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ceph/src/mon/Paxos.h
Adam C. Emerson 750ad8340c common: Unskew clock 
In preparation to deglobalizing CephContext, remove the CephContext*
parameter to ceph_clock_now() and ceph::real_clock::now() that carries
a configurable offset.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>
2016-12-22 13:55:37 -05:00

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// -*- 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.
*
*/
/*
time---->
cccccccccccccccccca????????????????????????????????????????
cccccccccccccccccca????????????????????????????????????????
cccccccccccccccccca???????????????????????????????????????? leader
cccccccccccccccccc?????????????????????????????????????????
ccccc??????????????????????????????????????????????????????
last_committed
pn_from
pn
a 12v
b 12v
c 14v
d
e 12v
*/
/**
* Paxos storage layout and behavior
*
* Currently, we use a key/value store to hold all the Paxos-related data, but
* it can logically be depicted as this:
*
* paxos:
* first_committed -> 1
* last_committed -> 4
* 1 -> value_1
* 2 -> value_2
* 3 -> value_3
* 4 -> value_4
*
* Since we are relying on a k/v store supporting atomic transactions, we can
* guarantee that if 'last_committed' has a value of '4', then we have up to
* version 4 on the store, and no more than that; the same applies to
* 'first_committed', which holding '1' will strictly meaning that our lowest
* version is 1.
*
* Each version's value (value_1, value_2, ..., value_n) is a blob of data,
* incomprehensible to the Paxos. These values are proposed to the Paxos on
* propose_new_value() and each one is a transaction encoded in a bufferlist.
*
* The Paxos will write the value to disk, associating it with its version,
* but will take a step further: the value shall be decoded, and the operations
* on that transaction shall be applied during the same transaction that will
* write the value's encoded bufferlist to disk. This behavior ensures that
* whatever is being proposed will only be available on the store when it is
* applied by Paxos, which will then be aware of such new values, guaranteeing
* the store state is always consistent without requiring shady workarounds.
*
* So, let's say that FooMonitor proposes the following transaction, neatly
* encoded on a bufferlist of course:
*
* Tx_Foo
* put(foo, last_committed, 3)
* put(foo, 3, foo_value_3)
* erase(foo, 2)
* erase(foo, 1)
* put(foo, first_committed, 3)
*
* And knowing that the Paxos is proposed Tx_Foo as a bufferlist, once it is
* ready to commit, and assuming we are now committing version 5 of the Paxos,
* we will do something along the lines of:
*
* Tx proposed_tx;
* proposed_tx.decode(Tx_foo_bufferlist);
*
* Tx our_tx;
* our_tx.put(paxos, last_committed, 5);
* our_tx.put(paxos, 5, Tx_foo_bufferlist);
* our_tx.append(proposed_tx);
*
* store_apply(our_tx);
*
* And the store should look like this after we apply 'our_tx':
*
* paxos:
* first_committed -> 1
* last_committed -> 5
* 1 -> value_1
* 2 -> value_2
* 3 -> value_3
* 4 -> value_4
* 5 -> Tx_foo_bufferlist
* foo:
* first_committed -> 3
* last_committed -> 3
* 3 -> foo_value_3
*
*/
#ifndef CEPH_MON_PAXOS_H
#define CEPH_MON_PAXOS_H
#include "include/types.h"
#include "mon_types.h"
#include "include/buffer.h"
#include "msg/msg_types.h"
#include "include/Context.h"
#include "common/perf_counters.h"
#include <errno.h>
#include "MonitorDBStore.h"
#include "mon/MonOpRequest.h"
class Monitor;
class MMonPaxos;
enum {
l_paxos_first = 45800,
l_paxos_start_leader,
l_paxos_start_peon,
l_paxos_restart,
l_paxos_refresh,
l_paxos_refresh_latency,
l_paxos_begin,
l_paxos_begin_keys,
l_paxos_begin_bytes,
l_paxos_begin_latency,
l_paxos_commit,
l_paxos_commit_keys,
l_paxos_commit_bytes,
l_paxos_commit_latency,
l_paxos_collect,
l_paxos_collect_keys,
l_paxos_collect_bytes,
l_paxos_collect_latency,
l_paxos_collect_uncommitted,
l_paxos_collect_timeout,
l_paxos_accept_timeout,
l_paxos_lease_ack_timeout,
l_paxos_lease_timeout,
l_paxos_store_state,
l_paxos_store_state_keys,
l_paxos_store_state_bytes,
l_paxos_store_state_latency,
l_paxos_share_state,
l_paxos_share_state_keys,
l_paxos_share_state_bytes,
l_paxos_new_pn,
l_paxos_new_pn_latency,
l_paxos_last,
};
// i am one state machine.
/**
* This libary is based on the Paxos algorithm, but varies in a few key ways:
* 1- Only a single new value is generated at a time, simplifying the recovery logic.
