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| -rw-r--r-- | src/mon/Elector.h | 416 |
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diff --git a/src/mon/Elector.h b/src/mon/Elector.h new file mode 100644 index 00000000..9ba52217 --- /dev/null +++ b/src/mon/Elector.h @@ -0,0 +1,416 @@ +// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*- +// vim: ts=8 sw=2 smarttab +/* + * Ceph - scalable distributed file system + * + * Copyright (C) 2004-2006 Sage Weil <sage@newdream.net> + * + * This is free software; you can redistribute it and/or + * modify it under the terms of the GNU Lesser General Public + * License version 2.1, as published by the Free Software + * Foundation. See file COPYING. + * + */ + + +#ifndef CEPH_MON_ELECTOR_H +#define CEPH_MON_ELECTOR_H + +#include <map> + +#include "include/types.h" +#include "include/Context.h" +#include "mon/MonOpRequest.h" +#include "mon/mon_types.h" + +class Monitor; + +/** + * This class is responsible for maintaining the local state when electing + * a new Leader. We may win or we may lose. If we win, it means we became the + * Leader; if we lose, it means we are a Peon. + */ +class Elector { + /** + * @defgroup Elector_h_class Elector + * @{ + */ + private: + /** + * @defgroup Elector_h_internal_types Internal Types + * @{ + */ + /** + * This struct will hold the features from a given peer. + * Features may both be the cluster's (in the form of a uint64_t), or + * mon-specific features. Instead of keeping maps to hold them both, or + * a pair, which would be weird, a struct to keep them seems appropriate. + */ + struct elector_info_t { + uint64_t cluster_features = 0; + mon_feature_t mon_features; + int mon_release = 0; + map<string,string> metadata; + }; + + /** + * @} + */ + + /** + * The Monitor instance associated with this class. + */ + Monitor *mon; + + /** + * Event callback responsible for dealing with an expired election once a + * timer runs out and fires up. + */ + Context *expire_event = nullptr; + + /** + * Resets the expire_event timer, by cancelling any existing one and + * scheduling a new one. + * + * @remarks This function assumes as a default firing value the duration of + * the monitor's lease interval, and adds to it the value specified + * in @e plus + * + * @post expire_event is set + * + * @param plus The amount of time to be added to the default firing value. + */ + void reset_timer(double plus=0.0); + /** + * Cancel the expire_event timer, if it is defined. + * + * @post expire_event is not set + */ + void cancel_timer(); + + /** + * Latest epoch we've seen. + * + * @remarks if its value is odd, we're electing; if it's even, then we're + * stable. + */ + epoch_t epoch; + + /** + * Indicates if we are participating in the quorum. + * + * @remarks By default, we are created as participating. We may stop + * participating if the Monitor explicitly calls + * Elector::stop_participating though. If that happens, it will + * have to call Elector::start_participating for us to resume + * participating in the quorum. + */ + bool participating; + + // electing me + /** + * @defgroup Elector_h_electing_me_vars We are being elected + * @{ + */ + /** + * Indicates if we are the ones being elected. + * + * We always attempt to be the one being elected if we are the ones starting + * the election. If we are not the ones that started it, we will only attempt + * to be elected if we think we might have a chance (i.e., the other guy's + * rank is lower than ours). + */ + bool electing_me; + /** + * Set containing all those that acked our proposal to become the Leader. + * + * If we are acked by everyone in the MonMap, we will declare + * victory. Also note each peer's feature set. + */ + map<int, elector_info_t> acked_me; + /** + * @} + */ + /** + * @defgroup Elector_h_electing_them_vars We are electing another guy + * @{ + */ + /** + * Indicates who we have acked + */ + int leader_acked; + /** + * @} + */ + + /** + * Update our epoch. + * + * If we come across a higher epoch, we simply update ours, also making + * sure we are no longer being elected (even though we could have been, + * we no longer are since we no longer are on that old epoch). + * + * @pre Our epoch is lower than @p e + * @post Our epoch equals @p e + * + * @param e Epoch to which we will update our epoch + */ + void bump_epoch(epoch_t e); + + /** + * Start new elections by proposing ourselves as the new Leader. + * + * Basically, send propose messages to all the monitors in the MonMap and + * then reset the expire_event timer so we can limit the amount of time we + * will be going at it. + * + * @pre participating is true + * @post epoch is an odd value + * @post electing_me is true + * @post we sent propose messages to all the monitors in the MonMap + * @post we reset the expire_event timer + */ + void start(); + /** + * Defer the current election to some other monitor. + * + * This means that we will ack some other monitor and drop out from the run + * to become the Leader. We will only defer an election if the monitor we + * are deferring to outranks us. + * + * @pre @p who outranks us (i.e., who < our rank) + * @pre @p who outranks any other monitor we have deferred to in the past + * @post electing_me is false + * @post leader_acked equals @p who + * @post we sent an ack message to @p who + * @post we reset the expire_event timer + * + * @param who Some other monitor's numeric identifier. + */ + void defer(int who); + /** + * The election has taken too long and has expired. + * + * This will happen when no one declared victory or started a new election + * during the time span allowed by the expire_event timer. + * + * When the election expires, we will check if we were the ones who won, and + * if so we will declare victory. If that is not the case, then we assume + * that the one we deferred to didn't declare victory quickly enough (in fact, + * as far as we know, we may even be dead); so, just propose ourselves as the + * Leader. + */ + void expire(); + /** + * Declare Victory. + * + * We won. Or at least we believe we won, but for all intentions and purposes + * that does not matter. What matters is that we Won. + * + * That said, we must now bump our epoch to reflect that the election is over + * and then we must let everybody in the quorum know we are their brand new + * Leader. And we will also cancel our