3 Low-level discovery

Overview

Low-level discovery provides a way to automatically create items, triggers, and graphs for different entities on a computer. For instance, Zabbix can automatically start monitoring file systems or network interfaces on your machine, without the need to create items for each file system or network interface manually. Additionally, it is possible to configure Zabbix to remove unneeded entities automatically based on the actual results of periodically performed discovery.

A user can define their own types of discovery, provided they follow a particular JSON protocol.

The general architecture of the discovery process is as follows.

First, a user creates a discovery rule in Data collection → Templates, in the Discovery column. A discovery rule consists of (1) an item that discovers the necessary entities (for instance, file systems or network interfaces) and (2) prototypes of items, triggers, and graphs that should be created based on the value of that item.

An item that discovers the necessary entities is like a regular item seen elsewhere: the server asks a Zabbix agent (or whatever the type of the item is set to) for a value of that item, the agent responds with a textual value. The difference is that the value the agent responds with should contain a list of discovered entities in a JSON format. While the details of this format are only important for implementers of custom discovery checks, it is necessary to know that the returned value contains a list of macro → value pairs. For instance, item "net.if.discovery" might return two pairs: "{#IFNAME}" → "lo" and "{#IFNAME}" → "eth0".

These macros are used in names, keys and other prototype fields where they are then substituted with the received values for creating real items, triggers, graphs or even hosts for each discovered entity. See the full list of options for using LLD macros.

When the server receives a value for a discovery item, it looks at the macro → value pairs and for each pair generates real items, triggers, and graphs, based on their prototypes. In the example with "net.if.discovery" above, the server would generate one set of items, triggers, and graphs for the loopback interface "lo", and another set for interface "eth0".

Note that since Zabbix 4.2, the format of the JSON returned by low-level discovery rules has been changed. It is no longer expected that the JSON will contain the "data" object. Low-level discovery will now accept a normal JSON containing an array, in order to support new features such as the item value preprocessing and custom paths to low-level discovery macro values in a JSON document.

Built-in discovery keys have been updated to return an array of LLD rows at the root of JSON document. Zabbix will automatically extract a macro and value if an array field uses the {#MACRO} syntax as a key. Any new native discovery checks will use the new syntax without the "data" elements. When processing a low-level discovery value first the root is located (array at $. or $.data).

While the "data" element has been removed from all native items related to discovery, for backward compatibility Zabbix will still accept the JSON notation with a "data" element, though its use is discouraged. If the JSON contains an object with only one "data" array element, then it will automatically extract the content of the element using JSONPath $.data. Low-level discovery now accepts optional user-defined LLD macros with a custom path specified in JSONPath syntax.

See also: Discovered entities

Configuring low-level discovery

We will illustrate low-level discovery based on an example of file system discovery.

To configure the discovery, do the following:

• Go to: Data collection → Templates or Hosts
• Click on Discovery in the row of an appropriate template/host

• Click on Create discovery rule in the upper-right corner of the screen
• Fill in the discovery rule form with the required details

Discovery rule

The discovery rule form contains five tabs, representing, from left to right, the data flow during discovery:

• Discovery rule - specifies, most importantly, the built-in item or custom script to retrieve discovery data
• Preprocessing - applies some preprocessing to the discovered data
• LLD macros - allows to extract some macro values to use in discovered items, triggers, etc
• Filters - allows to filter the discovered values
• Overrides - allows to modify items, triggers, graphs or host prototypes when applying to specific discovered objects

The Discovery rule tab contains the item key to use for discovery (as well as some general discovery rule attributes):

All mandatory input fields are marked with a red asterisk.

Preprocessing

The Preprocessing tab allows to define transformation rules to apply to the result of discovery. One or several transformations are possible in this step. Transformations are executed in the order in which they are defined. All preprocessing is done by Zabbix server.

See also:

• Preprocessing details
• Preprocessing testing

Note that if the discovery rule has been applied to the host via template then the content of this tab is read-only.

Custom macros

The LLD macros tab allows to specify custom low-level discovery macros.

Custom macros are useful in cases when the returned JSON does not have the required macros already defined. So, for example:

• The native vfs.fs.discovery key for filesystem discovery returns a JSON with some pre-defined LLD macros such as {#FSNAME}, {#FSTYPE}. These macros can be used in item, trigger prototypes (see subsequent sections of the page) directly; defining custom macros is not needed;
• The vfs.fs.get agent item also returns a JSON with filesystem data, but without any pre-defined LLD macros. In this case you may define the macros yourself, and map them to the values in the JSON using JSONPath:

The extracted values can be used in discovered items, triggers, etc. Note that values will be extracted from the result of discovery and any preprocessing steps so far.

Filter

A filter can be used to generate real items, triggers, and graphs only for entities that match the criteria. The Filters tab contains discovery rule filter definitions allowing to filter discovery values:

Override

The Override tab allows setting rules to modify the list of item, trigger, graph, host, and discovery prototypes or their attributes for discovered objects that meet given criteria.

Overrides (if any) are displayed in a reorderable drag-and-drop list and executed in the order in which they are defined.  To configure details of a new override, click on in the Overrides block. To edit an existing override, click on the override name. A popup window will open allowing to edit the override rule details.

All mandatory parameters are marked with red asterisks.

Configuring an operation

To configure details of a new operation, click on in the Operations block. To edit an existing operation, click on next to the operation. A popup window where you can edit the operation details will open.

Form buttons

Buttons at the bottom of the form allow to perform several operations.

Discovered entities

The screenshots below illustrate how discovered items, triggers, and graphs look like in the host's configuration. Discovered entities are prefixed with an orange link to a discovery rule they come from.

Note that discovered entities will not be created in case there are already existing entities with the same uniqueness criteria, for example, an item with the same key or graph with the same name. An error message is displayed in this case in the frontend that the low-level discovery rule could not create certain entities. The discovery rule itself, however, will not turn unsupported because some entity could not be created and had to be skipped. The discovery rule will go on creating/updating other entities.

If a discovered entity (host, file system, interface, etc) stops being discovered (or does not pass the filter anymore) the entities that were created based on it may be automatically disabled and eventually deleted.

Lost resources may be automatically disabled based on the value of the Disable lost resources parameter. This affects lost hosts, items and triggers.

Lost resources may be automatically deleted based on the value of the Delete lost resources parameter. This affects lost hosts, host groups, items, triggers, and graphs.

When discovered entities become 'Not discovered anymore', a lifetime indicator is displayed in the entity list. Move your mouse pointer over it and a message will be displayed indicating its status details.

If entities were marked for deletion, but were not deleted at the expected time (disabled discovery rule or item host), they will be deleted the next time the discovery rule is processed.

Entities containing other entities, which are marked for deletion, will not update if changed on the discovery rule level. For example, LLD-based triggers will not update if they contain items that are marked for deletion.

Other types of discovery

More detail and how-tos on other types of out-of-the-box discovery is available in the following sections:

• discovery of network interfaces;
• discovery of CPUs and CPU cores;
• discovery of SNMP OIDs;
• discovery of JMX objects;
• discovery using ODBC SQL queries;
• discovery of Windows services;
• discovery of host interfaces in Zabbix.

For more detail on the JSON format for discovery items and an example of how to implement your own file system discoverer as a Perl script, see creating custom LLD rules.