PMDAINIT(3)

NAME
pmdaInit, pmdaRehash, pmdaSetData, pmdaExtGetData, pmdaExtSetData, pmdaSetFlags, pmdaSetCommFlags, pmdaExtSetFlags - initialize a PMDA

C SYNOPSIS
#include <pcp/pmapi.h>
#include <pcp/pmda.h>

void pmdaInit(pmdaInterface *dispatch, pmdaIndom *indoms, int nindoms, pmdaMetric *metrics, int nmetrics);

void pmdaRehash(pmdaExt *pmda, pmdaMetric *metrics, int nmetrics);

void pmdaSetFlags(pmdaInterface *dispatch, int flags);
void pmdaSetCommFlags(pmdaInterface *dispatch, int flags);
void pmdaExtSetFlags(pmdaExt *pmda, int flags);

void pmdaSetData(pmdaInterface *dispatch, void *data);
void pmdaExtSetData(pmdaExt *pmda, void *data);
void *pmdaExtGetData(pmdaExt *pmda);

cc ... -lpcp_pmda -lpcp

DESCRIPTION
pmdaInit initializes a PMDA so that it is ready to receive PDUs from pmcd(1). The function expects as arguments the instance domain table (indoms) and the metric description table (metrics) that are initialized by the PMDA. The arguments nindoms and nmetrics should be set to the number of instances and metrics in the tables, respectively.

Much of the pmdaInterface structure can be automatically initialized with pmdaDaemon(3), pmdaGetOpt(3) and pmdaDSO(3). pmdaInit completes the PMDA initialization phase with three operations. The first operation adds the domain and instance numbers to the instance and metric tables. Singular metrics (metrics without an instance domain) should have the instance domain PM_INDOM_NULL set in the indom field of the pmDesc structure (see pmLookupDesc(3)). Metrics with an instance domain should set this field to be the serial number of the instance domain in the indoms table.

The instance domain table may be made empty by setting indoms to NULL and nindoms to 0. This allows the caller to provide custom Fetch and Instance callback functions. The metric table may be made empty by setting metrics to NULL and nmetrics to 0. This allows the caller to provide custom Fetch and Descriptor callback functions.

EXAMPLE
For example, a PMDA has three metrics: A, B and C, and two instance domains X and Y, with two instances in each instance domain. The instance domain and metrics description tables could be defined as:

static pmdaInstid _X[] = {
    { 0, "X1" }, { 1, "X2" }
};

static pmdaInstid _Y[] = {
    { 0, "Y1" }, { 1, "Y2" }
};

static pmdaIndom indomtab[] = {
#define X_INDOM 0
    { X_INDOM, 2, _X },
#define Y_INDOM 3
    { Y_INDOM, 2, _Y }
};

static pmdaMetric metrictab[] = {
/* A */
    { (void *)0,
      { PMDA_PMID(0,0), PM_TYPE_U32, PM_INDOM_NULL, PM_SEM_INSTANT,
        { 0,0,0,0,0,0} }, },
/* B */
    { (void *)0,
      { PMDA_PMID(0,1), PM_TYPE_U32, X_INDOM, PM_SEM_INSTANT,
        { 0,0,0,0,0,0} }, },
/* C */
    { (void *)0,
      { PMDA_PMID(0,2), PM_TYPE_DOUBLE, Y_INDOM, PM_SEM_INSTANT,
        { 0,1,0,0,PM_TIME_SEC,0} }, }
};

The metric description table defines metric A with no instance domain, metric B with instance domain X and metric C with instance domain Y. Metric C has units of seconds, while the other metrics have no units (simple counters). pmdaInit will take these structures and assign the PMDA(3) domain number to the it_indom field of each instance domain. This identifier also replaces the indom field of all metrics which have that instance domain, so that they are correctly associated.

The second stage opens the help text file, if one was specified with the -h command line option (see pmdaGetOpt(3)) or as a helptext argument to pmdaDSO(3) or pmdaDaemon(3).

The final stage involves preparing the metric table lookup strategy.

METRIC LOOKUP
When fetch and descriptor requests are made of the PMDA, each requested PMID must be mapped to a metric table entry. There are currently three strategies for performing this mapping - direct, linear and hashed. Each has its own set of tradeoffs and an appropriate strategy should be selected for each PMDA.

If all of the metric PMID item numbers correspond to the position in the metrics table, then direct mapping is used. This is the most efficient of the lookup functions as it involves a direct array index (no additional memory is required nor any additional processing overhead). If the PMID numbering requirement is met by the PMDA, it is ideal. This strategy can be explicitly requested by calling pmdaSetFlags (pmda, PMDA_EXT_FLAG_DIRECT) before calling pmdaInit. In this case, if the direct mapping is not possible (e.g. due to an oversight on the part of the PMDA developer), a warning is logged and the linear strategy is used instead.

