edu.umass.cs.reconfiguration.examples.noop

Class NoopApp

java.lang.Object

edu.umass.cs.reconfiguration.examples.noop.NoopApp

All Implemented Interfaces:

Application, AppRequestParser, AppRequestParserBytes, ClientMessenger, Replicable, Reconfigurable

public class NoopApp
extends java.lang.Object
implements Replicable, Reconfigurable, ClientMessenger

Author:

V. Arun A simple no-op application example.

Constructor Summary

Constructors

Constructor and Description
NoopApp() Default constructor used to create app replica via reflection.
NoopApp(java.lang.String id)

Method Summary

Modifier and Type	Method and Description
java.lang.String	checkpoint(java.lang.String name) Checkpoints the current application state and returns it.
boolean	deleteFinalState(java.lang.String name, int epoch) Garbage-collects the final state for name, epoch.
boolean	execute(Request request)
boolean	execute(Request request, boolean doNotReplyToClient) This method must handle the request atomically and return true if successful or throw an exception or return false.
java.lang.Integer	getEpoch(java.lang.String name) Returns the current (unique) epoch for name.
java.lang.String	getFinalState(java.lang.String name, int epoch) Return the final checkpointed state for name, epoch.
Request	getRequest(java.lang.String stringified) An application must support string-to-Request conversion and back.
java.util.Set<IntegerPacketType>	getRequestTypes()
ReconfigurableRequest	getStopRequest(java.lang.String name, int epoch) Returns a stop request that when executed by the app ensures that no subsequent request after the stop is executed in that epoch.
void	putInitialState(java.lang.String name, int epoch, java.lang.String state) Inserts the specified state as the initial state for name, epoch.
boolean	restore(java.lang.String name, java.lang.String state) Resets the current application state for name to state.
void	setClientMessenger(SSLMessenger<?,org.json.JSONObject> msgr) An interface to supply a messenger to the app so that it can directly send responses for executed requests back to the app.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface edu.umass.cs.gigapaxos.interfaces.AppRequestParser

getMutualAuthRequestTypes, getRequest, getServerAuthRequestTypes

Constructor Detail

NoopApp

public NoopApp()

Default constructor used to create app replica via reflection.

NoopApp

public NoopApp(java.lang.String id)

Parameters:

id -

Method Detail

setClientMessenger

public void setClientMessenger(SSLMessenger<?,org.json.JSONObject> msgr)

Description copied from interface: ClientMessenger

An interface to supply a messenger to the app so that it can directly send responses for executed requests back to the app.

Specified by:

setClientMessenger in interface ClientMessenger

execute

public boolean execute(Request request,
                       boolean doNotReplyToClient)

Description copied from interface: Replicable

This method must handle the request atomically and return true if successful or throw an exception or return false. It is the application's responsibility to ensure atomicity, i.e., it should return true iff the request was successfully executed; if it returns false or throws an exception, the application should ensure that the state of the application rolls back any partial execution of this request. If it returns false or throws an exception, the replica coordination protocol may try to re-attempt executing the request.

Specified by:

execute in interface Replicable

doNotReplyToClient - If true, the application is expected to not send a response back to the originating client (say, because this request is part of a post-crash roll-forward or only the "entry replica" that received the request from the client is expected to respond back. If false, the application is expected to either send a response (if any) back to the client via the ClientMessenger interface or delegate response messaging to paxos via the ClientRequest.getResponse() interface.

Returns:

Must return true if and only if the application handled the request successfully. For safety, executing a request must have a deterministic effect on the safety-critical state. If the request is bad and is to be discarded, the application must still return true (after "successfully" discarding it). If the application returns false, the replica coordination protocol (e.g., paxos) might try to repeatedly re-execute it until successful or kill this replica group ( request.getServiceName()) altogether after a limited number of retries, so the replica group may get stuck unless it returns true after a limited number of retries. Thus, with paxos as the replica coordination protocol, returning false is not really an option as paxos has no way to "roll back" a request whose global order has already been agreed upon.

With replica coordination protocols other than paxos, the boolean return value could be used in protocol-specific ways, e.g., a primary-backup protocol might normally execute a request at just one replica but relay the request to one or more backups if the return value of Application.execute(Request) is false.

getRequest

public Request getRequest(java.lang.String stringified)
                   throws RequestParseException

Description copied from interface: AppRequestParser

An application must support string-to-Request conversion and back. Furthermore, the conversion to a string and back must preserve the return values of all Request methods, i.e., Application.getRequest (request.toString())).equals(request) must be true.

