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MXS-2169 Allow a downed server to be selected as topology master

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This is required for the case when MaxScale is started when the master is
already down.
This commit is contained in:
Esa Korhonen
2019-05-10 11:23:41 +03:00
parent d2f3e56d0a
commit 4ee30f5c52
6 changed files with 290 additions and 184 deletions
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430
server/modules/monitor/mariadbmon/cluster_discovery.cc
View File

@ -300,100 +300,142 @@ MariaDBServer* MariaDBMonitor::find_best_reach_server(const ServerArray& candida
return best_reach;
}
static string disqualify_reasons_to_string(MariaDBServer* disqualified)
{
string reasons;
string separator;
const string word_and = " and ";
if (disqualified->is_in_maintenance())
{
reasons += separator + "in maintenance";
separator = word_and;
}
if (disqualified->is_down())
{
reasons += separator + "down";
separator = word_and;
}
if (disqualified->is_read_only())
{
reasons += separator + "in read_only mode";
}
return reasons;
}
/**
* Find the best master server in the cluster. This method should only be called when the monitor
* is starting, a cluster operation (e.g. failover) has occurred or the user has changed something on
* the current master making it unsuitable. Because of this, the method can be quite vocal and not
* consider the previous master.
*
* @param req_running Should non-running candidates be accepted
* @param msg_out Message output. Includes explanations on why potential candidates were not selected.
* @return The master with most slaves
*/
MariaDBServer* MariaDBMonitor::find_topology_master_server(string* msg_out)
MariaDBServer* MariaDBMonitor::find_topology_master_server(RequireRunning req_running, string* msg_out)
{
/* Finding the best master server may get somewhat tricky if the graph is complicated. The general
* criteria for the best master is that it reaches the most slaves (possibly in multiple layers and
* cycles). To avoid having to calculate this reachability (doable by a recursive search) to all nodes,
* let's use the knowledge that the best master is either a server with no masters (external ones don't
* count) or is part of a cycle with no out-cycle masters. The server must be running and writable
* to be eligible. */
string messages;
string separator;
const char disq[] = "is not a valid master candidate because it is ";
* count) or is part of a cycle with no out-cycle masters.
*
* Master candidates must be writable and not in maintenance to be eligible. Running candidates are
* preferred. A downed candidate is accepted only if no current master exists. A downed master can
* be failovered later. */
ServerArray master_candidates;
for (MariaDBServer* server : m_servers)
{
if (server->m_node.parents.empty())
{
if (server->is_usable() && !server->is_read_only())
{
master_candidates.push_back(server);
}
else
{
string reasons = disqualify_reasons_to_string(server);
messages += separator + "'" + server->name() + "' " + disq + reasons + ".";
separator = "\n";
}
}
}
// For each cycle, it's enough to take one sample server, as all members of a cycle have the same reach.
for (auto& iter : m_cycles)
{
ServerArray& cycle_members = iter.second;
// Check that no server in the cycle is replicating from outside the cycle. This requirement is
// analogous with the same requirement for non-cycle servers.
if (!cycle_has_master_server(cycle_members))
// Helper function for finding normal master candidates.
auto search_outside_cycles = [this, &master_candidates](RequireRunning req_running,
DelimitedPrinter& topo_messages) {
for (MariaDBServer* server : m_servers)
{
MariaDBServer* sample_server = find_master_inside_cycle(cycle_members);
if (sample_server)
if (server->m_node.parents.empty())
{
master_candidates.push_back(sample_server);
}
else
{
// No single server in the cycle was viable.
const char no_valid_servers[] = "No valid master server could be found in the cycle with "
"servers";
string server_names = monitored_servers_to_string(cycle_members);
messages += separator + no_valid_servers + " '" + server_names + "'.";
separator = "\n";
for (MariaDBServer* disqualified_server : cycle_members)
string why_not;
if (is_candidate_valid(server, req_running, &why_not))
{
string reasons = disqualify_reasons_to_string(disqualified_server);
