Title: Yac Object Cache
Author: laruence
Published: September 2, 2026
Last modified: September 2, 2026
---
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# Yac Object Cache
By [laruence](https://profiles.wordpress.org/laruence/)
[Download](https://downloads.wordpress.org/plugin/yac-object-cache.1.2.2.zip)
* [Details](https://wordpress.org/plugins/yac-object-cache/#description)
* [Reviews](https://wordpress.org/plugins/yac-object-cache/#reviews)
* [Installation](https://wordpress.org/plugins/yac-object-cache/#installation)
* [Development](https://wordpress.org/plugins/yac-object-cache/#developers)
[Support](https://wordpress.org/support/plugin/yac-object-cache/)
## Description
Yac uses the [Yac](https://github.com/laruence/yac) PHP extension as the backing
store for the WordPress object cache.
Unlike Memcached or Redis, Yac stores data in **shared memory inherited by every
PHP-FPM worker** on the machine. There is no cache server to install, no socket
to configure, and every cache read is a hash lookup in local memory — typically
microseconds, with no network round trip and no global lock.
**Highlights**
* **Fast.** A cache read is a shared-memory hash lookup — the status page measures
a 0.005 ms round trip. On a production site (laruence.com, PHP 8.1-FPM, 8 cores),
full homepage renders ran ~20% faster than the classic Memcached drop-in: 141.6
vs 118.7 req/s at 20 concurrent users, +18-20% across 20-100 concurrency, zero
failed requests.
* **No external server.** The cache lives in shared memory (`mmap`/SysV), fork-
inherited by FPM workers.
* **Lock-free.** Per-slot CAS arbitration; throughput scales with worker count.
No global lock.
* **Self-deploying.** Activating the plugin writes `object-cache.php` to `wp-content/`.
* **Simple keys, simple flush.** Keys are stored verbatim while they fit Yac’s
48-byte limit; `wp_cache_flush()` calls `Yac::flush()` and wipes the entire shared
memory on the machine — know that before you flush.
* **Graceful degradation.** If Yac is unavailable (extension missing), the drop-
in falls back to a per-request in-memory cache and WordPress keeps working.
* **Single-node focus.** Keys carry no per-blog prefix; installs sharing one PHP
pool isolate via `YAC_OCACHE_KEY_PREFIX`. Multisite blogs share one namespace.
* **Entry inspector.** On the admin page, click any top-entry key to see the deserialized
value, padded size, expiry and — on newer yac builds — last access, hit count
and whether the value is embedded in its slot; or delete the entry.
**Best fit**
Yac is a _local_ cache. It is ideal for single-node or few-node WordPress installs
where all PHP workers run on one machine. On large multi-server clusters with strict
cross-node consistency needs, a network cache (Memcached/Redis) may suit better.
## Screenshots
[⌊Cache health dashboard: hit-rate ring, cause-attributed metric bars and the live
shared-memory round trip (0.005 ms).⌉⌊Cache health dashboard: hit-rate ring, cause-
attributed metric bars and the live shared-memory round trip (0.005 ms).⌉[
Cache health dashboard: hit-rate ring, cause-attributed metric bars and the live
shared-memory round trip (0.005 ms).
[⌊Storage overview: key slots in use, lookups, hit rate, values-memory pool and
the memory-health verdict.⌉⌊Storage overview: key slots in use, lookups, hit rate,
values-memory pool and the memory-health verdict.⌉[
Storage overview: key slots in use, lookups, hit rate, values-memory pool and the
memory-health verdict.
[⌊Entry inspector: keys-by-group pie, occupancy totals and the largest entries by
content length.⌉⌊Entry inspector: keys-by-group pie, occupancy totals and the largest
entries by content length.⌉[
Entry inspector: keys-by-group pie, occupancy totals and the largest entries by
content length.
## Installation
1. Install the Yac extension, any of three ways (then enable `extension=yac.so` in
php.ini):
2. PECL
3. pecl install yac
4. PIE (the PHP Foundation’s PECL successor; yac is on Packagist)
5. pie install laruence/yac
6. Build from source
7. git clone https://github.com/laruence/yac.git && cd yac phpize && ./configure &&
make && sudo make install
8. Install and activate Yac Object Cache. Activation deploys `wp-content/object-cache.
php`.
