diff --git a/fencing/README.md b/fencing/README.md index 1f11542222..b5fd4b2bba 100644 --- a/fencing/README.md +++ b/fencing/README.md @@ -1,144 +1,143 @@ # Directory contents * `admin.c`, `stonith_admin.8`: `stonith_admin` command-line tool and its man page * `commands.c`, `internal.h`, `main.c`, `remote.c`, `stonithd.7`: stonithd and its man page * `fence_dummy`, `fence_legacy`, `fence_legacy.8`: Pacemaker-supplied fence agents and their man pages -* `regression.py(.in)`: regression tests for `stonithd` * `test.c`: `stonith-test` command-line tool # 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 `crmd`) asks the local `stonithd`, "Hey, can you fence this node?" 1. The local `stonithd` asks all the `stonithd's` in the cluster (including itself), "Hey, what fencing devices do you have access to that can fence this node?" 1. Each `stonithd` in the cluster replies with a list of available devices that it knows about. 1. Once the original `stonithd` gets all the replies, it asks the most appropriate `stonithd` 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. 1. The chosen `stonithd(s)` call the appropriate fencing resource agent(s) to do the fencing, then replies to the original `stonithd` with the result. 1. The original `stonithd` broadcasts the result to all `stonithd's`. 1. Each `stonithd` 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. * The cluster always initiates fencing via `crmd/te_actions.c:te_fence_node()` (which calls the `fence()` API). This occurs when a graph synapse contains a `CRM_OP_FENCE` XML operation. * The main external clients are `stonith_admin` and `stonith-test`. 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 `stonithd` 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 `stonithd` 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 `stonithd` receives the original `stonithd's STONITH_OP_QUERY` broadcast request via IPC or messaging layer callback, which calls: * `stonith_command()`, which (for requests) calls * `handle_request()`, which (for `STONITH_OP_QUERY` from a peer) calls * `stonith_query()`, which calls * `get_capable_devices()` with `stonith_query_capable_device_db()` to add device information to an XML reply and send it. (A message is considered a reply if it contains `T_STONITH_REPLY`, which is only set by `stonithd` peers, not clients.) The original `stonithd` 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 `stonithd` 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 `stonithd'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 stonithd'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 `stonithd` (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 `stonithd` 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.