* 2- Nodes track "committed" values, and share them generously (and trustingly)
* 3- A 'leasing' mechanism is built-in, allowing nodes to determine when it is
* safe to "read" their copy of the last committed value.
*
* This provides a simple replication substrate that services can be built on top of.
* See PaxosService.h
*/
class Paxos {
/**
* @defgroup Paxos_h_class Paxos
* @{
*/
/**
* The Monitor to which this Paxos class is associated with.
*/
Monitor *mon;
/// perf counter for internal instrumentations
PerfCounters *logger;
void init_logger();
// my state machine info
const string paxos_name;
friend class Monitor;
friend class PaxosService;
list<std::string> extra_state_dirs;
// LEADER+PEON
// -- generic state --
public:
/**
* @defgroup Paxos_h_states States on which the leader/peon may be.
* @{
*/
enum {
/**
* Leader/Peon is in Paxos' Recovery state
*/
STATE_RECOVERING,
/**
* Leader/Peon is idle, and the Peon may or may not have a valid lease.
*/
STATE_ACTIVE,
/**
* Leader/Peon is updating to a new value.
*/
STATE_UPDATING,
/*
* Leader proposing an old value
*/
STATE_UPDATING_PREVIOUS,
/*
* Leader/Peon is writing a new commit. readable, but not
* writeable.
*/
STATE_WRITING,
/*
* Leader/Peon is writing a new commit from a previous round.
*/
STATE_WRITING_PREVIOUS,
// leader: refresh following a commit
STATE_REFRESH,
};
/**
* Obtain state name from constant value.
*
* @note This function will raise a fatal error if @p s is not
* a valid state value.
*
* @param s State value.
* @return The state's name.
*/
static const string get_statename(int s) {
switch (s) {
case STATE_RECOVERING:
return "recovering";
case STATE_ACTIVE:
return "active";
case STATE_UPDATING:
return "updating";
case STATE_UPDATING_PREVIOUS:
return "updating-previous";
case STATE_WRITING:
return "writing";
case STATE_WRITING_PREVIOUS:
return "writing-previous";
case STATE_REFRESH:
return "refresh";
default:
return "UNKNOWN";
}
}
private:
/**
* The state we are in.
*/
int state;
/**
* @}
*/
public:
/**
* Check if we are recovering.
*
* @return 'true' if we are on the Recovering state; 'false' otherwise.
*/
bool is_recovering() const { return (state == STATE_RECOVERING); }
/**
* Check if we are active.
*
* @return 'true' if we are on the Active state; 'false' otherwise.
*/
bool is_active() const { return state == STATE_ACTIVE; }
/**
* Check if we are updating.
*
* @return 'true' if we are on the Updating state; 'false' otherwise.
*/
bool is_updating() const { return state == STATE_UPDATING; }
/**
* Check if we are updating/proposing a previous value from a
* previous quorum
*/
bool is_updating_previous() const { return state == STATE_UPDATING_PREVIOUS; }
/// @return 'true' if we are writing an update to disk
bool is_writing() const { return state == STATE_WRITING; }
/// @return 'true' if we are writing an update-previous to disk
bool is_writing_previous() const { return state == STATE_WRITING_PREVIOUS; }
/// @return 'true' if we are refreshing an update just committed
bool is_refresh() const { return state == STATE_REFRESH; }
private:
/**
* @defgroup Paxos_h_recovery_vars Common recovery-related member variables
* @note These variables are common to both the Leader and the Peons.
* @{
*/
/**
*
*/
version_t first_committed;
/**
* Last Proposal Number
*
* @todo Expand description
*/
version_t last_pn;
/**
* Last committed value's version.
*
* On both the Leader and the Peons, this is the last value's version that
* was accepted by a given quorum and thus committed, that this instance
* knows about.
*
* @note It may not be the last committed value's version throughout the
* system. If we are a Peon, we may have not been part of the quorum
* that accepted the value, and for this very same reason we may still
* be a (couple of) version(s) behind, until we learn about the most
* recent version. This should only happen if we are not active (i.e.,
* part of the quorum), which should not happen if we are up, running
* and able to communicate with others -- thus able to be part of the
* monmap and trigger new elections.
*/
version_t last_committed;
/**
* Last committed value's time.
*
* When the commit finished.
*/
utime_t last_commit_time;
/**
* The last Proposal Number we have accepted.
*
* On the Leader, it will be the Proposal Number picked by the Leader
* itself. On the Peon, however, it will be the proposal sent by the Leader
* and it will only be updated iif its value is higher than the one
* already known by the Peon.
*/
version_t accepted_pn;
/**
* The last_committed epoch of the leader at the time we accepted the last pn.
*
* This has NO SEMANTIC MEANING, and is there only for the debug output.
*/
version_t accepted_pn_from;
/**
* Map holding the first committed version by each quorum member.
