Recovery

The log files can be used to rebuild a database in the event of database corruption or to resize the root block. Recovery is quite fast, as each log file is processed as a single transaction. And since the small database uses only a single block, this means that there is a single write to the database for each log file processed.

##RecoveryTest

Step 1: Create a database and modify it.

val systemServices = SystemServices(new ServicesRootComponentFactory)
val dbName = "smallrecovery.db"
val logDirPathname = "smallRecovery"
val file = new java.io.File(dbName)
file.delete
EmptyLogDirectory(logDirPathname)
val properties = new Properties
properties.put("dbPathname", dbName)
properties.put("logDirPathname", logDirPathname)
val db = Subsystem(
  systemServices,
  new SmallComponentFactory,
  properties = properties,
  actorId = ActorId("db"))
val results = new Results
val chain = new Chain(results)
chain.op(systemServices, Register(db))
chain.op(db, SetRequest(db, "/$", IncDesInt(null)))
Future(systemServices, chain)
systemServices.close

Step 2: Perform 2 more updates.

val systemServices = SystemServices(new ServicesRootComponentFactory)
val dbName = "smallrecovery.db"
val logDirPathname = "smallRecovery"
val properties = new Properties
properties.put("dbPathname", dbName)
properties.put("logDirPathname", logDirPathname)
val db = Subsystem(
  systemServices,
  new SmallComponentFactory,
  properties = properties,
  actorId = ActorId("db"))
val results = new Results
val chain = new Chain(results)
chain.op(systemServices, Register(db))
chain.op(db, SetRequest(db, "/$", IncDesString(null)))
chain.op(db, SetRequest(db, "/$", IncDesBytes(null)))
Future(systemServices, chain)
systemServices.close

Step 3: Rebuild the database.

val systemServices = SystemServices(new ServicesRootComponentFactory)
val dbName = "smallrecovery.db"
val logDirPathname = "smallRecovery"
val file = new java.io.File(dbName)
file.delete
val properties = new Properties
properties.put("dbPathname", dbName)
properties.put("logDirPathname", logDirPathname)
val db = Subsystem(
  systemServices,
  new SmallRecoveryComponentFactory,
  properties = properties,
  actorId = ActorId("db"))
val results = new Results
val chain = new Chain(results)
chain.op(systemServices, Register(db))
chain.op(db, Recover())
Future(systemServices, chain)
systemServices.close

Output.

recovering smallRecovery\2011-10-04_02-02-11_897.jnl
recovering smallRecovery\2011-10-04_02-02-11_910.jnl

Step 4: Query the contents of the database.

val systemServices = SystemServices(new ServicesRootComponentFactory)
val dbName = "smallrecovery.db"
val logDirPathname = "smallRecovery"
val properties = new Properties
properties.put("dbPathname", dbName)
properties.put("logDirPathname", logDirPathname)
val db = Subsystem(
  systemServices,
  new SmallComponentFactory,
  properties = properties,
  actorId = ActorId("db"))
val results = new Results
val chain = new Chain(results)
chain.op(systemServices, Register(db))
chain.op(db, GetRequest(db, "/$"))
println(Future(systemServices, chain))
systemServices.close

Output.

org.agilewiki.incDes.IncDesBytes@38717323

RecoveryTest

##SmallRecoveryComponentFactory

This factory includes two components in the subsystem used for recovery: SmallDataStoreRecoveryComponentFactory and SmallTransactionsComponentFactory.

class SmallRecoveryComponentFactory
  extends ComponentFactory {
  addDependency(classOf[SmallDataStoreRecoveryComponentFactory])
  addDependency(classOf[SmallTransactionsComponentFactory])
}

SmallRecoveryComponentFactory

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tutorial