Chapman: Building a High-Performance Distributed Task Service with MongoDB
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When you're building a web application, you want to respond to every request as quickly as possible. The usual approach is to use an asynchronous job queue like Sidekiq, Resque, Celery, RQ, or a number of other frameworks to handle those tasks outside the request/response cycle in a separate 'worker' process. Unfortunately, many of these frameworks either require the deployment of Redis, RabbitMQ, or some other request broker, or they resort to polling a database for new work to do. Chapman is a distributed task queue built on MongoDB that avoids gratuitous polling, using tailable cursors with the oplog to provide notifications of incoming work. Inspired by Celery, Chapman also supports task graphs, where multiple tasks that depend on each other can be executed by the system asynchronously. Come learn how Synapp.io is using MongoDB and Chapman to handle its core data processing needs.
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Chapman: Building a High-Performance Distributed Task Service with MongoDB
- 1. Chapman: Building a Distributed Job Queue in MongoDB Rick Copeland @rick446 @synappio rick@synapp.io
- 2. @rick446 @synappio Getting to Know One Another
- 3. @rick446 @synappio Getting to Know One Another Rick
- 4. @rick446 @synappio Getting to Know One Another Rick
- 7. @rick446 @synappio WhatYou’ll Learn How to… Build a task queue in MongoDB
- 8. @rick446 @synappio WhatYou’ll Learn How to… Build a task queue in MongoDB
- 9. @rick446 @synappio WhatYou’ll Learn How to… Build a task queue in MongoDB Bring consistency to distributed systems (without transactions)
- 10. @rick446 @synappio WhatYou’ll Learn How to… Build a task queue in MongoDB Bring consistency to distributed systems (without transactions)
- 11. @rick446 @synappio WhatYou’ll Learn How to… Build a task queue in MongoDB Bring consistency to distributed systems (without transactions) Build low-latency reactive systems
- 12. @rick446 @synappio Why a Queue?
- 13. @rick446 @synappio Why a Queue? • Long-running task (or longer than the web can wait)
- 14. @rick446 @synappio Why a Queue? • Long-running task (or longer than the web can wait) • Farm out chunks of work for performance
- 15. @rick446 @synappio Things I Worry About
- 16. @rick446 @synappio Things I Worry About • Priority
- 17. @rick446 @synappio Things I Worry About • Priority • Latency
- 18. @rick446 @synappio Things I Worry About • Priority • Latency • Unreliable workers
- 20. @rick446 @synappio Queue Options • SQS? No priority
- 21. @rick446 @synappio Queue Options • SQS? No priority • Redis? Can’t overflow memory
- 22. @rick446 @synappio Queue Options • SQS? No priority • Redis? Can’t overflow memory • Rabbit-MQ? Lack of visibility
- 23. @rick446 @synappio Queue Options • SQS? No priority • Redis? Can’t overflow memory • Rabbit-MQ? Lack of visibility • ZeroMQ? Lack of persistence
- 24. @rick446 @synappio Queue Options • SQS? No priority • Redis? Can’t overflow memory • Rabbit-MQ? Lack of visibility • ZeroMQ? Lack of persistence • What about MongoDB?
- 25. @rick446 @synappio Chapman Graham Arthur Chapman 8 January 1941 – 4 October 1989
- 27. @rick446 @synappio Roadmap • Building a scheduled priority queue
- 28. @rick446 @synappio Roadmap • Building a scheduled priority queue • Handling unreliable workers
- 29. @rick446 @synappio Roadmap • Building a scheduled priority queue • Handling unreliable workers • Shared resources
- 30. @rick446 @synappio Roadmap • Building a scheduled priority queue • Handling unreliable workers • Shared resources • Managing Latency
- 31. @rick446 @synappio Building a Scheduled Priority Queue
- 32. @rick446 @synappio Step 1: Simple Queue db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z")! }! });! ! db.message.ensureIndex({'s.status': 1, 's.ts_enqueue': 1});! ! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.ts_enqueue': 1},! update: { '$set': {'s.status': 'reserved'} },! }! );!