*
* The versions kept in this map are updated during the collect phase.
* When the Leader starts the collect phase, each Peon will reply with its
* first committed version, which will then be kept in this map.
*/
map<int,version_t> peer_first_committed;
/**
* Map holding the last committed version by each quorum member.
*
* The versions kept in this map are updated during the collect phase.
* When the Leader starts the collect phase, each Peon will reply with its
* last committed version, which will then be kept in this map.
*/
map<int,version_t> peer_last_committed;
/**
* @}
*/
// active (phase 2)
/**
* @defgroup Paxos_h_active_vars Common active-related member variables
* @{
*/
/**
* When does our read lease expires.
*
* Instead of performing a full commit each time a read is requested, we
* keep leases. Each lease will have an expiration date, which may or may
* not be extended.
*/
utime_t lease_expire;
/**
* List of callbacks waiting for our state to change into STATE_ACTIVE.
*/
list<Context*> waiting_for_active;
/**
* List of callbacks waiting for the chance to read a version from us.
*
* Each entry on the list may result from an attempt to read a version that
* wasn't available at the time, or an attempt made during a period during
* which we could not satisfy the read request. The first case happens if
* the requested version is greater than our last committed version. The
* second scenario may happen if we are recovering, or if we don't have a
* valid lease.
*
* The list will be woken up once we change to STATE_ACTIVE with an extended
* lease -- which can be achieved if we have everyone on the quorum on board
* with the latest proposal, or if we don't really care about the remaining
* uncommitted values --, or if we're on a quorum of one.
*/
list<Context*> waiting_for_readable;
/**
* @}
*/
// -- leader --
// recovery (paxos phase 1)
/**
* @defgroup Paxos_h_leader_recovery Leader-specific Recovery-related vars
* @{
*/
/**
* Number of replies to the collect phase we've received so far.
*
* This variable is reset to 1 each time we start a collect phase; it is
* incremented each time we receive a reply to the collect message, and
* is used to determine whether or not we have received replies from the
* whole quorum.
*/
unsigned num_last;
/**
* Uncommitted value's version.
*
* If we have, or end up knowing about, an uncommitted value, then its
* version will be kept in this variable.
*
* @note If this version equals @p last_committed+1 when we reach the final
* steps of recovery, then the algorithm will assume this is a value
* the Leader does not know about, and trustingly the Leader will
* propose this version's value.
*/
version_t uncommitted_v;
/**
* Uncommitted value's Proposal Number.
*
* We use this variable to assess if the Leader should take into consideration
* an uncommitted value sent by a Peon. Given that the Peon will send back to
* the Leader the last Proposal Number it accepted, the Leader will be able
* to infer if this value is more recent than the one the Leader has, thus
* more relevant.
*/
version_t uncommitted_pn;
/**
* Uncommitted Value.
*
* If the system fails in-between the accept replies from the Peons and the
* instruction to commit from the Leader, then we may end up with accepted
* but yet-uncommitted values. During the Leader's recovery, it will attempt
* to bring the whole system to the latest state, and that means committing
* past accepted but uncommitted values.
*
* This variable will hold an uncommitted value, which may originate either
* on the Leader, or learnt by the Leader from a Peon during the collect
* phase.
*/
bufferlist uncommitted_value;
/**
* Used to specify when an on-going collect phase times out.
*/
Context *collect_timeout_event;
/**
* @}
*/
// active
/**
* @defgroup Paxos_h_leader_active Leader-specific Active-related vars
* @{
*/
/**
* Set of participants (Leader & Peons) that have acked a lease extension.
*
* Each Peon that acknowledges a lease extension will have its place in this
* set, which will be used to account for all the acks from all the quorum
* members, guaranteeing that we trigger new elections if some don't ack in
* the expected timeframe.
*/
set<int> acked_lease;
/**
* Callback responsible for extending the lease periodically.
*/
Context *lease_renew_event;
/**
* Callback to trigger new elections once the time for acks is out.
*/
Context *lease_ack_timeout_event;
/**
* @}
*/
/**
* @defgroup Paxos_h_peon_active Peon-specific Active-related vars
* @{
*/
/**
* Callback to trigger new elections when the Peon's lease times out.
*
* If the Peon's lease is extended, this callback will be reset (i.e.,
* we cancel the event and reschedule a new one with starting from the
* beginning).
*/
Context *lease_timeout_event;
/**
* @}
*/
// updating (paxos phase 2)
/**
* @defgroup Paxos_h_leader_updating Leader-specific Updating-related vars
* @{
*/
/**
* New Value being proposed to the Peons.
*
* This bufferlist holds the value the Leader is proposing to the Peons, and
* that will be committed if the Peons do accept the proposal.
*/
bufferlist new_value;
/**
* Set of participants (Leader & Peons) that accepted the new proposed value.
