Provided by: libpcp-pmda3-dev_5.0.3-1_amd64 
NAME
pmdaMain, pmdaGetContext, pmdaSetResultCallBack, pmdaSetCheckCallBack,
pmdaSetDoneCallBack, pmdaSetEndContextCallBack - generic PDU processing for a PMDA
C SYNOPSIS
#include <pcp/pmapi.h>
#include <pcp/pmda.h>
cc ... -lpcp_pmda -lpcp
void pmdaMain(pmdaInterface *dispatch);
void pmdaSetCheckCallBack(pmdaInterface *dispatch, pmdaCheckCallBack callback);
void pmdaSetDoneCallBack(pmdaInterface *dispatch, pmdaDoneCallBack callback);
void pmdaSetResultCallBack(pmdaInterface *dispatch, pmdaResultCallBack callback);
void pmdaSetEndContextCallBack(pmdaInterface *dispatch, pmdaEndContextCallBack callback);
int pmdaGetContext(void);
DESCRIPTION
For Performance Metric Domain Agents (PMDA(3)) using the binary PDU protocols to
communicate with pmcd(1), the routine pmdaMain provides a generic implementation of the
PDU-driven main loop.
dispatch describes how to process each incoming PDU. It is a vector of function pointers,
one per request PDU type, as used in the DSO interface for a PMDA, namely:
/*
* Interface Definitions for PMDA Methods
*/
typedef struct {
int domain; /* set/return performance metrics domain id here */
struct {
unsigned int pmda_interface: 8; /* PMDA DSO interface version */
unsigned int pmapi_version : 8; /* PMAPI version */
unsigned int flags : 16; /* optional feature flags */
} comm; /* set/return communication and version info */
int status; /* return initialization status here */
union {
struct { /* PMDA_INTERFACE_2 or _3 */
pmdaExt *ext;
int (*profile)(pmProfile *, pmdaExt *);
int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
int (*desc)(pmID, pmDesc *, pmdaExt *);
int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
int (*text)(int, int, char **, pmdaExt *);
int (*store)(pmResult *, pmdaExt *);
} two, three;
struct { /* PMDA_INTERFACE_4 or _5 */
pmdaExt *ext;
int (*profile)(pmProfile *, pmdaExt *);
int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
int (*desc)(pmID, pmDesc *, pmdaExt *);
int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
int (*text)(int, int, char **, pmdaExt *);
int (*store)(pmResult *, pmdaExt *);
int (*pmid)(char *, pmID *, pmdaExt *);
int (*name)(pmID, char ***, pmdaExt *);
int (*children)(char *, int, char ***, int **, pmdaExt *);
} four, five;
struct { /* PMDA_INTERFACE_6 */
pmdaExt *ext;
int (*profile)(pmProfile *, pmdaExt *);
int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
int (*desc)(pmID, pmDesc *, pmdaExt *);
int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
int (*text)(int, int, char **, pmdaExt *);
int (*store)(pmResult *, pmdaExt *);
int (*pmid)(char *, pmID *, pmdaExt *);
int (*name)(pmID, char ***, pmdaExt *);
int (*children)(char *, int, char ***, int **, pmdaExt *);
int (*attribute)(int, int, const char *, int, pmdaExt *);
} six;
} version;
} pmdaInterface;
This structure has been extended to incorporate the multiple interface versions that have
evolved over time. For pmdaMain, dispatch->domain and dispatch->status are ignored. The
comm.pmda_interface field is used to determine the interface used by the PMDA. Setting
this field to PMDA_INTERFACE_2 or PMDA_INTERFACE_3 will force pmdaMain to use the
callbacks in the version.two or version.three structure. A setting of PMDA_INTERFACE_4 or
PMDA_INTERFACE_5 will force pmdaMain to use the callbacks in the version.four or
version.five structure, and similarly a PMDA_INTERFACE_6 setting forces pmdaMain to use
the callbacks in the version.six structure. Any other value will result in an error and
termination of pmdaMain.
Note that the use of dispatch as the interface between the pmcd(1) and the methods of the
PMDA allows each PMDA to be implemented as though it were a DSO, with pmdaMain providing a
convenient wrapper that may be used to convert from the DSO interface to the binary PDU
(daemon PMDA) interface.
pmdaMain executes as a continuous loop, returning only when an end of file is encountered
on the PDU input file descriptor.
CALLBACKS
In addition to the individual PDU processing callbacks - pmdaProfile(3), pmdaFetch(3),
pmdaDesc(3), pmdaInstance(3), pmdaText(3), pmdaStore(3), pmdaPMID(3), pmdaName(3),
pmdaChildren(3), and pmdaAttribute(3) there are other callbacks that can affect or inform
all PDU processing within a PMDA, namely check, done and end. These callbacks should be
set with pmdaSetCheckCallBack, pmdaSetDoneCallBack and pmdaSetEndContextCallBack.
If not null, check is called after each PDU is received (but before it was processed), and
done is called after each PDU is sent. If check returns a value less than zero (typically
PM_ERR_AGAIN), the PDU processing is skipped and in most cases the function value is
returned as an error PDU to pmcd(1) - this may be used for PMDAs that require some sort of
deferred connection or reconnect protocols for the underlying sources of performance
metrics, e.g. a DBMS. The error indication from check is not passed back to pmcd(1) in
the cases where no acknowledgment is expected, e.g. for a PDU_PROFILE.
The end callback allows a PMDA to keep track of state for individual clients that are
requesting it to perform actions (PDU processing). Using pmdaGetContext a PMDA can
determine, at any point, an integer identifier that uniquely identifies the client tools
at the remote end of PMCD (for local context modes, this identifier is always zero). This
becomes very important for handling event metrics, where each event must be propagated
once only to each interested client. It also underlies the mechanism whereby connection
information is passed to the PMDA, such as the the credentials (user and group
identifiers) for the client tool.
One final callback mechanism is provided for handling the pmResult built for a PDU_RESULT
in response to a PDU_FETCH request. By default, pmdaMain will free the pmResult once the
result has been sent to the pmcd(1). For some PMDAs this is inappropriate, e.g. the
pmResult is statically allocated, or contains a hybrid of pinned PDU buffer information
and dynamically allocated information. pmdaSetResultCallBack may be used to define an
alternative callback from pmdaMain.
DIAGNOSTICS
These messages may be appended to the PMDA's log file:
PMDA interface version interface not supported
The interface version is not supported by pmdaMain.
Unrecognized pdu type The PMDA received a PDU from pmcd that it does not recognize.
This may indicate that the pmcd process is using a more advanced
interface than pmdaMain.
If the PMAPI(3) debugging control options have the ``libpmda'' option set then each PDU
that is received is reported in the PMDA's log file.
SEE ALSO
pmcd(1), PMAPI(3), PMDA(3), pmdaProfile(3), pmdaFetch(3), pmdaDesc(3), pmdaInstance(3),
pmdaText(3), pmdaStore(3), pmdaPMID(3), pmdaName(3), pmdaChildren(3), and
pmdaAttribute(3).