ProxySQL通过Monitor模块监控后端MySQL Server的read_only值来自动调整节点所属的组。所以，在配置读、写组之前，必须先配置好监控。
首先看下Monitor库中的表：
admin> show tables from monitor;
+------------------------------------+
| tables |
+------------------------------------+
| mysql_server_connect_log |
| mysql_server_group_replication_log |
| mysql_server_ping_log |
| mysql_server_read_only_log |
| mysql_server_replication_lag_log |
+------------------------------------+
Monitor监控4种指标：connect、ping、read_only和replication lag。
1.connect监控
ProxySQL连接到各后端是否成功，成功/失败的连接将记录到表mysql_server_connect_log中。
2.ping监控
这是一种心跳检测。Monitor模块向所有后端MySQL节点发起ping检查，ping成功/失败的情况将记录到表mysql_server_ping_log中。当ping某后端的失败次数达到了mysql-monitor_ping_max_failures时表示失去心跳，将发送一个信号给MySQL的主机组管理器来杀掉和该后端节点的所有连接。
请和connect监控区分开，connect监控是通过建立连接来检测和后端节点连接的连通性。ping监控是心跳检测，ProxySQL通过MySQL的一个ping API发送给后端MySQL服务上，然后等待ping回复，虽然是ping检测，但也是需要建立连接的。
所以，有两个确定连接不可用公式：
- mysql-monitor_ping_max_failures * mysql-monitor_connect_timeout
- mysql-monitor_ping_max_failures * mysql-monitor_ping_timeout
mysql> select * from global_variables;
+----------------------------------------------------------------------+--------------------------------------------+
| variable_name | variable_value |
+----------------------------------------------------------------------+--------------------------------------------+
| admin-stats_credentials | stats:stats |
| admin-stats_mysql_connections | 60 |
| admin-stats_mysql_connection_pool | 60 |
| admin-stats_mysql_query_cache | 60 |
| admin-stats_mysql_query_digest_to_disk | 0 |
| admin-stats_system_cpu | 60 |
| admin-stats_system_memory | 60 |
| admin-telnet_admin_ifaces | (null) |
| admin-telnet_stats_ifaces | (null) |
| admin-refresh_interval | 2000 |
| admin-read_only | false |
| admin-hash_passwords | true |
| admin-vacuum_stats | true |
| admin-version | 2.4.1-1-g1ea371d |
| admin-cluster_mysql_variables_diffs_before_sync | 3 |
| admin-cluster_admin_variables_diffs_before_sync | 3 |
| admin-cluster_ldap_variables_diffs_before_sync | 3 |
| admin-cluster_mysql_variables_save_to_disk | true |
| admin-cluster_admin_variables_save_to_disk | true |
| admin-cluster_ldap_variables_save_to_disk | true |
| admin-checksum_mysql_query_rules | true |
| admin-checksum_mysql_servers | true |
| admin-checksum_mysql_users | true |
| admin-checksum_mysql_variables | true |
| admin-checksum_admin_variables | true |
| admin-checksum_ldap_variables | true |
| admin-restapi_enabled | false |
| admin-restapi_port | 6070 |
| admin-web_enabled | true |
| admin-web_port | 6080 |
| admin-web_verbosity | 0 |
| admin-prometheus_memory_metrics_interval | 61 |
| admin-admin_credentials | admin:admin;cluster_20X:123456 |
| admin-mysql_ifaces | 0.0.0.0:6032 |
| admin-cluster_username | cluster_20X |
| admin-cluster_password | 123456 |
| admin-cluster_check_interval_ms | 200 |
| admin-cluster_check_status_frequency | 100 |
| admin-cluster_mysql_query_rules_save_to_disk | true |
| admin-cluster_mysql_servers_save_to_disk | true |
| admin-cluster_mysql_users_save_to_disk | true |
| admin-cluster_proxysql_servers_save_to_disk | true |
| admin-cluster_mysql_query_rules_diffs_before_sync | 3 |
| admin-cluster_mysql_servers_diffs_before_sync | 3 |
| admin-cluster_mysql_users_diffs_before_sync | 3 |
| admin-cluster_proxysql_servers_diffs_before_sync | 3 |
| mysql-default_charset | utf8 |
| mysql-default_collation_connection | utf8_general_ci |
| mysql-shun_on_failures | 5 |