*
* This set is used to keep track of those who have accepted the proposed
* value, so the leader may know when to issue a commit (when a majority of
* participants has accepted the proposal), and when to extend the lease
* (when all the quorum members have accepted the proposal).
*/
set<int> accepted;
/**
* Callback to trigger a new election if the proposal is not accepted by the
* full quorum within a given timeframe.
*
* If the full quorum does not accept the proposal, then it means that the
* Leader may no longer be recognized as the leader, or that the quorum has
* changed, and the value may have not reached all the participants. Thus,
* the leader must call new elections, and go through a recovery phase in
* order to propagate the new value throughout the system.
*
* This does not mean that we won't commit. We will commit as soon as we
* have a majority of acceptances. But if we do not have full acceptance
* from the quorum, then we cannot extend the lease, as some participants
* may not have the latest committed value.
*/
Context *accept_timeout_event;
/**
* List of callbacks waiting for it to be possible to write again.
*
* @remarks It is not possible to write if we are not the Leader, or we are
* not on the active state, or if the lease has expired.
*/
list<Context*> waiting_for_writeable;
/**
* List of callbacks waiting for a commit to finish.
*
* @remarks This may be used to a) wait for an on-going commit to finish
* before we proceed with, say, a new proposal; or b) wait for the
* next commit to be finished so we are sure that our value was
* fully committed.
*/
list<Context*> waiting_for_commit;
/**
* Pending proposal transaction
*
* This is the transaction that is under construction and pending
* proposal. We will add operations to it until we decide it is
* time to start a paxos round.
*/
MonitorDBStore::TransactionRef pending_proposal;
/**
* Finishers for pending transaction
*
* These are waiting for updates in the pending proposal/transaction
* to be committed.
*/
list<Context*> pending_finishers;
/**
* Finishers for committing transaction
*
* When the pending_proposal is submitted, pending_finishers move to
* this list. When it commits, these finishers are notified.
*/
list<Context*> committing_finishers;
/**
* @defgroup Paxos_h_sync_warns Synchronization warnings
* @todo Describe these variables
* @{
*/
utime_t last_clock_drift_warn;
int clock_drift_warned;
/**
* @}
*/
/**
* Should be true if we have proposed to trim, or are in the middle of
* trimming; false otherwise.
*/
bool trimming;
/**
* @defgroup Paxos_h_callbacks Callback classes.
* @{
*/
/**
* Callback class responsible for handling a Collect Timeout.
*/
class C_CollectTimeout;
/**
* Callback class responsible for handling an Accept Timeout.
*/
class C_AcceptTimeout;
/**
* Callback class responsible for handling a Lease Ack Timeout.
*/
class C_LeaseAckTimeout;
/**
* Callback class responsible for handling a Lease Timeout.
*/
class C_LeaseTimeout;
/**
* Callback class responsible for handling a Lease Renew Timeout.
*/
class C_LeaseRenew;
class C_Trimmed;
/**
*
*/
public:
class C_Proposal : public Context {
Context *proposer_context;
public:
bufferlist bl;
// for debug purposes. Will go away. Soon.
bool proposed;
utime_t proposal_time;
C_Proposal(Context *c, bufferlist& proposal_bl) :
proposer_context(c),
bl(proposal_bl),
proposed(false),
proposal_time(ceph_clock_now())
{ }
void finish(int r) {
if (proposer_context) {
proposer_context->complete(r);
proposer_context = NULL;
}
}
};
/**
* @}
*/
private:
/**
* @defgroup Paxos_h_election_triggered Steps triggered by an election.
*
* @note All these functions play a significant role in the Recovery Phase,
* which is triggered right after an election once someone becomes
* the Leader.
* @{
*/
/**
* Create a new Proposal Number and propose it to the Peons.
*
* This function starts the Recovery Phase, which can be directly mapped
* onto the original Paxos' Prepare phase. Basically, we'll generate a
* Proposal Number, taking @p oldpn into consideration, and we will send
* it to a quorum, along with our first and last committed versions. By
* sending these information in a message to the quorum, we expect to
* obtain acceptances from a majority, allowing us to commit, or be
* informed of a higher Proposal Number known by one or more of the Peons
* in the quorum.
*
* @pre We are the Leader.
* @post Recovery Phase initiated by sending messages to the quorum.
*
* @param oldpn A proposal number taken as the highest known so far, that
* should be taken into consideration when generating a new
* Proposal Number for the Recovery Phase.
*/
void collect(version_t oldpn);
/**
* Handle the reception of a collect message from the Leader and reply
* accordingly.
*
* Once a Peon receives a collect message from the Leader it will reply
* with its first and last committed versions, as well as information so
* the Leader may know if its Proposal Number was, or was not, accepted by
* the Peon. The Peon will accept the Leader's Proposal Number iif it is
* higher than the Peon's currently accepted Proposal Number. The Peon may
* also inform the Leader of accepted but uncommitted values.
