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#include <Common/GetPriorityForLoadBalancing.h>
#include <Common/Priority.h>
namespace DB
{
namespace ErrorCodes
{
extern const int LOGICAL_ERROR;
}
std::function<Priority(size_t index)> GetPriorityForLoadBalancing::getPriorityFunc(LoadBalancing load_balance, size_t offset, size_t pool_size) const
{
std::function<Priority(size_t index)> get_priority;
switch (load_balance)
{
case LoadBalancing::NEAREST_HOSTNAME:
if (hostname_differences.empty())
throw Exception(ErrorCodes::LOGICAL_ERROR, "It's a bug: hostname_differences is not initialized");
get_priority = [this](size_t i) { return Priority{static_cast<Int64>(hostname_differences[i])}; };
break;
case LoadBalancing::IN_ORDER:
get_priority = [](size_t i) { return Priority{static_cast<Int64>(i)}; };
break;
case LoadBalancing::RANDOM:
break;
case LoadBalancing::FIRST_OR_RANDOM:
get_priority = [offset](size_t i) { return i != offset ? Priority{1} : Priority{0}; };
break;
case LoadBalancing::ROUND_ROBIN:
if (last_used >= pool_size)
last_used = 0;
++last_used;
/* Consider pool_size equals to 5
* last_used = 1 -> get_priority: 0 1 2 3 4
* last_used = 2 -> get_priority: 4 0 1 2 3
* last_used = 3 -> get_priority: 4 3 0 1 2
* ...
* */
get_priority = [this, pool_size](size_t i)
{
++i; // To make `i` indexing start with 1 instead of 0 as `last_used` does
return Priority{static_cast<Int64>(i < last_used ? pool_size - i : i - last_used)};
};
break;
}
return get_priority;
}
}
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