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Concurrency Control in MongoDB
Kaloian ManassievSenior Engineer at MongoDB
[email protected]@kaloianm
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Audience
• Operations engineers• Application developers• Third-party storage engine developers• Anybody who is curious
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What is concurrency control?
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Locking
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Data consistency
$100 + $100 = $200 (not $100 )
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Performance
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Motivation
• Understand MongoDB’s concurrency control responsibilities
• Learn how MongoDB achieves document-level locking
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Talk outline
• MongoDB concurrency control• Multiple-granularity locking• WiredTiger vs MMAP V1• Questions
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Collection Collection
Storage Engine API
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MongoDB 2.0 (and before)
THETOP LOCK
R WR OK NOW NO NO
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MongoDB 2.2
Lock per database
db1.coll.insert({a:1}) db4.coll.find({c:5})
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MongoDB 2.2
THETOP LOCK + intents
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TOP
db1.coll.insert({a:1}) db4.coll.find({c:5})
IX IS
X S
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TOP
db.coll.insert({a:1}) db.coll.find({c:5})
IX IS
X S
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Why intents?
• Get rid of the global lock?
• Use read (shared) lock?
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TOP
db1.coll.insert({a:1}) db4.coll.find({c:5})
S S
X S
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TOP
db1.coll.insert({a:1}) db.fsyncLock({lock:1})
S S
X
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TOP
db1.coll.insert({a:1}) db.fsyncLock({lock:1})
IX S
X
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TOP
db1.coll.insert({a:1}) db.fsyncLock({lock:1})
IX S
XS S S S
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TOP
db.fsyncLock({lock:1})
S
S S S S
db1.coll.find({a:1})
IS
S
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Intents
• “Intention” to access one or more children of an item–Compatible with other intents–Need to acquire lock further down–No overhead
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Locks
• “Lock” an item for particular access–Item being locked–All items below it
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Intents compatibility
IS IX S XIS OK OK OK NOIX OK OK NO NOS OK NO OK NOX NO NO NO NO
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MongoDB 3.0
TOP
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Multiple-granularity locking
TOP
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Lock Manager
• Ensures the locking protocol is obeyed
• Very low overhead
• Testable
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Lock Manager
• Improved fairness
• Support for locking policies
• Lock statistics gathering
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TOP
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db.coll1.update({a:1}) db.coll1.update({a:4})
IX IX
IX IX
IX IX
X X
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Storage Engine API
TOP
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Storage Engine API
TOP
Storage Engine API
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How does WiredTiger work?
• Supports document concurrency
• Takes intents for the entire hierarchy
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TOP
db.coll1.update({a:1}) db.coll1.update({a:4})
IX IX
IX IX
IX IX
WiredTiger MVCC
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TOP
db.coll1.update({a:1}) db.coll1.drop()
IX IX
IX X
IX
WiredTiger MVCC
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How does MMAP V1 work?
• Supports collection concurrency
• Takes lock on the collection–Instead of intent
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TOP
db.coll1.update({a:1}) db.coll2.update({a:4})
IX IX
IX IX
X X
MMAP V1
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TOP
db.coll1.update({a:1}) db.coll1.find({a:4})
IX IS
IX IS
X S
MMAP V1
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WiredTiger
• Intents: Global, Database, Collection
• Document-level concurrency left to the storage engine
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MMAP V1
• Intents: Global, Database
• Locks: Collection
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Conclusion
• Storage engines have direct control over concurrency–Completely decoupled from top-level locking
–Enabled document-level control
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Conclusion
• Retrofitted MMAP V1 to use multi-granularity locks–Support for collection-level locking–Got rid of the global lock usage for journaling
QUESTIONS?
#MDBDaysmongodb.com
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