ScalaQuery_Nested

Summary

Safely manage nested sessions & transactions in Slick.

Details

After the first few weeks of using Slick (formerly ScalaQuery), our team at TIM Group was surprised to diagnose bugs caused by nested calls to the Slick methods Database#withSession and Database#withTransaction.

It turns out that, when you nest these calls, a different database session will be created in each scope. This can lead to race conditions. For example:

database.withSession {
  // insert object A in database table here
  database.withSession {
    // query returning object A here
    // --> race condition! may not see same A in different db connection
  }
}

This mini-library enriches Database with methods #withNestedSession and #withNestedTransaction, which behave in the way that we'd originally assumed it would work:

import com.timgroup.scalaquery_nested.NestedScalaQuerySessionsAndTransactions._

database.withNestedSession {
  // insert object A in database table here
  database.withNestedSession {
    // query returning object A here
    // --> no problem, same db connection used
  }
}

Caveat! Just like the Slick methods it is replacing, this implementation is ultimately based on storing the database session in a Java ThreadLocal. This means the following code may return unexpected results:

database.withNestedSession {
  // insert object A in database table here
  val future = database.withNestedSession {
    Future {
      // query returning object A here 
      // --> in a future which may be run in a different thread,
      //     different database connection might be used, leading to unexpected results
    }
  }
  Await.result(future, timeout)
}

Usage

To pull in the implicit conversion to add these two methods, you can extend the trait NestedScalaQuerySessionsAndTransactions:

import com.timgroup.scalaquery_nested.NestedScalaQuerySessionsAndTransactions

class Cars extends Table[Car]("cars") with NestedScalaQuerySessionsAndTransactions {
  database withNestedTransaction { /* can nest sessions and transactions... */ }    
}

Alternatively, you can also import directly from the object into any scope:

// inside some scope in which a ScalaQuery database is defined...
import com.timgroup.scalaquery_nested.NestedScalaQuerySessionsAndTransactions._

database withNestedTransaction { /* can nest sessions and transactions... */ }

How to Add as a Dependency

You can get scalaquery_nested with the following line in your build.sbt:

libraryDependencies += "com.timgroup" %% "scalaquery_nested" % "1.1.0-M1"

Versioning

ScalaQueryNested is available for multiple versions of Scala and Slick/ScalaQuery:

Library version	branch	Slick version	Scala version
1.0.x	1.0.x-for-scala_2_9-scalaquery-0_10	0.10.0-M1	2.9.x
1.1.x	master	1.0.x	2.10.x
1.2.x (planned)		2.0.x	2.10.x

Contributing

Please feel free to send pull requests. Any change to code must come with tests, of course.

tim-group / scalaquery_nested 1.1.0-M1