Differences between transactions and locking
Web applications can be used by multiple users at the same time. A typical application server like Passenger has multiple worker processes for a single app. In a distributed deployment setup like we use at makandra you will even have multiple application servers, each with their own worker pool.
This means that your code needs to deal with concurrent data access. The two main tools we use to cope with concurrency are database transactions and distributed locks. These two are not interchangeable. You can't use a transaction when you need a lock. You can't use a lock when you need a transaction.
This article explains the differences between transactions and locking. Examples will be given for the ActiveRecord ORM (from Ruby on Rails), but apply to all technologies.
Use transactions to group related changes
Use a transaction to ensure a list of changes is applied either completely or not at all.
Example: A method
copy_invoice should create a copy of a given
Invoice record. Here is a bad implementation that does not use transactions:
def copy_invoice(original) duplicate = Invoice.create!(original.recipient) original.items.each do |item| duplicate.items.create!(article: item.article, amount: item.amount) end end
The implementation above has two issues:
- If one of the items fails to create (e.g. due to a validation error), an incomplete
Invoicecopy will remain in the database.
- If another thread or process fetches invoices before
copy_invoicehas terminated, it will see an incomplete invoice (e.g. an invoice without items)
The method can be improved by wrapping the operation in a transaction:
def copy_invoice(original) Invoice.transaction do duplicate = Invoice.create!(original.recipient) original.items.each do |item| duplicate.items.create!(article: item.article, amount: item.amount) end end end
transaction(&block) method is available on all ActiveRecord classes or record instances. It doesn't matter on which object you call it, since they all delegate to the current thread's database connection. If you don't have an ActiveRecord class in your scope, you can always use
ActiveRecord::Base.transaction do ... end.
By using a transaction we have grouped together the creation of the copied invoice and the creation of its items. This list of changes (e.g. 12
INSERT statements) will be committed in a single atomic operation, or not at all. This has two desirable effects:
- Other threads or processes will not see an incomplete copy until
- The copy of the invoice (including all of its items) either occurs as a whole, or not at all. If an exception is raised at any point, the transaction is rolled back, and the exception is re-raised.
When should I use a transaction?
Whenever you make an operation that affects more than a single database row, you should always use a transaction. Note that
ActiveRecord::Base#save automatically opens a transaction, so changes you make in callbacks, nested attributes processing, etc. will automatically run inside a transaction.
What happens if two transactions run at the same time?
Nothing special happens if the "same" transaction (e.g. the
copy_invoice method above) runs at the same time. They will simply run at the same time. There is no guarantee of the order.
When two concurrently running transactions are done, they will each try to commit their changes in one, atomic commit. This may or may not succeed. E.g. if both transactions try to insert the same unique key, whatever transaction commits first will succeed. The other transaction will get an error from the database adapter and all of its changes will be rolled back.
Note that when two transaction run concurrently in two threads, each thread doesn't see the other transaction's changes until they are committed successfully. It does however see its own changes (this is a dumbed-down explanation, reality is much more complicated).
What happens if I raise an exception in a transaction?
Exceptions within a transactions are typically caused from a failed validation (
ActiveRecord::RecordInvalid) or from violating a database constraint (like an unique index).
If an exception is raised within an ActiveRecord transaction, all changes made within the transaction are rolled back and the exception is re-raised.
To better understand how ActiveRecord handles exceptions within a transaction, here is the implementation of
ActiveRecord::Base#transaction in pseudo-code:
class ActiveRecord::Base def transaction(&block) connection.start_transaction block.call connection.commit_transaction rescue Exception => e connection.rollback_transaction raise e end end
Why do I get deadlocks when using transactions?
Since the purpose of this article is to show that transactions and locks are not the same, you might be surprised to experience deadlocks when committing a transaction. E.g. you might see an error like this:
ActiveRecord::StatementInvalid: Mysql::Error: Lock wait timeout exceeded; try restarting transaction
This has nothing to do with the application-level locks that we talk about below. What happens here is that when MySQL makes changes, it locks rows in the database to ensure that the change will applied consistently. When two threads (two database connections) try to lock the same rows in different order, you might get a deadlock.
