ClickHouse/src/Common/PoolWithFailoverBase.h

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#pragma once
#include <time.h>
#include <cstdlib>
#include <climits>
#include <random>
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#include <functional>
#include <common/types.h>
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#include <ext/scope_guard.h>
#include <Core/Types.h>
#include <Common/PoolBase.h>
#include <Common/ProfileEvents.h>
#include <Common/NetException.h>
#include <Common/Exception.h>
#include <Common/randomSeed.h>
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namespace DB
{
namespace ErrorCodes
{
extern const int ALL_CONNECTION_TRIES_FAILED;
extern const int ALL_REPLICAS_ARE_STALE;
extern const int LOGICAL_ERROR;
}
}
namespace ProfileEvents
{
extern const Event DistributedConnectionFailTry;
extern const Event DistributedConnectionFailAtAll;
}
/// This class provides a pool with fault tolerance. It is used for pooling of connections to replicated DB.
/// Initialized by several PoolBase objects.
/// When a connection is requested, tries to create or choose an alive connection from one of the nested pools.
/// Pools are tried in the order consistent with lexicographical order of (error count, priority, random number) tuples.
/// Number of tries for a single pool is limited by max_tries parameter.
/// The client can set nested pool priority by passing a GetPriority functor.
///
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/// NOTE: if one of the nested pools blocks because it is empty, this pool will also block.
///
/// The client must provide a TryGetEntryFunc functor, which should perform a single try to get a connection from a nested pool.
/// This functor can also check if the connection satisfies some eligibility criterion (e.g. check if
/// the replica is up-to-date).
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template <typename TNestedPool>
class PoolWithFailoverBase : private boost::noncopyable
{
public:
using NestedPool = TNestedPool;
using NestedPoolPtr = std::shared_ptr<NestedPool>;
using Entry = typename NestedPool::Entry;
using NestedPools = std::vector<NestedPoolPtr>;
PoolWithFailoverBase(
NestedPools nested_pools_,
time_t decrease_error_period_,
size_t max_error_cap_,
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Poco::Logger * log_)
: nested_pools(std::move(nested_pools_))
, decrease_error_period(decrease_error_period_)
, max_error_cap(max_error_cap_)
, shared_pool_states(nested_pools.size())
, log(log_)
{
}
struct TryResult
{
TryResult() = default;
explicit TryResult(Entry entry_)
: entry(std::move(entry_))
, is_usable(true)
, is_up_to_date(true)
{
}
void reset()
{
entry = Entry();
is_usable = false;
is_up_to_date = false;
staleness = 0.0;
}
Entry entry;
bool is_usable = false; /// If false, the entry is unusable for current request
/// (but may be usable for other requests, so error counts are not incremented)
bool is_up_to_date = false; /// If true, the entry is a connection to up-to-date replica.
double staleness = 0.0; /// Helps choosing the "least stale" option when all replicas are stale.
};
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/// This functor must be provided by a client. It must perform a single try that takes a connection
/// from the provided pool and checks that it is good.
using TryGetEntryFunc = std::function<TryResult(NestedPool & pool, std::string & fail_message)>;
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/// The client can provide this functor to affect load balancing - the index of a pool is passed to
/// this functor. The pools with lower result value will be tried first.
using GetPriorityFunc = std::function<size_t(size_t index)>;
/// Returns a single connection.
Entry get(const TryGetEntryFunc & try_get_entry, const GetPriorityFunc & get_priority = GetPriorityFunc());
/// Returns at least min_entries and at most max_entries connections (at most one connection per nested pool).
/// The method will throw if it is unable to get min_entries alive connections or
/// if fallback_to_stale_replicas is false and it is unable to get min_entries connections to up-to-date replicas.
std::vector<TryResult> getMany(
size_t min_entries, size_t max_entries, size_t max_tries,
const TryGetEntryFunc & try_get_entry,
const GetPriorityFunc & get_priority = GetPriorityFunc(),
bool fallback_to_stale_replicas = true);
protected:
struct PoolState;
using PoolStates = std::vector<PoolState>;
/// This function returns a copy of pool states to avoid race conditions when modifying shared pool states.