- 33. @rick446 @synappio Step 1: Simple Queue db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z")! }! });! ! db.message.ensureIndex({'s.status': 1, 's.ts_enqueue': 1});! ! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.ts_enqueue': 1},! update: { '$set': {'s.status': 'reserved'} },! }! );! FIFO
- 34. @rick446 @synappio Step 1: Simple Queue db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z")! }! });! ! db.message.ensureIndex({'s.status': 1, 's.ts_enqueue': 1});! ! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.ts_enqueue': 1},! update: { '$set': {'s.status': 'reserved'} },! }! );! FIFO Get earliest message for processing
- 35. @rick446 @synappio Step 1: Simple Queue
- 36. @rick446 @synappio Step 1: Simple Queue Good
- 37. @rick446 @synappio Step 1: Simple Queue Good • Guaranteed FIFO
- 38. @rick446 @synappio Step 1: Simple Queue Good • Guaranteed FIFO Bad
- 39. @rick446 @synappio Step 1: Simple Queue Good • Guaranteed FIFO Bad • No priority (other than FIFO)
- 40. @rick446 @synappio Step 1: Simple Queue Good • Guaranteed FIFO Bad • No priority (other than FIFO) • No handling of worker problems
- 41. @rick446 @synappio Step 2: Scheduled Messages db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! “ts_after" : ISODate(…),! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z")! }! });! ! db.message.ensureIndex(! {'s.status': 1, 's.ts_enqueue': 1});! ! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready', ’s.ts_after': {$lt: now }},! sort: {'s.ts_enqueue': 1},! update: { '$set': {'s.status': 'reserved'} },! }! );
- 42. @rick446 @synappio Step 2: Scheduled Messages db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! “ts_after" : ISODate(…),! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z")! }! });! ! db.message.ensureIndex(! {'s.status': 1, 's.ts_enqueue': 1});! ! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready', ’s.ts_after': {$lt: now }},! sort: {'s.ts_enqueue': 1},! update: { '$set': {'s.status': 'reserved'} },! }! ); MinValid Time
- 43. @rick446 @synappio Step 2: Scheduled Messages db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! “ts_after" : ISODate(…),! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z")! }! });! ! db.message.ensureIndex(! {'s.status': 1, 's.ts_enqueue': 1});! ! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready', ’s.ts_after': {$lt: now }},! sort: {'s.ts_enqueue': 1},! update: { '$set': {'s.status': 'reserved'} },! }! ); MinValid Time Get earliest message for processing
- 44. @rick446 @synappio Step 2: Scheduled Messages
- 45. @rick446 @synappio Step 2: Scheduled Messages Good
- 46. @rick446 @synappio Step 2: Scheduled Messages Good • Easy to build periodic tasks
- 47. @rick446 @synappio Step 2: Scheduled Messages Good • Easy to build periodic tasks Bad
- 48. @rick446 @synappio Step 2: Scheduled Messages Good • Easy to build periodic tasks Bad • Be careful with the word “now”
- 49. @rick446 @synappio Step 3: Priority db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "pri": 30128,! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z")! }! });! ! db.message.ensureIndex({'s.status': 1, 's.pri': -1, 's.ts_enqueue': 1});! ! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.pri': -1, 's.ts_enqueue': 1},! update: { '$set': {'s.status': 'reserved'} },! }! );
- 50. @rick446 @synappio Step 3: Priority db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "pri": 30128,! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z")! }! });! ! db.message.ensureIndex({'s.status': 1, 's.pri': -1, 's.ts_enqueue': 1});! ! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.pri': -1, 's.ts_enqueue': 1},! update: { '$set': {'s.status': 'reserved'} },! }! ); Add Priority
- 51. @rick446 @synappio Step 3: Priority db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "pri": 30128,! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z")! }! });! ! db.message.ensureIndex({'s.status': 1, 's.pri': -1, 's.ts_enqueue': 1});! ! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.pri': -1, 's.ts_enqueue': 1},! update: { '$set': {'s.status': 'reserved'} },! }! ); Add Priority
- 52. @rick446 @synappio Step 3: Priority
- 53. @rick446 @synappio Step 3: Priority Good
- 54. @rick446 @synappio Step 3: Priority Good • Priorities are handled
- 55. @rick446 @synappio Step 3: Priority Good • Priorities are handled • Guaranteed FIFO within a priority
- 56. @rick446 @synappio Step 3: Priority Good • Priorities are handled • Guaranteed FIFO within a priority Bad
- 57. @rick446 @synappio Step 3: Priority Good • Priorities are handled • Guaranteed FIFO within a priority Bad • No handling of worker problems
- 59. @rick446 @synappio Approach 1 Timeouts db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "pri": 30128,! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z"),! "ts_timeout" : ISODate("2025-01-01T00:00:00.000Z")! }! });! ! db.message.ensureIndex({“s.status": 1, “s.ts_timeout": 1})! !