messages += separator + "'" + disqualified_server->name() + "' " + disq + reasons + ".";
separator = "\n";
master_candidates.push_back(server);
}
else
{
topo_messages.cat(why_not);
}
}
}
};
// Helper function for finding master candidates inside cycles.
auto search_inside_cycles = [this, &master_candidates](RequireRunning req_running,
DelimitedPrinter& topo_messages) {
// For each cycle, it's enough to take one sample server, as all members of a cycle have the
// same reach. The sample server needs to be valid, though.
for (auto& iter : m_cycles)
{
ServerArray& cycle_members = iter.second;
// Check that no server in the cycle is replicating from outside the cycle. This requirement is
// analogous with the same requirement for non-cycle servers.
if (!cycle_has_master_server(cycle_members))
{
// Find a valid candidate from the cycle.
MariaDBServer* cycle_cand = nullptr;
for (MariaDBServer* elem : cycle_members)
{
mxb_assert(elem->m_node.cycle != NodeData::CYCLE_NONE);
if (is_candidate_valid(elem, req_running))
{
cycle_cand = elem;
break;
}
}
if (cycle_cand)
{
master_candidates.push_back(cycle_cand);
}
else
{
// No single server in the cycle was viable. Go through the cycle again and construct
// a message explaining why.
string server_names = monitored_servers_to_string(cycle_members);
string msg_start = string_printf("No valid master server could be found in the cycle with "
"servers %s:", server_names.c_str());
DelimitedPrinter cycle_invalid_msg("\n");
cycle_invalid_msg.cat(msg_start);
for (MariaDBServer* elem : cycle_members)
{
string server_msg;
is_candidate_valid(elem, req_running, &server_msg);
cycle_invalid_msg.cat(server_msg);
}
cycle_invalid_msg.cat(""); // Adds a linebreak
topo_messages.cat(cycle_invalid_msg.message());
}
}
}
};
// Normally, do not accept downed servers as master.
DelimitedPrinter topo_messages_reject_down("\n");
search_outside_cycles(RequireRunning::REQUIRED, topo_messages_reject_down);
search_inside_cycles(RequireRunning::REQUIRED, topo_messages_reject_down);
MariaDBServer* best_candidate = nullptr;
string messages;
if (!master_candidates.empty())
{
// Found one or more candidates. Select the best one and output the messages.
best_candidate = find_best_reach_server(master_candidates);
messages = topo_messages_reject_down.message();
}
else if (req_running == RequireRunning::OPTIONAL)
{
// If no candidate was found and the caller allows, we get desperate and allow a downed server
// to be selected. This is required for the case when MaxScale is started while the master is
// already down. Failover may be able to fix the situation if settings allow it or
// if activated manually.
DelimitedPrinter topo_messages_accept_down("\n");
search_outside_cycles(RequireRunning::OPTIONAL, topo_messages_accept_down);
search_inside_cycles(RequireRunning::OPTIONAL, topo_messages_accept_down);
if (!master_candidates.empty())
{
// Found one or more candidates which are down. Select the best one. Instead of the original
// messages, output the messages without complaints that a server is down. The caller
// should detect and explain that a non-running server was selected.
best_candidate = find_best_reach_server(master_candidates);
messages = topo_messages_accept_down.message();
}
else
{
// Still no luck. Output the messages from the first run since these explain if any potential
// candidates were down.
messages = topo_messages_reject_down.message();
}
}
*msg_out = messages;
return master_candidates.empty() ? NULL : find_best_reach_server(master_candidates);
if (msg_out)
{
*msg_out = messages;
}
return best_candidate;
}
/**
@ -405,7 +447,6 @@ MariaDBServer* MariaDBMonitor::find_topology_master_server(string* msg_out)
*/
void MariaDBMonitor::calculate_node_reach(MariaDBServer* search_root)
{
mxb_assert(search_root && search_root->m_node.reach == NodeData::REACH_UNKNOWN);
// Reset indexes since they will be reused.
reset_node_index_info();
@ -450,27 +491,6 @@ int MariaDBMonitor::running_slaves(MariaDBServer* search_root)
return n_running_slaves;
}
/**
* Check which node in a cycle should be the master. The node must be running without read_only.
*
* @param cycle The cycle index
* @return The selected node
*/
MariaDBServer* MariaDBMonitor::find_master_inside_cycle(ServerArray& cycle_members)
{
/* For a cycle, all servers are equally good in a sense. The question is just if the server is up
* and writable. */
for (MariaDBServer* server : cycle_members)
{