*
* @invariant The message is an operation of type OP_COLLECT.
* @pre We are a Peon.
* @post Replied to the Leader, accepting or not accepting its PN.
*
* @param collect The collect message sent by the Leader to the Peon.
*/
void handle_collect(MonOpRequestRef op);
/**
* Handle a response from a Peon to the Leader's collect phase.
*
* The received message will state the Peon's last committed version, as
* well as its last proposal number. This will lead to one of the following
* scenarios: if the replied Proposal Number is equal to the one we proposed,
* then the Peon has accepted our proposal, and if all the Peons do accept
* our Proposal Number, then we are allowed to proceed with the commit;
* however, if a Peon replies with a higher Proposal Number, we assume he
* knows something we don't and the Leader will have to abort the current
* proposal in order to retry with the Proposal Number specified by the Peon.
* It may also occur that the Peon replied with a lower Proposal Number, in
* which case we assume it is a reply to an older value and we'll simply
* drop it.
* This function will also check if the Peon replied with an accepted but
* yet uncommitted value. In this case, if its version is higher than our
* last committed value by one, we assume that the Peon knows a value from a
* previous proposal that has never been committed, and we should try to
* commit that value by proposing it next. On the other hand, if that is
* not the case, we'll assume it is an old, uncommitted value, we do not
* care about and we'll consider the system active by extending the leases.
*
* @invariant The message is an operation of type OP_LAST.
* @pre We are the Leader.
* @post We initiate a commit, or we retry with a higher Proposal Number,
* or we drop the message.
* @post We move from STATE_RECOVERING to STATE_ACTIVE.
*
* @param last The message sent by the Peon to the Leader.
*/
void handle_last(MonOpRequestRef op);
/**
* The Recovery Phase timed out, meaning that a significant part of the
* quorum does not believe we are the Leader, and we thus should trigger new
* elections.
*
* @pre We believe to be the Leader.
* @post Trigger new elections.
*/
void collect_timeout();
/**
* @}
*/
/**
* @defgroup Paxos_h_updating_funcs Functions used during the Updating State
*
* These functions may easily be mapped to the original Paxos Algorithm's
* phases.
*
* Taking into account the algorithm can be divided in 4 phases (Prepare,
* Promise, Accept Request and Accepted), we can easily map Paxos::begin to
* both the Prepare and Accept Request phases; the Paxos::handle_begin to
* the Promise phase; and the Paxos::handle_accept to the Accepted phase.
* @{
*/
/**
* Start a new proposal with the intent of committing @p value.
*
* If we are alone on the system (i.e., a quorum of one), then we will
* simply commit the value, but if we are not alone, then we need to propose
* the value to the quorum.
*
* @pre We are the Leader
* @pre We are on STATE_ACTIVE
* @post We commit, iif we are alone, or we send a message to each quorum
* member
* @post We are on STATE_ACTIVE, iif we are alone, or on
* STATE_UPDATING otherwise
*
* @param value The value being proposed to the quorum
*/
void begin(bufferlist& value);
/**
* Accept or decline (by ignoring) a proposal from the Leader.
*
* We will decline the proposal (by ignoring it) if we have promised to
* accept a higher numbered proposal. If that is not the case, we will
* accept it and accordingly reply to the Leader.
*
* @pre We are a Peon
* @pre We are on STATE_ACTIVE
* @post We are on STATE_UPDATING iif we accept the Leader's proposal
* @post We send a reply message to the Leader iif we accept its proposal
*
* @invariant The received message is an operation of type OP_BEGIN
*
* @param begin The message sent by the Leader to the Peon during the
* Paxos::begin function
*
*/
void handle_begin(MonOpRequestRef op);
/**
* Handle an Accept message sent by a Peon.
*
* In order to commit, the Leader has to receive accepts from a majority of
* the quorum. If that does happen, then the Leader may proceed with the
* commit. However, the Leader needs the accepts from all the quorum members
* in order to extend the lease and move on to STATE_ACTIVE.
*
* This function handles these two situations, accounting for the amount of
* received accepts.
*
* @pre We are the Leader
* @pre We are on STATE_UPDATING
* @post We are on STATE_ACTIVE iif we received accepts from the full quorum
* @post We extended the lease iif we moved on to STATE_ACTIVE
* @post We are on STATE_UPDATING iif we didn't received accepts from the
* full quorum
* @post We have committed iif we received accepts from a majority
*
* @invariant The received message is an operation of type OP_ACCEPT
*
* @param accept The message sent by the Peons to the Leader during the
* Paxos::handle_begin function
*/
void handle_accept(MonOpRequestRef op);
/**
* Trigger a fresh election.
*
* During Paxos::begin we set a Callback of type Paxos::C_AcceptTimeout in
* order to limit the amount of time we spend waiting for Accept replies.
* This callback will call Paxos::accept_timeout when it is fired.
