HazptrObserver implements a read-optimized Observer which caches an Observer's snapshot and protects access to it using hazptrs. The cached snapshot is kept up to date using a callback which fires when the original observer changes. This implementation incurs an additional allocation on updates making it less suitable for write-heavy workloads.
Declared in <folly/observer/HazptrObserver.h>
template<
typename T,
template<typename> typename Atom = atomic>
class HazptrObserver;There are 2 main APIs: 1) getSnapshot: Returns a Snapshot containing a const pointer to T and guards access to it using folly::hazptr_holder. The pointer is only safe to use while the returned Snapshot object is alive. 2) getLocalSnapshot: Same as getSnapshot but backed by folly::hazptr_local. This API is ~3ns faster than getSnapshot but is unsafe for the current thread to construct any other hazptr holder type objects (hazptr_holder, hazptr_array and other hazptr_local) while the returned snapshot exists.
See folly/synchronization/Hazptr.h for more details on hazptrs.
| Name | Description |
|---|---|
DefaultSnapshot | Snapshot type backed by a hazptr_holder. |
LocalSnapshot | Snapshot type backed by a hazptr_local. |
| Name | Description |
|---|---|
HazptrObserver [constructor] | Constructors |
~HazptrObserver [destructor] | Marks the instance dead and retires the cached state. |
operator= | Copy-assigns by destroying and reconstructing in place. |
getLocalSnapshot | Returns a hazptr_local-backed snapshot of the cached value. |
getSnapshot | Returns a hazptr_holder-backed snapshot of the cached value. |
getUnderlyingObserver | Returns the underlying observer being cached. |
with | Invoke a function with the current observed value. The snapshot is held alive for the duration of the call, preventing read-after-free when accessing members of the observed object. |
| Name | Description |
|---|---|
makeHazptrObserver | Same as makeObserver(...), but creates HazptrObserver. |