Ubuntu Manpage: pmdaInit, pmdaRehash, pmdaSetData, pmdaExtGetData, pmdaExtSetData, pmdaSetFlags,

Provided by: libpcp-pmda3-dev_6.2.0-1.1build4_amd64

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.