DatabaseReader

public protocol DatabaseReader : AnyObject, Sendable

DatabaseReader is the protocol for all types that can fetch values from an SQLite database.

It is adopted by DatabaseQueue, DatabasePool, and DatabaseSnapshot.

The protocol comes with isolation guarantees that describe the behavior of adopting types in a multithreaded application.

Types that adopt the protocol can provide in practice stronger guarantees. For example, DatabaseQueue provides a stronger isolation level than DatabasePool.

Warning: Isolation guarantees stand as long as there is no external connection to the database. Should you have to cope with external connections, protect yourself with transactions, and be ready to setup a busy handler.

  • The database configuration

    Declaration

    Swift

    var configuration: Configuration { get }
  • Closes the database connection with the sqlite3_close() function.

    Note: You DO NOT HAVE to call this method, and you SHOULD NOT call it unless the correct execution of your program depends on precise database closing. Database connections are automatically closed when they are deinitialized, and this is sufficient for most applications.

    If this method does not throw, then the database is properly closed, and every future database access will throw a DatabaseError of code SQLITE_MISUSE.

    Otherwise, there exists concurrent database accesses or living prepared statements that prevent the database from closing, and this method throws a DatabaseError of code SQLITE_BUSY. See https://www.sqlite.org/c3ref/close.html for more information.

    After an error has been thrown, the database may still be opened, and you can keep on accessing it. It may also remain in a “zombie” state, in which case it will throw SQLITE_MISUSE for all future database accesses.

    Declaration

    Swift

    func close() throws

Interrupting Database Operations

  • This method causes any pending database operation to abort and return at its earliest opportunity.

    It can be called from any thread.

    A call to interrupt() that occurs when there are no running SQL statements is a no-op and has no effect on SQL statements that are started after interrupt() returns.

    A database operation that is interrupted will throw a DatabaseError with code SQLITE_INTERRUPT. If the interrupted SQL operation is an INSERT, UPDATE, or DELETE that is inside an explicit transaction, then the entire transaction will be rolled back automatically. If the rolled back transaction was started by a transaction-wrapping method such as DatabaseWriter.write or Database.inTransaction, then all database accesses will throw a DatabaseError with code SQLITE_ABORT until the wrapping method returns.

    For example:

    try dbQueue.write { db in
        // interrupted:
        try Player(...).insert(db)     // throws SQLITE_INTERRUPT
        // not executed:
        try Player(...).insert(db)
    }                                  // throws SQLITE_INTERRUPT
    
    try dbQueue.write { db in
        do {
            // interrupted:
            try Player(...).insert(db) // throws SQLITE_INTERRUPT
        } catch { }
        try Player(...).insert(db)     // throws SQLITE_ABORT
    }                                  // throws SQLITE_ABORT
    
    try dbQueue.write { db in
        do {
            // interrupted:
            try Player(...).insert(db) // throws SQLITE_INTERRUPT
        } catch { }
    }                                  // throws SQLITE_ABORT
    

    When an application creates transaction without a transaction-wrapping method, no SQLITE_ABORT error warns of aborted transactions:

    try dbQueue.inDatabase { db in // or dbPool.writeWithoutTransaction
        try db.beginTransaction()
        do {
            // interrupted:
            try Player(...).insert(db) // throws SQLITE_INTERRUPT
        } catch { }
        try Player(...).insert(db)     // success
        try db.commit()                // throws SQLITE_ERROR "cannot commit - no transaction is active"
    }
    

    Both SQLITE_ABORT and SQLITE_INTERRUPT errors can be checked with the DatabaseError.isInterruptionError property.

    Declaration

    Swift

    func interrupt()

Read From Database

  • Synchronously executes a read-only function that accepts a database connection, and returns its result.

