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MXS-2300: Add session command pruning

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This commit adds a new parameter that, when enabled, prunes the session
command history to a known length. This makes it possible to keep a
client-side pooled connection open indefinitely at the cost of making
reconnections theoretically unsafe. In practice the maximum history length
can be set to a value that encompasses a single session using the pooled
connection with no risk to session state integrity. The default history
length of 50 commands is quite likely to be adequate for the majority of
use-cases.
This commit is contained in:
Markus Mäkelä 2019-01-30 09:14:13 +02:00
parent bf4aa1ab2c
commit 260ce9b8b8
No known key found for this signature in database
GPG Key ID: 72D48FCE664F7B19
5 changed files with 66 additions and 12 deletions
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33
Documentation/Routers/ReadWriteSplit.md
View File

@ -22,6 +22,7 @@ Table of Contents
* [Interaction Between slave_selection_criteria and max_slave_connections](#interaction-between-slave_selection_criteria-and-max_slave_connections)
* [max_sescmd_history](#max_sescmd_history)
* [disable_sescmd_history](#disable_sescmd_history)
* [prune_sescmd_history](#prune_sescmd_history)
* [master_accept_reads](#master_accept_reads)
* [strict_multi_stmt](#strict_multi_stmt)
* [strict_sp_calls](#strict_sp_calls)
@ -318,6 +319,38 @@ default of 50 session commands after which the history is disabled.
disable_sescmd_history=true
```
### `prune_sescmd_history`
This option prunes the session command history when it exceeds the value
configured in `max_sescmd_history`. When this option is enabled, only a set
number of statements are stored in the history. This limits the per-session
memory use while still allowing safe reconnections. This parameter was added in
MaxScale 2.3.4 and is disabled by default.
This parameter is intended to be used with pooled connections that remain in use
for a very long time. Most connection pool implementations do not reset the
session state and instead re-initialize it with new values. This causes the
session command history to grow at roughly a constant rate for the lifetime of
the pooled connection.
Each client-side session that uses a pooled connection only executes a finite
amount of session commands. By retaining a shorter history that encompasses all
session commands the individual clients execute, the session state of a pooled
connection can be accurately recreated on another server.
If the session command history pruning is enabled, there is a theoretical
possibility that upon server reconnection the session states of the connections
are inconsistent. This can only happen if the length of the stored history is
shorter than the list of relevant statements that affect the session state. In
practice the default value of 50 session commands is a fairly reasonable value
and the risk of inconsistent session state is relatively low.
In case the default history length is too short for safe pruning, set the value
of `max_sescmd_history` to the total number of commands that affect the session
state plus a safety margin of 10. The safety margin reserves some extra space
for new commands that might be executed due to changes in the client side
application.
### `master_accept_reads`
**`master_accept_reads`** allows the master server to be used for reads. This is

4
server/modules/routing/readwritesplit/readwritesplit.cc
View File

@ -273,6 +273,9 @@ void RWSplit::diagnostics(DCB* dcb)
dcb_printf(dcb,
"\tstrict_sp_calls: %s\n",
cnf.strict_sp_calls ? "true" : "false");
dcb_printf(dcb,
"\tprune_sescmd_history: %s\n",
cnf.prune_sescmd_history ? "true" : "false");
dcb_printf(dcb,
"\tdisable_sescmd_history: %s\n",
cnf.disable_sescmd_history ? "true" : "false");
@ -504,6 +507,7 @@ extern "C" MXS_MODULE* MXS_CREATE_MODULE()
{"max_slave_replication_lag", MXS_MODULE_PARAM_INT, "-1" },
{"max_slave_connections", MXS_MODULE_PARAM_STRING, MAX_SLAVE_COUNT},
{"retry_failed_reads", MXS_MODULE_PARAM_BOOL, "true" },
{"prune_sescmd_history", MXS_MODULE_PARAM_BOOL, "false" },
{"disable_sescmd_history", MXS_MODULE_PARAM_BOOL, "false" },
{"max_sescmd_history", MXS_MODULE_PARAM_COUNT, "50" },
{"strict_multi_stmt", MXS_MODULE_PARAM_BOOL, "false" },

2
server/modules/routing/readwritesplit/readwritesplit.hh
View File

@ -151,6 +151,7 @@ struct Config
(enum failure_mode)config_get_enum(
params, "master_failure_mode", master_failure_mode_values))
, max_sescmd_history(config_get_integer(params, "max_sescmd_history"))
, prune_sescmd_history(config_get_bool(params, "prune_sescmd_history"))
, disable_sescmd_history(config_get_bool(params, "disable_sescmd_history"))
, master_accept_reads(config_get_bool(params, "master_accept_reads"))
, strict_multi_stmt(config_get_bool(params, "strict_multi_stmt"))
@ -205,6 +206,7 @@ struct Config
* master or all nodes */
failure_mode master_failure_mode; /**< Master server failure handling mode */
uint64_t max_sescmd_history; /**< Maximum amount of session commands to store */
bool prune_sescmd_history; /**< Prune session command history */
bool disable_sescmd_history;/**< Disable session command history */
bool master_accept_reads; /**< Use master for reads */
bool strict_multi_stmt; /**< Force non-multistatement queries to be routed to

38
server/modules/routing/readwritesplit/rwsplit_route_stmt.cc
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@ -393,6 +393,23 @@ void RWSplitSession::continue_large_session_write(GWBUF* querybuf, uint32_t type
}
}
void RWSplitSession::prune_to_position(uint64_t pos)
{
/** Prune all completed responses before a certain position */
ResponseMap::iterator it = m_sescmd_responses.lower_bound(pos);
if (it != m_sescmd_responses.end())
{
// Found newer responses that were returned after this position
m_sescmd_responses.erase(m_sescmd_responses.begin(), it);
}
else
{
// All responses are older than the requested position
m_sescmd_responses.clear();
}
}
/**
* Execute in backends used by current router session.
* Save session variable commands to router session property
@ -522,20 +539,17 @@ bool RWSplitSession::route_session_write(GWBUF* querybuf, uint8_t command, uint3
m_sescmd_list.clear();
}
if (m_config.prune_sescmd_history && !m_sescmd_list.empty()
&& m_sescmd_list.size() + 1 >= m_config.max_sescmd_history)
{
// Close to the history limit, remove the oldest command
prune_to_position(m_sescmd_list.front()->get_position());
m_sescmd_list.pop_front();
}
if (m_config.disable_sescmd_history)
{
/** Prune stored responses */
ResponseMap::iterator it = m_sescmd_responses.lower_bound(lowest_pos);
if (it != m_sescmd_responses.end())
{
m_sescmd_responses.erase(m_sescmd_responses.begin(), it);
}
else
{
// All responses processed
m_sescmd_responses.clear();
}
prune_to_position(lowest_pos);
}
else
{

1
server/modules/routing/readwritesplit/rwsplitsession.hh
View File

@ -182,6 +182,7 @@ private:
void process_sescmd_response(mxs::SRWBackend& backend, GWBUF** ppPacket);
void compress_history(mxs::SSessionCommand& sescmd);
void prune_to_position(uint64_t pos);
bool route_session_write(GWBUF* querybuf, uint8_t command, uint32_t type);
void continue_large_session_write(GWBUF* querybuf, uint32_t type);
bool route_single_stmt(GWBUF* querybuf);