Ubuntu Manpage: pmNewContext - establish a new PMAPI context

name
c synopsis
description
caveats
diagnostics
environment
see also

NAME

       pmNewContext - establish a new PMAPI context

C SYNOPSIS

       #include <pcp/pmapi.h>

       int pmNewContext(int type, const char *name);

       cc ... -lpcp

DESCRIPTION

An application using the Performance Metrics Application Programming Interface (PMAPI) may manipulate
several concurrent contexts, each associated with a source of performance metrics, e.g. pmcd(1) on some
host, or a set of archives of performance metrics as created by pmlogger(1), or a stand-alone connection
on the local host that does not involve pmcd(1).

pmNewContext may be used to establish a new context. The source of the metrics is identified by name,
and may be either a host name (type is PM_CONTEXT_HOST), or a comma-separated list of names referring to
a set of archives (type is PM_CONTEXT_ARCHIVE). Each element of the list may either be the base name
common to all of the physical files of an archive or the name of a directory containing archives.

For a type of PM_CONTEXT_HOST, in addition to identifying a host the name may also be used to encode
additional optional information in the form of a pmcd(1) port number, a pmproxy(1) hostname and a proxy
port number. For example the name "app23:14321,4321@firewall.example.com:11111" specifies a connection on
port 14321 (or port 4321 if 14321 is unavailable) to pmcd(1) on the host app23 via port 11111 to
pmproxy(1) on the host firewall.example.com.

For a type of PM_CONTEXT_ARCHIVE, each element of the list of names in name may also be the name of any
of the physical files of an archive, e.g. myarchive.meta (the metadata file) or myarchive.index (the
temporal index) or myarchive.0 (the first data volume of the archive) or myarchive.0.bz2 or
myarchive.0.bz (the first data volume compressed with bzip2(1)) or myarchive.0.gz or myarchive.0.Z or
myarchive.0.z (the first data volume compressed with gzip(1)), myarchive.1 or myarchive.3.bz2 or
myarchive.42.gz etc.

If more than one archive is specified for a type of PM_CONTEXT_ARCHIVE, there are some restrictions on
the archives within the set:
• The archives must all have been generated on the same host.
• The archives must not overlap in time.
• The archives must all have been created using the same time zone.
• The PMID of each metric should be the same in all of the archives. Multiple PMIDs are currently
tolerated by using the first PMID defined for each metric and ignoring subsequent PMIDs.
• The type of each metric must be the same in all of the archives.
• The semantics of each metric must be the same in all of the archives.
• The units of each metric must be the same in all of the archives.
• The instance domain of each metric must be the same in all of the archives.

In the case where type is PM_CONTEXT_LOCAL, name is ignored, and the context uses a stand-alone
connection to the PMDA methods used by pmcd(1). The mechanism in the library uses the same ``plugin''
architecture that operates between pmcd(1) and DSO PMDAs, so operations involve function calls rather
than IPC message passing - for PM_CONTEXT_LOCAL contexts this may mean lower latency for operations like
pmFetch(3), but at the cost of longer initialization time and possible access control differences
compared to pmcd(1). When this type of context is used, the range of accessible performance metrics is
constrained to those from the DSO PMDAs defined in the pmcd(1) configuration file
/etc/pcp/pmcd/pmcd.conf, so those reported by the command
$ awk '$3 == "dso" {print}' /etc/pcp/pmcd/pmcd.conf
or alternatively reported by the command
$ pminfo -f pmcd.agent.type | grep 'value 0'
This usually means the PMDA exporting metrics from the operating system and the ``pmcd'', ``pmproxy'' and
may includes some others like ``mmv''. Alternate DSO PMDAs can be used, refer to pmSpecLocalPMDA(3).

In the case where type is PM_CONTEXT_HOST, additional flags can be added to the type to indicate if the
connection to pmcd(1) should be encrypted (PM_CTXFLAG_SECURE), deferred (PM_CTXFLAG_SHALLOW) and if the
file descriptor used to communicate with pmcd(1), should not be shared across contexts
(PM_CTXFLAG_EXCLUSIVE). Both the PM_CTXFLAG_SHALLOW and PM_CTXFLAG_EXCLUSIVE flags are now deprecated
and ignored.