    For example:

    let count = try reader.read { db in
        try Player.fetchCount(db)
    }
    

    The value function runs in an isolated fashion: eventual concurrent database updates are not visible from the function:

    try reader.read { db in
        // Those two values are guaranteed to be equal, even if the
        // `player` table is modified, between the two requests, by
        // some other database connection or some other thread.
        let count1 = try Player.fetchCount(db)
        let count2 = try Player.fetchCount(db)
    }
    
    try reader.read { db in
        // Now this value may be different:
        let count = try Player.fetchCount(db)
    }
    

    Attempts to write in the database throw a DatabaseError with resultCode SQLITE_READONLY.

    It is a programmer error to call this method from another database access method:

    try reader.read { db in
        // Raises a fatal error
        try reader.read { ... )
    }
    

    Throws

    The error thrown by value, or any DatabaseError that would happen while establishing the read access to the database.

    Declaration

    Swift

    @_disfavoredOverload
    func read<T>(_ value: (Database) throws -> T) throws -> T

    Parameters

    value

    A function that accesses the database.

  • Asynchronously executes a read-only function that accepts a database connection.

    The value function runs in an isolated fashion: eventual concurrent database updates are not visible from the function:

    reader.asyncRead { dbResult in
        do {
            let db = try dbResult.get()
            // Those two values are guaranteed to be equal, even if the
            // `player` table is modified, between the two requests, by
            // some other database connection or some other thread.
            let count1 = try Player.fetchCount(db)
            let count2 = try Player.fetchCount(db)
        } catch {
            // handle error
        }
    }
    

    Attempts to write in the database throw a DatabaseError with resultCode SQLITE_READONLY.

    Declaration

    Swift

    func asyncRead(_ value: @escaping (Result<Database, Error>) -> Void)

    Parameters

    value

    A function that accesses the database. Its argument is a Result that provides the database connection, or the failure that would prevent establishing the read access to the database.

  • Synchronously executes a function that accepts a database connection, and returns its result.

    For example:

    let count = try reader.unsafeRead { db in
        try Player.fetchCount(db)
    }
    

    The guarantees of the read method are lifted:

    the value function is not isolated: eventual concurrent database updates are visible from the function:

    try reader.unsafeRead { db in
        // Those two values can be different, because some other
        // database connection or some other thread may modify the
        // database between the two requests.
        let count1 = try Player.fetchCount(db)
        let count2 = try Player.fetchCount(db)
    }
    

    The value function is not prevented from writing (DatabaseQueue, in particular, will accept database modifications in unsafeRead).

    It is a programmer error to call this method from another database access method:

    try reader.read { db in
        // Raises a fatal error
        try reader.unsafeRead { ... )
    }
    

    Throws

    The error thrown by value, or any DatabaseError that would happen while establishing the read access to the database.

    Declaration

    Swift

    @_disfavoredOverload
    func unsafeRead<T>(_ value: (Database) throws -> T) throws -> T

    Parameters

    value

    A function that accesses the database.

  • Asynchronously executes a function that accepts a database connection.

    The guarantees of the asyncRead method are lifted:

    the value function is not isolated: eventual concurrent database updates are visible from the function:

    reader.asyncUnsafeRead { dbResult in
        do {
            let db = try dbResult.get()
            // Those two values can be different, because some other
            // database connection or some other thread may modify the
            // database between the two requests.
            let count1 = try Player.fetchCount(db)
            let count2 = try Player.fetchCount(db)
        } catch {
            // handle error
        }
    }
    

    The value function is not prevented from writing (DatabaseQueue, in particular, will accept database modifications in asyncUnsafeRead).

    Declaration

    Swift

    func asyncUnsafeRead(_ value: @escaping (Result<Database, Error>) -> Void)

    Parameters

    value

    A function that accesses the database. Its argument is a Result that provides the database connection, or the failure that would prevent establishing the read access to the database.

  • Synchronously executes a function that accepts a database connection, and returns its result.

    The guarantees of the safe read method are lifted:

    the value function is not isolated: eventual concurrent database updates are visible from the function:

    try reader.unsafeReentrantRead { db in
        // Those two values can be different, because some other
        // database connection or some other thread may modify the
        // database between the two requests.
        let count1 = try Player.fetchCount(db)
        let count2 = try Player.fetchCount(db)
    }
    

    The value function is not prevented from writing (DatabaseQueue, in particular, will accept database modifications in unsafeRead).