Specified by:

getRequest in interface AppRequestParser

Returns:

Request corresponding to stringified.

Throws:

RequestParseException

getRequestTypes

public java.util.Set<IntegerPacketType> getRequestTypes()

Specified by:

getRequestTypes in interface AppRequestParser

Returns:

The set of request types that the application expects to process.

execute

public boolean execute(Request request)

Specified by:

execute in interface Application

Returns:

True if the request is executed successfully.

checkpoint

public java.lang.String checkpoint(java.lang.String name)

Description copied from interface: Replicable

Checkpoints the current application state and returns it.

Specified by:

checkpoint in interface Replicable

Returns:

Returns the checkpoint state. If the application encounters an error while creating the checkpoint, it must retry until successful or throw a RuntimeException. Returning a null value will be interpreted to mean that the application state is indeed null.

Note that state may simply be an app-specific handle, e.g., a file name, representing the state as opposed to the actual state. The application is responsible for interpreting the state returned by Replicable.checkpoint(String) and that supplied in Replicable.restore(String, String) in a consistent manner.

restore

public boolean restore(java.lang.String name,
                       java.lang.String state)

Description copied from interface: Replicable

Resets the current application state for name to state.

Note that state may simply be an app-specific handle, e.g., a file name, representing the state as opposed to the actual state. The application is responsible for interpreting the state returned by Replicable.checkpoint(String) and that supplied in Replicable.restore(String, String) in a consistent manner.

Specified by:

restore in interface Replicable

Returns:

True if the app atomically updated the state successfully. Else, it must throw an exception. If it returns false, the replica coordination protocol may try to repeat the operation until successful causing the application to get stuck.

getStopRequest

public ReconfigurableRequest getStopRequest(java.lang.String name,
                                            int epoch)

Description copied from interface: Reconfigurable

Returns a stop request that when executed by the app ensures that no subsequent request after the stop is executed in that epoch. We need the app to return a stop request as opposed to just exposing a stop() method because, in general, the stop may need to be coordinated across app replicas. If nothing special has to be done by the app upon an epoch stop, it is okay for this method to just return null or a no-op request. If an app is using PaxosReplicaCoordinator, nothing special needs to be done by the app upon an epoch stop, so it can return either null or a no-op request.

Specified by:

getStopRequest in interface Reconfigurable

Returns:

InterfaceReconfigurableRequest corresponding to the stop request for the replica group name:epoch.

getFinalState

public java.lang.String getFinalState(java.lang.String name,
                                      int epoch)

Description copied from interface: Reconfigurable

Return the final checkpointed state for name, epoch. If a checkpoint does not yet exist, it has to be created. This method should return null if the app has moved on, i.e., its current epoch number is higher than epoch and it has no final state for the requested epoch (possibly because it has been garbage-collected). This method will typically get called right after a stop request has been supplied for execution to the app, so it is a good idea for the application to create the checkpoint immediately after executing an epoch stop request.

Specified by:

getFinalState in interface Reconfigurable

Returns:

The final state, or a concise handle thereof, as a String.

putInitialState

public void putInitialState(java.lang.String name,
                            int epoch,
                            java.lang.String state)

Description copied from interface: Reconfigurable

Inserts the specified state as the initial state for name, epoch. This method marks the creation of an epoch. All lower epoch numbers must be marked as inactive at this point. After this method has been called, the app can only accept requests for "name" in the epoch "epoch".

Specified by:

putInitialState in interface Reconfigurable

deleteFinalState

public boolean deleteFinalState(java.lang.String name,
                                int epoch)

Description copied from interface: Reconfigurable

Garbage-collects the final state for name, epoch.

Specified by:

deleteFinalState in interface Reconfigurable

Returns:

True if final state for name:epoch was deleted or did not exist to begin with.

getEpoch

public java.lang.Integer getEpoch(java.lang.String name)

Description copied from interface: Reconfigurable

Returns the current (unique) epoch for name. At most a single epoch can actively exist at any time at a replica. The app replica may maintain the final state for lower (inactive) epochs, but may not accept requests in any epoch other than the currently active epoch if one exists. This method should return null if no active replica for the name exists at this node.

Specified by:

getEpoch in interface Reconfigurable

Returns:

The current epoch number for name.