diff --git a/doc/sphinx/Pacemaker_Development/components.rst b/doc/sphinx/Pacemaker_Development/components.rst index 30b4191089..bc39197e1e 100644 --- a/doc/sphinx/Pacemaker_Development/components.rst +++ b/doc/sphinx/Pacemaker_Development/components.rst @@ -1,182 +1,366 @@ Coding Particular Pacemaker Components -------------------------------------- The Pacemaker code can be intricate and difficult to follow. This chapter has some high-level descriptions of how individual components work. .. index:: single: fencer single: pacemaker-fenced Fencer ###### ``pacemaker-fenced`` is the Pacemaker daemon that handles fencing requests. In the broadest terms, fencing works like this: #. The initiator (an external program such as ``stonith_admin``, or the cluster itself via the controller) asks the local fencer, "Hey, could you please fence this node?" #. 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?" #. Each fencer in the cluster replies with a list of available devices that it knows about. #. 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. #. The chosen fencer(s) call the appropriate fencing resource agent(s) to do the fencing, then reply to the original fencer with the result. #. The original fencer broadcasts the result to all fencers. #. Each fencer sends the result to each of its local clients (including, at some point, the initiator). A more detailed description follows. .. index:: single: libstonithd Initiating a fencing request ____________________________ A fencing request can be initiated by the cluster or externally, using the libstonithd API. * The cluster always initiates fencing via ``daemons/controld/controld_te_actions.c:te_fence_node()`` (which calls the ``fence()`` API method). This occurs when a transition graph synapse contains a ``CRM_OP_FENCE`` XML operation. * The main external clients are ``stonith_admin`` and ``cts-fence-helper``. The ``DLM`` project also uses Pacemaker for fencing. 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 fencing request go something like this: 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_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 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: 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. + + +.. index:: + single: scheduler + single: pacemaker-schedulerd + single: libpe_status + single: libpe_rules + single: libpacemaker + +Scheduler +######### + +``pacemaker-schedulerd`` is the Pacemaker daemon that runs the Pacemaker +scheduler for the controller, but "the scheduler" in general refers to related +library code in ``libpe_status`` and ``libpe_rules`` (``lib/pengine/*.c``), and +some of ``libpacemaker`` (``lib/pacemaker/pcmk_sched_*.c``). + +The purpose of the scheduler is to take a CIB as input and generate a +transition graph (list of actions that need to be taken) as output. + +The controller invokes the scheduler by contacting the scheduler daemon via +local IPC. Tools such as ``crm_simulate``, ``crm_mon``, and ``crm_resource`` +can also invoke the scheduler, but do so by calling the library functions +directly. This allows them to run using a ``CIB_file`` without the cluster +needing to be active. + +The main entry point for the scheduler code is +``lib/pacemaker/pcmk_sched_messages.c:pcmk__schedule_actions()``. It sets +defaults and calls a bunch of "stage *N*" functions. Yes, there is a stage 0 +and no stage 1. :) The code has evolved over time to where splitting the stages +up differently and renumbering them would make sense. + +* ``stage0()`` "unpacks" most of the CIB XML into data structures, and + determines the current cluster status. It also creates implicit location + constraints for the node health feature. +* ``stage2()`` applies factors that make resources prefer certain nodes (such + as shutdown locks, location constraints, and stickiness). +* ``stage3()`` creates internal constraints (such as the implicit ordering for + group members, or start actions being implicitly ordered before promote + actions). +* ``stage4()`` "checks actions", which means processing resource history + entries in the CIB status section. This is used to decide whether certain + actions need to be done, such as deleting orphan resources, forcing a restart + when a resource definition changes, etc. +* ``stage5()`` allocates resources to nodes and creates actions (which might or + might not end up in the final graph). +* ``stage6()`` creates implicit ordering constraints for resources running + across remote connections, and schedules fencing actions and shutdowns. +* ``stage7()`` "updates actions", which means applying ordering constraints in + order to modify action attributes such as optional or required. +* ``stage8()`` creates the transition graph. + +Challenges +__________ + +Working with the scheduler is difficult. Challenges include: + +* It is far too much code to keep more than a small portion in your head at one + time. +* Small changes can have large (and unexpected) effects. This is why we have a + large number of regression tests (``cts/cts-scheduler``), which should be run + after making code changes. +* It produces an insane amount of log messages