Missed Events

In a system with a high level of event activity, a client mav subscribe for many events. In such a scenario, it may not be possible for the client to receive all the posted events. This happens if the client takes a long time to process the events — for example, the client that has subscribed needs to write each event to disk. An example of such a client is the EVM logger, which recognizes when it has missed events and reports the failure by logging a special “missed events” event. If there is a danger that your application might miss events, you must ensure that the system is configured to minimize the risk, and write your programs to handle appropriately.

When the EVM daemon sends an event to a subscribing client, it does so with a connection that depends upon fixed-size communication (send and receive) buffers. If the client does not handle the incoming events within a reasonable time, the buffers may be filled because of the heavy event load. As a result, the EVM daemon cannot send further events. The default receive buffer size is set to a constant value equal to 1024*128.

As the EVM daemon is a critical resource for many system components and applications, it cannot block the events while waiting for a client to clear its buffer. Consequently, if it fails to write to a connection because the buffer is full, the daemon marks the connection as blocked and continues with its other activities. When the client eventually reads its input and frees space in the buffer, the EVM daemon completes the failed write and the client will receive the event. However, if in the meantime, any other events arrive that should be sent to the blocked subscriber, the daemon does not attempt to send them; instead, it counts how many have been missed and reports the number to the subscriber when the connection becomes unblocked. The subscriber must take an appropriate action, but there is no way for it to know which events were missed.

Your application can miss incoming events under extreme circumstances. The likelihood of this happening depends on the following factors.

To minimize the risk of missing incoming events, design your application to handle the incoming events as quickly and efficiently as possible. The application must also take appropriate action when it is notified that it has missed events. For a sample program that deals with missed events, see “Dealing with Missed Events”.