PoolStates updatePoolStates();
PoolStates getPoolStates() const;
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NestedPools nested_pools;
const time_t decrease_error_period;
const size_t max_error_cap;
mutable std::mutex pool_states_mutex;
PoolStates shared_pool_states;
/// The time when error counts were last decreased.
time_t last_error_decrease_time = 0;
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Poco::Logger * log;
};
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template <typename TNestedPool>
typename TNestedPool::Entry
PoolWithFailoverBase<TNestedPool>::get(const TryGetEntryFunc & try_get_entry, const GetPriorityFunc & get_priority)
{
std::vector<TryResult> results = getMany(1, 1, 1, try_get_entry, get_priority);
if (results.empty() || results[0].entry.isNull())
throw DB::Exception(
"PoolWithFailoverBase::getMany() returned less than min_entries entries.",
DB::ErrorCodes::LOGICAL_ERROR);
return results[0].entry;
}
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template <typename TNestedPool>
std::vector<typename PoolWithFailoverBase<TNestedPool>::TryResult>
PoolWithFailoverBase<TNestedPool>::getMany(
size_t min_entries, size_t max_entries, size_t max_tries,
const TryGetEntryFunc & try_get_entry,
const GetPriorityFunc & get_priority,
bool fallback_to_stale_replicas)
{
/// Update random numbers and error counts.
PoolStates pool_states = updatePoolStates();
if (get_priority)
{
for (size_t i = 0; i < pool_states.size(); ++i)
pool_states[i].priority = get_priority(i);
}
struct ShuffledPool
{
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NestedPool * pool{};
const PoolState * state{};
size_t index = 0;
size_t error_count = 0;
};
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/// Sort the pools into order in which they will be tried (based on respective PoolStates).
std::vector<ShuffledPool> shuffled_pools;
shuffled_pools.reserve(nested_pools.size());
for (size_t i = 0; i < nested_pools.size(); ++i)
shuffled_pools.push_back(ShuffledPool{nested_pools[i].get(), &pool_states[i], i, 0});
std::sort(
shuffled_pools.begin(), shuffled_pools.end(),
[](const ShuffledPool & lhs, const ShuffledPool & rhs)
{
return PoolState::compare(*lhs.state, *rhs.state);
});
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/// We will try to get a connection from each pool until a connection is produced or max_tries is reached.
std::vector<TryResult> try_results(shuffled_pools.size());
size_t entries_count = 0;
size_t usable_count = 0;
size_t up_to_date_count = 0;
size_t failed_pools_count = 0;
/// At exit update shared error counts with error counts occurred during this call.
SCOPE_EXIT(
{
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std::lock_guard lock(pool_states_mutex);
for (const ShuffledPool & pool: shuffled_pools)
{
auto & pool_state = shared_pool_states[pool.index];
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pool_state.error_count = std::min<UInt64>(max_error_cap, pool_state.error_count + pool.error_count);
}
});
std::string fail_messages;
bool finished = false;
while (!finished)
{
for (size_t i = 0; i < shuffled_pools.size(); ++i)
{
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if (up_to_date_count >= max_entries /// Already enough good entries.
|| entries_count + failed_pools_count >= nested_pools.size()) /// No more good entries will be produced.
