Operations

Running Recached in production: what it exports, what to alert on, and which limits fail closed.

Metrics endpoint

Recached serves Prometheus metrics on a separate port from the cache itself, so you can expose it to your monitoring network without exposing the data plane.

RECACHED_METRICS_PORT=9090 recached-server
curl http://127.0.0.1:9090/metrics

The port is set by RECACHED_METRICS_PORT and binds to the same host as RECACHED_BIND.

The metrics port has no authentication

It inherits RECACHED_BIND but not RECACHED_PASSWORD. Anything that can reach the port can read your metrics — including key hit/miss volume and per-command traffic. Bind it to a private interface or firewall it.

What is exported

Traffic metrics are event-driven:

Metric	Type	Labels	Meaning
recached_commands_total	counter	command	Commands executed, by command name.
recached_command_errors_total	counter	command	Commands that returned an error, by command name.
recached_connections_total	counter	type = tcp | ws	Connections accepted since start, split by transport.
recached_connections_active	gauge	—	Connections currently open (TCP and WebSocket combined).
recached_keyspace_hits_total	counter	—	Reads that found a live key.
recached_keyspace_misses_total	counter	—	Reads that found nothing or an expired key.
recached_command_duration_seconds	histogram	command	End-to-end store execution time by command.

Persistence and queue health

Metric	Type	Labels	Meaning
recached_persistence_healthy	gauge	—	1 while persistence is healthy; 0 after an AOF or checkpoint failure. Writes then return MISCONF until SAVE succeeds.
recached_persistence_errors_total	counter	operation	Persistence failures by operation; operation="all" is the aggregate.
recached_snapshot_saves_total	counter	status	Completed and failed checkpoints.
recached_snapshot_duration_seconds	histogram	—	Time a checkpoint holds the save and all-write barriers.
recached_last_successful_save_timestamp_seconds	gauge	—	Unix timestamp of the last successful checkpoint.
recached_last_save_age_seconds	gauge	—	Seconds since the last successful checkpoint.
recached_aof_bytes	gauge	—	Current AOF file size when AOF is enabled.
recached_notification_overflows_total	counter	—	WATCH/QSUB clients disconnected because their bounded notification queue filled.
recached_sync_lag_disconnects_total	counter	—	WebSocket sync clients closed (code 4001) because they fell behind the mutation fan-out far enough to miss frames. They reconnect and resynchronise via qstate, so this is a backpressure signal, not data loss — but a sustained rate means clients cannot drain as fast as you are writing.
recached_pubsub_overflows_total	counter	—	Pub/sub clients disconnected because their bounded delivery queue filled.

Capacity and sync

Sampled every 5 seconds, because these are levels rather than events.

Metric	Type	Meaning
recached_memory_bytes	gauge	Incremental logical bytes for stored keys and values. This is not process RSS. Compare it with RECACHED_MAX_MEMORY.
recached_keys	gauge	Maintained stored-key count. Expired entries remain until bounded active expiry removes them. Compare it with RECACHED_MAX_KEYS.
recached_evictions_total	counter	Keys evicted since start. A rising rate means the cache is working at its cap.
recached_replicas_connected	gauge	Replicas currently attached to this primary.
recached_live_queries	gauge	Registered QSUB patterns across all connections.
recached_watched_keys	gauge	Keys under WATCH.
recached_dedup_clients_tracked	gauge	Clients with duplicate-suppression high-water marks in memory.
recached_replication_queue_depth	gauge	Deepest replica send queue, in frames — work the primary has not yet put on the wire.
recached_replication_queue_bytes	gauge	Encoded bytes in the deepest replica send queue.
recached_replication_lag_frames	gauge	Frames the furthest-behind replica has been sent but has not acknowledged applying. Zero means every replica is caught up.
recached_replication_backlog_bytes	gauge	Bytes retained for partial replica resynchronization.
recached_replication_syncs_total	counter	Full and partial synchronizations, labeled by type.
recached_replication_sync_duration_seconds	histogram	Initial full or partial synchronization time, labeled by type.
recached_replication_disconnects_total	counter	Replica disconnects caused by the frame or byte queue limit, labeled by reason.

