98’; PREPARE TRANSACTION ‘citus_...98’; Sumedh Pathak | Citus Data | QCon London 2018 Coordinator
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    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 Coordinator PREPARE TRANSACTION ‘citus_...98’; ROLLBACK TRANSACTION ‘citus_...98’; Sumedh Pathak | Citus Data | QCon London 2018
  34. 44.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 PREPARE TRANSACTION ‘citus_...98’; PREPARE TRANSACTION ‘citus_...98’; Sumedh Pathak | Citus Data | QCon London 2018 Coordinator
  35. 45.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 COMMIT PREPARED ‘citus_...98’; COMMIT PREPARED ‘citus_...98’; Sumedh Pathak | Citus Data | QCon London 2018 Coordinator
  36. 46.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 COMMIT PREPARED ‘citus_...98’; COMMIT PREPARED ‘citus_...98’; Sumedh Pathak | Citus Data | QCon London 2018 Coordinator
  37. 47.

    A 1 A 2 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; COMMIT; A 3 A 4 COMMIT PREPARED ‘citus_...98’; COMMIT PREPARED ‘citus_...98’; Sumedh Pathak | Citus Data | QCon London 2018 Coordinator
  38. 49.

    Example // SESSION 1 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; (LOCK on ROW with id ‘ALICE’) // SESSION 2 Sumedh Pathak | Citus Data | QCon London 2018
  39. 50.

    Example // SESSION 1 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; (LOCK on ROW with id ‘ALICE’) // SESSION 2 BEGIN; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; (LOCK on ROW with id ‘BOB’) Sumedh Pathak | Citus Data | QCon London 2018
  40. 51.

    Example // SESSION 1 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; (LOCK on ROW with id ‘ALICE’) UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; // SESSION 2 BEGIN; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; (LOCK on ROW with id ‘BOB’) Sumedh Pathak | Citus Data | QCon London 2018
  41. 52.

    Example // SESSION 1 BEGIN; UPDATE accounts SET balance =

    balance - 100 WHERE id = ‘ALICE’; (LOCK on ROW with id ‘ALICE’) UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; // SESSION 2 BEGIN; UPDATE accounts SET balance = balance + 100 WHERE id = ‘BOB’; (LOCK on ROW with id ‘BOB’) UPDATE accounts SET balance = balance - 100 WHERE id = ‘ALICE’ Sumedh Pathak | Citus Data | QCon London 2018
  42. 53.

    How do Relational DB’s solve this? S1 S2 - Construct

    a Directed Graph - Each node is a session/transaction - Edge represents a wait on a lock Waiting on ‘Bob’ Waiting on ‘Alice’
  43. 54.

    S1 S2 S1 S2 S2 S1 S2 Waits on S1

    S1 Waits on S2 Sumedh Pathak | Citus Data | QCon London 2018
  44. 55.

    S1 S2 S1 S2 S2 S1 S2 Waits on S1

    S1 Waits on S2 Distributed Deadlock Detector S1 S2 S2 S1 Sumedh Pathak | Citus Data | QCon London 2018
  45. 56.

    keys to scaling transactions - 2PC to ensure atomic transactions

    across nodes - Deadlock Detection—to scale complex & concurrent transaction workloads - MVCC - Failure Handling 4
  46. 57.

    It’s 2018. Distributed can be Relational - Scale tables—via sharding

    - Scale SQL—via distributed relational algebra - Scale transactions—via 2PC & Deadlock Detection Sumedh Pathak | Citus Data | QCon London 2018 Now, how do we implement all of this?
  47. 59.

    PostgreSQL Planner Executor Custom scan Commit / abort Extension (.so)

    Access methods Foreign tables Functions ... ... ... ... ... ... ... CREATE EXTENSION ...
  48. 60.

    PostgreSQL PostgreSQL PostgreSQL shards shards shard shards shards shard SELECT

    … FROM distributed_table … SELECT … FROM shard… SELECT … FROM shard… Citus
  49. 61.

    PostgreSQL Citus PostGIS PL/Python JSONB 2PC Replication ... Sequences Indexes

    Full-Text Search Transactions … dblink Sumedh Pathak | Citus Data | QCon London 2018 Rich ecosystem of tooling, data types, & extensions in PostgreSQL
  50. 64.

    Thank you! Sumedh Pathak sumedh@citusdata.com @moss_toss | @citusdata | citusdata.com

    QCon London 2018 | The Future of Distributed Databases is Relational