State Retention Routines

Ton Voon, June 16, 2010

The aim is to create a set of library routines that can be used for saving state information between invocations of a plugin. This way, it is possible to calculate the rate of change and provide threshold calculations on this, rather than just the current state.

This is based on a patch submitted by Alain Williams, Nagios::Monitoring::Plugin::Differences by Jose Luis Martinez and comments on the mailing list (see references).

Lots of discussion between Holger and I ended up with this.


Use ./configure --sharedstatedir to define, default $PREFIX/var. Override with NAGIOS_PLUGIN_STATE_DIRECTORY envvar at runtime if set. Add plugin name to end.
Is used as the filename of the store. Default to state.dat. Recommend that this is set to the string returned by np_state_generate_key(), to be unique per plugin call. Key can only consist of alphanumerics and underscore.


Example format:

# NP state file
1 [file format version]
{data version}



char *name
char *plugin_name
int data_version
char *_filename


time_t time
void *data
int length (of binary data)



Returns a string to use as a key_name, based on an MD5 hash of argv, thus hopefully a unique key per service/plugin invocation. Use the Extra-Opts parse of argv, so that uniqueness in parameters are reflected there.

np_state_init(plugin_name, key_name, data_version)

Sets variables. Generates filename. Returns np_state_key. Die with UNKNOWN if exception.


Returns np_state_data. Will return NULL if no data is available (first run). If key currently exists, read data. If state file format version is not expected, return as if no data. Get state data version number and compare to expected. If numerically lower, then return as no previous state. Die with UNKNOWN if exceptional error.

np_state_write_string(np_state_key, time, string)

If time==NULL, use current time. Create state file, with state file format version, default text. Write version, time, and data. Avoid locking problems - use mv to write and then swap. Possible loss of state data if two things writing to same key at same time.

np_state_write_binary(np_state_key, time, start, length)

Same as np_state_write_string(), but writes binary data.