- 60. @rick446 @synappio Approach 1 Timeouts db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "pri": 30128,! "ts_enqueue" : ISODate("2015-03-02T15:27:29.228Z"),! "ts_timeout" : ISODate("2025-01-01T00:00:00.000Z")! }! });! ! db.message.ensureIndex({“s.status": 1, “s.ts_timeout": 1})! ! Far-future placeholder
- 61. @rick446 @synappio // Reserve message! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.pri': -1, 's.ts_enqueue': 1},! update: { '$set': {! 's.status': 'reserved',! 's.ts_timeout': now + processing_time } }! }! );! ! // Timeout message ("unlock")! db.message.update(! {'s.ts_status': 'reserved', 's.ts_timeout': {'$lt': now}},! {'$set': {'s.status': 'ready'}},! {'multi': true}); Approach 1 Timeouts
- 62. @rick446 @synappio // Reserve message! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.pri': -1, 's.ts_enqueue': 1},! update: { '$set': {! 's.status': 'reserved',! 's.ts_timeout': now + processing_time } }! }! );! ! // Timeout message ("unlock")! db.message.update(! {'s.ts_status': 'reserved', 's.ts_timeout': {'$lt': now}},! {'$set': {'s.status': 'ready'}},! {'multi': true}); Client sets timeout Approach 1 Timeouts
- 65. @rick446 @synappio Approach 1 Timeouts Good • Worker failure handled via timeout
- 66. @rick446 @synappio Approach 1 Timeouts Good • Worker failure handled via timeout Bad
- 67. @rick446 @synappio Approach 1 Timeouts Good • Worker failure handled via timeout Bad • Requires periodic “unlock” task
- 68. @rick446 @synappio Approach 1 Timeouts Good • Worker failure handled via timeout Bad • Requires periodic “unlock” task • Slow (but “live”) workers can cause spurious timeouts
- 69. @rick446 @synappio db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "pri": 30128,! "cli": "--------------------------"! "ts_enqueue" : ISODate("2015-03-02T..."),! "ts_timeout" : ISODate("2025-...")! }! }); Approach 2 Worker Identity
- 70. @rick446 @synappio db.message.insert({! "_id" : NumberLong("3784707300388732067"),! "data" : BinData(...),! "s" : {! "status" : "ready",! "pri": 30128,! "cli": "--------------------------"! "ts_enqueue" : ISODate("2015-03-02T..."),! "ts_timeout" : ISODate("2025-...")! }! }); Client / worker placeholder Approach 2 Worker Identity
- 71. @rick446 @synappio // Reserve message! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.pri': -1, 's.ts_enqueue': 1},! update: { '$set': {! 's.status': 'reserved',! 's.cli': ‘client_name:pid',! 's.ts_timeout': now + processing_time } }! }! );! ! // Unlock “dead” client messages! db.message.update(! {'s.status': 'reserved', ! 's.cli': {'$nin': active_clients} },! {'$set': {'s.status': 'ready'}},! {'multi': true});! Approach 2 Worker Identity
- 72. @rick446 @synappio // Reserve message! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.pri': -1, 's.ts_enqueue': 1},! update: { '$set': {! 's.status': 'reserved',! 's.cli': ‘client_name:pid',! 's.ts_timeout': now + processing_time } }! }! );! ! // Unlock “dead” client messages! db.message.update(! {'s.status': 'reserved', ! 's.cli': {'$nin': active_clients} },! {'$set': {'s.status': 'ready'}},! {'multi': true});! Mark the worker who reserved the message Approach 2 Worker Identity
- 73. @rick446 @synappio // Reserve message! db.runCommand(! {! findAndModify: "message",! query: { 's.status': 'ready' },! sort: {'s.pri': -1, 's.ts_enqueue': 1},! update: { '$set': {! 's.status': 'reserved',! 's.cli': ‘client_name:pid',! 's.ts_timeout': now + processing_time } }! }! );! ! // Unlock “dead” client messages! db.message.update(! {'s.status': 'reserved', ! 's.cli': {'$nin': active_clients} },! {'$set': {'s.status': 'ready'}},! {'multi': true});! Mark the worker who reserved the message Messages reserved by dead workers are unlocked Approach 2 Worker Identity
- 74. @rick446 @synappio Approach 2 Worker Identity
- 75. @rick446 @synappio Approach 2 Worker Identity Good
- 76. @rick446 @synappio Approach 2 Worker Identity Good • Worker failure handled via out- of-band detection of live workers
- 77. @rick446 @synappio Approach 2 Worker Identity Good • Worker failure handled via out- of-band detection of live workers • Handles slow workers
- 78. @rick446 @synappio Approach 2 Worker Identity Good • Worker failure handled via out- of-band detection of live workers • Handles slow workers
- 79. @rick446 @synappio Approach 2 Worker Identity Good • Worker failure handled via out- of-band detection of live workers • Handles slow workers Bad
- 80. @rick446 @synappio Approach 2 Worker Identity Good • Worker failure handled via out- of-band detection of live workers • Handles slow workers Bad • Requires periodic “unlock” task
- 81. @rick446 @synappio Approach 2 Worker Identity Good • Worker failure handled via out- of-band detection of live workers • Handles slow workers Bad • Requires periodic “unlock” task • Unlock updates can be slow
- 85. @rick446 @synappio Semaphores • Some services perform connection- throttling (e.g. Mailchimp)
- 86. @rick446 @synappio Semaphores • Some services perform connection- throttling (e.g. Mailchimp) • Some services just have a hard time with 144 threads hitting them simultaneously