Recached does not implement SLOWLOG. The command histogram identifies which command class is slow; use client-side tracing when you need individual request attribution. Process RSS remains the capacity metric for allocator overhead, network buffers, and fragmentation.

Reading the two replication gauges

They fail differently, which is why both exist:

• Queue depth high, lag high — the primary cannot hand frames off fast enough. The replica's channel is backing up, usually a slow or saturated network link. A replica whose queue fills is disconnected outright. It resumes from the retained backlog when possible and otherwise receives a fresh snapshot.
• Queue depth zero, lag high — everything was written to the socket and the replica is not acknowledging it. The frames are in flight, or the replica is applying them slowly, or it is wedged. This is the case queue depth alone cannot see, and it is the one worth alerting on.

Lag is measured in frames, not bytes or seconds: one frame is one replicated write command. The RCP1 replication handshake rejects incompatible peers; upgrade primary and replicas together.

What is still not exported

• Client outbox depth. That state lives in the browser — read it there with cache.pendingWrites().

Useful queries

# Command throughput by command
rate(recached_commands_total[1m])

# Error ratio — the single most useful health signal
sum(rate(recached_command_errors_total[5m]))
  / sum(rate(recached_commands_total[5m]))

# Cache hit ratio
sum(rate(recached_keyspace_hits_total[5m]))
  / (sum(rate(recached_keyspace_hits_total[5m])) + sum(rate(recached_keyspace_misses_total[5m])))

# Connection headroom against RECACHED_MAX_CONNECTIONS (default 1024)
recached_connections_active

# WebSocket sync clients specifically
rate(recached_connections_total{type="ws"}[5m])

Suggested alerts

Thresholds are starting points — tune to your traffic.

Alert	Condition	Why it matters
Error-rate spike	error ratio > 1% for 5m	Usually a client sending unsupported commands or malformed args after a deploy.
Connection saturation	recached_connections_active > 80% of RECACHED_MAX_CONNECTIONS	New connections are rejected once the semaphore is exhausted — this fails hard, not gracefully.
Hit ratio collapse	hit ratio drops sharply vs baseline	Keys expiring faster than expected, an eviction storm, or a cold restart.
Traffic flatline	rate(recached_commands_total[5m]) == 0 while clients are up	The process is alive enough to scrape but not serving.
Memory pressure	recached_memory_bytes > 80% of RECACHED_MAX_MEMORY	Eviction is about to start, or already has.
Eviction churn	rate(recached_evictions_total[5m]) climbing	The working set no longer fits; results will start missing.
Replica lost	recached_replicas_connected drops	Failover risk — the standby is no longer following.
Replica falling behind	recached_replication_lag_frames > 1000 for 5m	The standby is not keeping up; a failover now would lose those writes.
Persistence unhealthy	recached_persistence_healthy == 0	Writes are being refused after an AOF or checkpoint failure. Fix storage, then run SAVE.
Save stalled	high recached_snapshot_duration_seconds or rising recached_last_save_age_seconds	Checkpoints pause writers and may be blocked on storage.
Slow consumers	increase in either overflow counter	A pub/sub, WATCH, or QSUB client cannot drain its bounded queue.

Health checking

There is no dedicated HTTP health endpoint. Use the protocol itself:

# Liveness — is the cache answering?
redis-cli -p 6379 ping        # → PONG

# With auth enabled
redis-cli -p 6379 -a "$RECACHED_PASSWORD" ping

For container orchestration:

livenessProbe:
  exec:
    command: ["redis-cli", "-p", "6379", "ping"]
  initialDelaySeconds: 5
  periodSeconds: 10

The /metrics endpoint returning 200 proves the metrics listener is up, not that the cache is healthy — they are separate listeners. Probe the cache port for liveness and require recached_persistence_healthy == 1 for write readiness when persistence is enabled.