*
* This is essential to the algorithm because there may be the chance that
* we are no longer the Leader (i.e., others don't believe in us) and we
* are getting ignored, or we dropped out of the quorum and haven't realised
* it. So, our only option is to trigger fresh elections.
*
* @pre We are the Leader
* @pre We are on STATE_UPDATING
* @post Triggered fresh elections
*/
void accept_timeout();
/**
* @}
*/
utime_t commit_start_stamp;
friend struct C_Committed;
/**
* Commit a value throughout the system.
*
* The Leader will cancel the current lease (as it was for the old value),
* and will store the committed value locally. It will then instruct every
* quorum member to do so as well.
*
* @pre We are the Leader
* @pre We are on STATE_UPDATING
* @pre A majority of quorum members accepted our proposal
* @post Value locally stored
* @post Quorum members instructed to commit the new value.
*/
void commit_start();
void commit_finish(); ///< finish a commit after txn becomes durable
/**
* Commit the new value to stable storage as being the latest available
* version.
*
* @pre We are a Peon
* @post The new value is locally stored
* @post Fire up the callbacks waiting on waiting_for_commit
*
* @invariant The received message is an operation of type OP_COMMIT
*
* @param commit The message sent by the Leader to the Peon during
* Paxos::commit
*/
void handle_commit(MonOpRequestRef op);
/**
* Extend the system's lease.
*
* This means that the Leader considers that it should now safe to read from
* any node on the system, since every quorum member is now in possession of
* the latest version. Therefore, the Leader will send a message stating just
* this to each quorum member, and will impose a limited timeframe during
* which acks will be accepted. If there aren't as many acks as expected
* (i.e, if at least one quorum member does not ack the lease) during this
* timeframe, then we will force fresh elections.
*
* @pre We are the Leader
* @pre We are on STATE_ACTIVE
* @post A message extending the lease is sent to each quorum member
* @post A timeout callback is set to limit the amount of time we will wait
* for lease acks.
* @post A timer is set in order to renew the lease after a certain amount
* of time.
*/
void extend_lease();
/**
* Update the lease on the Peon's side of things.
*
* Once a Peon receives a Lease message, it will update its lease_expire
* variable, reply to the Leader acknowledging the lease update and set a
* timeout callback to be fired upon the lease's expiration. Finally, the
* Peon will fire up all the callbacks waiting for it to become active,
* which it just did, and all those waiting for it to become readable,
* which should be true if the Peon's lease didn't expire in the mean time.
*
* @pre We are a Peon
* @post We update the lease accordingly
* @post A lease timeout callback is set
* @post Move to STATE_ACTIVE
* @post Fire up all the callbacks waiting for STATE_ACTIVE
* @post Fire up all the callbacks waiting for readable iif we are readable
* @post Ack the lease to the Leader
*
* @invariant The received message is an operation of type OP_LEASE
*
* @param lease The message sent by the Leader to the Peon during the
* Paxos::extend_lease function
*/
void handle_lease(MonOpRequestRef op);
/**
* Account for all the Lease Acks the Leader receives from the Peons.
*
* Once the Leader receives all the Lease Acks from the Peons, it will be
* able to cancel the Lease Ack timeout callback, thus avoiding calling
* fresh elections.
*
* @pre We are the Leader
* @post Cancel the Lease Ack timeout callback iif we receive acks from all
* the quorum members
*
* @invariant The received message is an operation of type OP_LEASE_ACK
*
* @param ack The message sent by a Peon to the Leader during the
* Paxos::handle_lease function
*/
void handle_lease_ack(MonOpRequestRef op);
/**
* Call fresh elections because at least one Peon didn't acked our lease.
*
* @pre We are the Leader
* @pre We are on STATE_ACTIVE
* @post Trigger fresh elections
*/
void lease_ack_timeout();
/**
* Extend lease since we haven't had new committed values meanwhile.
*
* @pre We are the Leader
* @pre We are on STATE_ACTIVE
* @post Go through with Paxos::extend_lease
*/
void lease_renew_timeout();
/**
* Call fresh elections because the Peon's lease expired without being
* renewed or receiving a fresh lease.
*
* This means that the Peon is no longer assumed as being in the quorum
* (or there is no Leader to speak of), so just trigger fresh elections
* to circumvent this issue.
*
* @pre We are a Peon
* @post Trigger fresh elections
*/
void lease_timeout(); // on peon, if lease isn't extended
/// restart the lease timeout timer
void reset_lease_timeout();
/**
* Cancel all of Paxos' timeout/renew events.
*/
void cancel_events();
/**
* Shutdown this Paxos machine
*/
void shutdown();
/**
* Generate a new Proposal Number based on @p gt
*
* @todo Check what @p gt actually means and what its usage entails
* @param gt A hint for the geration of the Proposal Number
* @return A globally unique, monotonically increasing Proposal Number
*/
version_t get_new_proposal_number(version_t gt=0);
/**
* @todo document sync function
*/
void warn_on_future_time(utime_t t, entity_name_t from);
/**
* Begin proposing the pending_proposal.
