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Now that the event handler API is pretty mature, we can expose it in the lua API. Introducing the core.event_sub(<event_types>, <cb>) lua function that takes an array of event types <event_types> as well as a callback function <cb> as argument. The function returns a subscription <sub> on success. Subscription <sub> allows you to manage the subscription from anywhere in the script. To this day only the sub->unsub method is implemented. The following event types are currently supported: - "SERVER_ADD": when a server is added - "SERVER_DEL": when a server is removed from haproxy - "SERVER_DOWN": server states goes from up to down - "SERVER_UP": server states goes from down to up As for the <cb> function: it will be called when one of the registered event types occur. The function will be called with 3 arguments: cb(<event>,<data>,<sub>) <event>: event type (string) that triggered the function. (could be any of the types used in <event_types> when registering the subscription) <data>: data associated with the event (specific to each event family). For "SERVER_" family events, server details such as server name/id/proxy will be provided. If the server still exists (not yet deleted), a reference to the live server is provided to spare you from an additionnal lookup if you need to have direct access to the server from lua. <sub> refers to the subscription. In case you need to manage it from within an event handler. (It refers to the same subscription that the one returned from core.event_sub()) Subscriptions are per-thread: the thread that will be handling the event is the one who performed the subscription using core.event_sub() function. Each thread treats events sequentially, it means that if you have, let's say SERVER_UP, then SERVER_DOWN in a short timelapse, then your cb function will first be called with SERVER_UP, and once you're done handling the event, your function will be called again with SERVER_DOWN. This is to ensure event consitency when it comes to logging / triggering logic from lua. Your lua cb function may yield if needed, but you're pleased to process the event as fast as possible to prevent the event queue from growing up To prevent abuses, if the event queue for the current subscription goes over 100 unconsumed events, the subscription will pause itself automatically for as long as it takes for your handler to catch up. This would lead to events being missed, so a warning will be emitted in the logs to inform you about that. This is not something you want to let happen too often, it may indicate that you subscribed to an event that is occurring too frequently or/and that your callback function is too slow to keep up the pace and you should review it. If you want to do some parallel processing because your callback functions are slow: you might want to create subtasks from lua using core.register_task() from within your callback function to perform the heavy job in a dedicated task and allow remaining events to be processed more quickly. Please check the lua documentation for more information. |
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The HAProxy documentation has been split into a number of different files for ease of use. Please refer to the following files depending on what you're looking for : - INSTALL for instructions on how to build and install HAProxy - BRANCHES to understand the project's life cycle and what version to use - LICENSE for the project's license - CONTRIBUTING for the process to follow to submit contributions The more detailed documentation is located into the doc/ directory : - doc/intro.txt for a quick introduction on HAProxy - doc/configuration.txt for the configuration's reference manual - doc/lua.txt for the Lua's reference manual - doc/SPOE.txt for how to use the SPOE engine - doc/network-namespaces.txt for how to use network namespaces under Linux - doc/management.txt for the management guide - doc/regression-testing.txt for how to use the regression testing suite - doc/peers.txt for the peers protocol reference - doc/coding-style.txt for how to adopt HAProxy's coding style - doc/internals for developer-specific documentation (not all up to date)