ChallengeDetecting livelihood in a distributed cluster

time to read 2 min | 286 words

I’m teaching a course at university about cloud computing. That can be a lot of fun, but quite frustrating at time. The key issue for me is that I occasionally need to provide students with some way to do something that I know how to do properly, but I can’t.

Case in point, assuming that I have a distributed cluster of nodes, and we need to detect what nodes are up or down, how do you do that?

With RavenDB, we assign an observer to the cluster whose job is to do health monitoring. I can explain that to the students, but I can’t expect them to utilize this technique in their exercises, there is too much detail there. The focus of the lesson or exercise is not to build a distributed system but to make use of one, after all.

As a rule, I try to ensure that all projects that we are working on can be done in under 200 lines of Python code. That puts a hard limit to the amount of behavior I can express. Because of that, I find myself looking for ways to rely on existing infrastructure to deal with the situation. 

Each node is running the same code, and they are setup so they can talk to one another, if needed. It is important that all the live nodes will converge to agree on the active nodes in relatively short order.

The task is to find the list of active nodes in a cluster, where nodes may go up or down dynamically. We are running in AWS cloud so you can use its resources, how would you do that?

The situation should be as simple as possible and easy to explain to students.

More posts in "Challenge" series:

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