9. In `wp-config.php`, above the “That’s all, stop editing!” line, add:
10. define( ‘WP_CACHE’, true ); define( ‘YAC_OCACHE_KEY_PREFIX’, ‘ab_’ ); // unique
per install when sites share one PHP pool
That is it. Visit **Tools Yac Object Cache** to verify status.
**Optional php.ini tuning**
```
yac.enable = 1
yac.keys_memory_size = 4M ; ~32K slots
yac.values_memory_size = 64M ; raise for large sites (big alloptions)
```
**Optional wp-config switches**
```
define( 'YAC_OCACHE_EMPTY_TTL', 0 ); // default 21600: lifetime cap (seconds) on empty-array negative cache results; set 0 to disable
define( 'YAC_OCACHE_DISABLE', true ); // emergency escape hatch: force runtime-only mode
```
## FAQ
### Do I need a Memcached or Redis server?
No. That is the point — the cache lives in shared memory on the machine.
### What happens if the Yac extension is not installed?
The drop-in degrades to a per-request in-memory cache. Your site keeps working;
you just lose cross-request persistence. The status page flags the missing extension.
### How are keys stored, given Yac’s 48-byte key limit?
Storage keys are `::` (default prefix `wp`),
kept verbatim while they fit the 48-byte budget; over-long keys keep the group verbatim
and hash (crc32b) only the key part. Keys carry no per-blog prefix: use a different
YAC_OCACHE_KEY_PREFIX per install when sites share one PHP pool; multisite blogs
share the namespace.
### Does flush() clear other Yac users on the same machine?
Yes. `wp_cache_flush()` calls `Yac::flush()`, which wipes the entire shared memory
on the machine, including data written by other Yac users sharing the PHP pool.
The admin page asks for confirmation before flushing.
### Multisite?
Yes, with a caveat: blogs of one install share the cache namespace (keys carry no
blog prefix), which is fine for most sites; `switch_to_blog()` does not re-namespace
keys. Run separate installs with different `YAC_OCACHE_KEY_PREFIX` values when blogs
must not share entries.
### What about wp_cache_flush_group()?
Yac cannot delete entries by prefix, so a group flush clears the request-level copy
of that group; shared entries then expire via TTL. The plugin reports `flush_group`
as unsupported so WordPress core does not rely on it.
## Reviews
There are no reviews for this plugin.
## Contributors & Developers
“Yac Object Cache” is open source software. The following people have contributed
to this plugin.
Contributors
* [ laruence ](https://profiles.wordpress.org/laruence/)
[Translate “Yac Object Cache” into your language.](https://translate.wordpress.org/projects/wp-plugins/yac-object-cache)
### Interested in development?
[Browse the code](https://plugins.trac.wordpress.org/browser/yac-object-cache/),
check out the [SVN repository](https://plugins.svn.wordpress.org/yac-object-cache/),
or subscribe to the [development log](https://plugins.trac.wordpress.org/log/yac-object-cache/)
by [RSS](https://plugins.trac.wordpress.org/log/yac-object-cache/?limit=100&mode=stop_on_copy&format=rss).
## Changelog
#### 1.2.2
* Fixed stale shared-memory writes: within one request, `wp_cache_set()` on a key
already written by `wp_cache_add()` in that same request skipped the shared-memory
write, so other requests kept reading the value the add stored (the `update_option()`
pattern — `add()` inside `get_option()`, then `set()` — left the old option behind).
* Replaced `YAC_OCACHE_SKIP_EMPTY` with `YAC_OCACHE_EMPTY_TTL` (default 21600s):
empty results (bot-probed `get_page_by_path()` paths, comment query misses) now
share as usual but expire after that lifetime instead of occupying a slot forever.
Set 0 to disable the cap.
* The health panel’s “keys used” now reports the number of live entries rather
than the `slots_used` high-water mark.
#### 1.2.1
* Placed the PHPCS EscapeOutput annotations around the health-ring output so WordPress.
org Plugin Check recognizes the exemption (the previous inline annotation sat
on the wrong line of the multi-line statement).
