HydraCache 0.47.0

HydraCache 0.47.0

0.47.0 adds the cross-region session consistency slice: session context,
read-your-writes, monotonic reads and writes, writes-follow-reads via bounded
causal dependencies, convergence plus explicit bounded staleness, and session
lifecycle observability.

The release does not claim cross-region linearizability or distributed
transactions. It makes the application-session contract explicit and testable:
requests may move between regions, but a verified session token carries the
bounded context needed to avoid stale self-reads, backwards reads, reordered
session writes, and effect-before-cause visibility.

Highlights

• Added SessionId, SessionWatermark, SessionToken, SessionRequest, and
  VersionStamp for bounded, tamper-evident session context propagation.
• Added SessionReadBudget, ReadEscalation, resolve_session_read, and
  serve_session_read so read-your-writes either serves safely, escalates, or
  fails loud instead of serving below the session watermark.
• Added monotonic guards: SessionSequence, SessionWriteStamp,
  resolve_monotonic_read, apply_monotonic_read,
  resolve_monotonic_write, and apply_monotonic_write.
• Added CausalSummary, AppliedSet, CausalWrite, causal_apply, and
  apply_causal_write. Overflow coarsens conservatively through a stable floor
  instead of silently dropping dependencies.
• Added convergence and freshness helpers: converge_replicated_values,
  SessionReadMode, StalenessBound, resolve_session_read_mode, and
  within_staleness_bound.
• Added session lifecycle helpers: SessionTtl,
  validate_session_lifecycle, rebuild_expired_sessionless, and
  recover_session_after_failover.
• Added aggregate session observability with SessionStats, registered
  session metric descriptors, and session alert rules that obey the bounded
  cardinality rule.

Verification

Focused gates:

cargo test -p hydracache --locked session_context
cargo test -p hydracache --locked session_ryw
cargo test -p hydracache --locked session_monotonic
cargo test -p hydracache --locked causal_consistency
cargo test -p hydracache --locked convergence_staleness
cargo test -p hydracache-observability --locked session_observability
cargo test -p hydracache --locked fault_injector_selftest

Implementation checks used during the release work:

cargo fmt --all
cargo test -p hydracache --locked session_context
cargo test -p hydracache --locked session_ryw
cargo test -p hydracache --locked session_monotonic
cargo test -p hydracache --locked causal_consistency
cargo test -p hydracache --locked convergence_staleness
cargo test -p hydracache-observability --locked session_observability

Scope Boundary

This is causal+ session consistency, not a global transaction layer. It orders a
session's causally related operations and keeps concurrent writes convergent via
the existing CRDT and merge-policy machinery. Operators still own topology,
durability, WAN transport, repair budgets, token-secret management, and any
stronger consistency guarantees above the library surface.

Release Status

Implemented as the 0.47.0 release slice after the focused gates above. The
workspace version remains on the current published line until the explicit
publish step.