Speaker: Samantha Ritter, Software Engineer, MongoDB Level: 200 (Intermediate) Track: How We Build MongoDB MongoDB 3.6 delivers three new features to help you develop resilient applications: retriable writes, a cluster-wide killOp command, and zombie cursor cleanup. These features share a common base, an idea called a logical session. This new cluster-wide concept of user state is the quiet magic that allows you to know, with certainty, the status of your operations. MongoDB engineer Samantha Ritter will describe the above features in-depth, discuss when and how logical sessions can be used by applications and administrators, and show you how we implemented sessions for large, distributed systems. What You Will Learn: - What logical sessions are and how they are implemented in the server - How to leverage logical sessions for retriable writes - How to pull the new cluster-wide killOp emergency break

4. > db.coll.updateOne(
{'_id ' : 12 },
{'$inc' : {'counter': 1 }}
);

5. > RUNNING OP...
> Oh No!
> Network Error.

6. > Retry op?
> Type yes/no:

7. No way to retrieve op state

8. lack of system-wide resilience

9. 1. RetrYable writes
2. zombie cursor cleanup
3. cluster-wide kilLop

10. SAMANTHA RITTER
MONGODB ENGINEER

11. > Retry op?
> Type yes/no:

12. NO WAY TO RETRIEVE OP STATE.
WHY?

13. mongoD
client
state tied to
this connection
object
state is gone.
o

14. INTRODUCING:
LOGICAL
SESSIONS!

15. mongoD
client
14
14
o
state tied to
this session
object
state is still there!

16. > var session = startSession(
{ retry_writes : True }
);
> session.db.coll.updateOne(
{'_id ‘: 12 },
{'$inc': {'counter': 1 }}
);

17. > RUNNING OP...
> Oh No!
> Network Error.

18. > Reconnected!
> donT WORRY
> IVE GOT THIS
> ItS 10PM
> go watch tv or
something

19. Logical sessions:
retrYable
Writes!

20. client
sessions work
across failover
mongoD
mongoD
mongoD
14
14

21. client
mongoS
shard
mongoS
sessions work
across mongos
14
14

22. Logical sessions:
retrYable
Writes!

23. 1UP

25. WANT TO FIND LOTS OF DOCUMENTS
...AND DO LENGTHY PROCESSING ON
THEM

26. // turn off cursor timeout
> var cursor = coll.find(
no_cursor_timeout = True
);

27. > GOT RESULTS!
> LENGTHY PROCESSING...
> Oh No!
> Network Error

28. > NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR> TERMINATING.

29. > db.serverStatus();
> {"cursors" :
{“open" :
{"noTimeout" : 1 }
}
}

30. NO WAY TO KILL ABANDONED
CURSORS.
WHY?

31. client mongoD1212 1212
ok, see you
later then
please keep my
cursor alive forever

32. ...WE DON’T KNOW IT’S ABANDONED

33. Logical sessions:
zombie cursor
clean up!

34. client mongoD1212
cursors run inside
of sessions
14
14

35. HOW IS THIS DIFFERENT?

36. client mongoD
14
14
please keep my
session alive forever
...no.

37. IDLE SESSIONS
EXPIRE

38. client mongoD1212
cursors run inside
of sessions
14
14
70
45

39. > NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR
> NETWORK ERROR> TERMINATING.

40. Logical sessions:
zombie cursor
clean up!

41. 1UP

43. // Run an op that takes FOREVER
> var cursor = coll.find(
someQueryThatRunsTableScansOnAllShards
);

44. I ReGRET
EVERYTHING

45. DANGER

46. // Find our operation's id
> db.currentOp();
{
"desc" : "conn",
"threadId" : "139722470946560",
"connectionId" : 41,
"client_s" : "127.0.0.1:49704",
"appName" : "MongoDB Shell",
"clientMetadata" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},
"active" : true,
"opid" : "c1a:596612",
"secs_running" : 0,
"microsecs_running" : NumberLong(25),
"op" : "command",
"ns" : "admin.$cmd",
"command" : {
"currentOp" : 1,
"$client" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},
"$configServerState" : {
"opTime" : {
"ts" : Timestamp(1496862002, 1041),
"t" : NumberLong(1)
}
},
"desc" : "conn",
"threadId" : "398934083",
"connectionId" : 40,
"client_s" : "127.0.0.1:49704",
"appName" : "MongoDB Shell",
"clientMetadata" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},
"active" : true,
"opid" : "c1a:596600",
"secs_running" : 0,
"microsecs_running" : NumberLong(5),
"op" : "command",
"ns" : "admin.$cmd",
"command" : {
"currentOp" : 1,
"$logicalTime" : {
"clusterTime" : Timestamp(1496862091, 101),
"signature" : {
"hash" : BinData(0,"znBnOQbapwrefOTE+9345875="),
"keyId" : NumberLong("00001232239834388037")
}
},
"$client" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},
"desc" : "conn",
"threadId" : "398934083",
"connectionId" : 40,
"client_s" : "127.0.0.1:49704",
"appName" : "MongoDB Shell",
"clientMetadata" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
"$configServerState" : {
"opTime" : {
"ts" : Timestamp(1496862002, 1041),
"t" : NumberLong(1)
}
}, "threadId" : "139722470946560",
"connectionId" : 41,
"client_s" : “127.0.0.1:49704",
"$db" : "admin"
},
"numYields" : 0,
"locks" : {
}
},
{
"desc" : "conn",
"threadId" : "398934083",
"connectionId" : 40,
"client_s" : "127.0.0.1:49704",
"appName" : "MongoDB Shell",
"clientMetadata" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},
"active" : true,
"opid" : "12345",
"secs_running" : 0,
"microsecs_running" : NumberLong(5),
"$client" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},
"$configServerState" : {
"opTime" : {
"ts" : Timestamp(1496862002, 1041),
"t" : NumberLong(1)
}
}, "threadId" : "139722470946560",
"connectionId" : 41,
"client_s" : “127.0.0.1:49704",
"$db" : "admin"
},
"numYields" : 0,
"locks" : {
}
},
{
"desc" : "conn",
"threadId" : "398934083",
"connectionId" : 40,
"client_s" : "127.0.0.1:49704",
"appName" : "MongoDB Shell",
"clientMetadata" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},
"active" : true,
"opid" : "12345",
"secs_running" : 0,
"microsecs_running" : NumberLong(5),
"$client" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},
"threadId" : "398934083",
"connectionId" : 40,
"client_s" : "127.0.0.1:49704",
"appName" : "MongoDB Shell",
"clientMetadata" : {
"application" : {
"active" : true,
"opid" : "c1a:596612",
"secs_running" : 0,
"microsecs_running" : NumberLong(25),
"op" : "command",
"ns" : "admin.$cmd",
"command" : {
"currentOp" : 1,
"$client" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},
"$configServerState" : {
"opTime" : {
"ts" : Timestamp(1496862002, 1041),
"t" : NumberLong(1)
}
},
{
"desc" : "conn",
"threadId" : "139722470946560",
"connectionId" : 41,
"client_s" : "127.0.0.1:49704",
"appName" : "MongoDB Shell",
"clientMetadata" : {
"application" : {
"name" : "MongoDB Shell"
},
"driver" : {
"name" : "MongoDB Internal Client",
"version" : "3.5.5-518-gb5147cd"
},
"os" : {
"type" : "Linux",
"name" : "Ubuntu",
"architecture" : "x86_64",
"version" : "16.04"
}
},

47. // Kill it!!!!
> db.killOp( 123456 );

48. On a sharded cluster,
Queries Fan out

49. shard
client
o shard
shard
shard
mongoS