When type is PM_CONTEXT_ARCHIVE, additional flags can be added to the type for restricted handling of the
archive suited to applications that are aware of the structure of PCP archives, namely
PM_CTXFLAG_NO_FEATURE_CHECK (do not check feature compatibility for archive label records) and
PM_CTXFLAG_METADATA_ONLY (open only the metadata, not the data volume(s) nor the index). Currently these
additional flags are only used by pmlogrewrite(1) and pmlogdump(1).

The initial instance profile is set up to select all instances in all instance domains. In the case of a
set of archives, the initial collection time is also set to zero, so that an initial pmFetch(3) will
result in the earliest set of metrics being returned from the set of archives.

Once established, the association between a context and a source of metrics is fixed for the life of the
context, however routines are provided to independently manipulate both the instance profile (see
pmAddProfile(3) and pmDelProfile(3)) and the collection time for archives (see pmSetMode(3)).

pmNewContext returns a handle that may be used with subsequent calls to pmUseContext(3).

The new context remains the current PMAPI context for all subsequent calls across the PMAPI, until
another call to pmNewContext(3) is made, or the context is explicitly changed with a call to
pmDupContext(3) or pmUseContext(3), or destroyed using pmDestroyContext(3).

When attempting to connect to a remote pmcd(1) on a machine that is booting, pmNewContext could
potentially block for a long time until the remote machine finishes its initialization. pmNewContext
will abort and return an error if the connection has not been established after some specified interval
has elapsed. The default interval is 5 seconds. This may be modified by setting PMCD_CONNECT_TIMEOUT in
the environment to a real number of seconds for the desired timeout. This is most useful in cases where
the remote host is at the end of a slow network, requiring longer latencies to establish the connection
correctly.

CAVEATS

       When using a type of PM_CONTEXT_LOCAL, the operating system PMDA may export data structures directly from
       the  kernel,  which  means  that the pmNewContext caller should be an executable program compiled for the
       same object code format as the booted kernel.

       In addition, applications using a PM_CONTEXT_LOCAL context must be single-threaded  because  the  various
       DSO  PMDAs  may  not  be  thread-safe.   This restriction is enforced at the PMAPI(3), where routines may
       return the error code PM_ERR_THREAD if the library detects calls from more than one thread.

       Applications that use gethostbyname(3) should exercise  caution  because  the  static  fields  in  struct
       hostent   may  not  be  preserved  across  some  PMAPI(3)  calls.   In  particular,  pmNewContext(3)  and
       pmReconnectContext(3) both may call gethostbyname(3) internally.

DIAGNOSTICS

       PM_ERR_PERMISSION

              No permission to perform requested operation

       PM_ERR_CONNLIMIT

              PMCD connection limit for this host exceeded

       PM_ERR_NOCONTEXT

              Requested context type was not PM_CONTEXT_LOCAL, PM_CONTEXT_HOST or PM_CONTEXT_ARCHIVE.

       PM_ERR_LOGOVERLAP

              Archives overlap in time

       PM_ERR_LOGHOST

              Archives differ by host

       PM_ERR_LOGCHANGETYPE

              The type of a metric differs among archives

       PM_ERR_LOGCHANGESEM

              The semantics of a metric differs among archives

       PM_ERR_LOGCHANGEINDOM

              The instance domain of a metric differs among archives

       PM_ERR_LOGCHANGEUNITS

              The units of a metric differs among archives

ENVIRONMENT

       PMCD_CONNECT_TIMEOUT
              Timeout period (in seconds) for pmcd(1) connection attempts.

       PMCD_PORT
              TCP/IP port(s) for connecting to pmcd(1), historically was 4321 and more recently  the  officially
              registered  port 44321; in the current release, pmcd listens on both these ports as a transitional
              arrangement.  If used, should be set to a comma-separated list of numerical port numbers.