    This method is reentrant. It should be avoided because it fosters dangerous concurrency practices.

    Throws

    The error thrown by value, or any DatabaseError that would happen while establishing the read access to the database.

    Declaration

    Swift

    func unsafeReentrantRead<T>(_ value: (Database) throws -> T) throws -> T

    Parameters

    value

    A function that accesses the database.

Backup

  • backup(to:) Extension method

    Copies the database contents into another database.

    The backup method blocks the current thread until the destination database contains the same contents as the source database.

    When the source is a DatabasePool, concurrent writes can happen during the backup. Those writes may, or may not, be reflected in the backup, but they won’t trigger any error.

    Declaration

    Swift

    public func backup(to writer: DatabaseWriter) throws

Asynchronous Database Access

  • read(_:) Extension method, asynchronous

    Asynchronously executes a read-only function that accepts a database connection, and returns its result.

    Experimental

    For example:

    let count = try await reader.read { db in
        try Player.fetchCount(db)
    }
    

    The value function runs in an isolated fashion: eventual concurrent database updates are not visible from the function:

    try await reader.read { db in
        // Those two values are guaranteed to be equal, even if the
        // `player` table is modified, between the two requests, by
        // some other database connection or some other thread.
        let count1 = try Player.fetchCount(db)
        let count2 = try Player.fetchCount(db)
    }
    
    try await reader.read { db in
        // Now this value may be different:
        let count = try Player.fetchCount(db)
    }
    

    Attempts to write in the database throw a DatabaseError with resultCode SQLITE_READONLY.

    Throws

    The error thrown by value, or any DatabaseError that would happen while establishing the read access to the database.

    Declaration

    Swift

    @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
    public func read<T>(_ value: @escaping (Database) throws -> T) async throws -> T

    Parameters

    value

    A function that accesses the database.

  • unsafeRead(_:) Extension method, asynchronous

    Asynchronously executes a function that accepts a database connection.

    Experimental

    For example:

    let count = try await reader.unsafeRead { db in
        try Player.fetchCount(db)
    }
    

    The guarantees of the read method are lifted:

    the value function is not isolated: eventual concurrent database updates are visible from the function:

    try await reader.asyncRead { db in
        // Those two values can be different, because some other
        // database connection or some other thread may modify the
        // database between the two requests.
        let count1 = try Player.fetchCount(db)
        let count2 = try Player.fetchCount(db)
    }
    

    The value function is not prevented from writing (DatabaseQueue, in particular, will accept database modifications in asyncUnsafeRead).

    Throws

    The error thrown by value, or any DatabaseError that would happen while establishing the read access to the database.

    Declaration

    Swift

    @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
    public func unsafeRead<T>(_ value: @escaping (Database) throws -> T) async throws -> T

    Parameters

    value

    A function that accesses the database.

Publishing Database Values

  • readPublisher(value:) Extension method

    Returns a Publisher that asynchronously completes with a fetched value.

    // DatabasePublishers.Read<[Player]>
    let players = dbQueue.readPublisher { db in
        try Player.fetchAll(db)
    }
    

    Its value and completion are emitted on the main dispatch queue.

    Declaration

    Swift

    @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
    public func readPublisher<Output>(
        value: @escaping (Database) throws -> Output)
    -> DatabasePublishers.Read<Output>

    Parameters

    value

    A closure which accesses the database.

  • Returns a Publisher that asynchronously completes with a fetched value.

    // DatabasePublishers.Read<[Player]>
    let players = dbQueue.readPublisher(
        receiveOn: DispatchQueue.global(),
        value: { db in try Player.fetchAll(db) })
    

    Its value and completion are emitted on scheduler.

    Declaration

    Swift

    @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
    public func readPublisher<S, Output>(
        receiveOn scheduler: S,
        value: @escaping (Database) throws -> Output)
    -> DatabasePublishers.Read<Output>
    where S: Scheduler

    Parameters

    scheduler

    A Combine Scheduler.

    value

    A closure which accesses the database.