ChallengeEfficient snapshotable state
time to read 4 min | 778 words

At the heart of RavenDB, there is a data structure that we call the Page Translation Table. It is one of the most important pieces inside RavenDB.

The page translation table is basically a Dictionary<long, Page>, mapping between a page number and the actual page. The critical aspect of this data structure is that it is both concurrent and multi-version. That is, at a single point, there may be multiple versions of the table, representing different versions of the table at given points in time.

The way it works, a transaction in RavenDB generates a page translation table as part of its execution and publishes the table on commit. However, each subsequent table builds upon the previous one, so things become more complex. Here is a usage example (in Python pseudo-code):

table = {}


with wtx1 = write_tx(table):
  wtx1.put(2, 'v1')
  wtx1.put(3, 'v1')
  wtx1.publish(table)


# table has (2 => v1, 3 => v1)


with wtx2 = write_tx(table):
  wtx2.put(2, 'v2')
  wtx2.put(4, 'v2')
  wtx2.publish(table)


# table has (2 => v2, 3 => v1, 4 => v2)

This is pretty easy to follow, I think. The table is a simple hash table at this point in time.

The catch is when we mix read transactions as well, like so:

# table has (2 => v2, 3 => v1, 4 => v2)


with rtx1 = read_tx(table):


        with wtx3 = write_tx(table):
                wtx3.put(2, 'v3')
                wtx3.put(3, 'v3')
                wtx3.put(5, 'v3')


                with rtx2 = read_tx(table):
                        rtx2.read(2) # => gives, v2
                        rtx2.read(3) # => gives, v1
                        rtx2.read(5) # => gives, None


                wtx3.publish(table)


# table has (2 => v3, 3 => v3, 4 => v2, 5 => v3)
# but rtx2 still observe the value as they were when
# rtx2 was created


        rtx2.read(2) # => gives, v2
        rtx2.read(3) # => gives, v1
        rtx2.read(5) # => gives, None

In other words, until we publish a transaction, its changes don’t take effect. And any read translation that was already started isn’t impacted. We also need this to be concurrent, so we can use the table in multiple threads (a single write transaction at a time, but potentially many read transactions). Each transaction may modify hundreds or thousands of pages, and we’ll only clear the table of old values once in a while (so it isn’t infinite growth, but may certainly reach respectable numbers of items).

The implementation we have inside of RavenDB for this is complex! I tried drawing that on the whiteboard to explain what was going on, and I needed both the third and fourth dimensions to illustrate the concept.

Given these requirements, how would you implement this sort of data structure?

Tweet Share Share 1 comments
Tags:

More posts in "Challenge" series:

  1. (01 Jul 2024) Efficient snapshotable state
  2. (13 Oct 2023) Fastest node selection metastable error state–answer
  3. (12 Oct 2023) Fastest node selection metastable error state
  4. (19 Sep 2023) Spot the bug
  5. (04 Jan 2023) what does this code print?
  6. (14 Dec 2022) What does this code print?
  7. (01 Jul 2022) Find the stack smash bug… – answer
  8. (30 Jun 2022) Find the stack smash bug…
  9. (03 Jun 2022) Spot the data corruption
  10. (06 May 2022) Spot the optimization–solution
  11. (05 May 2022) Spot the optimization
  12. (06 Apr 2022) Why is this code broken?
  13. (16 Dec 2021) Find the slow down–answer
  14. (15 Dec 2021) Find the slow down
  15. (03 Nov 2021) The code review bug that gives me nightmares–The fix
  16. (02 Nov 2021) The code review bug that gives me nightmares–the issue
  17. (01 Nov 2021) The code review bug that gives me nightmares
  18. (16 Jun 2021) Detecting livelihood in a distributed cluster
  19. (21 Apr 2020) Generate matching shard id–answer
  20. (20 Apr 2020) Generate matching shard id
  21. (02 Jan 2020) Spot the bug in the stream
  22. (28 Sep 2018) The loop that leaks–Answer
  23. (27 Sep 2018) The loop that leaks
  24. (03 Apr 2018) The invisible concurrency bug–Answer
  25. (02 Apr 2018) The invisible concurrency bug
  26. (31 Jan 2018) Find the bug in the fix–answer
  27. (30 Jan 2018) Find the bug in the fix
  28. (19 Jan 2017) What does this code do?
  29. (26 Jul 2016) The race condition in the TCP stack, answer
  30. (25 Jul 2016) The race condition in the TCP stack
  31. (28 Apr 2015) What is the meaning of this change?
  32. (26 Sep 2013) Spot the bug
  33. (27 May 2013) The problem of locking down tasks…
  34. (17 Oct 2011) Minimum number of round trips
  35. (23 Aug 2011) Recent Comments with Future Posts
  36. (02 Aug 2011) Modifying execution approaches
  37. (29 Apr 2011) Stop the leaks
  38. (23 Dec 2010) This code should never hit production
  39. (17 Dec 2010) Your own ThreadLocal
  40. (03 Dec 2010) Querying relative information with RavenDB
  41. (29 Jun 2010) Find the bug
  42. (23 Jun 2010) Dynamically dynamic
  43. (28 Apr 2010) What killed the application?
  44. (19 Mar 2010) What does this code do?
  45. (04 Mar 2010) Robust enumeration over external code
  46. (16 Feb 2010) Premature optimization, and all of that…
  47. (12 Feb 2010) Efficient querying
  48. (10 Feb 2010) Find the resource leak
  49. (21 Oct 2009) Can you spot the bug?
  50. (18 Oct 2009) Why is this wrong?
  51. (17 Oct 2009) Write the check in comment
  52. (15 Sep 2009) NH Prof Exporting Reports
  53. (02 Sep 2009) The lazy loaded inheritance many to one association OR/M conundrum
  54. (01 Sep 2009) Why isn’t select broken?
  55. (06 Aug 2009) Find the bug fixes
  56. (26 May 2009) Find the bug
  57. (14 May 2009) multi threaded test failure
  58. (11 May 2009) The regex that doesn’t match
  59. (24 Mar 2009) probability based selection
  60. (13 Mar 2009) C# Rewriting
  61. (18 Feb 2009) write a self extracting program
  62. (04 Sep 2008) Don't stop with the first DSL abstraction
  63. (02 Aug 2008) What is the problem?
  64. (28 Jul 2008) What does this code do?
  65. (26 Jul 2008) Find the bug fix
  66. (05 Jul 2008) Find the deadlock
  67. (03 Jul 2008) Find the bug
  68. (02 Jul 2008) What is wrong with this code
  69. (05 Jun 2008) why did the tests fail?
  70. (27 May 2008) Striving for better syntax
  71. (13 Apr 2008) calling generics without the generic type
  72. (12 Apr 2008) The directory tree
  73. (24 Mar 2008) Find the version
  74. (21 Jan 2008) Strongly typing weakly typed code
  75. (28 Jun 2007) Windsor Null Object Dependency Facility