at debug and trace levels. + You can put resource ID(s) in the ``PCMK_trace_tags`` environment variable to + enable trace-level messages only when related to specific resources. +* Different parts of the main ``pe_working_set_t`` structure are finalized at + different points in the scheduling process, so you have to keep in mind + whether information you're using at one point of the code can possibly change + later. For example, data unpacked from the CIB can safely be used anytime + after stage0(), but actions may become optional or required anytime before + stage8(). There's no easy way to deal with this. +* Many names of struct members, functions, etc., are suboptimal, but are part + of the public API and cannot be changed until an API backward compatibility + break. + + +.. index:: + single: pe_working_set_t + +Cluster Working Set +___________________ + +The main data object for the scheduler is ``pe_working_set_t``, which contains +all information needed about nodes, resources, constraints, etc., both as the +raw CIB XML and parsed into more usable data structures, plus the resulting +transition graph XML. The variable name is usually ``data_set``. + +.. index:: + single: pe_resource_t + +Resources +_________ + +``pe_resource_t`` is the data object representing cluster resources. A resource +has a variant: primitive (a.k.a. native), group, clone, or bundle. + +The resource object has members for two sets of methods, +``resource_object_functions_t`` from the ``libpe_status`` public API, and +``resource_alloc_functions_t`` whose implementation is internal to +``libpacemaker``. The actual functions vary by variant. + +The object functions have basic capabilities such as unpacking the resource +XML, and determining the current or planned location of the resource. + +The allocation functions have more obscure capabilities needed for scheduling, +such as processing location and ordering constraints. For example, +``stage3()``, which creates internal constraints, simply calls the +``internal_constraints()`` method for each top-level resource in the working +set. + +.. index:: + single: pe_node_t + +Nodes +_____ + +Allocation of resources to nodes is done by choosing the node with the highest +score for a given resource. The scheduler does a bunch of processing to +generate the scores, then the actual allocation is straightforward. + +Node lists are frequently used. For example, ``pe_working_set_t`` has a +``nodes`` member which is a list of all nodes in the cluster, and +``pe_resource_t`` has a ``running_on`` member which is a list of all nodes on +which the resource is (or might be) active. These are lists of ``pe_node_t`` +objects. + +The ``pe_node_t`` object contains a ``struct pe_node_shared_s *details`` member +with all node information that is independent of resource allocation (the node +name, etc.). + +The working set's ``nodes`` member contains the original of this information. +All other node lists contain copies of ``pe_node_t`` where only the ``details`` +member points to the originals in the working set's ``nodes`` list. In this +way, the other members of ``pe_node_t`` (such as ``weight``, which is the node +score) may vary by node list, while the common details are shared. + +.. index:: + single: pe_action_t + single: pe_action_flags + +Actions +_______ + +``pe_action_t`` is the data object representing actions that might need to be +taken. These could be resource actions, cluster-wide actions such as fencing a +node, or "pseudo-actions" which are abstractions used as convenient points for +ordering other actions against. + +It has a ``flags`` member which is a bitmask of ``enum pe_action_flags``. The +most important of these are ``pe_action_runnable`` (if not set, the action is +"blocked" and cannot be added to the transition graph) and +``pe_action_optional`` (actions with this set will not be added to the +transition graph; actions often start out as optional, and may become required +later). + + +.. index:: + single: pe__ordering_t + single: pe_ordering + +Orderings +_________ + +Ordering constraints are simple in concept, but they are one of the most +important, powerful, and difficult to follow aspects of the scheduler code. + +``pe__ordering_t`` is the data object representing an ordering, better thought +of as a relationship between two actions, since the relation can be more +complex than just "this one runs after that one". + +For an ordering "A then B", the code generally refers to A as "first" or +"before", and B as "then" or "after". + +Much of the power comes from ``enum pe_ordering``, which are flags that +determine how an ordering behaves. There are many obscure flags with big +effects. A few examples: + +* ``pe_order_none`` means the ordering is disabled and will be ignored. It's 0, + meaning no flags set, so it must be compared with equality rather than + ``pcmk_is_set()``. +* ``pe_order_optional`` means the ordering does not make either action + required, so it only applies if they both become required for other reasons. +* ``pe_order_implies_first`` means that if action B becomes required for any + reason, then action A will become required as well.