* Annotated `pre_wp_cache_get` in the drop-in as a WordPress core hook.
#### 1.2.0
* Fixed the health-ring donut on the admin dashboard disappearing: the SVG was
piped through `wp_kses_post()`, whose allowed-tag list strips SVG elements and
left only the center text visible.
* Renamed every plugin-owned symbol from the `wp_yac_`/`WP_YAC_`/`wp-yac-` prefixes
to `yac_ocache_`/`YAC_OCACHE_`/`yac-ocache-` for WordPress.org prefix policy (
the `wp_` prefix is reserved for WordPress core). The wp-config switches were
renamed accordingly: `YAC_OCACHE_KEY_PREFIX`, `YAC_OCACHE_SKIP_EMPTY`, `YAC_OCACHE_DISABLE`,`
YAC_OCACHE_WARMUP_LOOKUPS`.
* The `wp_cache_*` functions and the `$wp_object_cache` global are the standard
WordPress object cache API and keep their names.
* WP-CLI: the commands stay `wp yac status` / `wp yac flush`; the command class
is now `YAC_OCACHE_CLI_Command`.
#### 1.1.1
* A stored `false` is now written to shared memory as `0`. Yac’s `get()` returns`
false` for both a miss and a stored `false`, so false negatives never survived
past the request; with `0` they persist like any other value (readers comparing
by value see `0` instead of `false`).
* Admin page: top entries grew a Hottest tab (by access count) alongside Largest,
shown whenever this yac build reports per-entry hits; the entry inspector gains
Hits, Embedded-in-slot and compressed-payload (`c_len`) metadata.
* Admin page: entry listings now page through `Yac::dump()` 1000 entries at a time
instead of one `dump(-1)`, which could exhaust the PHP memory limit on a busy
cache; builds older than 2.4.0 fall back to the single full dump.
#### 1.1.0
* Dashboard rebuilt around a cache-health verdict: hit-rate ring, cause-attributed
metric bars, keys-by-group pie, largest entries by content length, configuration
reference and runtime diagnostics.
* Key format rework: `::` with no per-blog prefix (default
prefix `wp`); over-long keys keep the group verbatim and hash only the key part;`
switch_to_blog()` no longer re-namespaces keys.
* `YAC_OCACHE_SKIP_EMPTY` (on by default): empty `get_page_by_path:` negatives (
bot 404 probes) stay request-local instead of filling slots.
* `YAC_OCACHE_DISABLE` escape hatch forces runtime-only mode.
#### 1.0.0
* Initial release. Yac-backed object cache with self-deploying drop-in, verbatim
keys with hashed fallback, multisite support, and graceful runtime-only fallback.
* Storage key prefix configurable via `YAC_OCACHE_KEY_PREFIX` (0-6 chars, default`
wp_`); shorter is better.
* `wp_cache_flush()` calls `Yac::flush()` and clears the entire shared memory on
the machine — the admin page asks for confirmation and states the scope.
* Admin dashboard: status bar, key-slot donut, counters, memory-health advice,
diagnostics, self-test, memory-contents snapshot, and the configuration reference
inside the Environment panel.
## Meta
* Version **1.2.2**
* Last updated **1 day ago**
* Active installations **Fewer than 10**
* WordPress version ** 5.6 or higher **
* Tested up to **7.1**
* PHP version ** 7.0 or higher **
* Tags
* [cache](https://wordpress.org/plugins/tags/cache/)[object cache](https://wordpress.org/plugins/tags/object-cache/)
[performance](https://wordpress.org/plugins/tags/performance/)
* [Advanced View](https://wordpress.org/plugins/yac-object-cache/advanced/)
## Ratings
No reviews have been submitted yet.
[Your review](https://wordpress.org/support/plugin/yac-object-cache/reviews/#new-post)
[See all reviews](https://wordpress.org/support/plugin/yac-object-cache/reviews/)
## Contributors
* [ laruence ](https://profiles.wordpress.org/laruence/)
## Support
Got something to say? Need help?
[View support forum](https://wordpress.org/support/plugin/yac-object-cache/)