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Explicit define what first replica is.

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This commit is contained in:
Amos Bird 2020-09-16 14:36:55 +08:00
parent c2f762e20a
commit 38d53c38f6
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3 changed files with 11 additions and 2 deletions
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2
docs/en/operations/settings/settings.md
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@ -940,6 +940,8 @@ This algorithm chooses the first replica in the set or a random replica if the f
The `first_or_random` algorithm solves the problem of the `in_order` algorithm. With `in_order`, if one replica goes down, the next one gets a double load while the remaining replicas handle the usual amount of traffic. When using the `first_or_random` algorithm, the load is evenly distributed among replicas that are still available.
It's possible to explicitly define what the first replica is by using the setting `load_balancing_first_offset`. This gives more control to rebalance query workloads among replicas.
### Round Robin {#load_balancing-round_robin}
``` sql

10
src/Client/ConnectionPoolWithFailover.cpp
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@ -56,6 +56,9 @@ IConnectionPool::Entry ConnectionPoolWithFailover::get(const ConnectionTimeouts
return tryGetEntry(pool, timeouts, fail_message, settings);
};
size_t offset = 0;
if (settings)
offset = settings->load_balancing_first_offset % nested_pools.size();
GetPriorityFunc get_priority;
switch (settings ? LoadBalancing(settings->load_balancing) : default_load_balancing)
{
@ -68,7 +71,7 @@ IConnectionPool::Entry ConnectionPoolWithFailover::get(const ConnectionTimeouts
case LoadBalancing::RANDOM:
break;
case LoadBalancing::FIRST_OR_RANDOM:
get_priority = [](size_t i) -> size_t { return i >= 1; };
get_priority = [offset](size_t i) -> size_t { return i != offset; };
break;
case LoadBalancing::ROUND_ROBIN:
if (last_used >= nested_pools.size())
@ -190,6 +193,9 @@ std::vector<ConnectionPoolWithFailover::TryResult> ConnectionPoolWithFailover::g
else
throw DB::Exception("Unknown pool allocation mode", DB::ErrorCodes::LOGICAL_ERROR);
size_t offset = 0;
if (settings)
offset = settings->load_balancing_first_offset % nested_pools.size();
GetPriorityFunc get_priority;
switch (settings ? LoadBalancing(settings->load_balancing) : default_load_balancing)
{
@ -202,7 +208,7 @@ std::vector<ConnectionPoolWithFailover::TryResult> ConnectionPoolWithFailover::g
case LoadBalancing::RANDOM:
break;
case LoadBalancing::FIRST_OR_RANDOM:
get_priority = [](size_t i) -> size_t { return i >= 1; };
get_priority = [offset](size_t i) -> size_t { return i != offset; };
break;
case LoadBalancing::ROUND_ROBIN:
if (last_used >= nested_pools.size())

1
src/Core/Settings.h
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@ -88,6 +88,7 @@ class IColumn;
M(UInt64, replication_alter_columns_timeout, 60, "Wait for actions to change the table structure within the specified number of seconds. 0 - wait unlimited time.", 0) \
\
M(LoadBalancing, load_balancing, LoadBalancing::RANDOM, "Which replicas (among healthy replicas) to preferably send a query to (on the first attempt) for distributed processing.", 0) \
M(UInt64, load_balancing_first_offset, 0, "Which replica to preferably send a query when FIRST_OR_RANDOM load balancing strategy is used.", 0) \
\
M(TotalsMode, totals_mode, TotalsMode::AFTER_HAVING_EXCLUSIVE, "How to calculate TOTALS when HAVING is present, as well as when max_rows_to_group_by and group_by_overflow_mode = ‘any’ are present.", IMPORTANT) \
M(Float, totals_auto_threshold, 0.5, "The threshold for totals_mode = 'auto'.", 0) \