MongoDB has separate drivers for ten programming languages, and all are responsible for connecting to replica sets or groups of mongos. What algorithms do they use to find servers, connect to them, discover their state, choose which ones to use, and survive failovers? A. Jesse Jiryu Davis, a Staff Engineer on the MongoDB Drivers Team, is the author of the Server Discovery And Monitoring Spec. He will explain the inner workings of MongoDB drivers and their behavior within complex server topologies.

1. MongoDB Drivers
&
High Availability
A. Jesse Jiryu Davis
Staff Engineer, MongoDB
@jessejiryudavis

2. Client
Side

3. • It’s a MongoDB driver specification.
• (It’s not MMS.)
• Written by me.
Advisors: David Golden, Craig Wilson, Jeff Yemin.
Overseen by Bernie Hackett.
Server Discovery
&
Monitoring Spec

4. Good News
&
Bad News

5. Agenda
•Startup
•Steady State
•Crisis
•Resolution
•Practical Applications
•Further Study

6. Setup
•3-node replica set
•PyMongo 3

7. client = MongoClient(!
'mongodb://host1,host2/?replicaSet=my-rs')!

8. primary
host1
secondary
host2
secondary
host3
Python Program
monitor
thread
monitor
thread
MongoClient
{secondary: true,
hosts: ["host1",
"host2",
"host3"]}
"mongodb://host1,host2/?replicaSet=my-rs"

9. ismaster
(from secondary)
{"setName": "rs-name",!
"ismaster": false,!
"secondary": true,!
"hosts": ["host1", "host2", "host3"]}!

10. {"setName": "rs-name",!
"ismaster": true,!
"secondary": false,!
"hosts": ["host1", "host2", "host3"]}!
ismaster
(from primary)

11. Updating Topology
Description
Current description
+
ismaster response
→ new description

12. Updating Topology
Description

13. primary
host1
secondary
host2
secondary
host3
Python Program
monitor
thread
monitor
thread
monitor
thread
MongoClient
{secondary: true,
hosts: ["host1",
"host2",
"host3"]}
"new info!"
Your
Code?
✓

14. client.db.collection.insert({'my': 'document'})!
client = MongoClient(!
'mongodb://host1,host2/?replicaSet=my-rs')!

15. primary
host1
secondary
host2
secondary
host3
Python Program
monitor
thread
monitor
thread
monitor
thread
MongoClient
{ismaster: true,
hosts: ["host1",
"host2",
"host3"]}
{secondary: true,
hosts: ["host1",
"host2",
"host3"]}
Your
Code
insert
ok
✓
✓
"new info!"
insert
ok
✓

16. Steady State

17. primary
host1
secondary
host2
secondary
host3
monitor
thread
monitor
thread
monitor
thread
MongoClient
Your
Code
insert
ok
insert
ok
✓
✓
✓
Steady State

18. Crisis

19. primary
host1
secondary
host2
secondary
host3
monitor
thread
monitor
thread
monitor
thread
MongoClient
Your
Code
insert
ok
insert
ok
✓
✓
✓

20. primary
host1
secondary
host2
secondary
host3
monitor
thread
monitor
thread
monitor
thread
Your
Code
insert
ConnectionFailure!
insert
✓
✓
✓
MongoClient
?
Crisis

21. primary
host1
secondary
host2
secondary
host3
monitor
thread
monitor
thread
monitor
thread
Your
Code
insert
✓ ✓
MongoClient
"Wake up!"
Crisis
?

22. Resolution

23. primary
host1
secondary
host2
secondary
host3
monitor
thread
monitor
thread
monitor
thread
Your
Code
✓
insert
"Wake up!"
(every half second)
MongoClient
primary
host2
✓
insert
ok
ok
Resolution
?

24. So What?

25. client = MongoClient(!
'mongodb://host1,host2/?replicaSet=my-rs')!
!
try:!
client.admin.command('ismaster')!
except pymongo.errors.ConnectionFailure as e:!
print("Can't connect: %s" % e)!
Non-Blocking
Client Construction
#1

26. Error Handling
#2
def find_my_document():!
try:!
return collection.find_one(query)!
except ConnectionFailure:!
logging.exception("finding document")!
return collection.find_one(query)!

27. Error Handling
#2
def insert_doc():!
doc = {'_id': ObjectId(), 'a': 1}!
try:!
collection.insert(doc)!
except ConnectionFailure:!
logging.exception("inserting document")!
# Try again.!
collection.insert(doc)!
except DuplicateKeyError:!
# Previous try actually succeeded.!
pass!

28. #2
Error Handling
collection.update(!
{'_id': 1},!
{'$inc': {'counter': 1}})!

29. #2
Only idempotent
operations can
be retried
Error Handling

30. Knobs
#3
•connectTimeoutMS
•serverSelectionTimeoutMS

31. ConnectionFailure!
monitor
thread
monitor
thread
monitor
thread
primary
host1
secondary
host2
secondary
host3
Your
Code
insert insertMongoClient
"Wake up!"
wait for
connectTimeoutMS
wait for
serverSelectionTimeoutMS
(30 seconds)

32. bit.ly/server-discovery
Further Study: Server Discovery
&
Monitoring Spec