Data Input Methods

Data input methods allow Cacti to retrieve data to insert into data sources and ultimately put on a graph. There are different ways for Cacti to retrieve data, the most popular being through an external script or from SNMP.

Creating a Data Input Method

To create a new data input method, select the Data Input Methods option under the Management heading. Once on that screen, click Add on the right. You will be presented with a few fields to populate on the following screen.

Table 11-1. Field Description: Data Input Methods

Name	Description
Name	Give the data query a name that you will use to identify it
Input Type	Select the type of data input method you are trying to create
Input String	This field is only used when the Input Type is set to 'Script/Command'
Output String	This field is only here for legacy purposes and will probably go away in a future version of Cacti

The Name specified will be used throughout Cacti to identify the data input method. Careful consideration should be made to help uniquely identify the data source. Having very similar names can lead to confusion when utilising them.

Valid options for Input Type are 'Script/Command', 'SNMP', 'SNMP Query', and 'Script Query'.

When the type is set to Script/Command, the Input String specifies the full path to the script including any per data source variables inside <>'s from the user. For instance, if you are passing an IP address to a script, your input string might look something like: /path/to/script.pl <ip> When the user creates a data source based on this data input method, they will be prompted for an IP address to pass onto the script.

When you are finished filling in all necessary fields, click the Create button to continue. You will be redirected back to the same page, but this time with two new boxes, Input Fields and Output Fields.

The Input Fields box is used to define any fields that require information from the user. Any input fields referenced to in the input string must be defined here.

The Output Fields box is used to define each field that you expect back from the script.

All data input methods must have at least one output field defined, but may have more than one depending on the type.

Data Input Fields

To define a new field, click Add next to the input or output field boxes. You will be presented with some or all of the fields below depending on whether you are adding an input or output field.

Table 11-2. Field Description: Data Input Fields

Name	Description
Field/Field Name	Either select or type in the name of the field you want to define
Friendly Name	Enter a more descriptive name for this field
Regular Expression Match (Input Only)	Enter a valid regular expression
Allow Empty Input (Input Only)	Can the input value of this field blank or not
Special Type Code (Input Only)	Reference to an internal field
Update RRD File (Output Only)	Checked to store the result

The Field Name must contain no spaces or other non-alphanumeric characters (except '-' or '_').

If you want to enforce a certain regular expression pattern when the user enters a value into Regular Expression Match (Input Only) for this Data Input Field, it must follow POSIX syntax as it will be passed to PHP's preg() functions.

If the Data Input Field needs to reference another field internally, you can enter this into the Special Type Code. For instance, if your field requires an IP address from the user, you can enter 'management_ip' here and Cacti will fill this field in with the current IP address of the selected host.

Table 11-3. Special Type Codes

Field Name	Description
hostname	The hostname
management_ip	The ip
snmp_community	The SNMP community
snmp_username	The SNMP username
snmp_password	The SNMP version

If you enable the Update RRD File, Cacti will insert the return value from this field into the RRD file. Obviously, this box needs to be checked for at least one output field per data input source, but can be left blank to have Cacti store the value only in the database instead.

When you are finished filling in all necessary fields, click the Create button to continue. You will be redirected back to the data input method edit page. From here you can continue to add additional fields, or click Save on this screen when finished.

Making Your Scripts Work With Cacti

The simplest way to extend Cacti's data gathering functionality is through external scripts. Cacti comes with a number of scripts out of the box which are located in the scripts/ directory. These scripts are used by the data input methods that are present in a new installation of Cacti.

To have Cacti call an external script to gather data you must create a new data input method, making sure to specify Script/Command for the Input Type field. See the previous section, Creating a Data Input Method for more information about how to create a data input method. To gather data using your data input method, Cacti simply executes the shell command specified in the Input String field. Because of this, you can have Cacti run any shell command or call any script which can be written in almost any language.

What Cacti is concerned with is the output of the script. When you define your data input method, you are required to define one or more output fields. The number of output fields that you define here is important to your script's output. For a data input method with only one output field, your script should output its value in the following format:

<value_1>

So if I wrote a script that outputs the number of running processes, its output might look like the following:

67

Data input methods with more than one output field are handled a bit differently when writing scripts. Scripts that output more than one value should be formatted like the following:

<fieldname_1>:<value_1> <fieldname_2>:<value_2> ... <fieldname_n>:<value_n>

Lets say that I write a script that outputs the 1, 5, and 10 minute load average of a Unix machine. In Cacti, I name the output fields '1min', '5min', and '10min', respectively. Based on these two things, the output of the script should look like the following:

1min:0.40 5min:0.32 10min:0.01

One last thing to keep in mind when writing scripts for Cacti is that they will be executed as the user the data gatherer runs as. Sometimes a script may work correctly when executed as root, but fails due to permissions problems when executed as a less privileged user.

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