mxb_assert(server->m_node.cycle != NodeData::CYCLE_NONE);
if (server->is_usable() && !server->is_read_only())
{
return server;
}
}
return NULL;
}
/**
* Assign replication role status bits to the servers in the cluster. Starts from the cluster master server.
* Also updates replication lag.
@ -779,13 +799,21 @@ bool MariaDBMonitor::cycle_has_master_server(ServerArray& cycle_servers)
void MariaDBMonitor::update_topology()
{
m_servers_by_id.clear();
for (auto server : m_servers)
if (m_cluster_topology_changed)
{
m_servers_by_id[server->m_server_id] = server;
// Server IDs may have changed.
m_servers_by_id.clear();
for (auto server : m_servers)
{
if (server->m_server_id != SERVER_ID_UNKNOWN)
{
m_servers_by_id[server->m_server_id] = server;
}
}
build_replication_graph();
find_graph_cycles();
}
build_replication_graph();
find_graph_cycles();
/* Check if a failover/switchover was performed last loop and the master should change.
* In this case, update the master and its cycle info here. */
@ -795,114 +823,134 @@ void MariaDBMonitor::update_topology()
m_next_master = NULL;
}
// Find the server that looks like it would be the best master. It does not yet overwrite the
// current master.
string topology_messages;
MariaDBServer* master_candidate = find_topology_master_server(&topology_messages);
// If the 'master_candidate' is a valid server but different from the current master,
// a change may be necessary. It will only happen if the current master is no longer usable.
bool have_better = (master_candidate && master_candidate != m_master);
bool current_still_best = (master_candidate && master_candidate == m_master);
if (m_cluster_topology_changed || !m_master || !m_master->is_usable())
{
update_master();
}
}
void MariaDBMonitor::update_master()
{
// Check if current master is still valid.
string reason_not_valid;
bool current_is_ok = master_is_valid(&reason_not_valid);
if (current_is_ok)
{
// Current master is still ok. If topology has changed, check which server looks like the best master
// and alert if it's not the current master. Don't change yet.
m_warn_current_master_invalid = true;
// Update master cycle info in case it has changed.
update_master_cycle_info();
if (have_better)
if (m_cluster_topology_changed)
{
// Master is still valid but it is no longer the best master. Print a warning. This
// may be a continuous situation so only print once.
if (m_warn_have_better_master)
// Update master cycle info in case it has changed.
update_master_cycle_info();
MariaDBServer* master_cand = find_topology_master_server(RequireRunning::REQUIRED);
// master_cand can be null if e.g. current master is down.
if (master_cand && (master_cand != m_master))
{
// This is unlikely to be printed continuously because of the topology-change requirement.
MXS_WARNING("'%s' is a better master candidate than the current master '%s'. "
"Master will change when '%s' is no longer a valid master.",
master_candidate->name(),
m_master->name(),
m_master->name());
m_warn_have_better_master = false;
master_cand->name(), m_master->name(), m_master->name());
}
}
}
else
else if (m_master)
{
// Current master is faulty or does not exist
m_warn_have_better_master = true;
if (have_better)
// Current master is faulty and swapping to a better, running server is allowed.
string topology_messages;
MariaDBServer* master_cand = find_topology_master_server(RequireRunning::REQUIRED,
&topology_messages);
m_warn_cannot_find_master = true;
if (master_cand)
{
// We have an alternative. Swap master. The messages give the impression
// that new master selection has not yet happened, but this is just for clarity.
const char sel_new_master[] = "Selecting new master server.";
if (m_master)
if (master_cand != m_master)
{
// We have another master to swap to. The messages give the impression that new master
// selection has not yet happened, but this is just for clarity for the user.
mxb_assert(!reason_not_valid.empty());
MXS_WARNING("The current master server '%s' is no longer valid because %s. %s",
m_master->name(),
reason_not_valid.c_str(),
sel_new_master);
}
else
{
// This typically happens only when starting from scratch.
MXS_NOTICE("%s", sel_new_master);
}
MXS_WARNING("The current master server '%s' is no longer valid because %s. "
"Selecting new master server.",
m_master->name(), reason_not_valid.c_str());
// At this point, print messages explaining why any/other possible master servers weren't picked.