- 87. @rick446 @synappio Semaphores • Some services perform connection- throttling (e.g. Mailchimp) • Some services just have a hard time with 144 threads hitting them simultaneously • Need a way to limit our concurrency
- 88. @rick446 @synappio Semaphores Semaphore Active: msg1, msg2, msg3, … Capacity: 16 Queued: msg17, msg18, msg19, …
- 89. @rick446 @synappio Semaphores Semaphore Active: msg1, msg2, msg3, … Capacity: 16 Queued: msg17, msg18, msg19, … • Keep active and queued messages in arrays
- 90. @rick446 @synappio Semaphores Semaphore Active: msg1, msg2, msg3, … Capacity: 16 Queued: msg17, msg18, msg19, … • Keep active and queued messages in arrays • Releasing the semaphore makes queued messages available for dispatch
- 91. @rick446 @synappio Semaphores Semaphore Active: msg1, msg2, msg3, … Capacity: 16 Queued: msg17, msg18, msg19, … • Keep active and queued messages in arrays • Releasing the semaphore makes queued messages available for dispatch • Use $slice (2.6) to keep arrays the right size
- 92. @rick446 @synappio Semaphores:Acquire db.semaphore.insert({! '_id': 'semaphore-name',! 'value': 16,! 'active': [],! 'queued': []});! ! def acquire(sem_id, msg_id, sem_size):! sem = db.semaphore.find_and_modify(! {'_id': sem_id},! update={'$push': {! 'active': {! '$each': [msg_id], ! '$slice': sem_size},! 'queued': msg_id}},! new=True)! if msg_id in sem['active']:! db.semaphore.update(! {'_id': 'semaphore-name'},! {'$pull': {'queued': msg_id}})! return True! return False
- 93. @rick446 @synappio Semaphores:Acquire db.semaphore.insert({! '_id': 'semaphore-name',! 'value': 16,! 'active': [],! 'queued': []});! ! def acquire(sem_id, msg_id, sem_size):! sem = db.semaphore.find_and_modify(! {'_id': sem_id},! update={'$push': {! 'active': {! '$each': [msg_id], ! '$slice': sem_size},! 'queued': msg_id}},! new=True)! if msg_id in sem['active']:! db.semaphore.update(! {'_id': 'semaphore-name'},! {'$pull': {'queued': msg_id}})! return True! return False Pessimistic update
- 94. @rick446 @synappio Semaphores:Acquire db.semaphore.insert({! '_id': 'semaphore-name',! 'value': 16,! 'active': [],! 'queued': []});! ! def acquire(sem_id, msg_id, sem_size):! sem = db.semaphore.find_and_modify(! {'_id': sem_id},! update={'$push': {! 'active': {! '$each': [msg_id], ! '$slice': sem_size},! 'queued': msg_id}},! new=True)! if msg_id in sem['active']:! db.semaphore.update(! {'_id': 'semaphore-name'},! {'$pull': {'queued': msg_id}})! return True! return False Pessimistic update Compensation
- 95. @rick446 @synappio Semaphores: Release def release(sem_id, msg_id, sem_size):! sem = db.semaphore.find_and_modify(! {'_id': sem_id},! update={'$pull': {! 'active': msg_id, ! 'queued': msg_id}},! new=True)! ! while len(sem['active']) < sem_size and sem['queued']:! wake_msg_ids = sem['queued'][:sem_size]! updated = self.cls.m.find_and_modify(! {'_id': sem_id},! update={'$pullAll': {'queued': wake_msg_ids}},! new=True)! for msgid in wake_msg_ids:! make_dispatchable(msgid)! sem = updated
- 96. @rick446 @synappio Semaphores: Release def release(sem_id, msg_id, sem_size):! sem = db.semaphore.find_and_modify(! {'_id': sem_id},! update={'$pull': {! 'active': msg_id, ! 'queued': msg_id}},! new=True)! ! while len(sem['active']) < sem_size and sem['queued']:! wake_msg_ids = sem['queued'][:sem_size]! updated = self.cls.m.find_and_modify(! {'_id': sem_id},! update={'$pullAll': {'queued': wake_msg_ids}},! new=True)! for msgid in wake_msg_ids:! make_dispatchable(msgid)! sem = updated Actually release
- 97. @rick446 @synappio Semaphores: Release def release(sem_id, msg_id, sem_size):! sem = db.semaphore.find_and_modify(! {'_id': sem_id},! update={'$pull': {! 'active': msg_id, ! 'queued': msg_id}},! new=True)! ! while len(sem['active']) < sem_size and sem['queued']:! wake_msg_ids = sem['queued'][:sem_size]! updated = self.cls.m.find_and_modify(! {'_id': sem_id},! update={'$pullAll': {'queued': wake_msg_ids}},! new=True)! for msgid in wake_msg_ids:! make_dispatchable(msgid)! sem = updated Actually release Awaken queued message(s)
- 98. @rick446 @synappio Semaphores: Release def release(sem_id, msg_id, sem_size):! sem = db.semaphore.find_and_modify(! {'_id': sem_id},! update={'$pull': {! 'active': msg_id, ! 'queued': msg_id}},! new=True)! ! while len(sem['active']) < sem_size and sem['queued']:! wake_msg_ids = sem['queued'][:sem_size]! updated = self.cls.m.find_and_modify(! {'_id': sem_id},! update={'$pullAll': {'queued': wake_msg_ids}},! new=True)! for msgid in wake_msg_ids:! make_dispatchable(msgid)! sem = updated Actually release Awaken queued message(s) Some magic (covered later)
- 100. @rick446 @synappio Message States ready acquirequeued busy • Reserve the message
- 101. @rick446 @synappio Message States ready acquirequeued busy • Reserve the message • Acquire resources
- 102. @rick446 @synappio Message States ready acquirequeued busy • Reserve the message • Acquire resources • Process the message
- 103. @rick446 @synappio Message States ready acquirequeued busy • Reserve the message • Acquire resources • Process the message • Release resources
