Struct mz_persist::location::LockInfo

pub struct LockInfo { /* private fields */ }

The partially structured information stored in an exclusive-writer lock.

To allow for restart without operator intervention in situations where we’ve crashed and are unable to cleanly unlock (s3 has no flock equivalent, for example), this supports reentrance. We define the “same” process as any that have the same reentrance id, which is an opaque user-provided string.

This, in essence, delegates the problem of ensuring writer-exclusivity to the persist user, which may have access to better (possibly distributed) locking primitives than we do.

Concretely, MZ Cloud will initially depend on the exclusivity of attaching an EBS volume. We’ll store the reentrant_id somewhere on the EBS volume that holds the catalog. Any process that starts and has access to this volume is guaranteed that the previous machine is no longer available and thus the previous mz process is no longer running. Similarly, a second process cannot accidentally start up pointed at the same storage locations because it will not have the reentrant_id available.

Violating writer-exclusivity will cause undefined behavior including data loss. It’s always correct, and MUCH safer, to provide a random reentrant_id here (e.g. a UUID). It will simply require operator intervention to verify that the previous process is no longer running in the event of a crash and to confirm this by manually removing the previous lock.

Implementations

impl LockInfo

pub fn new(reentrance_id: String, details: String) -> Result<Self, Error>

Returns a new LockInfo from its component parts.

Errors if reentrance_id contains a newline.

pub fn new_no_reentrance(details: String) -> Self

Constructs a new, empty LockInfo with a unique reentrance id.

Helper for tests that don’t care about locking reentrance (which is most of them).

pub fn check_reentrant_for<D: Debug, R: Read>(
    &self,
    location: &D,
    existing: R
) -> Result<(), Error>

Returns Ok if this lock information represents a process that may proceed in the presence of some existing lock.

Trait Implementations

impl Clone for LockInfo

fn clone(&self) -> LockInfo

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for LockInfo

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

Formats the value using the given formatter.

impl Display for LockInfo

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

Formats the value using the given formatter.

impl FromStr for LockInfo

type Err = Error

The associated error which can be returned from parsing.

fn from_str(s: &str) -> Result<Self, Self::Err>

Parses a string s to return a value of this type.

impl PartialEq<LockInfo> for LockInfo

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

This method tests for self and other values to be equal, and is used by ==.

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

This method tests for !=.

impl Eq for LockInfo

impl StructuralEq for LockInfo

impl StructuralPartialEq for LockInfo