# 5.3.7 The Sequence Counter

_Peios / Advanced Peios / PKM / LCS / Layers_

> The single global monotonic counter stamped on every layer-qualified entry — allocation, initialisation, and what it is not.

LCS keeps one global monotonic counter. Every mutation that creates a
layer-qualified entry — a path entry, a value write, a key hide, a
blanket tombstone — takes the next number from it. The counter is never
decremented and never reset.

It provides deterministic tiebreaking within a precedence tier
(§5.3.6). Wall-clock time is tracked separately, as each key's last
write time, for humans.

## 5.3.7.1 Allocation

Allocating a number increments the counter. **Allocated numbers are
never reused**, even if the operation later fails, times out, or is
part of a transaction that aborts. Gaps in the sequence space are
normal and carry no meaning.

A transactional mutation is assigned its number when the operation is
**accepted into the transaction**, not at commit. That preserves the
order in which the caller performed the operations, which is what layer
tiebreaking and watch ordering need if it commits (§5.7.2).

## 5.3.7.2 Initialisation

Each source reports the highest sequence number it has persisted in its
registration handshake, and LCS raises the counter to one above the
maximum reported. New writes therefore always outrank anything already
in storage, even after a restart.

A source registering later advances the counter the same way, to
`max(current, source_max + 1)`. If that addition would overflow 64 bits
the registration fails with `EOVERFLOW` and the source is not made
Active — the failure happens before the slot becomes usable.

The counter itself refuses to hand out `U64_MAX`, so the value is never
allocated.

## 5.3.7.3 What a sequence number is not

A sequence number is not a hive generation number.

A **sequence number** orders layer-qualified entries for resolution and
is persisted by sources.

A **hive generation number** is a volatile, per-hive, kernel-owned
change epoch, exposed by `REG_IOC_QUERY_KEY_INFO` (§5.5.3) so that a
watcher recovering from `OVERFLOW` can tell whether it actually missed
anything. Sources never see it and never persist it. Its baseline is
initialised from the source's reported maximum sequence at
registration, purely so that observed generations are monotonic
relative to persisted entries; after that the two are unrelated.

## 5.3.7.4 Restore

A restore does not write the backup's sequence numbers. It reserves a
fresh range and remaps into it, so restored entries are newer than
everything that was there before while keeping their relative order
from the backup (§5.9.3).