For a human-readable snapshot at a terminal — uptime, connected clients, keyspace size, replication role — use INFO:

redis-cli -p 6379 INFO              # all default sections
redis-cli -p 6379 INFO replication  # just the topology

INFO and Prometheus serve different jobs and neither replaces the other: INFO is a point-in-time snapshot for an operator or a client's ready-check, while /metrics carries the per-command counters, error counts, and history that dashboards and alerts need. Alert on the metrics, not on scraped INFO output.

Capacity limits

Hard limits compiled into the server. Exceeding them produces errors rather than degradation, so it is worth knowing where the walls are:

Limit	Default	Configurable
Max connections	1024	RECACHED_MAX_CONNECTIONS
Consecutive auth failures before disconnect	5	No
Read buffer per TCP connection	64 MB	No
Queued commands per MULTI	10,000	RECACHED_MAX_MULTI_QUEUE
WATCHed keys per connection	1,024	RECACHED_MAX_WATCHES_PER_CONN
Live queries (QSUB) per connection	64	RECACHED_MAX_LIVE_QUERIES
Pub/sub channel and pattern subscriptions per connection	1,024	RECACHED_MAX_PUBSUB_SUBSCRIPTIONS
Keys allowed in a complete live-query initial state	10,000	RECACHED_MAX_QSUB_INITIAL_KEYS
Keys sampled per eviction pass	10	RECACHED_EVICTION_SAMPLE
Replication frame	512 MB	No
WATCH/QSUB delivery queue	256 messages and 8 MiB per connection	No
Pub/sub delivery queue	256 messages and 8 MiB per connection	No
Replica delivery queue	4,096 frames and 8 MiB per replica	RECACHED_REPL_BUFFER / RECACHED_REPL_BUFFER_BYTES
Partial-resync backlog	16 MiB per primary	RECACHED_REPL_BACKLOG_BYTES
Glob pattern length (KEYS, SCAN MATCH, PSUBSCRIBE, sync scopes)	1,024 bytes	No
Elements reserved up front for an aggregate	1,024	No
Client outbox (browser, offline writes)	10,000 writes	via sync-client

The keyspace cap (RECACHED_MAX_KEYS) and memory cap (RECACHED_MAX_MEMORY) are configured rather than compiled — see Configuration.

Backup and restore

Snapshots are MessagePack files at RECACHED_SAVE_PATH, written atomically (temp file, fsync, rename, and directory fsync). A single snapshot read therefore sees a complete old or new file, but a durable deployment is a checkpoint set: snapshot, .dedup sidecar, and—when enabled—AOF. Do not copy those files one by one while writes or another save can run; that can combine files from different checkpoints.

# Take a snapshot on demand, then confirm it completed
redis-cli -p 6379 BGSAVE
redis-cli -p 6379 LASTSAVE     # timestamp advances when the save lands

# Stop the server after SAVE, then copy the snapshot, its .dedup sidecar,
# and the AOF (when configured), or capture them with one atomic filesystem
# snapshot while the server is paused.

A sidecar file sits next to the snapshot with a .dedup extension, holding duplicate-suppression high-water marks. Back it up as part of the same checkpoint set: without it a restarted server can re-apply a write a client replays. A missing sidecar is a clean first boot; a present but corrupt sidecar is a startup error.

To restore, stop the server, put the snapshot (and its .dedup sidecar) at RECACHED_SAVE_PATH, and start it — both load at boot. There is no import path from a Redis RDB file; the formats are unrelated.

If AOF is enabled, the AOF replays on top of the snapshot. Losing the AOF while keeping the snapshot costs you every write since the last save.

Upgrades

Recached is pre-1.0 and the wire protocol is not frozen — see the protocol spec. Read the changelog before upgrading a minor version, and upgrade server and browser SDK together. The replication protocol identifies itself as RCP1; mixed replication protocol versions fail explicitly and are not supported.

The current versioned snapshot envelope accepts legacy bare-entry snapshots. The reverse is not guaranteed, so keep a complete copy of the pre-upgrade checkpoint set if you may need to roll back.