pmdaGetContext(3) generic PDU processing for a PMDA

Other Alias

pmdaMain, pmdaSetResultCallBack, pmdaSetCheckCallBack, pmdaSetDoneCallBack, pmdaSetEndContextCallBack


#include <pcp/pmapi.h>
#include <pcp/impl.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);


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.


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.


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) debug control variable (pmdebug) has the DBG_TRACE_LIBPMDA flag set then each PDU that is received is reported in the PMDA's log file.