- 104. @rick446 @synappio Reserve a Message msg = db.message.find_and_modify(! {'s.status': 'ready'},! sort=[('s.sub_status', -1), ('s.pri', -1), ('s.ts', 1)],! update={'$set': {'s.w': worker, 's.status': 'acquire'}},! new=True) message.s == {! pri: 10,! semaphores: ['foo'],! status: 'ready',! sub_status: 0,! w: '----------',! ...} message.s == {! pri: 10,! semaphores: ['foo'],! status: 'acquire! sub_status: 0,! w: worker,! ...}
- 105. @rick446 @synappio Reserve a Message msg = db.message.find_and_modify(! {'s.status': 'ready'},! sort=[('s.sub_status', -1), ('s.pri', -1), ('s.ts', 1)],! update={'$set': {'s.w': worker, 's.status': 'acquire'}},! new=True) message.s == {! pri: 10,! semaphores: ['foo'],! status: 'ready',! sub_status: 0,! w: '----------',! ...} Required semaphores message.s == {! pri: 10,! semaphores: ['foo'],! status: 'acquire! sub_status: 0,! w: worker,! ...}
- 106. @rick446 @synappio Reserve a Message msg = db.message.find_and_modify(! {'s.status': 'ready'},! sort=[('s.sub_status', -1), ('s.pri', -1), ('s.ts', 1)],! update={'$set': {'s.w': worker, 's.status': 'acquire'}},! new=True) message.s == {! pri: 10,! semaphores: ['foo'],! status: 'ready',! sub_status: 0,! w: '----------',! ...} Required semaphores # semaphores acquired message.s == {! pri: 10,! semaphores: ['foo'],! status: 'acquire! sub_status: 0,! w: worker,! ...}
- 107. @rick446 @synappio Reserve a Message msg = db.message.find_and_modify(! {'s.status': 'ready'},! sort=[('s.sub_status', -1), ('s.pri', -1), ('s.ts', 1)],! update={'$set': {'s.w': worker, 's.status': 'acquire'}},! new=True) message.s == {! pri: 10,! semaphores: ['foo'],! status: 'ready',! sub_status: 0,! w: '----------',! ...} Required semaphores # semaphores acquired message.s == {! pri: 10,! semaphores: ['foo'],! status: 'acquire! sub_status: 0,! w: worker,! ...} Prefer partially-acquired messages
- 108. @rick446 @synappio Acquire Resources def acquire_resources(msg):! for i, sem_id in enumerate(msg['s']['semaphores']):! if i < msg['sub_status']: # already acquired! continue! sem = db.semaphore.find_one({'_id': 'sem_id'})! if try_acquire_resource(sem_id, msg['_id'], sem['value']):! db.message.update(! {'_id': msg['_id']}, {'$set': {'s.sub_status': i}})! else:! return False! db.message.update(! {'_id': msg['_id']}, {'$set': {'s.status': 'busy'}})! return True
- 109. @rick446 @synappio Acquire Resources def acquire_resources(msg):! for i, sem_id in enumerate(msg['s']['semaphores']):! if i < msg['sub_status']: # already acquired! continue! sem = db.semaphore.find_one({'_id': 'sem_id'})! if try_acquire_resource(sem_id, msg['_id'], sem['value']):! db.message.update(! {'_id': msg['_id']}, {'$set': {'s.sub_status': i}})! else:! return False! db.message.update(! {'_id': msg['_id']}, {'$set': {'s.status': 'busy'}})! return True Save forward progress
- 110. @rick446 @synappio Acquire Resources def acquire_resources(msg):! for i, sem_id in enumerate(msg['s']['semaphores']):! if i < msg['sub_status']: # already acquired! continue! sem = db.semaphore.find_one({'_id': 'sem_id'})! if try_acquire_resource(sem_id, msg['_id'], sem['value']):! db.message.update(! {'_id': msg['_id']}, {'$set': {'s.sub_status': i}})! else:! return False! db.message.update(! {'_id': msg['_id']}, {'$set': {'s.status': 'busy'}})! return True Save forward progress Failure to acquire (already queued)
- 111. @rick446 @synappio Acquire Resources def acquire_resources(msg):! for i, sem_id in enumerate(msg['s']['semaphores']):! if i < msg['sub_status']: # already acquired! continue! sem = db.semaphore.find_one({'_id': 'sem_id'})! if try_acquire_resource(sem_id, msg['_id'], sem['value']):! db.message.update(! {'_id': msg['_id']}, {'$set': {'s.sub_status': i}})! else:! return False! db.message.update(! {'_id': msg['_id']}, {'$set': {'s.status': 'busy'}})! return True Save forward progress Failure to acquire (already queued) Resources acquired, message ready to be processed
- 112. @rick446 @synappio Acquire Resources def try_acquire_resource(sem_id, msg_id, sem_size):! '''Version 1 (race condition)'''! if reserve(sem_id, msg_id, sem_size):! return True! else:! db.message.update(! {'_id': msg_id},! {'$set': {'s.status': 'queued'}})! return False
- 113. @rick446 @synappio Acquire Resources def try_acquire_resource(sem_id, msg_id, sem_size):! '''Version 1 (race condition)'''! if reserve(sem_id, msg_id, sem_size):! return True! else:! db.message.update(! {'_id': msg_id},! {'$set': {'s.status': 'queued'}})! return False Here be dragons!
- 114. @rick446 @synappio Release Resources (v1) “magic” def make_dispatchable(msg_id):! '''Version 1 (race condition)'''! db.message.update(! {'_id': msg_id, 's.status': 'queued'},! {'$set': {'s.status': 'ready'}})
- 115. @rick446 @synappio Release Resources (v1) “magic” def make_dispatchable(msg_id):! '''Version 1 (race condition)'''! db.message.update(! {'_id': msg_id, 's.status': 'queued'},! {'$set': {'s.status': 'ready'}}) But what if s.status == ‘acquire’?