if (!topology_messages.empty())
{
MXS_WARNING("%s", topology_messages.c_str());
}
MXS_NOTICE("Setting '%s' as master.", master_candidate->name());
// Change the master, even though this may break replication.
assign_new_master(master_candidate);
}
else if (current_still_best)
{
// Tried to find another master but the current one is still the best.
MXS_WARNING("Attempted to find a replacement for the current master server '%s' because %s, "
"but '%s' is still the best master server.",
m_master->name(),
reason_not_valid.c_str(),
m_master->name());
if (!topology_messages.empty())
{
MXS_WARNING("%s", topology_messages.c_str());
}
// The following updates some data on the master.
assign_new_master(master_candidate);
}
else
{
// No alternative master. Keep current status and print warnings.
// This situation may stick so only print the messages once.
if (m_warn_current_master_invalid)
{
if (m_master)
// At this point, print messages explaining why any/other possible master servers
// weren't picked.
if (!topology_messages.empty())
{
mxb_assert(!reason_not_valid.empty());
MXS_WARNING("The current master server '%s' is no longer valid because %s, "
"but there is no valid alternative to swap to.",
m_master->name(),
reason_not_valid.c_str());
}
else
{
MXS_WARNING("No valid master server found.");
MXS_WARNING("%s", topology_messages.c_str());
}
MXS_NOTICE("Setting '%s' as master.", master_cand->name());
// Change the master, even though this may break replication.
assign_new_master(master_cand);
}
else if (m_cluster_topology_changed)
{
// Tried to find another master but the current one is still the best. This is typically
// caused by a topology change. The check on 'm_cluster_topology_changed' should stop this
// message from printing repeatedly.
MXS_WARNING("Attempted to find a replacement for the current master server '%s' because %s, "
"but '%s' is still the best master server.",
m_master->name(), reason_not_valid.c_str(), m_master->name());
if (!topology_messages.empty())
{
MXS_WARNING("%s", topology_messages.c_str());
}
m_warn_current_master_invalid = false;
// The following updates some data on the master.
assign_new_master(master_cand);
}
}
else if (m_warn_current_master_invalid)
{
// No alternative master. Keep current status and print warnings.
// This situation may stick so only print the messages once.
mxb_assert(!reason_not_valid.empty());
MXS_WARNING("The current master server '%s' is no longer valid because %s, "
"but there is no valid alternative to swap to.",
m_master->name(), reason_not_valid.c_str());
if (!topology_messages.empty())
{
MXS_WARNING("%s", topology_messages.c_str());
}
m_warn_current_master_invalid = false;
}
}
else
{
// This happens only when starting from scratch before a master has been selected.
// Accept either a running or downed master, preferring running servers.
string topology_messages;
MariaDBServer* master_cand = find_topology_master_server(RequireRunning::OPTIONAL,
&topology_messages);
if (master_cand)
{
MXS_NOTICE("Selecting new master server.");
if (master_cand->is_down())
{
const char msg[] = "No running master candidates detected and no master currently set. "
"Accepting a non-running server as master.";
MXS_WARNING("%s", msg);
}
if (!topology_messages.empty())
{
MXS_WARNING("%s", topology_messages.c_str());
}
MXS_NOTICE("Setting '%s' as master.", master_cand->name());
assign_new_master(master_cand);
}
else if (m_warn_cannot_find_master)
{
// No current master and could not select another. This situation may stick so only print once.
MXS_WARNING("Tried to find a master but no valid master server found.");
if (!topology_messages.empty())
{
MXS_WARNING("%s", topology_messages.c_str());
}
m_warn_cannot_find_master = false;
}
}
}
@ -920,3 +968,33 @@ void MariaDBMonitor::set_low_disk_slaves_maintenance()
}
}
}
bool MariaDBMonitor::is_candidate_valid(MariaDBServer* cand, RequireRunning req_running, string* why_not)
{
bool is_valid = true;
DelimitedPrinter reasons(" and ");
if (cand->is_in_maintenance())
{
is_valid = false;
reasons.cat("in maintenance");
}
if (cand->is_read_only())
{
is_valid = false;
reasons.cat("in read_only mode");
}
if (req_running == RequireRunning::REQUIRED && cand->is_down())
{
is_valid = false;
reasons.cat("down");
}
if (!is_valid && why_not)
{
*why_not = string_printf("'%s' is not a valid master candidate because it is %s.",
cand->name(), reasons.message().c_str());
}
return is_valid;
};

