title: State Retention parent: Documentation --- # 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::Plugin::Differences by Jose Luis Martinez and comments on the mailing list (see [references](#references)). Lots of discussion between Holger and I ended up with this. ## Terms **Location** : Use `./configure` `--sharedstatedir` to define, default `$PREFIX/var`. Override with `MP_STATE_DIRECTORY` envvar at runtime if set. Add plugin name to end. **Key** : 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. ## Format Example format: # NP state file 1 [file format version] {data version} {time} {data} ## Structs ### np\_state\_key char *name char *plugin_name int data_version char *_filename ### np\_state\_data time_t time void *data int length (of binary data) ## Calls ### np\_state\_generate\_key(argv) 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][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. ### np\_state\_read(np\_state\_key) 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. ### np\_state\_data\_cleanup(np\_state\_data) Cleanup. ### np\_state\_key\_cleanup(np\_state\_key) Cleanup. ## Notes - All opens and close within these functions, retaining atomicity. - Libtap tests for library. - Update [Development Guidelines][guidelines] with library usage. - This has problems if a remote host is checked from different Nagios instances. - Binary data may not restore on a program compiled with different options from the program that saved it; e.g., 32 or 64 bit. - Binary data may include a structure containing a pointer. Pointer values may not be used in the reading program - i.e., you need to overwrite the value with something `malloc()`ed in the current run of the program. - State files could be left lying around. We recommend you run a regular job to remove unmodified state files older than 1 week. ## References - for the initial patch. - for a conversation about the patch. - for Nagios::Plugin::Differences. [extra-opts]: doc/extra-opts.html "Extra-Opts" [guidelines]: doc/guidelines.html "Monitoring Plugin Development Guidelines"