- 116. @rick446 @synappio Release Resources (v1) “magic” def make_dispatchable(msg_id):! '''Version 1 (race condition)'''! db.message.update(! {'_id': msg_id, 's.status': 'queued'},! {'$set': {'s.status': 'ready'}}) But what if s.status == ‘acquire’?
- 117. @rick446 @synappio Release Resources (v1) “magic” def make_dispatchable(msg_id):! '''Version 1 (race condition)'''! db.message.update(! {'_id': msg_id, 's.status': 'queued'},! {'$set': {'s.status': 'ready'}}) But what if s.status == ‘acquire’? That’s the dragon.
- 118. @rick446 @synappio Release Resources (v2) def make_dispatchable(msg_id):! res = db.message.update(! {'_id': msg_id, 's.status': 'acquire'},! {'$set': {'s.event': True}})! if not res['updatedExisting']:! db.message.update(! {'_id': msg_id, 's.status': 'queued'},! {'$set': {'s.status': 'ready'}})
- 119. @rick446 @synappio Release Resources (v2) def make_dispatchable(msg_id):! res = db.message.update(! {'_id': msg_id, 's.status': 'acquire'},! {'$set': {'s.event': True}})! if not res['updatedExisting']:! db.message.update(! {'_id': msg_id, 's.status': 'queued'},! {'$set': {'s.status': 'ready'}}) Hey, something happened!
- 120. @rick446 @synappio Acquire Resources (v2) def try_acquire_resource(sem_id, msg_id, sem_size):! '''Version 2'''! while True:! db.message.update(! {'_id': msg_id}, {'$set': {'event': False}})! if reserve(sem_id, msg_id, sem_size):! return True! else:! res = db.message.update(! {'_id': msg_id, 's.event': False},! {'$set': {'s.status': 'queued'}})! if not res['updatedExisting']:! # Someone released this message; try again! continue! return False
- 121. @rick446 @synappio Acquire Resources (v2) def try_acquire_resource(sem_id, msg_id, sem_size):! '''Version 2'''! while True:! db.message.update(! {'_id': msg_id}, {'$set': {'event': False}})! if reserve(sem_id, msg_id, sem_size):! return True! else:! res = db.message.update(! {'_id': msg_id, 's.event': False},! {'$set': {'s.status': 'queued'}})! if not res['updatedExisting']:! # Someone released this message; try again! continue! return False Nothing’s happened yet!
- 122. @rick446 @synappio Acquire Resources (v2) def try_acquire_resource(sem_id, msg_id, sem_size):! '''Version 2'''! while True:! db.message.update(! {'_id': msg_id}, {'$set': {'event': False}})! if reserve(sem_id, msg_id, sem_size):! return True! else:! res = db.message.update(! {'_id': msg_id, 's.event': False},! {'$set': {'s.status': 'queued'}})! if not res['updatedExisting']:! # Someone released this message; try again! continue! return False Nothing’s happened yet! Check if something happened
- 123. @rick446 @synappio One More Race…. def release(sem_id, msg_id, sem_size):! sem = db.semaphore.find_and_modify(! {'_id': sem_id},! update={'$pull': {! 'active': msg_id, ! 'queued': msg_id}},! new=True)! ! while len(sem['active']) < sem_size and sem['queued']:! wake_msg_ids = sem['queued'][:sem_size]! updated = self.cls.m.find_and_modify(! {'_id': sem_id},! update={'$pullAll': {'queued': wake_msg_ids}},! new=True)! for msgid in wake_msg_ids:! make_dispatchable(msgid)! sem = updated
- 124. @rick446 @synappio One More Race…. def release(sem_id, msg_id, sem_size):! sem = db.semaphore.find_and_modify(! {'_id': sem_id},! update={'$pull': {! 'active': msg_id, ! 'queued': msg_id}},! new=True)! ! while len(sem['active']) < sem_size and sem['queued']:! wake_msg_ids = sem['queued'][:sem_size]! updated = self.cls.m.find_and_modify(! {'_id': sem_id},! update={'$pullAll': {'queued': wake_msg_ids}},! new=True)! for msgid in wake_msg_ids:! make_dispatchable(msgid)! sem = updated
- 125. @rick446 @synappio Compensate! def fixup_queued_messages():! for msg in db.message.find({'s.status': 'queued'}):! sem_id = msg['semaphores'][msg['s']['sub_status']]! sem = db.semaphore.find_one(! {'_id': sem_id, 'queued': msg['_id']})! if sem is None:! db.message.m.update(! {'_id': msg['_id'], ! 's.status': 'queued', ! 's.sub_status': msg['sub_status']},! {'$set': {'s.status': 'ready'}})
- 127. @rick446 @synappio Managing Latency • Reserving messages is expensive • Use Pub/Sub system instead • Publish to the channel whenever a message is ready to be handled • Each worker subscribes to the channel • Workers only ‘poll’ when they have a chance of getting work
- 128. Capped Collections Capped Collection • Fixed size • Fast inserts • “Tailable” cursors Tailable Cursor
- 129. Capped Collections Capped Collection • Fixed size • Fast inserts • “Tailable” cursors Tailable Cursor
- 130. Capped Collections Capped Collection • Fixed size • Fast inserts • “Tailable” cursors Tailable Cursor
- 131. Capped Collections Capped Collection • Fixed size • Fast inserts • “Tailable” cursors Tailable Cursor
- 132. Capped Collections Capped Collection • Fixed size • Fast inserts • “Tailable” cursors Tailable Cursor
- 133. Capped Collections Capped Collection • Fixed size • Fast inserts • “Tailable” cursors Tailable Cursor
