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How fencing requests are handled

Bird’s eye view

In the broadest terms, stonith works like this:

  1. The initiator (an external program such as stonith_admin, or the cluster itself via the controller) asks the local fencer, “Hey, can you fence this node?”
  2. The local fencer asks all the fencers in the cluster (including itself), “Hey, what fencing devices do you have access to that can fence this node?”
  3. Each fencer in the cluster replies with a list of available devices that it knows about.
  4. Once the original fencer gets all the replies, it asks the most appropriate fencer peer to actually carry out the fencing. It may send out more than one such request if the target node must be fenced with multiple devices.
  5. The chosen fencer(s) call the appropriate fencing resource agent(s) to do the fencing, then replies to the original fencer with the result.
  6. The original fencer broadcasts the result to all fencers.
  7. Each fencer sends the result to each of its local clients (including, at some point, the initiator).

Detailed view

Initiating a fencing request

A fencing request can be initiated by the cluster or externally, using the libfencing API.

Highlights of the fencing API: * stonith_api_new() creates and returns a new stonith_t object, whose cmds member has methods for connect, disconnect, fence, etc. * the fence() method creates and sends a STONITH_OP_FENCE XML request with the desired action and target node. Callers do not have to choose or even have any knowledge about particular fencing devices.

Fencing queries

The function calls for a stonith request go something like this as of this writing:

The local fencer receives the client’s request via an IPC or messaging layer callback, which calls * stonith_command(), which (for requests) calls * handle_request(), which (for STONITH_OP_FENCE from a client) calls * initiate_remote_stonith_op(), which creates a STONITH_OP_QUERY XML request with the target, desired action, timeout, etc.. then broadcasts the operation to the cluster group (i.e. all fencer instances) and starts a timer. The query is broadcast because (1) location constraints might prevent the local node from accessing the stonith device directly, and (2) even if the local node does have direct access, another node might be preferred to carry out the fencing.

Each fencer receives the original fencer’s STONITH_OP_QUERYbroadcast request via IPC or messaging layer callback, which calls: *stonith_command(), which (for requests) calls *handle_request(), which (forSTONITH_OP_QUERYfrom a peer) calls *stonith_query(), which calls *get_capable_devices()withstonith_query_capable_device_db()to add device information to an XML reply and send it. (A message is considered a reply if it containsT_STONITH_REPLY`, which is only set by fencer peers, not clients.)

The original fencer receives all peers’ STONITH_OP_QUERY replies via IPC or messaging layer callback, which calls: * stonith_command(), which (for replies) calls * handle_reply() which (for STONITH_OP_QUERY) calls * process_remote_stonith_query(), which allocates a new query result structure, parses device information into it, and adds it to operation object. It increments the number of replies received for this operation, and compares it against the expected number of replies (i.e. the number of active peers), and if this is the last expected reply, calls * call_remote_stonith(), which calculates the timeout and sends STONITH_OP_FENCE request(s) to carry out the fencing. If the target node has a fencing “topology” (which allows specifications such as “this node can be fenced either with device A, or devices B and C in combination”), it will choose the device(s), and send out as many requests as needed. If it chooses a device, it will choose the peer; a peer is preferred if it has “verified” access to the desired device, meaning that it has the device “running” on it and thus has a monitor operation ensuring reachability.

Fencing operations

Each STONITH_OP_FENCE request goes something like this as of this writing:

The chosen peer fencer receives the STONITH_OP_FENCE request via IPC or messaging layer callback, which calls: * stonith_command(), which (for requests) calls * handle_request(), which (for STONITH_OP_FENCE from a peer) calls * stonith_fence(), which calls * schedule_stonith_command() (using supplied device if F_STONITH_DEVICE was set, otherwise the highest-priority capable device obtained via get_capable_devices() with stonith_fence_get_devices_cb()), which adds the operation to the device’s pending operations list and triggers processing.

The chosen peer fencer’s mainloop is triggered and calls * stonith_device_dispatch(), which calls * stonith_device_execute(), which pops off the next item from the device’s pending operations list. If acting as the (internally implemented) watchdog agent, it panics the node, otherwise it calls * stonith_action_create() and stonith_action_execute_async() to call the fencing agent.

The chosen peer fencer’s mainloop is triggered again once the fencing agent returns, and calls * stonith_action_async_done() which adds the results to an action object then calls its * done callback (st_child_done()), which calls schedule_stonith_command() for a new device if there are further required actions to execute or if the original action failed, then builds and sends an XML reply to the original fencer (via stonith_send_async_reply()), then checks whether any pending actions are the same as the one just executed and merges them if so.

Fencing replies

The original fencer receives the STONITH_OP_FENCE reply via IPC or messaging layer callback, which calls: * stonith_command(), which (for replies) calls * handle_reply(), which calls * process_remote_stonith_exec(), which calls either call_remote_stonith() (to retry a failed operation, or try the next device in a topology is appropriate, which issues a new STONITH_OP_FENCE request, proceeding as before) or remote_op_done() (if the operation is definitively failed or successful). * remote_op_done() broadcasts the result to all peers.

Finally, all peers receive the broadcast result and call * remote_op_done(), which sends the result to all local clients.