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StickyLocking

StickyLocking is a general purpose embedded lock manager which allows for locking any resource hierarchy. Installable Lock modes allow for customization of the locking system that can meet the needs of almost any locking scenario.

Documentation

Sticky Locking provides the Locker class which is a high-level locking system designed to facilitate many different concurrency use cases including simple readers-writer locks which provide shared access for read operations and exclusive access for write operations to more complex hierarchical locking schemes used to power database file, database, page, and row level locking.

Sticky Locking also provides a low-level mutual exclusion lock through the Mutex class to protect critical sections of your code. In addition, wait conditions (Condition) are provided to allow for threads to wait for a mutex to become available.

The mutual exclusion lock is provided through the Mutex class while wait conditions can be created with the Condition class. Internally, the higher level components are implemented using these two primitives, and other modules in the Sticky Tools suite of libraries also use the mutex for protecting various critical sections of code.

Hierarchical Locker

Sticky Locking provides the Locker class which is a high-level locking system designed to facilitate many different concurrency use cases including simple readers-writer locks which provide shared access for read operations and exclusive access for write operations to more complex hierarchical locking schemes used to power database file, database, page, and row level locking.

The Locker is highly configurable to your specific use case using three interconnected constructs.

1. The LockMode defines the symbols your application will use to specify the lock modes various resources can be locked in.
2. The CompatibilityMatrix defines whether two modes can be granted concurrently for the same resource (is it shared or exclusive).
3. The GroupModeMatrix defines the composite mode a group of granted locks take on when granted together.

Sticky has two built-in sets of these values in the following enums.

SharedExclusiveLockMode which is a simple readers-writer system used to provide shared read access and exclusive write access.

An example use case for this mode may be to protect access to a file or many files which require all readers to be able to share access to the file and writers to be granted exclusive access forcing readers and writers to wait until the write operation is complete before they proceed.

ExtendedLockMode an extended mode that includes intention and update modes which can be used for advanced database type use cases. This LockMode set was designed to be used by other models in the Sticky Tools suite of libraries.

You are free to define your own LockMode set depending on your use case, from simpler mode structures to more complex, Sticky Locking will adapt to the mode given.

Defining Locker Behavior

The Lockers behavior is defined by the LockMode, CompatibilityMatrix, and GroupModeMatrix. These types and structures define how the Locker will grant requests for lock modes.

A LockMode is an enum entry that defines a specific mode of the lock. These modes are user defined

Lock Modes

Lock modes determine the symbols used to define the modes a lock can be in.

Here is an example of a simple lock mode definition:

    enum MyLockMode: LockMode {
        case S  /// Shared
        case X  /// Exclusive
    }

The mode on it's own only defines the symbols that can be used. You must define a CompatibilityMatrix and GroupModeMatrix to describe how the various modes interact with each other.

Note: Sticky Locking defines two built in LockMode enums along with corresponding compatibility and group mode matrix's. A simple readers-writer type named SharedExclusiveLockMode and an extended mode named ExtendedLockMode which is suitable for advanced data storage applications.

Lock Mode Compatibility

The ability to share a lock mode with another request/thread is determined by the lock mode compatibility matrix supplied for the lock mode. For every new lock request, the matrix is checked and if the value at the index of the current lock mode and the requested lock mode is true the lock will be granted concurrently, otherwise the request will queue and wait.

    let compatibilityMatrix: CompatibilityMatrix<MyLockMode>
            = [
                /*               Shared, Exclusive  */
                /* Shared    */  [true,    false],
                /* Exclusive */  [false,   false],
              ]

Lock Group Mode

When multiple requests are compatible and granted concurrently the lock mode of the group must be calculated. This is called the group mode. A new request is compatible with the members of the group if it is compatible with the group mode.

Sticky Locking uses the GroupModeMatrix to determine the group mode when a new request joins the group.

    let groupModeMatrix: GroupModeMatrix<MyLockMode>
            = [
                /* Requested     Shared, Exclusive  */
                /* Shared    */  [S,     X],
                /* Exclusive */  [X,     X],
              ]

See Lock Wait State & Grant Behavior for a more detailed description and examples of how the locker behaves during locking.

Built-in Lock Modes

Sticky Locking contains two pre-defined LockMode enums and associated matrix's for use with various use cases.

Shared-Exclusive Lock Mode

A Shared Exclusive lock (a.k.a. readers-writer or multi-reader) allows concurrent access for read-only operations, while write operations gain exclusive access.

This allows multiple readers to gain shared access to a resource blocking all writers until no more readers are reading. Writers gain exclusive access to the resource blocking all other readers and writers until the operation is complete.

The defined modes are:

• S - Shared (Read)
• X - Exclusive (Write)

The default CompatibilityMatrix is defined as:

The default GroupModeMatrix is defined as:

Extended Lock Mode

The ExtendedLockMode is a predefined LockMode implementation that can be used for complex database type applications. It defines an extended set of lock modes including Update and Intention modes.

The defined modes are:

• IS - Intention Shared
• IX - Intention Exclusive
• S - Shared
• SIX - Shared Intention Exclusive
• U - Update
• X - Exclusive

The default CompatibilityMatrix is defined as:

The default GroupModeMatrix is defined as:

Resources & Hashing

The Locker will lock and unlock any Hashable resource and it distinguishes the lock resources by the hash value, therefore care must be taken to create a hashing algorithm that ensure uniqueness between individual objects of the same type as well as the hash values between different types.

If two resources hash to the same hash value, and the two requested modes are incompatible, then the collision may cause spurious waits.

Also keep in mind that the hashValue should never change for the resource. If the hashValue changes over the life of the resource, the locker will consider it a different resource each time the hashValue changes. For instance, an Array<> hashValue changes with each element that is added or removed from the array, therefore the Array<> instance itself could not be used as a lock resource on its own. You would have to use a surrogate Resource such as a fixed String or integer as the Resource identifier.

Mutexes & Conditions

Sticky Locking also provides a low-level mutual exclusion lock through the Mutex class to protect critical sections of your code. In addition, wait conditions (Condition) are provided to allow for threads to wait for a mutex to become available.

The mutual exclusion lock is provided through the Mutex class while wait conditions can be created with the Condition class. Internally, the higher level components are implemented using these two primitives, and other modules in the Sticky Tools suite of libraries also use the mutex for protecting various critical sections of code.

Sources and Binaries

You can find the latest sources and binaries on github.

Communication and Contributions

• If you found a bug, and can provide steps to reliably reproduce it, open an issue.
• If you have a feature request, open an issue.
• If you want to contribute
  • Fork it! StickyLocking repository
  • Create your feature branch: git checkout -b my-new-feature
  • Commit your changes: git commit -am 'Add some feature'
  • Push to the branch: git push origin my-new-feature
  • Submit a pull request :-)

Installation

Swift Package Manager

StickyLocking supports dependency management via Swift Package Manager on All Apple OS variants as well as Linux.

Please see Swift Package Manager for further information.

CocoaPods

StickyLocking is available through CocoaPods. To install it, simply add the following line to your Podfile:

   pod "StickyLocking"

Minimum Requirements

Build Environment

Minimum Runtime Version

Note:

To build and run on Linux we have a a preconfigure Vagrant file located at https://github.com/tonystone/vagrant-swift

See the README for instructions.

Author

Tony Stone (https://github.com/tonystone)

License

StickyLocking is released under the Apache License, Version 2.0