GOBLIN.HCAD

GOBLIN.HCAD key field expected

Compare-and-delete a hash field. Delete field and reply 1 when it holds a string equal to expected; otherwise leave it in place and reply 0. It is the field-level form of GOBLIN.CAD: the atomic "delete only if it still holds what I put there" check, applied to one field of a hash instead of a whole key.

The idiom it replaces

Deleting a field only when it still holds an expected token — a per-field lock release, a conditional retraction — is a read-compare-delete that must be atomic, so in Redis it is a Lua script:

if redis.call("hget", KEYS[1], ARGV[1]) == ARGV[2] then
  return redis.call("hdel", KEYS[1], ARGV[1])
end
return 0

GOBLIN.HCAD key field expected does exactly this in one atomic C++ op — an HGET, a byte-wise compare, and a conditional HDEL. The redis.calls become direct calls to the store's primitives, with no interpreter and no command re-entry.

Semantics

• The comparison is an exact byte-for-byte string match (like GOBLIN.CAD), not numeric — "1" does not match "1.0".
• A mismatch, an absent field, or an absent key all reply 0 and change nothing.
• Deleting the last field of a hash drops the key (an empty hash does not linger), matching HDEL.
• On a match the reply is 1 (one field deleted), so it is a drop-in for the script's return redis.call("hdel", ...).

Return value

Examples

A per-field lock release — a worker deletes its slot only if it still owns it:

> HSET locks job:7 worker-a
(integer) 1
> GOBLIN.HCAD locks job:7 worker-b
(integer) 0          # not the owner -> the lock is left alone
> GOBLIN.HCAD locks job:7 worker-a
(integer) 1          # owner matches -> released
> HEXISTS locks job:7
(integer) 0

Because it deletes only on an exact match, two workers cannot both release the slot: the second HCAD sees a changed (or absent) value and returns 0, the same safety GOBLIN.CAD gives a whole-key lock.

See also

• GOBLIN.CAD — compare-and-delete on a whole string key (the key-level version of this command).
• GOBLIN.HSETGT — set-if-greater on a hash field, the conditional-write companion.
• Goblin extension commands — the rest of the GOBLIN.* family.
• The idiom scripted in each embedded language: EVAL (Lua) and the other interpreters — GOBLIN.HCAD replaces the hget + hdel pair with one native op.