*/
void propose_pending();
/**
* refresh state from store
*
* Called when we have new state for the mon to consume. If we return false,
* abort (we triggered a bootstrap).
*
* @returns true on success, false if we are now bootstrapping
*/
bool do_refresh();
void commit_proposal();
void finish_round();
public:
/**
* @param m A monitor
* @param name A name for the paxos service. It serves as the naming space
* of the underlying persistent storage for this service.
*/
Paxos(Monitor *m, const string &name)
: mon(m),
logger(NULL),
paxos_name(name),
state(STATE_RECOVERING),
first_committed(0),
last_pn(0),
last_committed(0),
accepted_pn(0),
accepted_pn_from(0),
num_last(0),
uncommitted_v(0), uncommitted_pn(0),
collect_timeout_event(0),
lease_renew_event(0),
lease_ack_timeout_event(0),
lease_timeout_event(0),
accept_timeout_event(0),
clock_drift_warned(0),
trimming(false) { }
const string get_name() const {
return paxos_name;
}
void dispatch(MonOpRequestRef op);
void read_and_prepare_transactions(MonitorDBStore::TransactionRef tx,
version_t from, version_t last);
void init();
/**
* dump state info to a formatter
*/
void dump_info(Formatter *f);
/**
* This function runs basic consistency checks. Importantly, if
* it is inconsistent and shouldn't be, it asserts out.
*
* @return True if consistent, false if not.
*/
bool is_consistent();
void restart();
/**
* Initiate the Leader after it wins an election.
*
* Once an election is won, the Leader will be initiated and there are two
* possible outcomes of this method: the Leader directly jumps to the active
* state (STATE_ACTIVE) if it believes to be the only one in the quorum, or
* will start recovering (STATE_RECOVERING) by initiating the collect phase.
*
* @pre Our monitor is the Leader.
* @post We are either on STATE_ACTIVE if we're the only one in the quorum,
* or on STATE_RECOVERING otherwise.
*/
void leader_init();
/**
* Initiate a Peon after it loses an election.
*
* If we are a Peon, then there must be a Leader and we are not alone in the
* quorum, thus automatically assume we are on STATE_RECOVERING, which means
* we will soon be enrolled into the Leader's collect phase.
*
* @pre There is a Leader, and it?s about to start the collect phase.
* @post We are on STATE_RECOVERING and will soon receive collect phase's
* messages.
*/
void peon_init();
/**
* Include an incremental state of values, ranging from peer_first_committed
* to the last committed value, on the message m
*
* @param m A message
* @param peer_first_committed Lowest version to take into account
* @param peer_last_committed Highest version to take into account
*/
void share_state(MMonPaxos *m, version_t peer_first_committed,
version_t peer_last_committed);
/**
* Store on disk a state that was shared with us
*
* Basically, we received a set of version. Or just one. It doesn't matter.
* What matters is that we have to stash it in the store. So, we will simply
* write every single bufferlist into their own versions on our side (i.e.,
* onto paxos-related keys), and then we will decode those same bufferlists
* we just wrote and apply the transactions they hold. We will also update
* our first and last committed values to point to the new values, if need
* be. All this is done tightly wrapped in a transaction to ensure we
* enjoy the atomicity guarantees given by our awesome k/v store.
*
* @param m A message
* @returns true if we stored something new; false otherwise
*/
bool store_state(MMonPaxos *m);
void _sanity_check_store();
/**
* Helper function to decode a bufferlist into a transaction and append it
* to another transaction.
*
* This function is used during the Leader's commit and during the
* Paxos::store_state in order to apply the bufferlist's transaction onto
* the store.
*
* @param t The transaction to which we will append the operations
* @param bl A bufferlist containing an encoded transaction
*/
static void decode_append_transaction(MonitorDBStore::TransactionRef t,
bufferlist& bl) {
MonitorDBStore::TransactionRef vt(new MonitorDBStore::Transaction);
bufferlist::iterator it = bl.begin();
vt->decode(it);
t->append(vt);
}
/**
* @todo This appears to be used only by the OSDMonitor, and I would say
* its objective is to allow a third-party to have a "private"
* state dir. -JL
*/
void add_extra_state_dir(string s) {
extra_state_dirs.push_back(s);
}
// -- service interface --
/**
* Add c to the list of callbacks waiting for us to become active.
*
* @param c A callback
*/
void wait_for_active(MonOpRequestRef op, Context *c) {
if (op)
op->mark_event("paxos:wait_for_active");
waiting_for_active.push_back(c);
}
void wait_for_active(Context *c) {
MonOpRequestRef o;
wait_for_active(o, c);
}
/**
* Trim the Paxos state as much as we can.
