PMNEWCONTEXT(3)

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.

Alternatively, for a type of PM_CONTEXT_HOST, name may be unix: for an authenticated Unix domain socket connection to pmcd(1) on the localhost or local: for an authenticated connection to pmcd(1) on the localhost via a Unix domain socket if available, else via an internet socket connection to localhost. local: is the default choice for most applications when calling pmNewContext to establish a context for pmcd(1) on the local host.

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.

SEE ALSO
pmcd(1), pminfo(1), pmproxy(1), PMAPI(3), pmAddProfile(3), pmDelProfile(3), pmDestroyContext(3), pmDupContext(3), pmFetch(3), pmGetConfig(3), pmReconnectContext(3), pmSetMode(3), pmSpecLocalPMDA(3), pmUseContext(3), pmWhichContext(3), pcp.conf(5) and pcp.env(5).