For instance, transaction A wants to change row #1 and row #2. Transaction B wants to change the same rows, but in different order (row #2, then row #1). Here is what hapepens:
- Transaction A locks row #1
- Transaction B locks row #2
- Transaction A tries to lock row #1, but can't
- Transaction B tries to lock row #2, but can't
- Both transactions wait forever
- MySQL times out after 50 seconds and raises an error in both database connections
- Both Rails worker processes raise
ActiveRecord::StatementInvalidwith a confusing error message
The internal locking model that MySQL uses to ensure consistency is very complex. Also MySQL might lock many more rows that you might expect, especially when you do not have proper database indexes for the relevant statement. For updates without indexes, every row in a table gets locked, for example.
An application-level lock as described below can help you prevent deadlocks caused by concurrent transactions.
We also some hints for debugging deadlocks in MySQL.
Use locks to prevent concurrent data access
Use locks to ensure that a critical piece of code only runs in a single thread at the same time. A lock is also called a "mutex".
Example: A method
transfer_money should transfer an amount of money from one account to another. Here is a bad implementation that does not use locking:
def transfer_money(source_id, target_id, amount) source_account = Account.find(source_id) target_account = Account.find(target_id) source_account.balance -= amount target_account.balance += amount source_account.save! target_account.save! end
The example above has all the issues that the transaction-less
copy_invoice example at the beginning of this article had. In addition, it might lose money transactions.
Let's say two users try to transfer 5 units of currency from the same source account to the same target account. Here is what can happen:
- Thread A retrieves both accounts. It sees that
target_accountboth have a balance of
- Thread B retrieves both accounts. It sees that
target_accountboth have a balance of
- Thread A sets
source_account.balanceto 95 and
- Thread A saves both accounts and terminates.
- Thread B also sets
source_account.balanceto 95 and increases
- Thread B saves both accounts and terminates.
We just lost the second money transfer. The account balances should be
110, but they are
105. We call this the "lost update problem". You cannot fix this with a transaction. In fact, if you think that wrapping
transfer_money in a transaction will help, you should re-read this article from the top.
What you should do instead is to wrap critical code in a lock. The lock ensures only a single thread may access it at a single time:
def transfer_money(source_id, target_id, amount) Lock.acquire('money-transfer-lock') do source_account = Account.find(source_id) target_account = Account.find(target_id) source_account.balance -= amount target_account.balance += amount source_account.save! target_account.save! end end
The code example above uses the
Lock class from our simple database mutex to synchronize multiple threads or worker processes on a given name (
'transfer-money-lock' in the example). Only a single thread can be inside the lock at any given time. All other threads will block on
Lock.acquire until the first thread exits the block and releases the lock.
Note that this is very different from transactions, which can run concurrently.
But I can fix the code without using a lock!
You really can't. Please don't try things like reloading the accounts before saving to make sure they're really totally positively up to date.
Also please don't write your own application-level protocol for detecting other threads manipulating the same resource. Just use a lock.
But I can write my a better lock implementation using files / memcached / Redis / INSERT_TECHNOLOGY_HERE!
Please don't unless you've been working with distributed code for a number of years.
Locks are hard, especially in a distributed deployment setup where you have multiple worker processes, or multiple app servers with their own worker pool each. Unless you really know what you're doing, prefer to lock against a database cluster as shown above.
But if this is true it would mean all my code is broken!
It probably is. Most web application code is not thread-safe due to missing locks. It's just that you require a lot of concurrent users to make failures become frequent enough that they will notify you, the developer.
Another reason why you're not seeing these failures in development that you are using a development server like Webrick that has a single worker process. Thus you are never experiencing concurrency anyway. Because of this you should instead use Passenger for development.