Struct Position 

pub struct Position(/* private fields */);

A global coordinate: the order an event was committed in, across every stream of one database.

Positions are dense and gap-free as read: a reader never observes n + 1 while n is still uncommitted, so a subscription is a plain position > cursor range read with no grace window.

The mechanism is narrower than “one writer”, and worth stating precisely because the narrower version is also stronger. The position is allocated inside the transaction that commits it, and that transaction holds SQLite’s write lock from BEGIN, because every write the backend makes is IMMEDIATE. Allocation order and commit order therefore cannot diverge — and that argument does not depend on there being one connection. It holds for two processes on one file, and it is measured: 120 appends through two separately-opened logs come back as exactly 1..=120, in order.

What a second connection does cost is the wake-up, not the order — see crate::log::EventLog::subscribe.

Implementations

impl Position

pub const BEGINNING: Position

Before the first event. Where a consumer with no checkpoint starts.

pub const fn new(value: u64) -> Self

pub const fn get(self) -> u64

pub fn as_stored(self) -> Option<i64>

The value as SQLite stores it, or None when it does not fit.

A position is a rowid, so every position the store ever assigns is in range. Position::new is public, though, and a u64 above i64::MAX would bind as a negative number: position > -1 reads the whole log rather than nothing, and a checkpoint written from one would silently replay everything through the exactly-once path. Callers that bind a position use this and refuse rather than wrap.

Trait Implementations

impl Clone for Position

fn clone(&self) -> Position

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for Position

impl Debug for Position

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Position

fn default() -> Position

Returns the “default value” for a type.

impl Display for Position

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for Position

impl Hash for Position

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Ord for Position

fn cmp(&self, other: &Position) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

fn clamp_to<R>(self, range: R) -> Self

where Self: Sized, R: ClampBounds<Self>,

🔬This is a nightly-only experimental API. (clamp_to)

Restrict a value to a certain range.

impl PartialEq for Position

fn eq(&self, other: &Position) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl PartialOrd for Position

fn partial_cmp(&self, other: &Position) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl StructuralPartialEq for Position