EvmConnection(5)EvmConnection(5)NAME
EvmConnection - connection to the EVM (Event Management) daemon
DESCRIPTION
An EVM connection is the context through which data is passed to and
from the EVM daemon. A connection has the following attributes:
· Connection Type
· Response Mode
· Transport Type
· Callback, and associated Callback Argument
· Connection Context
These attributes are described in the following sections.
An EVM client is any program that processes events through the EVM dae‐
mon. EVM supports three distinct types of client: posting clients,
subscribing (listening) clients, and service clients. Regardless of
type, all clients connect to the EVM daemon in the same way.
Connection Type
The type of connection established determines the client type. There
are associated constants to be used when the connection is created.
The client uses this connection to post events to the daemon for dis‐
tribution.
The client uses this connection to listen for events distributed by the
daemon.
The client uses this connection to request that the daemon provide a
service, such as retrieve events from a log.
A client may use all three types of connection, but must
establish each connection separately.
Response Mode
The response mode associated with a connection determines the manner in
which certain API functions will deal with the daemon's responses to
request messages. See the EvmConnCreate(3) reference page for full
details of each mode. The modes are:
The API functions will return as soon as the request has been sent to
the daemon, and the caller will not receive the daemon's
response.
The API functions will send the request to the daemon, and wait until a
response has been received before returning to the caller.
The returned status code will reflect the response.
The API functions will return as soon as the request has been sent to
the daemon, and the caller must monitor the connection for a
response. When the response is received, the connection's
callback function will be invoked to handle it.
Transport Type
The transport type specifies the type of connection to be made to the
daemon. The only valid connection that can be made is:
Connection is through a domain socket to a daemon running on the local
host.
Callback
This attribute specifies the function you want to handle any incoming
responses resulting from activity on the connection. Callbacks are dis‐
cussed in more detail in the EvmCallback(5) reference page. This
attribute is valid only when the Response Mode is
Connection Context
This attribute is a handle to a connection returned when a connection
is created. You must pass this handle to all other functions you call
which need to access the connection.
Connection Monitoring
Once you have established a connection, the API functions will take
care of all communication activity with the daemon. However, you must
make sure that these functions get an opportunity to do their work when
activity occurs. EVM provides several ways for you to do this. The
method you choose depends on the model of your program.
· If your program is I/O driven, waiting for I/O to occur on one or
more file descriptors, handling activity as it occurs and then
returning to wait for more I/O, it probably spends most of its time
in a call. In this case, you should use to establish which file
descriptor the EVM connection is using, and then include that file
descriptor in your read mask. When you detect activity on the con‐
nection, invoke to handle the activity.
· If your program is driven purely by activity on a single EVM connec‐
tion, you can let the API handle the I/O entirely by using to wait
for activity to occur on the connection. When the function returns,
dispatch the I/O using and then return to
· If your program is driven in some other way, and there are certain
points (for example, at the end of some control loop) at which you
want to handle EVM activity, you can call to check for any outstand‐
ing activity. If this function indicates that there is something to
do, you can call otherwise you can continue immediately with normal
processing.
Destroying a Connection
When you are finished with the connection, use to disconnect from the
daemon. It is important to check the return status for any failure
each time you call a connection function and destroy the connection if
a failure occurred. The file descriptor associated with the connection
remains open until you destroy the connection even if I/O errors have
been detected on the connection.
SEE ALSO
Functions
select(2).
Routines
EvmConnControl(3), EvmConnCreate(3), EvmConnFdGet(3), EvmConnCheck(3),
EvmConnWait(3), EvmConnDispatch(3), EvmConnDestroy(3).
Event Management
EVM(5).
Event Callback
EvmCallback(5).
EVM Events
EvmEvent(5).
EvmConnection(5)