The second strategy (linear search) is the default, when a direct mapping cannot be established. This provides greater flexibility in the PMID numbering scheme, as the PMDA item numbers do not have to be unique (hence, the PMID cluster numbers can be used more freely, which is often extremely convenient for the PMDA developer). However, lookup involves a linear walk from the start of the metric table until a matching PMID is found, for each requested PMID in a request.

The third strategy (hash lookup) can be requested by calling pmdaSetFlags (pmda, PMDA_EXT_FLAG_HASHED) before calling pmdaInit. This strategy is most useful for PMDAs with large numbers of metrics (many hundreds, or thousands). Such PMDAs will almost always use the cluster numbering scheme, so the direct lookup scheme becomes inappropriate. They may also be prepared to sacrifice a small amount of additional memory for a hash table, mapping PMID to metric table offsets, to speed up lookups in their vast metric tables.

This final strategy can also be used by PMDAs serving up dynamically numbered metrics. For this case, the pmdaRehash function should be used to replace the metric table when new metrics become available, or existing metrics are removed. The PMID hash mapping will be recomputed at the same time that the new metric table is installed.

METRIC CHANGES
It should be well understood by PMDA authors that metric metadata for individual metrics is fixed, and ideally would not ever change. In the situation where metadata is incorrect and is updated, such a change requires correction to logged metrics using pmlogrewrite(1), and as a result should be avoided whenever possible.

However, a PMDA may become aware of new domain metrics at runtime, and in this case it is ideal to export them immediately (without any collector system restart). In this situation, the PMDA can inform all running PMAPI clients that may have already explored the metric namespace (for example, using pmTraversePMNS(3)) of the change to the metric namespace.

This is achieved using pmdaSetFlags (pmda, PMDA_EXT_NAMES_CHANGE) which will result in the PMCD_NAMES_CHANGE state change notification being sent to each PMAPI client on next fetch. If the newly discovered metrics have label metadata associated, then the PMDA_EXT_LABEL_CHANGE flag may also be set, which will result in the PMCD_LABEL_CHANGE notification being sent as well.

pmdaExtSetFlags is equivalent to pmdaSetFlags, and is provided as a convenience interface in situations where the pmdaExt is more readily available than the pmdaInterface structure.

COMMUNICATION ATTRIBUTES
Agents that make use of authentication or container attributes should indicate this using the pmdaSetCommFlags interface. This indicates the need for these attributes to be communicated on the channel between the PMDA and pmcd or local context client. Valid flags are PMDA_FLAG_AUTHORIZE (for authentication related attributes) and PMDA_FLAG_CONTAINER (for container name related attributes).

PRIVATE DATA
A facility for associating private PMDA data with the pmdaExt structure is available. This allows a PMDA to associate an arbitrary (and typically not global) pointer with the pmdaExt such that it can be later obtained during callbacks. The interfaces for setting this pointer are pmdaSetData and pmdaExtSetData, and pmdaExtGetData for subsequently retrieving it.

CAVEAT
The PMDA must be using PMDA_INTERFACE_2 or later, as specified in the call to pmdaDSO(3) or pmdaDaemon(3) to use pmdaInit.

The PMDA must use PMDA_INTERFACE_7 or later to issue state change notifications using pmdaSetFlags or pmdaExtSetFlags.

DIAGNOSTICS
pmdaInit will set dispatch->status to a value less than zero if there is an error that would prevent the PMDA(3) from successfully running. pmcd(1) will terminate the connection to the PMDA(3) if this occurs.

pmdaInit may issue any of these messages:

PMDA interface version interface not supported
    The interface version is not supported by pmdaInit.

Using pmdaFetch() but fetch call back not set
    The fetch callback, pmdaFetch(3), requires an additional callback to be provided using pmdaSetFetchCallBack(3).

Illegal instance domain inst for metric pmid
    The instance domain inst that was specified for metric pmid is not within the range of the instance domain table.

No help text path specified
    The help text callback, pmdaText(3), requires a help text file for the metrics to have been opened, however no path to the help text was specified as a command line option, or as an argument to pmdaDSO(3) or pmdaDaemon(3). This message is only a warning.

Direct mapping for metrics disabled @ num
    The unit numbers of the metrics did not correspond to the index in the metric description table. The direct mapping failed for metric number num in the metrics table. This is less efficient but is not fatal and the message is only a warning.

Hashed mapping for metrics disabled @ num
    A memory allocation failure occurred while building the hash table to index the metric description table. This is a non-fatal warning message - a fallback to linear searching will be automatically performed should this situation arise.

SEE ALSO
newhelp(1), pmcd(1), pmlogrewrite(1), PMAPI(3), PMDA(3), pmdaDaemon(3), pmdaDSO(3), pmdaFetch(3), pmdaGetOpt(3), pmdaText(3), pmLookupDesc(3) and pmTraversePMNS(3).