Counters, statistics and watchdogs

Some blocks remember earlier messages: a count, the previous value, a window of values, the time of the last message. This page lists those blocks and explains how their memory behaves, which matters when you deploy changes.

How block memory works

Rule	Detail
One memory per device	Unless you set key, these blocks keep a separate memory per message originator: each device has its own count, previous value or watch. Set key to msg.topic or another path to group differently.
Survives restarts	Memory and waiting messages are kept by the platform; a restart or a move to another worker does not lose them. Waits re-arm at their original due time.
Reset on settings change	A block keeps its memory across a deploy when its settings did not change. A block whose settings changed starts with an empty memory (messages it was holding are kept). A removed block, or one whose type changed, loses everything.
Kept while paused	A switched-off or paused flow keeps its blocks’ memory; a deleted flow loses it.
Size	At most 10,000 keys per block; beyond that the oldest key is forgotten. Held messages count against 16 MiB per block.
Tests	A test starts from an empty memory, or from a read-only copy of the live memory when you tick Start from the live state and context. A test never changes the live memory.

So: when you change a Statistics window, its averages start from scratch; when you only rewire around it, they continue.

Counter

Counts messages per key, over all time or within a sliding window, and adds the count to the message.

Setting	Meaning	Default	Values
n	The count that sends a copy to reached; empty or 0 = never	empty	0 or more
window_ms	Count only the last this long; 0 = all	0	0 to 7 days
reset_at_n	Start again from 0 when n is reached	off	on, off
key	Each key has its own count	msg.originator	a path
store	memory, or persist (kept in the flow’s persistent context)	memory	memory, persist

Outputs: out (every message, with msg.count), reached (a copy when the count is n or more). msg.reset = true sets the key’s count to 0. Without reset_at_n, every message from the n-th on also goes to reached.

Example: “3 door openings within 10 minutes”: n 3, window_ms 10 min, reset_at_n on, reached → Raise alarm.

Delta

The difference between a number and the previous number of the same key: consumption from a meter, pulses since the last report.

Setting	Meaning	Default	Values
input	The number	msg.payload	a path
output	Where the difference is written	msg.payload.delta	a path
period	Where to write the milliseconds since the previous value	none	a path
round	Decimals, applied first	none	0 to 15
on_negative	A drop (counter restart)	error	error (negative_delta), pass (written with its sign), zero
skip_zero	Pass a zero delta on unchanged, without writing it	off	on, off
key	Each key has its own previous value	msg.originator	a path
seed	The first message’s previous value	none (delta 0)	none, latest (the device’s stored latest value)
seed_key	The stored key to seed from (seed = latest)		up to 128 characters
ts_path	Where the reading’s time is	the record time	a path

Outputs: out, other (no value at the input path), error (not_a_number, negative_delta). After a drop, the new value becomes the baseline in every mode. Delta reads no database per message, so it is the efficient way to compute consumption.

Statistics

Count, sum, average, minimum, maximum, standard deviation, last value and rate over the last values or the last period, per key.

Setting	Meaning	Default	Values
property	The number	msg.payload	a path
window	count (the last size values) or time (the last size ms)	count	count, time
size	Number of values, or the window in ms	10	count: 1 to 10,000; time: 1 ms to 7 days
outputs	Which figures	count, avg, min, max	any of count, sum, avg, min, max, stddev, last, rate
emit	each message, or once per full window (end_of_window)	each	each, end_of_window
key	Each key has its own window	msg.originator	a path

msg.payload is replaced by the figure (one output chosen) or an object of figures. rate is values per second. A time window holds at most 10,000 values. After switching window to time, set size in milliseconds: the default 10 would be 10 ms. Error invalid_value “not a number”.

Example: hourly average sent once an hour: window time, size 3600000, outputs avg, emit end_of_window.

Watchdog

Notices when messages stop: if no message of a key arrives within the time, it sends a timeout message; when messages come back it reports that too.

Setting	Meaning	Default	Values
ms	How long a key may be silent	1 min	100 ms to 7 days
key	Each key watched on its own	msg.originator	a path
repeat	Send timeout again every ms while silent	off	on, off

Outputs:

• timeout: a new message (a new run) with msg.payload = the key, msg.topic of the last message, msg.timeout_ms, and the originator of the last message, so Raise alarm works on it.
• ok: the first message of a key after a timeout, unchanged.

Other messages are not passed on. The watch of a key starts with its first message: a device that never sends is not noticed (use an offline alarm rule for that; see Offline alarms). A late message (after an outage) neither feeds nor fires the watchdog, so a silence alarm is not cleared on old readings. Watching per device needs key set to the device: msg.originator (the default) or msg.metadata.deviceId.

Start / end

Marks the start and the end of a period of activity.

Setting	Meaning	Default	Values
ms	Quiet time after the last message that ends the period	1 min	0 to 7 days
key	Each key has its own period	msg.originator	a path

Outputs: start (a copy of the first message of a period, at once), end (the last message, ms after it arrived). Messages in between do not go out.

Variables as memory

For values several blocks or flows share, write variables with Set variable or Change and read them with paths:

Store	Scope	Limit
flow.x	This flow	1 MiB per flow
global.x	Every flow of the organisation	4 MiB per organisation
flow.persist:x, global.persist:x	The persistent store of the same scope	within the same limits

A write over a limit fails with limit and keeps the old value. The editor’s Activity > Data tab shows the values and how full the stores are. See Action blocks.

Timers

For “if nothing cancels it within 10 minutes, do this” across flows, use the Timer action block with a Named timer trigger. See Action blocks and Trigger blocks.