Table of Contents

Interface ISessionLock

Namespace
PushTechnology.ClientInterface.Client.Session
Assembly
Diffusion.Client.dll

A ISession lock.

public interface ISessionLock

Examples

Acquiring a session lock.

// session is a previously obtained ISession instance.
var sessionLock = await session.LockAsync( "lock name" );

Remarks

A session lock is a server-managed resource that can be used to coordinate exclusive access to shared resources across sessions. For example, to ensure a single session has the right to update a topic; to ensure at most one session responds to an event; or to select a single session to perform a housekeeping task. Session locks support general collaborative locking schemes. The application architect is responsible for designing a suitable locking scheme and for ensuring each application component follows the scheme appropriately.

Session locks are identified by a lock name. Lock names are arbitrary and chosen at will to suit the application. Each lock is owned by at most one session. Locks are established on demand; there is no separate operation to create or destroy a lock.

A session lock is acquired using the LockAsync(string) method. If no other session owns the lock, the server will assign the lock to the calling session immediately. Otherwise, the server will record that the session is waiting to acquire the lock. A session can call LockAsync(string) more than once for a given session lock – if the lock is acquired, all calls will complete successfully with equal ISessionLock values.

If a session closes, the session locks it owns are automatically released. A session can also release a lock by calling UnlockAsync(). When a session lock is released and other sessions are waiting to acquire the lock, the server will arbitrarily select one of the waiting sessions and notify it that it has acquired the lock. All of the newly selected session's pending LockAsync(string) calls will complete normally. Other sessions will continue to wait.

The LockAsync(string, SessionLockScope) variant of this method takes a scope parameter that provides the further option of automatically releasing the lock when the session loses its connection to the server.

Race conditions

This session lock API has inherent race conditions. Even if an application is coded correctly to protect a shared resource using session locks, there may be a period where two or more sessions concurrently access the resource. The races arise for several reasons including

  • Due to the check-then-act approach of polling IsOwned, the lock can be lost after the check has succeeded but before the resource is accessed;
  • The server can detect a session is disconnected and assign the lock to another session before the original session has detected the disconnection.

Despite this imprecision, session locks provide a useful way to coordinate session actions.

Note

This interface does not require user implementation and is only used to hide implementation details.

Added in 6.2.

Properties

IsOwned

Gets whether this session lock is still owned.

bool IsOwned { get; }

Property Value

bool

true if this session lock is still owned by the session. Otherwise false.

Examples

Checking if a session lock is still owned by the session.

// session is a previously obtained ISession instance.
var sessionLock = await session.LockAsync( "lock name" );

// ...

if ( sessionLock.IsOwned ) {
    Console.WriteLine( "Session lock is still owned by the session." );
} else {
    Console.WriteLine( "Session lock is still owned by the session." );
}

Remarks

Added in 6.2.

Name

Gets the name of this session lock.

string Name { get; }

Property Value

string

The name of this session lock.

Examples

Printing the name of a session lock to the terminal.

// session is a previously obtained ISession instance.
var sessionLock = await session.LockAsync( "lock name" );
Console.WriteLine( sessionLock.Name ); // Prints "lock name"

Remarks

Added in 6.2.

Scope

Gets the scope of this session lock.

SessionLockScope Scope { get; }

Property Value

SessionLockScope

The scope of this session lock.

Examples

Printing the scope of a session lock to the terminal.

// session is a previously obtained ISession instance.
var sessionLock = await session.LockAsync( "lock name" );
Console.WriteLine( sessionLock.Scope );

Remarks

The scope determines when the lock will be released automatically.

If a session makes multiple requests for a lock using different scopes, and the server assigns the lock to the session fulfilling the requests, the lock will be given the weakest scope (UNLOCK_ON_CONNECTION_LOSS). Consequently, an individual request can complete with a lock that has a different scope to that requested.

Added in 6.2.

See Also

Sequence

Gets the sequence number of this session lock.

long Sequence { get; }

Property Value

long

The sequence number.

Examples

Printing the sequence number of a session lock to the terminal.

// session is a previously obtained ISession instance.
var sessionLock = await session.LockAsync( "lock name" );
Console.WriteLine( sessionLock.Sequence );

Remarks

This is a value identifying the acquisition of the lock with the given Name. Session locks that are acquired later are guaranteed to have bigger sequence values, allowing the sequence number to be used as a fencing token.

Added in 6.2.

Methods

UnlockAsync()

Releases this session lock if it is owned by the session.

Task<bool> UnlockAsync()

Returns

Task<bool>

The Task representing the current operation.

Examples

Unlocking a previously obtained session lock.

// sessionLock is a previously obtained ISessionLock instance.
try {
    var owned = await sessionLock.UnlockAsync();
    if ( owned ) {
        Console.WriteLine( "Previously owned session lock released." );
    } else {
        Console.WriteLine( "Session lock was not owned by the current session." );
    }
} catch (SessionClosedException ex ) {
    Console.WriteLine( "Couldn't release session lock because the session was closed." );
}

Remarks

If the operation completes successfully, the Task result will be a bool value and the calling session will no longer own the named session lock. A result of true indicates the calling session previously owned the lock and false indicates it did not.

This method is the same as calling UnlockAsync(CancellationToken) with System.Threading.CancellationToken.None.

Added in 6.2.

Exceptions

SessionClosedException

The calling session is closed. Thrown by the returned Task.

See Also

UnlockAsync(CancellationToken)

Releases this session lock if it is owned by the session.

Task<bool> UnlockAsync(CancellationToken cancellationToken)

Parameters

cancellationToken CancellationToken

The cancellation token used to cancel the current operation.

Returns

Task<bool>

The Task representing the current operation.

Examples

Unlocking a previously obtained session lock.

// sessionLock is a previously obtained ISessionLock instance.
var cancellationTokenSource = new CancellationTokenSource();
try {
    var owned = await sessionLock.UnlockAsync( cancellationTokenSource.Token );
    if ( owned ) {
        Console.WriteLine( "Previously owned session lock released." );
    } else {
        Console.WriteLine( "Session lock was not owned by the current session." );
    }
} catch (SessionClosedException ex ) {
    Console.WriteLine( "Couldn't release session lock because the session was closed." );
}

Remarks

If the operation completes successfully, the Task result will be a bool value and the calling session will no longer own the named session lock. A result of true indicates the calling session previously owned the lock and false indicates it did not.

Added in 6.2.

Exceptions

SessionClosedException

The calling session is closed. Thrown by the returned Task.

See Also