Drivers and High Availability
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How do MongoDB drivers discover and monitor your servers? How do they determine your replica set config, and how do they resolve a crisis like the loss of a primary? And finally, what's the best way to write your application to handle errors and outages? Staff Engineer A. Jesse Jiryu Davis covers these topics for the first time at any MongoDB conference, in his talk at MongoDB World 2015 about the new Server Discovery And Monitoring spec.
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Drivers and High Availability
- 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
- 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
- 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
- 18. 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
- 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)