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InnoDB Locking Explained with Stick Figures

When does InnoDB lock a row? Multiple rows? Why would it lock a gap? How do transactions affect these scenarios? Locking is one of the more opaque features of MySQL, but it’s very important for both developers and DBA’s to understand if they want their applications to work with high performance and concurrency. This is a creative presentation to illustrate the scenarios for locking in InnoDB and make these scenarios easier to visualize. I'll cover: key locks, table locks, gap locks, shared locks, exclusive locks, intention locks, insert locks, auto-inc locks, and also conditions for deadlocks.

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INNODB LOCKING
EXPLAINED
WITH STICK FIGURES
Bill Karwin
Bill Karwin
• Senior Database Architect at School Messenger
(West Corporation)
• Author of SQL Antipatterns: Avoiding the Pitfalls of
Database Programming
• 20+ years experience with SQL databases, software
development, MySQL consulting and training
why locking?
• When multiple clients
access the same data,
they have to avoid
clobbering each others’
work.
• Databases must restrict
access to one client at a
time for a given table or
row.
https://commons.wikimedia.org/wiki/File:New_York_City_Gridlock.jpg  
why locking?
• The DBMS creates locks
against tables and rows, and
gives them to clients, first-
come, first-serve.
• When a client requests a
exclusive lock, but a different
client currently holds it, the
requestor waits until the
holder releases its lock.
• Most locks last until the end of
the transaction.
https://commons.wikimedia.org/wiki/Traffic_lights#/media/File:LED_traffic_light.jpg
the analogy
a museum
• Many people can visit the museum (a database
table) to view art (rows of data).

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InnoDB Locking Explained with Stick Figures

  • 2. Bill Karwin • Senior Database Architect at School Messenger (West Corporation) • Author of SQL Antipatterns: Avoiding the Pitfalls of Database Programming • 20+ years experience with SQL databases, software development, MySQL consulting and training
  • 3. why locking? • When multiple clients access the same data, they have to avoid clobbering each others’ work. • Databases must restrict access to one client at a time for a given table or row. https://commons.wikimedia.org/wiki/File:New_York_City_Gridlock.jpg  
  • 4. why locking? • The DBMS creates locks against tables and rows, and gives them to clients, first- come, first-serve. • When a client requests a exclusive lock, but a different client currently holds it, the requestor waits until the holder releases its lock. • Most locks last until the end of the transaction. https://commons.wikimedia.org/wiki/Traffic_lights#/media/File:LED_traffic_light.jpg
  • 6. a museum • Many people can visit the museum (a database table) to view art (rows of data).
  • 8. reads • Many visitors can view paintings at the same time— no locking required. https://commons.wikimedia.org/wiki/File:L%27%C3%A9glise_d%27Auvers-­‐‑sur-­‐‑Oise.jpg
  • 9. writes • A curator can change the paintings—while casual visitors are viewing them. https://commons.wikimedia.org/wiki/File:Vincent_Willem_van_Gogh_128.jpg
  • 10. writes • A curator can change the paintings—while casual visitors are viewing them.
  • 11. repeatable reads • The viewers still see the prior painting in spite of the change, because they used their tablets to capture the image.* *  Please  do  not  take  photographs  of  the  art  in  a  real  museum.
  • 12. read committed • If the viewers are okay allowing their view to change, they can simply say so. READ   COMMITTED
  • 13. read committed • If the viewers are okay allowing their view to change, they can simply say so. READ   COMMITTED I  don’t  mind
  • 15. exclusive locks • The curator can change the painting if they are the exclusive person working on it.
  • 16. exclusive locks • Casual viewers don’t block the curator. X-­‐‑lock!
  • 17. exclusive locks • A second curator who tries to change the same painting must wait for the first curator to finish. X-­‐‑lock!… https://commons.wikimedia.org/wiki/Vincent_van_Gogh#/media/File:Vincent_Willem_van_Gogh_107.jpg
  • 18. what is an exclusive lock? • A lock that does not share. • It must be the only lock on the resource. • Request for an exclusive lock waits for the release of any other shared or exclusive lock on that resource.
  • 20. shared locks • The curator cannot make changes while the art critic is viewing a painting. S-­‐‑lock! .  .  .
  • 21. shared locks • Art critics can share—they do not block each other. S-­‐‑lock! .  .  . S-­‐‑lock!
  • 22. shared locks • Once the art critics leave, the curator can proceed to change the art. X-­‐‑lock!
