what is the difference between zabbix passive checks & active checks ?

Zabbix agents support both passive (polling) and active checks (trapping). Zabbix may perform checks based on an interval, however, it is also possible to schedule specific times for item polling.

Passive checks (polling):

• Zabbix server (or proxy) requests a value from Zabbix agent
• Agent processes the request and returns the value to Zabbix server (or proxy)

Zabbix server or proxy asks for some data (for example, CPU load) and Zabbix agent sends back the result to the server.

Example: agent ping

1. Server opens a TCP connection
2. Server sends agent.ping\n
3. Agent reads the request and response with <HEADER><DATALEN>1
4. Server processes data to get the value, ‘1’ in our case
5. TCP connection is closed

Active checks (trapping):

• Zabbix agent requests from Zabbix server (or proxy) a list of active checks
• Agent sends the results in periodically

Active checks require more complex processing. The agent must first retrieve from the server(s) a list of items for independent processing.

For example:

1. Agent opens a TCP connection
2. Agent asks for the list of checks
3. Server responds with a list of items (item key, delay)
4. Agent parses the response
5. TCP connection is closed
6. Agent starts periodical collection of data

Zabbix Trapper

Trapper items accept incoming data instead of querying for it.It is useful for any data you might want to “push” into Zabbix.

To use a trapper item you must:

1. have a trapper item set up in Zabbix
2. send in the data into Zabbix(with zabbix_sender)

Reference : https://www.zabbix.com/zabbix_agent#active_passive_