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138 lines
6.0 KiB
Markdown
# Data Source Templates
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Data sources serve an important role in Cacti data sources are data points
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collected from the devices you monitor in Cacti.
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Data sources take the data that has been collected by the poller.
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The Data source then holds the value to be presented to the graph.
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The Data source template is then associated with a graph template.
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These data sources can be for example incoming traffic on an interface. Cacti
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allows you to create data source templates to associate to data queries the
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template allows you to describe what type of data Cacti should expect.
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To create a Data source you will need to select the proper data input there are
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several options but the most popular ones would be GET SNMP data or GET SNMP
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data (indexed.)
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You would use Get SNMP data (indexed) for interfaces for example as Cacti would
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need to query the device to find the interface index. You would use Get SNMP
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data and put in a single OID for an OID that won't change for example CPU usage
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generally does not have an index and the OID won't change
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You will also need to tell the template what the data source type is. The most
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common ones being Gauge and Counter. These data types come from the RRDtool its
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self. Below are the supported data types and their uses. For more information
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see
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[RRD Tool Create Function](https://oss.oetiker.ch/rrdtool/doc/rrdcreate.en.html)
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## GAUGE
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GAUGE is for things like temperatures or number of people in a room or the value
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of a RedHat share.
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## COUNTER
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COUNTER is for continuous incrementing counters like the ifInOctets counter in a
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router. The COUNTER data source assumes that the counter never decreases, except
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when a counter overflows. The update function takes the overflow into account.
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The counter is stored as a per-second rate. When the counter overflows, RRDtool
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checks if the overflow happened at the 32bit or 64bit border and acts
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accordingly by adding an appropriate value to the result.
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## DCOUNTER
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Available when the configured RRDtool version is 1.5 or later.
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DCOUNTER the same as COUNTER, but for quantities expressed as double-precision
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floating point number. It could be used to track quantities that increment by
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non-integer numbers, i.e. number of seconds that some routine has taken to run,
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total weight processed by some technology equipment etc.
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The only substantial difference is that DCOUNTER can either be upward counting
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or downward counting, but not both at the same time. The current direction is
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detected automatically on the second non-undefined counter update and any
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further change in the direction is considered a reset. The new direction is
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determined and locked in by the second update after reset and its difference to
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the value at reset.
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## DERIVE
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DERIVE will store the derivative of the line going from the last to the current
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value of the data source. This can be useful for gauges, for example, to measure
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the rate of people entering or leaving a room. Internally, derive works exactly
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like COUNTER but without overflow checks. So if your counter does not reset at
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32 or 64 bit you might want to use DERIVE and combine it with a MIN value of 0.
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## DDERIVE
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Available when the configured RRDtool version is 1.5 or later.
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DDERIVE the same as DERIVE, but for quantities expressed as double-precision
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floating point number.
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> **NOTE**: on COUNTER vs DERIVE
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>
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> by Don Baarda <don.baarda@baesystems.com>
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>
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> If you cannot tolerate ever mistaking the occasional counter reset for a
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> legitimate counter wrap, and would prefer "Unknowns" for all legitimate
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> counter wraps and resets, always use DERIVE with min=0. Otherwise, using
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> COUNTER with a suitable max will return correct values for all legitimate
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> counter wraps, mark some counter resets as "Unknown", but can mistake some
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> counter resets for a legitimate counter wrap.
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>
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> For a 5 minute step and 32-bit counter, the probability of mistaking a counter
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> resetfor a legitimate wrap is arguably about 0.8% per 1Mbps of maximum
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> bandwidth. Note that this equates to 80% for 100Mbps interfaces, so for high
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> bandwidth interfaces and a 32bit counter, DERIVE with min=0 is probably
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> preferable. If you are using a 64bit counter, just about any max setting will
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> eliminate the possibility of mistaking a reset for a counter wrap.
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## ABSOLUTE
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ABSOLUTE is for counters which get reset upon reading. This is used for fast
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counters which tend to overflow. So instead of reading them normally you reset
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them after every read to make sure you have a maximum time available before the
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next overflow. Another usage is for things you count like number of messages
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since the last update.
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## COMPUTE
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RRDtool supports computed data sources using an RPN expression derived from
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other data sources in the same file. That is distinct from a graph-time CDEF.
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The current Cacti type list includes `COMPUTE`, but the ordinary data-template
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editor has no expression field and the normal RRD creation path emits the
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heartbeat/minimum/maximum form used by collected data sources. That form is not
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a valid COMPUTE definition. Do not select it for a new data source through this
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workflow. Use a [CDEF](CDEFs.md) for graph-time calculations, or an explicitly
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managed external RRD workflow when persistent computed data is required.
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Advanced RRD/XML helpers do not supply the missing template editor and creation
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support.
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You are also able to use Variables to set the name for the data source. The
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available Variables can be seen on the
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[Variables](https://github.com/Cacti/documentation/blob/develop/Variables.md)
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documentation page.
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What is also an important setting is the internal data source name. This will
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be used to associate the data source with the graph template. The template
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editor accepts 1–19 letters, digits, or underscores. Use a recognizable name,
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such as `traffic_in`. Do not rename an existing item without also planning the
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matching RRD file change; a metadata edit alone does not rename the stored
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series.
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---
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Copyright (c) 2004-2026 The Cacti Group
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