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# snmpa_discovery_handler behaviour (snmp v5.20.2)

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Behaviour module for the SNMP agent discovery handler.

This module defines the behaviour of the agent discovery handler. A `snmpa_discovery_handler` compliant module must export the following functions:

- `stage1_finish/3`

The semantics of them and their exact signatures are explained below.

## Summary

### Callbacks

stage1_finish(TargetName, ManagerEngineID, ExtraInfo)

This function is called at the end of stage 1 of the discovery process. It should return either the atom `ignore` or `{ok, usm_entry() | [usm_entry()]}`. See usm_entry() and usm_entry/1,3 for more info.

## Callbacks

# stage1_finish(TargetName, ManagerEngineID, ExtraInfo)

    -callback stage1_finish(TargetName, ManagerEngineID, ExtraInfo) ->
                               ignore | {ok, UsmEntry | [UsmEntry]} | {ok, UsmEntry | [UsmEntry], NewExtraInfo}
                               when
                                   TargetName :: snmp_target_mib:name(),
                                   ManagerEngineID :: snmp_framework_mib:engine_id(),
                                   ExtraInfo :: term(),
                                   UsmEntry :: snmp_user_based_sm_mib:usm_entry(),
                                   NewExtraInfo :: term().

This function is called at the end of stage 1 of the discovery process. It should return either the atom `ignore` or `{ok, usm_entry() | [usm_entry()]}`. See usm_entry() and usm_entry/1,3 for more info.

If the function returns `ignore`, then it is assumed that either:

- The caller (of the discovery function) will make the needed updates later.
- The callback function itself did the updates.

In either case, the agent will do nothing, but return the retrieved ManagerEngineID (see `snmpa:discovery/6` for more info) and possible continue with stage 2 of the discovery process.

The `ExtraInfo` argument is passed on from the `snmpa:discovery/6` function.

This function may return an updated `NewExtraInfo` that will be used in subsequent calls to the callback functions. Intended for future use.

The purpose of this function is to generate the usm- related security data needed for usm processing in the agent. Specifically, updating the usmUserTable.

When an `usm_entry()` tuple (or a list of such tuples) is returned, this data is then added to the `usmUserTable` by the (master-) agent.

When an `usm_entry()` tuple (or a list of such tuples) is returned, this data is then added to the `usmUserTable` by the (master-) agent.

#### Note

Note that the function does not check if this entry already exists.

#### Note

Note that this function is executed in the context of the master-agent process.