SnmpKit.Agent.Store (snmpkit v2.0.1)

Scalars kept in an ETS table: the handler behind SnmpKit.Agent.put/4.

Every agent has one store covering the whole tree; objects put into it are served unless a handler registered at a more specific prefix shadows them. A value may be a zero-arity function, which is called on every read, so a live gauge is just:

SnmpKit.Agent.put(agent, "hrSystemNumUsers.0", :gauge32, fn -> MyApp.Sessions.count() end)

Objects are read-only unless put with writable: true, in which case a SET of a value with the same type replaces it (type aliases such as :string/:octet_string are treated as equal). A store can also be registered on its own at a prefix, with table: naming an ETS ordered_set you own.

Concurrency

The table is public and every request runs in its own process, so put/5, delete/2 and SETs interleave. Each is a single ETS operation: put/5 and delete/2 are one insert or delete, and SnmpKit.Agent.Handler.set/3 is one :ets.select_replace/2 that succeeds only if the object still exists, is still writable and still has a compatible type at the moment of the write. SnmpKit.Agent.Handler.check_set/3 is a separate, read-only phase run by SnmpKit.Agent.Request before any set/3; an object deleted or made read-only between the two phases makes the SET fail with noCreation or notWritable rather than crash. There is no undo across the varbinds of one SET (see SnmpKit.Agent.Handler).

Summary

Types

table()

Functions

delete(table, oid)

Removes the object at oid.

fetch(table, oid)

Reads the object at oid, calling a function value.

new_table()

Creates the ETS table a store uses (a public ordered_set).

put(table, oid, type, value, opts \\ [])

Puts {type, value} at oid; writable: true lets SET replace the value.

to_list(table)

Every object in the store as {oid, type, value}, in OID order.