{
finished = true;
break;
}
ShuffledPool & shuffled_pool = shuffled_pools[i];
TryResult & result = try_results[i];
if (shuffled_pool.error_count >= max_tries || !result.entry.isNull())
continue;
std::string fail_message;
result = try_get_entry(*shuffled_pool.pool, fail_message);
if (!fail_message.empty())
fail_messages += fail_message + '\n';
if (!result.entry.isNull())
{
++entries_count;
if (result.is_usable)
{
++usable_count;
if (result.is_up_to_date)
++up_to_date_count;
}
}
else
{
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LOG_WARNING(log, "Connection failed at try №{}, reason: {}", (shuffled_pool.error_count + 1), fail_message);
ProfileEvents::increment(ProfileEvents::DistributedConnectionFailTry);
shuffled_pool.error_count = std::min(max_error_cap, shuffled_pool.error_count + 1);
if (shuffled_pool.error_count >= max_tries)
{
++failed_pools_count;
ProfileEvents::increment(ProfileEvents::DistributedConnectionFailAtAll);
}
}
}
}
if (usable_count < min_entries)
throw DB::NetException(
"All connection tries failed. Log: \n\n" + fail_messages + "\n",
DB::ErrorCodes::ALL_CONNECTION_TRIES_FAILED);
try_results.erase(
std::remove_if(
try_results.begin(), try_results.end(),
[](const TryResult & r) { return r.entry.isNull() || !r.is_usable; }),
try_results.end());
/// Sort so that preferred items are near the beginning.
std::stable_sort(
try_results.begin(), try_results.end(),
[](const TryResult & left, const TryResult & right)
{
return std::forward_as_tuple(!left.is_up_to_date, left.staleness)
< std::forward_as_tuple(!right.is_up_to_date, right.staleness);
});
if (fallback_to_stale_replicas)
{
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/// There is not enough up-to-date entries but we are allowed to return stale entries.
/// Gather all up-to-date ones and least-bad stale ones.
size_t size = std::min(try_results.size(), max_entries);
try_results.resize(size);
}
else if (up_to_date_count >= min_entries)
{
/// There is enough up-to-date entries.
try_results.resize(up_to_date_count);
}
else
throw DB::Exception(
"Could not find enough connections to up-to-date replicas. Got: " + std::to_string(up_to_date_count)
+ ", needed: " + std::to_string(min_entries),
DB::ErrorCodes::ALL_REPLICAS_ARE_STALE);
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return try_results;
}
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template <typename TNestedPool>
struct PoolWithFailoverBase<TNestedPool>::PoolState
{
UInt64 error_count = 0;
Int64 priority = 0;
UInt32 random = 0;
void randomize()
{
random = rng();
}
static bool compare(const PoolState & lhs, const PoolState & rhs)
{
return std::forward_as_tuple(lhs.error_count, lhs.priority, lhs.random)
< std::forward_as_tuple(rhs.error_count, rhs.priority, rhs.random);
}
private:
std::minstd_rand rng = std::minstd_rand(randomSeed());
};
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template <typename TNestedPool>
typename PoolWithFailoverBase<TNestedPool>::PoolStates
PoolWithFailoverBase<TNestedPool>::updatePoolStates()
{
PoolStates result;
result.reserve(nested_pools.size());
{
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std::lock_guard lock(pool_states_mutex);
for (auto & state : shared_pool_states)
state.randomize();
time_t current_time = time(nullptr);
if (last_error_decrease_time)
{
time_t delta = current_time - last_error_decrease_time;
if (delta >= 0)
{
const UInt64 MAX_BITS = sizeof(UInt64) * CHAR_BIT;
size_t shift_amount = MAX_BITS;
/// Divide error counts by 2 every decrease_error_period seconds.
if (decrease_error_period)
shift_amount = delta / decrease_error_period;
/// Update time but don't do it more often than once a period.
/// Else if the function is called often enough, error count will never decrease.
if (shift_amount)
last_error_decrease_time = current_time;
if (shift_amount >= MAX_BITS)
{
for (auto & state : shared_pool_states)
state.error_count = 0;
}
else if (shift_amount)
{
for (auto & state : shared_pool_states)
state.error_count >>= shift_amount;
}
}
}
else
last_error_decrease_time = current_time;
result.assign(shared_pool_states.begin(), shared_pool_states.end());
}
return result;
}
template <typename TNestedPool>
typename PoolWithFailoverBase<TNestedPool>::PoolStates
PoolWithFailoverBase<TNestedPool>::getPoolStates() const
{
std::lock_guard lock(pool_states_mutex);
return shared_pool_states;
}