Caching

Open Configure → Application Setup → Accelerators → Caching. This section sits under the Accelerators menu group alongside Caching and Prefetching — they interact, since a prefetched object is only useful if Caching later serves it. The Cache section controls in-memory HTTP caching, refresh policy, and ICP parameters for forwarding.

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Core mechanics

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Refresh — first match wins

Walk Refresh rows top-down; first enabled row whose profiles match sets min/max age, validate, and cachable flag, then stops. cachable=FALSE sets CACHE_INVALID unless CACHE_FORCE.

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Freshness semantics

What each Refresh age field actually does, once an entry has matched:

• Minimum age — guards against serving something that changed very recently: if the origin’s Last-Modified is more recent than this many seconds ago, the object is not treated as safe to serve without revalidating.
• Maximum age — hard ceiling; SafeSquid expires the object this many seconds after caching regardless of the freshness the origin’s own headers promised. 0 disables the ceiling.
• Revalidate age — applies only when the origin gave no explicit freshness guidance at all (no Expires, no max-age): fresh for this many seconds since caching, then must revalidate.
• Last-Modified time factor — last-resort heuristic, used only when the origin gave no freshness guidance but did supply a Last-Modified date: the longer ago the object was modified relative to when it was fetched, the longer it is assumed to stay fresh, scaled by this percentage.

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Violate RFC

Objects stored despite no-cache / no-store get CACHE_VIOLATION. When Violate RFC is off, reads return NULL (not served). When on, violation objects may be served.

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Object size limits

• Minimum file size — an object smaller than this is discarded after the write completes rather than kept.
• Maximum file size — an object exceeding this during writing is discarded unless the matching Refresh entry forced caching regardless of size.

0 disables either check.

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Memory management

When memory used by cached objects exceeds Memory cache size, a cleanup pass evicts least-recently-used, currently-unread entries first, targeting Memory cache size minus Memory free extra. Clean interval is the minimum time between passes so cleanup does not run continuously under sustained pressure.

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Cache bypass

If the connection was flagged elsewhere to bypass the cache, no cache lookup happens at all.

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Manage cached objects

A regular-expression pattern plus “Delete matches” invalidates and removes every cached entry whose key matches, except an entry a client is actively reading at that moment, which is left alone rather than pulled out from under the reader.

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Schema fields

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Global fields

• Enabled (enabled) — master switch. Off means no cache lookup and no write for any request.
• Violate RFC (violate_rfc) — see above.
• Memory cache size (maxmemsize) — accepts K / M / G suffixes.
• Memory free extra (memextra) — how far below the cache size a cleanup pass frees.
• Minimum file size (minsize) / Maximum file size (maxsize) — see Object size limits above.
• Prefetch window (prefetchwindow) — seconds after a Prefetching-driven fetch during which a second prefetch write for the same URL is rejected.
• ICP port (icpport) — UDP port for ICP query/reply used by Proxy chain peer selection; 0 leaves ICP unbound.
• ICP timeout (icptimeout) — milliseconds to wait for a peer’s ICP reply; a slow peer is treated as down for that query.
• Store balance method (sbalancemethod) — FILL SIZE picks the store holding the least data; FILL PERCENT picks the store least full by percentage of its own quota. No effect while disk caching is disabled.
• journal size (journalsize) — 0 uses a default of 128,000 records.
• Clean interval (cleaninterval) — 0 uses a default of 30 seconds.

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Store entry fields (inactive — disk caching is disabled)

• Enabled (enabled), Comment (comment), Profiles (profiles) — as elsewhere.
• Path (path) — blank skips the entry.
• Maximum disk size (maxdisksize) — 0 is treated as unconfigured and skipped in selection.
• Disk free extra (diskextra) — free space targeted below the quota during cleanup.
• MD5 integrity check (md5) — verifies each object’s content hash on read and invalidates on mismatch when disk caching is active. No effect today.

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Refresh entry fields

• Cachable (cachable) — off marks new matching objects invalid on arrival.
• Minimum age (minage), Maximum age (maxage), Revalidate age (validate), Last-Modified time factor (lmfactor) — see Freshness semantics above.
• Profiles (profiles) — blank matches every connection.
• Comment (comment) — administrator note.

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Examples

Open Configure → Application Setup → Accelerators → Caching → Refresh. Row fields are Enabled, Comment, Profiles, Cachable, Minimum/Maximum age, Revalidate age, and Last-Modified time factor.

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How to verify

1. Enable CACHE in LOG_LEVEL.
2. Repeat URL request; check Detailed logs for hit/miss.
3. Use Manage cached objects to inspect or purge entries.

4. If an object you expect to be cached never shows as a hit, read the Refresh entries top to bottom — a broader entry above the intended one can be winning, exactly as in any other first-match list.