  • 23. shared locks • A new art critic will not begin his viewing while the curator is still working on changing the painting. X-­‐‑lock!…
  • 24. what is a shared lock? • A lock that allows other shared locks on the same table or rows. • Shared locks blocks exclusive locks. • Exclusive locks block shared locks.
  • 26. table locks • A construction worker needs to remodel the museum, but not while visitors are inside.
  • 27. table locks • Each person is given a special badge as they enter the museum, showing their intention to view or change the paintings. IS!IX!
  • 28. table locks • A construction worker requests exclusive access to the museum, but can not get it. .  .  .IS! IX!
  • 29. table locks • When all the visitors have left, the worker can finally get his exclusive access to do his work. LOCK   TABLE!
  • 30. table locks • While the construction is going on, visitors cannot get their badges, and therefore cannot enter the museum. … https://commons.wikimedia.org/wiki/Category:Under_construction_icons#/media/File:Enobras.PNG LOCK   TABLE! …
  • 31. what is an intention lock? • Before SELECT…LOCK IN SHARE MODE or other shared lock, request an intention shared lock (“IS”) on the table. • Before SELECT…FOR UPDATE, INSERT, UPDATE, DELETE, request an intention exclusive lock (“IX”). • IS and IX locks allow access by multiple clients. They won’t necessarily conflict until they try to get real locks on the same rows. • But a table lock (ALTER TABLE, DROP TABLE, LOCK TABLES) blocks both IS and IX, and vice-versa.
  • 33. gap locks • The art critic needs to view the whole collection without changes and no new inserts. gap .  .  . S-­‐‑lock!
  • 34. what is a gap lock? • A lock on a painting locks the “space” before the painting. • The gap lock prevents inserts of new paintings before the locked painting (within the space). • This happens automatically, to prevent “phantom reads”—i.e. the view of data changes during a transaction.
  • 35. what is a gap lock? • Exception: if the art critic is okay with seeing the latest additions among the paintings, he can choose to use the READ COMMITTED transaction isolation level. • Another exception: no gap lock is needed for a UNIQUE or PRIMARY KEY index.
  • 36. gap locks • Optionally be more permissive about inserts. gap S-­‐‑lock! READ   COMMITTED
  • 37. gap locks • Optionally be more permissive about inserts. X-­‐‑lock! S-­‐‑lock! READ   COMMITTED I  don’t  mind
  • 38. traveling exhibit • Some art exhibits move to another museum.
  • 39. installation plans • The curators must keep records of how to install the art, so the show is the same at each museum. Label  all   paintings  on   this  wall  with   “Van  Gogh”
  • 40. installation plans • The curators must keep records of how to install the art, so the show is the same at each museum. Van  Gogh Van  Gogh
  • 41. installation plans • Another painting is added. Van  Gogh Van  Gogh Install  this  in   the  gap  and   label  it   “Lautrec” https://commons.wikimedia.org/wiki/Moulin_Rouge#/media/File:Lautrec_at_the_moulin_rouge_two_women_waltzing_1892.jpg
  • 42. installation plans • Another painting is installed in the gap. Van  Gogh Van  GoghLautrec
  • 43. installation plans • The latter curator commits his installation plans first. Van  Gogh Van  GoghLautrec COMMIT
  • 44. installation plans • Then the first curator commits his installation plans. Van  Gogh Van  GoghLautrec COMMIT
  • 45. traveling exhibit • The exhibit is installed in the second museum.
  • 46. order matters • The installation instructions are carried out in the wrong order! Install  this  in   the  gap  and   label  it   “Lautrec”
  • 47. order matters • The installation instructions are carried out in the wrong order! Lautrec Label  all   paintings  on   this  wall  with   “Van  Gogh”
  • 48. order matters • The installation instructions are carried out in the wrong order! Van  GoghVan  Gogh Van  Gogh
  • 49. what happened? • Installation instructions are recorded in the order they were committed, not the order they were originally done. • Therefore the installation at the next museum may repeat the steps in an incorrect order.
