# snmp_local_db

### MODULE

snmp_local_db

### MODULE SUMMARY

The SNMP built-in database

### DESCRIPTION

The module `snmp_local_db` contains functions for implementing tables (and variables) using the SNMP built-in database. The database exists in two instances, one volatile and one persistent. The volatile database is implemented with ets. The persistent database is implemented with a module, pets (Persistent ets (Erlang Term Store)), that keeps an ets database in memory for speed, and on disk for persistent storage. At start-up, the ets database is initialized from disk. All readings from the database go to the ets. Writings go also to the ets, and a note is made in a log file. When the database is closed, the entire ets database is dumped, and the log file is cleared. If a crash occurs, the log file will always contain the latest changes. At start-up, the dumped ets is read, and then the database is recovered using the log file. It is also possible to manually dump the database, in which case the entire ets is dumped, and the log file is cleared.

There are three scaling problems with this database.

- If the database is never dumped, there are a lot of modifications to the database and the log file will grow rapidly. This can be solved by regularly dumping the database.
- The second problem occurs if the database is large, dumping the entire database may take some considerable time and it may slow down the system.
- The third problem is that insertions and deletions are inefficient for large tables.

All these problems are best solved by using Mnesia instead.

In order to know when the content of the database should be dumped, it is possible to register/unregister a notification client. This client will be notified of all persistent/permanent changes to the database by a call to:

`Module:notify(Client,What)`

Where the arguments are:

- `Client = term()`
- `What = insert | delete | close`

| The snmp local db does not initiate the dump! Instead, this could be initiated by the "notify client" as a result of the received db change information!

See `register_notify_client/2` and `unregister_notify_client/2` below for further information.

The following functions describe the interface to `snmp_local_db`. Each function has a Mnesia equivalent. The argument `NameDb` is a tuple `{Name, Db}` where `Name` is the symbolic name of the managed object (as defined in the MIB), and `Db` is either `volatile` or `persistent`. `mnesia` is not possible since all these functions are `snmp_local_db` specific.

### Common Data Types

In the functions defined below, the following types are used:

- `NameDb = {Name, Db}`
- `Name = atom(), Db = volatile | persistent`
- `RowIndex = [int()]`
- `Cols = [Col] | [{Col, Value}], Col = int(), Value = term()`

where `RowIndex` denotes the last part of the OID, that specifies the index of the row in tha table. `Cols` is a list of column numbers in case of a get operation, and a list of column numbers and values in case of a set operation.

### EXPORTS

`dump() -> ok | {error, Reason}`

This function can be used to dump the database at any time. The entire ets database is dumped to disk, and the log file is cleared. This might be useful if the log file grows rapidly. Returns `{error, Reason}` if a file system error occured.

`match(NameDb,Pattern)`

Performs an ets matching on the table. See Stdlib documentation, module ets, for a description of `Pattern` and the return values.

`print()`
`print(TableName)`
`print(TableName, Db)`

Types:

`TableName = atom()`

Prints the contents of the database on screen. This is useful for debugging since the `STANDARD-MIB` and `OTP-SNMPEA-MIB` (and maybe your own MIBs) are stored in `snmp_local_db`.

`TableName` is an atom for a table in the database. When no name is supplied, the whole database is shown.

`table_create(NameDb) -> bool()`

Creates a table. If the table already exist, the old copy is destroyed.

Returns `false` if the `NameDb` argument is incorrectly specified, `true` otherwise.

`table_create_row(NameDb,RowIndex,Row) -> bool()`

Types:

`Row = {Val1, Val2, ..., ValN}`
`Val1 = Val2 = ... = ValN = term()`

Creates a row in a table. `Row` is a tuple with values for all columns, including the index columns.

`table_delete(NameDb) -> void()`

Deletes a table.

`table_delete_row(NameDb,RowIndex) -> bool()`

Deletes the row in the table.

`table_exists(NameDb) -> bool()`

Checks if a table exists.

`table_get_row(NameDb,RowIndex) -> Row | undefined`

Types:

`Row = {Val1, Val2, ..., ValN}`
`Val1 = Val2 = ... = ValN = term()`

`Row` is a tuple with values for all columns, including the index columns.

`register_notify_client(Client,Module) -> ok | {error,Reason}`

Types:

`Client = term()`
`Module = atom()`
`Reason = {already_registered,CurrentModule}`
`CurrentModule = atom()`

Register `Client` as notification client to snmp_local_db. Client is actually just used as an identity, but could e.g. be a `pid()`. When changes are made to the database (`insert`, `delete` or `stop`) notify clients will be notified.

`unregister_notify_client(Client) -> ok | {error,Reason}`

Types:

`Client = term()`
`Reason = not_registered`

Unregister `Client` as notification client to snmp_local_db.

### See Also

ets(3), snmp_generic(3)

### AUTHORS

Martin Björklund - support@erlang.ericsson.se
Klas Eriksson - support@erlang.ericsson.se

snmp 3.4
Copyright © 1991-2003 Ericsson Utvecklings AB