- 134. @rick446 @synappio Getting a Tailable Cursor def get_cursor(collection, topic_re, await_data=True):! options = { 'tailable': True }! if await_data:! options['await_data'] = True! cur = collection.find(! { 'k': topic_re },! **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! return cur
- 135. @rick446 @synappio Getting a Tailable Cursor def get_cursor(collection, topic_re, await_data=True):! options = { 'tailable': True }! if await_data:! options['await_data'] = True! cur = collection.find(! { 'k': topic_re },! **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! return cur Make cursor tailable
- 136. @rick446 @synappio Getting a Tailable Cursor def get_cursor(collection, topic_re, await_data=True):! options = { 'tailable': True }! if await_data:! options['await_data'] = True! cur = collection.find(! { 'k': topic_re },! **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! return cur Holds open cursor for a while Make cursor tailable
- 137. @rick446 @synappio Getting a Tailable Cursor def get_cursor(collection, topic_re, await_data=True):! options = { 'tailable': True }! if await_data:! options['await_data'] = True! cur = collection.find(! { 'k': topic_re },! **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! return cur Holds open cursor for a while Make cursor tailable Don’t use indexes
- 138. @rick446 @synappio Getting a Tailable Cursor def get_cursor(collection, topic_re, await_data=True):! options = { 'tailable': True }! if await_data:! options['await_data'] = True! cur = collection.find(! { 'k': topic_re },! **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! return cur import re, time! while True:! cur = get_cursor(! db.capped_collection, ! re.compile('^foo'), ! await_data=True)! for msg in cur:! do_something(msg)! time.sleep(0.1) Holds open cursor for a while Make cursor tailable Don’t use indexes
- 139. @rick446 @synappio Getting a Tailable Cursor def get_cursor(collection, topic_re, await_data=True):! options = { 'tailable': True }! if await_data:! options['await_data'] = True! cur = collection.find(! { 'k': topic_re },! **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! return cur import re, time! while True:! cur = get_cursor(! db.capped_collection, ! re.compile('^foo'), ! await_data=True)! for msg in cur:! do_something(msg)! time.sleep(0.1) Holds open cursor for a while Make cursor tailable Don’t use indexes Still some polling when no producer, so don’t spin too fast
- 140. @rick446 @synappio Building in retry... def get_cursor(collection, topic_re, last_id=-1, await_data=True):! options = { 'tailable': True }! spec = { ! 'id': { '$gt': last_id }, # only new messages! 'k': topic_re }! if await_data:! options['await_data'] = True! cur = collection.find(spec, **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! return cur
- 141. @rick446 @synappio Building in retry... def get_cursor(collection, topic_re, last_id=-1, await_data=True):! options = { 'tailable': True }! spec = { ! 'id': { '$gt': last_id }, # only new messages! 'k': topic_re }! if await_data:! options['await_data'] = True! cur = collection.find(spec, **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! return cur Integer autoincrement “id”
- 142. @rick446 @synappio Ludicrous Speed from pymongo.cursor import _QUERY_OPTIONS! ! def get_cursor(collection, topic_re, last_id=-1, await_data=True):! options = { 'tailable': True }! spec = { ! 'ts': { '$gt': last_id }, # only new messages! 'k': topic_re }! if await_data:! options['await_data'] = True! cur = collection.find(spec, **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! if await:! cur = cur.add_option(_QUERY_OPTIONS['oplog_replay'])! return cur
- 143. @rick446 @synappio Ludicrous Speed from pymongo.cursor import _QUERY_OPTIONS! ! def get_cursor(collection, topic_re, last_id=-1, await_data=True):! options = { 'tailable': True }! spec = { ! 'ts': { '$gt': last_id }, # only new messages! 'k': topic_re }! if await_data:! options['await_data'] = True! cur = collection.find(spec, **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! if await:! cur = cur.add_option(_QUERY_OPTIONS['oplog_replay'])! return cur id ==> ts
- 144. @rick446 @synappio Ludicrous Speed from pymongo.cursor import _QUERY_OPTIONS! ! def get_cursor(collection, topic_re, last_id=-1, await_data=True):! options = { 'tailable': True }! spec = { ! 'ts': { '$gt': last_id }, # only new messages! 'k': topic_re }! if await_data:! options['await_data'] = True! cur = collection.find(spec, **options)! cur = cur.hint([('$natural', 1)]) # ensure we don't use any indexes! if await:! cur = cur.add_option(_QUERY_OPTIONS['oplog_replay'])! return cur id ==> ts Co-opt the oplog_replay option
- 145. @rick446 @synappio The Oplog • Capped collection that records all operations for replication • Includes a ‘ts’ field suitable for oplog_replay • Does not require a separate publish operation (all changes are automatically “published”)