  • 50. how to solve this? • Labeling “Van Gogh” should have first locked all the spaces on that wall. Van  Gogh Van  Gogh gap
  • 51. how to solve this? • The addition would be forced to wait for the locks to be released. Van  Gogh Van  Gogh gap …
  • 53. insert intention locks • One curator wants to update paintings where year > 1886 X-­‐‑lock! gap
  • 54. insert intention locks • The second curator wants to insert an 1887 painting, but it would fall within the existing gap lock. X-­‐‑lock! … gap
  • 55. insert intention locks ---TRANSACTION 32070411, ACTIVE 6 sec inserting mysql tables in use 1, locked 1 LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s), undo log entries 1 MySQL thread id 26, OS thread handle 0x7f2ba845f700, query id 1423 192.168.50.1 root update insert into Museum (year) values (1887) ------- TRX HAS BEEN WAITING 6 SEC FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 3337 page no 4 n bits 72 index `year` of table `test`.`Museum` trx id 32070411 lock_mode X insert intention waiting
  • 56. insert intention locks • But if the first curator doesn’t care about new paintings entering his view… X-­‐‑lock! gap READ   COMMITTED
  • 57. insert intention locks • Then the second curator gets an insert intention lock, and is then free to insert. insert-­‐‑ lock! gap X-­‐‑lock! READ   COMMITTED I  don’t  mind
  • 58. insert intention locks • Then the second curator gets an insert intention lock, and is then free to insert. X-­‐‑lock!X-­‐‑lock! READ   COMMITTED I  don’t  mind
  • 59. what is an insert intention lock? • A special kind of gap lock, requested before a client tries to insert a row. • Insert locks are shared, not exclusive—multiple clients can acquire insert locks on the same gap. • But insert locks conflict with other exclusive locks.
  • 60. why is insert intention lock shared? • Multiple clients prepare to insert into the same gap. • They may be inserting different rows within the same gap, so they don’t conflict with each other. • But the insert intention lock blocks other clients from requesting exclusive locks on the same gap.
  • 62. auto-inc locks • Two curators are installing paintings. They both need to post a unique number for self-guided tours. ? ?
  • 63. auto-inc locks • There must be one number generator per table. • One curator at a time can request the next value. http://www.istockphoto.com/photo/ticket-­‐‑dispenser-­‐‑isolated-­‐‑9396862
  • 64. auto-inc locks • The first curator gets a number. auto-­‐‑inc! 1 …
  • 65. auto-inc locks • As soon as the first curator gets his number, the second curator can proceed. 1 2 auto-­‐‑inc!
  • 66. auto-inc locks • They may both keep locks on the paintings, but they’re done allocating numbers. X-­‐‑lock! 1 2 X-­‐‑lock!
  • 67. what is an auto-inc lock? • A table lock, used when a client requests the next unique id for a given table. • Ensures that each id is given to one client. • Brief—it is released as soon as the id is generated, instead of lasting to the end of the transaction like other locks. • Because the lock is so brief, neither client can “undo”— i.e. they cannot return their id to the stack for someone else to use.
  • 69. deadlocks • Two curators are updating the art, but they start from different ends of the collection. X-­‐‑lock!X-­‐‑lock!
  • 70. deadlocks • A curator requests a lock on the second painting, which is already locked. He waits. X-­‐‑lock!X-­‐‑lock! …
  • 71. deadlocks • The other curator requests a lock on the first painting, which is already locked. He also waits. X-­‐‑lock!X-­‐‑lock! … …
  • 72. deadlocks • Neither will give up the lock they have, so they are doomed to wait until one or both of them dies. X-­‐‑lock! X-­‐‑lock! … …
  • 73. what is a deadlock? • When two or more concurrent clients wait for each other to release their locks, but since they are both waiting, they will never give up the lock they have. • In other words, a “catch-22” of lock-waits. • Many people use the term “deadlock” incorrectly— when they are describing a simple one-way lock wait.
  • 74. resolving deadlocks • MySQL detects cycles in lock waits, and kills one of the transactions immediately. X-­‐‑lock!X-­‐‑lock! … …
  • 75. resolving deadlocks • MySQL detects cycles in lock waits, and automatically kills one of the transactions. X-­‐‑lock! X-­‐‑lock!
  • 76. avoiding deadlocks • All clients request locks in the same order. X-­‐‑lock! … X-­‐‑lock!
  • 77. avoiding deadlocks • Each client locks everything they need in one atomic request. X-­‐‑lock! …
  • 79. read committed all the things? • Yes … and no. • READ COMMITTED avoids gap locks, therefore reduces lock waits and allows greater throughput. • But using READ COMMITTED allows more cases where you could get deadlocks.
  • 80. thank you • I hope your trip to the museum was educational!
  • 81. license and copyright Copyright 2015-2016 Bill Karwin http://www.slideshare.net/billkarwin Released under a Creative Commons 3.0 License: http://creativecommons.org/licenses/by-nc-nd/3.0/ You are free to share—to copy, distribute, and transmit this work, under the following conditions: Attribution. You  must  attribute  this   work  to  Bill  Karwin. Noncommercial. You  may  not  use  this   work  for  commercial   purposes. No  Derivative  Works. You may  not  alter,   transform,  or  build  upon   this  work.