expire_event timer. + * + * Actually, the quorum will be now defined as the group of monitors that + * acked us during the election process. + * + * @pre Election is on-going + * @pre electing_me is true + * @post electing_me is false + * @post epoch is bumped up into an even value + * @post Election is not on-going + * @post We have a quorum, composed of the monitors that acked us + * @post We sent a message of type OP_VICTORY to each quorum member. + */ + void victory(); + + /** + * Handle a message from some other node proposing itself to become it + * the Leader. + * + * If the message appears to be old (i.e., its epoch is lower than our epoch), + * then we may take one of two actions: + * + * @li Ignore it because it's nothing more than an old proposal + * @li Start new elections if we verify that it was sent by a monitor from + * outside the quorum; given its old state, it's fair to assume it just + * started, so we should start new elections so it may rejoin + * + * If we did not ignore the received message, then we know that this message + * was sent by some other node proposing itself to become the Leader. So, we + * will take one of the following actions: + * + * @li Ignore it because we already acked another node with higher rank + * @li Ignore it and start a new election because we outrank it + * @li Defer to it because it outranks us and the node we previously + * acked, if any + * + * + * @invariant The received message is an operation of type OP_PROPOSE + * + * @param m A message sent by another participant in the quorum. + */ + void handle_propose(MonOpRequestRef op); + /** + * Handle a message from some other participant Acking us as the Leader. + * + * When we receive such a message, one of three thing may be happening: + * @li We received a message with a newer epoch, which means we must have + * somehow lost track of what was going on (maybe we rebooted), thus we + * will start a new election + * @li We consider ourselves in the run for the Leader (i.e., @p electing_me + * is true), and we are actually being Acked by someone; thus simply add + * the one acking us to the @p acked_me set. If we do now have acks from + * all the participants, then we can declare victory + * @li We already deferred the election to somebody else, so we will just + * ignore this message + * + * @pre Election is on-going + * @post Election is on-going if we deferred to somebody else + * @post Election is on-going if we are still waiting for further Acks + * @post Election is not on-going if we are victorious + * @post Election is not on-going if we must start a new one + * + * @param m A message with an operation type of OP_ACK + */ + void handle_ack(MonOpRequestRef op); + /** + * Handle a message from some other participant declaring Victory. + * + * We just got a message from someone declaring themselves Victorious, thus + * the new Leader. + * + * However, if the message's epoch happens to be different from our epoch+1, + * then it means we lost track of something and we must start a new election. + * + * If that is not the case, then we will simply update our epoch to the one + * in the message, cancel our @p expire_event timer and inform our Monitor + * that we lost the election and provide it with the new quorum. + * + * @pre Election in on-going + * @post Election is not on-going + * @post Updated @p epoch + * @post We have a new quorum if we lost the election + * + * @param m A message with an operation type of OP_VICTORY + */ + void handle_victory(MonOpRequestRef op); + /** + * Send a nak to a peer who's out of date, containing information about why. + * + * If we get a message from a peer who can't support the required quorum + * features, we have to ignore them. This function will at least send + * them a message about *why* they're being ignored -- if they're new + * enough to support such a message. + * + * @param m A message from a monitor not supporting required features. We + * take ownership of the reference. + */ + void nak_old_peer(MonOpRequestRef op); + /** + * Handle a message from some other participant declaring + * we cannot join the quorum. + * + * Apparently the quorum requires some feature that we do not implement. Shut + * down gracefully. + * + * @pre Election is on-going. + * @post We've shut down. + * + * @param m A message with an operation type of OP_NAK + */ + void handle_nak(MonOpRequestRef op); + + public: + /** + * Create an Elector class + * + * @param m A Monitor instance + */ + explicit Elector(Monitor *m) : mon(m), + epoch(0), + participating(true), + electing_me(false), + leader_acked(-1) { } + + /** + * Initiate the Elector class. + * + * Basically, we will simply read whatever epoch value we have in our stable + * storage, or consider it to be 1 if none is read. + * + * @post @p epoch is set to 1 or higher. + */ + void init(); + /** + * Inform this class it is supposed to shutdown. + * + * We will simply cancel the @p expire_event if any exists. + * + * @post @p expire_event is cancelled + */ + void shutdown(); + + /** + * Obtain our epoch + * + * @returns Our current epoch number + */ + epoch_t get_epoch() { return epoch; } + + /** + * advance_epoch + * + * increase election epoch by 1 + */ + void advance_epoch() { + bump_epoch(epoch + 1); + } + + /** + * Handle received messages. + * + * We will ignore all messages that are not of type @p MSG_MON_ELECTION + * (i.e., messages whose interface is not of type @p MMonElection). All of + * those that are will then be dispatched to their operation-specific + * functions. + * + * @param m A received message + */ + void dispatch(MonOpRequestRef op); + + /** + * Call an election. + * + * This function simply calls Elector::start. + */ + void call_election() { + start(); + } + + /** + * Stop participating in subsequent Elections. + * + * @post @p participating is false + */ + void stop_participating() { participating = false; } + /** + * Start participating in Elections. + * + * If we are already participating (i.e., @p participating is true), then + * calling this function is moot. + * + * However, if we are not participating (i.e., @p participating is false), + * then we will start participating by setting @p participating to true and + * we will call for an Election. + * + * @post @p participating is true + */ + void start_participating(); + + /** + * @} + */ +}; + +#endif |