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MongoDB Transaction Pattern
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Ian Yang
May 20, 2013
Technology
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MongoDB Transaction Pattern
Ian Yang
May 20, 2013
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Transcript
MongoDB Transaction Pattern Who? Ian Yang From? Intridea Inc. When?
2013-05-15 Wed
Outline 1 Overview 2 Write Operation Properties 3 Two Phrase
Commit 4 Recovering 5 Rollback 6 Multiple Applications 7 References
Transaction (ACID) Atomicity all or nothing. Consistency does not violate
any integrity constraints Isolation allow concurrent execution of transactions Durability Transaction is persistent after commit
MongoDB Features Atomicity Only on the level of a single
document. Write Concern Return write operation result
MongoDB Transaction Atomicity done or rollback Consistency rollback may leave
system in inconsistent state. Isolation other transactions can see intermediate result. Durability write concern, replica set, journal.
Requirements of Write Operations in Transaction Revertible for recover and
rollback Idempotent for resume, recover and rollback
Not Revertible db.users.save(user); db.users.update ({ username: "ian"}, user); db.users.update ({
username: "ian"}, {$set: {balance: 120}});
Revertible db.users.update ({ username: "ian"}, {$inc: {balance: 20}}); db.users.update ({
username: "ian"}, {$inc: {balance: -20}}); db.users.update ({ username: "ian"}, {$push: {tx: 20}}); db.users.update ({ username: "ian"}, {$pull: {tx: 20}});
Not Idempotent db.users.update ({ username: "ian"}, {$inc: {balance: 20}}); db.users.update
({ username: "ian"}, {$push: {tx: 20}}); db.users.update( {username: "ian", balance: 100}, {$inc: {balance: 20}} );
Idempotent db.users.update( {username: "ian", tx: {$ne: tx._id}}, {$inc: {balance: 20},
$push: {tx: tx._id}} ); db.users.update( {username: "ian", tx: tx._id}, {$inc: {balance: -20}, $pull: {tx: tx._id}} );
Sample Transfer 20 dollars from Ian to Daniel
Create Transaction Transaction provides a unique identifier, enough information to
recover and rollback, a document for atomic write operation.
var tx = { _id: ObjectId (), source: ian._id ,
dest: daniel._id , amount: 20 }; db.transactions.create(tx);
Transaction State Machine State represents transaction stages Different actions should
be taken in different stages to recover or rollback
Step 1: Create Transaction var tx = { _id: ObjectId
(), source: ian._id , dest: daniel._id , amount: 20, state: 'initial ' }; db.transactions.create(tx);
Step 2: initial → pending db.transactions.update( {_id: tx._id , state:
'initial '}, {$set: {state: 'pending '}} );
Step 3: Apply Transaction db.users.update( {_id: tx.source , tx: {$ne:
tx._id}}, {$push: {tx: tx._id}, $inc: { balance: - tx.amount }} ); db.users.update( {_id: tx.dest , tx: {$ne: tx._id}}, {$push: {tx: tx._id}, $inc: { balance: tx.amount }} );
Step 4: pending → committed db.transactions.update( {_id: tx._id , state:
'pending '}, {$set: {state: 'committed '}} );
Step 5: Remove Pending Transaction db.users.update( {_id: tx.source , tx:
tx._id}, {$pull: {tx: tx._id}} ); db.users.update( {_id: tx.dest , tx: tx._id}, {$pull: {tx: tx._id}} );
Step 6: committed → done db.transactions.update( {_id: tx._id , state:
'committed '}, {$set: {state: 'done'}} );
Recover Initial Transactions Recover from Step 2: initial → pending
Recover Pending Transactions Recover from Step 3: Apply Transactions
Recover Committed Transactions Recover from Step 5: Remove Pending Transaction
General Rule Recover from the Step just following the step
transaction just entered current state. State transition creates a check point
Rollback Operations Better to create a new transaction in reverse
direction for Committed Transactions and Done Transactions Use following steps to rollback initial and pending transactions
Rollback 1: state → canceling db.transactions.update( {_id: t._id , state:
{$in: ['initial ', ' pending ']}}, {$set: {state: 'canceling '}} );
Rollback 2: Undo the Transaction db.users.update( {_id: tx.dest , tx:
{$ne: tx._id}}, {$pull: {tx: tx._id}, $inc: { balance: - tx.amount }} ); db.users.update( {_id: tx.source , tx: tx._id}, {$pull: {tx: tx._id}, $inc: { balance: tx.amount }} );
Rollback 3: canceling → canceled db.transactions.update( {_id: t._id , state:
{$in: ['canceling ']}}, {$set: {state: 'canceled '}} );
Recover Rollback Restart from Rollback 2 for canceling transactions
Conclusion Create a document for any transaction Store transaction info
in the document Apply only Revertible and Idempotent operations
Conclusion (Cont.) Transit state to save current work Transit state
if several operation branches exists
Enough? Only 1 application can handle a give transaction at
any point in time.
Background A picks a pending transaction to recover B picks
also a pending transaction to recover
Scenario 1 A is so slow, when it starts apply
transaction, B already has done the transaction. A must rollback. If A exists abnormally, the transaction is in inconsistent state.
Scenario 2 B just finished Step 5: Removing Pending Transaction
A applied transactions and exited abnormally B change state to done without error
Solution findAndModify lock db.transactions.findAndModify ({ query: {state: "initial", application: {$exists:
0}}, update: {$set: {state: "pending", application: "A1"}}, new: true });
Lock Release db.transactions.findAndModify ({ query: {application: "A1"}, update: {$unset: {application:
1}}, new: true });
Lock Forever? Application exists abnormally before release the lock Solution:
timeout
Timeout Lock now = new Date(); timeout = new Date(now.valueOf
() + 60 * 10 * 1000); db.transactions.findAndModify ({ query: {state: "initial", lockUntil: {$not: {$gte: now}}}, update: {$set: {state: "pending", lockUntil: timeout}}, new: true });
Client Requirements Client timeout to ensure the operation is finished
in expected time. Check time after return from block IO Defer transactions cleanup
References http://docs.mongodb.org/ manual/tutorial/ perform-two-phase-commits/ http://docs.mongodb.org/ manual/tutorial/ isolate-sequence-of-operations/
Q & A