общая обработка PDU для PMDA (generic PDU processing for a PMDA)
Имя (Name)
pmdaMain, pmdaGetContext, pmdaSetResultCallBack,
pmdaSetCheckCallBack, pmdaSetDoneCallBack,
pmdaSetEndContextCallBack - generic PDU processing for a PMDA
Синопсис C (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);
Описание (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.
Отзывы (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.
Диагностика (Diagnostic)
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.
Смотри также (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).