*/
void trim();
/**
* Check if we should trim.
*
* If trimming is disabled, we must take that into consideration and only
* return true if we are positively sure that we should trim soon.
*
* @returns true if we should trim; false otherwise.
*/
bool should_trim() {
int available_versions = get_version() - get_first_committed();
int maximum_versions = g_conf->paxos_min + g_conf->paxos_trim_min;
if (trimming || (available_versions <= maximum_versions))
return false;
return true;
}
// read
/**
* @defgroup Paxos_h_read_funcs Read-related functions
* @{
*/
/**
* Get latest committed version
*
* @return latest committed version
*/
version_t get_version() { return last_committed; }
/**
* Get first committed version
*
* @return the first committed version
*/
version_t get_first_committed() { return first_committed; }
/**
* Check if a given version is readable.
*
* A version may not be readable for a myriad of reasons:
* @li the version @e v is higher that the last committed version
* @li we are not the Leader nor a Peon (election may be on-going)
* @li we do not have a committed value yet
* @li we do not have a valid lease
*
* @param seen The version we want to check if it is readable.
* @return 'true' if the version is readable; 'false' otherwise.
*/
bool is_readable(version_t seen=0);
/**
* Read version @e v and store its value in @e bl
*
* @param[in] v The version we want to read
* @param[out] bl The version's value
* @return 'true' if we successfully read the value; 'false' otherwise
*/
bool read(version_t v, bufferlist &bl);
/**
* Read the latest committed version
*
* @param[out] bl The version's value
* @return the latest committed version if we successfully read the value;
* or 0 (zero) otherwise.
*/
version_t read_current(bufferlist &bl);
/**
* Add onreadable to the list of callbacks waiting for us to become readable.
*
* @param onreadable A callback
*/
void wait_for_readable(MonOpRequestRef op, Context *onreadable) {
assert(!is_readable());
if (op)
op->mark_event("paxos:wait_for_readable");
waiting_for_readable.push_back(onreadable);
}
void wait_for_readable(Context *onreadable) {
MonOpRequestRef o;
wait_for_readable(o, onreadable);
}
/**
* @}
*/
/**
* Check if we have a valid lease.
*
* @returns true if the lease is still valid; false otherwise.
*/
bool is_lease_valid();
// write
/**
* @defgroup Paxos_h_write_funcs Write-related functions
* @{
*/
/**
* Check if we are writeable.
*
* We are writeable if we are alone (i.e., a quorum of one), or if we match
* all the following conditions:
* @li We are the Leader
* @li We are on STATE_ACTIVE
* @li We have a valid lease
*
* @return 'true' if we are writeable; 'false' otherwise.
*/
bool is_writeable();
/**
* Add c to the list of callbacks waiting for us to become writeable.
*
* @param c A callback
*/
void wait_for_writeable(MonOpRequestRef op, Context *c) {
assert(!is_writeable());
if (op)
op->mark_event("paxos:wait_for_writeable");
waiting_for_writeable.push_back(c);
}
void wait_for_writeable(Context *c) {
MonOpRequestRef o;
wait_for_writeable(o, c);
}
/**
* Get a transaction to submit operations to propose against
*
* Apply operations to this transaction. It will eventually be proposed
* to paxos.
*/
MonitorDBStore::TransactionRef get_pending_transaction();
/**
* Queue a completion for the pending proposal
*
* This completion will get triggered when the pending proposal
* transaction commits.
*/
void queue_pending_finisher(Context *onfinished);
/**
* (try to) trigger a proposal
*
* Tell paxos that it should submit the pending proposal. Note that if it
* is not active (e.g., because it is already in the midst of committing
* something) that will be deferred (e.g., until the current round finishes).
*/
bool trigger_propose();
/**
* Add oncommit to the back of the list of callbacks waiting for us to
* finish committing.
*
* @param oncommit A callback
*/
void wait_for_commit(Context *oncommit) {
waiting_for_commit.push_back(oncommit);
}
/**
* Add oncommit to the front of the list of callbacks waiting for us to
* finish committing.
*
* @param oncommit A callback
*/
void wait_for_commit_front(Context *oncommit) {
waiting_for_commit.push_front(oncommit);
}
/**
* @}
*/
/**
* @}
*/
protected:
MonitorDBStore *get_store();
};
inline ostream& operator<<(ostream& out, Paxos::C_Proposal& p)
{
string proposed = (p.proposed ? "proposed" : "unproposed");
out << " " << proposed
<< " queued " << (ceph_clock_now() - p.proposal_time)
<< " tx dump:\n";
MonitorDBStore::TransactionRef t(new MonitorDBStore::Transaction);
bufferlist::iterator p_it = p.bl.begin();
t->decode(p_it);
JSONFormatter f(true);
t->dump(&f);
f.flush(out);
return out;
}
#endif
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