Ubuntu Manpage: pmReconnectContext - reconnect to a PMAPI context

NAME

       pmReconnectContext - reconnect to a PMAPI context

C SYNOPSIS

       #include <pcp/pmapi.h>

       int pmReconnectContext(int handle);

       cc ... -lpcp

DESCRIPTION

As a consequence of network, host or Performance Metrics Collector Daemon (PMCD) failures, an
application's connection to a PMCD may be established and then subsequently lost.

The routine pmReconnectContext allows an application to request that the context identified by handle
should be re-established, provided the associated metrics source is accessible.

When the source of metrics associated with the context handle is pmcd(1), then to avoid flooding the
system with reconnect requests, pmReconnectContext will only attempt a reconnection after a suitable
delay from the previous unsuccessful attempt to reconnect this context. This imposed restriction on the
reconnect re-try time interval uses an exponential back-off so that the initial delay is 5 seconds after
the first unsuccessful attempt, then 10 seconds, then 20 seconds, then 40 seconds and then 80 seconds
thereafter.

The environment variable PMCD_RECONNECT_TIMEOUT may be used to redefine the back-off intervals, see
PMAPI(3).

Calling pmReconnectContext with a handle identifying a currently connected pmcd(1) context will cause the
connection to be broken before any reconnection is attempted.

If handle identifies a context associated with an archive source of metrics, pmReconnectContext returns
without delay.

If the reconnection succeeds, pmReconnectContext returns handle.

As a side-effect of reconnecting, any derived metrics that have previously been defined using
pmRegisterDerived(3), pmRegisterDerivedMetric(3) or pmLoadDerivedConfig(3) will be re-processed and re-
bound to the available metrics from the reconnected source. The support of dynamic definition for
derived metrics provides one use case where pmReconnectContext may be called even if the connection to
the metrics source has not been lost.

Note that even in the case of a successful reconnection, pmReconnectContext does not change the current
Performance Metrics Application Programming Interface (PMAPI) context, so handle remains valid.

When attempting to connect to a remote pmcd(1) on a machine that is booting, pmReconnectContext could
potentially block for a long time until the remote machine finishes its initialization.
pmReconnectContext 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.

CAVEAT

       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_NOCONTEXT

              handle does not identify a valid PMAPI context

       -ETIMEDOUT

              The re-try time has not elapsed, or the reconnection is attempted and fails.

ENVIRONMENT