- 146. @rick446 @synappio Using the Oplog def oplog_await(oplog, spec):! '''Await the very next message on the oplog satisfying the spec'''! last = oplog.find_one(spec, sort=[('$natural', -1)])! if last is None:! return # Can't await unless there is an existing message satisfying spec! await_spec = dict(spec)! last_ts = last['ts']! await_spec['ts'] = {'$gt': bson.Timestamp(last_ts.time, last_ts.inc - 1)}! curs = oplog.find(await_spec, tailable=True, await_data=True)! curs = curs.hint([('$natural', 1)])! curs = curs.add_option(_QUERY_OPTIONS['oplog_replay'])! curs.next() # should always find 1 element! try:! return curs.next()! except StopIteration:! return None
- 147. @rick446 @synappio Using the Oplog def oplog_await(oplog, spec):! '''Await the very next message on the oplog satisfying the spec'''! last = oplog.find_one(spec, sort=[('$natural', -1)])! if last is None:! return # Can't await unless there is an existing message satisfying spec! await_spec = dict(spec)! last_ts = last['ts']! await_spec['ts'] = {'$gt': bson.Timestamp(last_ts.time, last_ts.inc - 1)}! curs = oplog.find(await_spec, tailable=True, await_data=True)! curs = curs.hint([('$natural', 1)])! curs = curs.add_option(_QUERY_OPTIONS['oplog_replay'])! curs.next() # should always find 1 element! try:! return curs.next()! except StopIteration:! return None most recent oplog entry
- 148. @rick446 @synappio Using the Oplog def oplog_await(oplog, spec):! '''Await the very next message on the oplog satisfying the spec'''! last = oplog.find_one(spec, sort=[('$natural', -1)])! if last is None:! return # Can't await unless there is an existing message satisfying spec! await_spec = dict(spec)! last_ts = last['ts']! await_spec['ts'] = {'$gt': bson.Timestamp(last_ts.time, last_ts.inc - 1)}! curs = oplog.find(await_spec, tailable=True, await_data=True)! curs = curs.hint([('$natural', 1)])! curs = curs.add_option(_QUERY_OPTIONS['oplog_replay'])! curs.next() # should always find 1 element! try:! return curs.next()! except StopIteration:! return None most recent oplog entry finds most recent plus following entries
- 149. @rick446 @synappio Using the Oplog def oplog_await(oplog, spec):! '''Await the very next message on the oplog satisfying the spec'''! last = oplog.find_one(spec, sort=[('$natural', -1)])! if last is None:! return # Can't await unless there is an existing message satisfying spec! await_spec = dict(spec)! last_ts = last['ts']! await_spec['ts'] = {'$gt': bson.Timestamp(last_ts.time, last_ts.inc - 1)}! curs = oplog.find(await_spec, tailable=True, await_data=True)! curs = curs.hint([('$natural', 1)])! curs = curs.add_option(_QUERY_OPTIONS['oplog_replay'])! curs.next() # should always find 1 element! try:! return curs.next()! except StopIteration:! return None most recent oplog entry finds most recent plus following entries skip most recent
- 150. @rick446 @synappio Using the Oplog def oplog_await(oplog, spec):! '''Await the very next message on the oplog satisfying the spec'''! last = oplog.find_one(spec, sort=[('$natural', -1)])! if last is None:! return # Can't await unless there is an existing message satisfying spec! await_spec = dict(spec)! last_ts = last['ts']! await_spec['ts'] = {'$gt': bson.Timestamp(last_ts.time, last_ts.inc - 1)}! curs = oplog.find(await_spec, tailable=True, await_data=True)! curs = curs.hint([('$natural', 1)])! curs = curs.add_option(_QUERY_OPTIONS['oplog_replay'])! curs.next() # should always find 1 element! try:! return curs.next()! except StopIteration:! return None most recent oplog entry finds most recent plus following entries skip most recent return on anything new
- 151. @rick446 @synappio What We’ve Learned
- 152. @rick446 @synappio What We’ve Learned How to…
- 153. @rick446 @synappio What We’ve Learned How to… Build a task queue in MongoDB
- 154. @rick446 @synappio What We’ve Learned How to… Build a task queue in MongoDB
- 155. @rick446 @synappio What We’ve Learned How to… Build a task queue in MongoDB Bring consistency to distributed systems (without transactions)
- 156. @rick446 @synappio What We’ve Learned How to… Build a task queue in MongoDB Bring consistency to distributed systems (without transactions)
- 157. @rick446 @synappio What We’ve Learned How to… Build a task queue in MongoDB Bring consistency to distributed systems (without transactions) Build low-latency reactive systems
- 159. @rick446 @synappio Tips • findAndModify is ideal for queues
- 160. @rick446 @synappio Tips • findAndModify is ideal for queues
- 161. @rick446 @synappio Tips • findAndModify is ideal for queues • Atomic update + compensation brings consistency to your distributed system
- 162. @rick446 @synappio Tips • findAndModify is ideal for queues • Atomic update + compensation brings consistency to your distributed system
- 163. @rick446 @synappio Tips • findAndModify is ideal for queues • Atomic update + compensation brings consistency to your distributed system • Use the oplog to build reactive, low-latency systems