8
server/modules/monitor/mariadbmon/mariadbmon.cc
View File

@ -446,10 +446,10 @@ void MariaDBMonitor::tick()
}
}
// Topology needs to be rechecked if it has changed or if master is down.
if (m_cluster_topology_changed || (m_master && m_master->is_down()))
update_topology();
if (m_cluster_topology_changed)
{
update_topology();
m_cluster_topology_changed = false;
// If cluster operations are enabled, check topology support and disable if needed.
if (m_settings.auto_failover || m_settings.switchover_on_low_disk_space || m_settings.auto_rejoin)
@ -675,7 +675,7 @@ void MariaDBMonitor::assign_new_master(MariaDBServer* new_master)
mxb::atomic::store(&m_master, new_master, mxb::atomic::RELAXED);
update_master_cycle_info();
m_warn_current_master_invalid = true;
m_warn_have_better_master = true;
m_warn_cannot_find_master = true;
}
/**

13
server/modules/monitor/mariadbmon/mariadbmon.hh
View File

@ -119,6 +119,12 @@ private:
ON
};
enum class RequireRunning
{
REQUIRED,
OPTIONAL
};
class SwitchoverParams
{
public:
@ -239,7 +245,7 @@ private:
// Fields controlling logging of various events. TODO: Check these
bool m_log_no_master = true; /* Should it be logged that there is no master? */
bool m_warn_current_master_invalid = true; /* Print warning if current master is not valid? */
bool m_warn_have_better_master = true; /* Print warning if the current master is not the best one? */
bool m_warn_cannot_find_master = true; /* Print warning if a master cannot be found? */
bool m_warn_master_down = true; /* Print warning that failover may happen soon? */
bool m_warn_failover_precond = true; /* Print failover preconditions error message? */
bool m_warn_switchover_precond = true; /* Print switchover preconditions error message? */
@ -267,6 +273,7 @@ private:
// Cluster discovery and status assignment methods, top levels
void update_topology();
void build_replication_graph();
void update_master();
void assign_new_master(MariaDBServer* new_master);
void find_graph_cycles();
bool master_is_valid(std::string* reason_out);
@ -274,8 +281,7 @@ private:
void assign_slave_and_relay_master(MariaDBServer* start_node);
void check_cluster_operations_support();
MariaDBServer* find_topology_master_server(std::string* msg_out);
MariaDBServer* find_master_inside_cycle(ServerArray& cycle_servers);
MariaDBServer* find_topology_master_server(RequireRunning req_running, std::string* msg_out = nullptr);
MariaDBServer* find_best_reach_server(const ServerArray& candidates);
// Cluster discovery and status assignment methods, low level
@ -286,6 +292,7 @@ private:
void update_gtid_domain();
void update_external_master();
void update_master_cycle_info();
bool is_candidate_valid(MariaDBServer* cand, RequireRunning req_running, std::string* why_not = nullptr);
// Cluster operation launchers
bool manual_switchover(SERVER* new_master, SERVER* current_master, json_t** error_out);

11
server/modules/monitor/mariadbmon/mariadbmon_common.cc
View File

@ -32,3 +32,14 @@ void DelimitedPrinter::cat(string& target, const string& addition)
target += m_current_separator + addition;
m_current_separator = m_separator;
}
void DelimitedPrinter::cat(const std::string& addition)
{
cat(m_message, addition);
m_current_separator = m_separator;
}
std::string DelimitedPrinter::message() const
{
return m_message;
}

11
server/modules/monitor/mariadbmon/mariadbmon_common.hh
View File

@ -61,7 +61,18 @@ public:
* @param addition String to add. The delimiter is printed before the addition.
*/
void cat(std::string& target, const std::string& addition);
/**
* Add to internal string.
*
* @param addition The string to add.
*/
void cat(const std::string& addition);
std::string message() const;
private:
const std::string m_separator;
std::string m_current_separator;
std::string m_message;
};

1
server/modules/monitor/mariadbmon/mariadbserver.cc
View File

@ -52,7 +52,6 @@ NodeData::NodeData()
void NodeData::reset_results()
{
cycle = CYCLE_NONE;
reach = REACH_UNKNOWN;
parents.clear();
children.clear();
external_masters.clear();