| mysql-shun_recovery_time_sec | 10 |
| mysql-unshun_algorithm | 0 |
| mysql-query_retries_on_failure | 1 |
| mysql-client_host_cache_size | 0 |
| mysql-client_host_error_counts | 0 |
| mysql-connect_retries_delay | 1 |
| mysql-connection_delay_multiplex_ms | 0 |
| mysql-connection_max_age_ms | 0 |
| mysql-connect_timeout_client | 10000 |
| mysql-connect_timeout_server_max | 10000 |
| mysql-enable_client_deprecate_eof | true |
| mysql-enable_server_deprecate_eof | true |
| mysql-enable_load_data_local_infile | false |
| mysql-eventslog_filename | |
| mysql-eventslog_filesize | 104857600 |
| mysql-eventslog_default_log | 0 |
| mysql-eventslog_format | 1 |
| mysql-auditlog_filename | |
| mysql-auditlog_filesize | 104857600 |
| mysql-handle_unknown_charset | 1 |
| mysql-free_connections_pct | 10 |
| mysql-connection_warming | false |
| mysql-session_idle_ms | 1 |
| mysql-have_ssl | false |
| mysql-log_mysql_warnings_enabled | false |
| mysql-monitor_enabled | true |
| mysql-monitor_connect_timeout | 600 |
| mysql-monitor_ping_max_failures | 3 |
| mysql-monitor_ping_timeout | 1000 |
| mysql-monitor_read_only_max_timeout_count | 3 |
| mysql-monitor_replication_lag_group_by_host | false |
| mysql-monitor_replication_lag_interval | 10000 |
| mysql-monitor_replication_lag_timeout | 1000 |
| mysql-monitor_replication_lag_count | 1 |
| mysql-monitor_groupreplication_healthcheck_interval | 5000 |
| mysql-monitor_groupreplication_healthcheck_timeout | 800 |
| mysql-monitor_groupreplication_healthcheck_max_timeout_count | 3 |
| mysql-monitor_groupreplication_max_transactions_behind_count | 3 |
| mysql-monitor_groupreplication_max_transactions_behind_for_read_only | 1 |
| mysql-monitor_galera_healthcheck_interval | 5000 |
| mysql-monitor_galera_healthcheck_timeout | 800 |
| mysql-monitor_galera_healthcheck_max_timeout_count | 3 |
| mysql-monitor_replication_lag_use_percona_heartbeat | |
| mysql-monitor_query_interval | 60000 |
| mysql-monitor_query_timeout | 100 |
| mysql-monitor_slave_lag_when_null | 60 |
| mysql-monitor_threads_min | 8 |
| mysql-monitor_threads_max | 128 |
| mysql-monitor_threads_queue_maxsize | 128 |
| mysql-monitor_wait_timeout | true |
| mysql-monitor_writer_is_also_reader | true |
| mysql-max_allowed_packet | 67108864 |
| mysql-tcp_keepalive_time | 0 |
| mysql-use_tcp_keepalive | false |
| mysql-automatic_detect_sqli | false |
| mysql-firewall_whitelist_enabled | false |
| mysql-firewall_whitelist_errormsg | Firewall blocked this query |
| mysql-throttle_connections_per_sec_to_hostgroup | 1000000 |
| mysql-max_transaction_idle_time | 14400000 |
| mysql-max_transaction_time | 14400000 |
| mysql-multiplexing | true |
| mysql-log_unhealthy_connections | true |
| mysql-enforce_autocommit_on_reads | false |
| mysql-autocommit_false_not_reusable | false |
| mysql-autocommit_false_is_transaction | false |
| mysql-verbose_query_error | false |
| mysql-hostgroup_manager_verbose | 1 |
| mysql-binlog_reader_connect_retry_msec | 3000 |
| mysql-threshold_query_length | 524288 |
| mysql-threshold_resultset_size | 4194304 |
| mysql-query_digests_max_digest_length | 2048 |
| mysql-query_digests_max_query_length | 65000 |
| mysql-query_digests_grouping_limit | 3 |
| mysql-query_digests_groups_grouping_limit | 0 |
| mysql-wait_timeout | 28800000 |
| mysql-throttle_max_bytes_per_second_to_client | 0 |
| mysql-throttle_ratio_server_to_client | 0 |
| mysql-max_stmts_per_connection | 20 |
| mysql-max_stmts_cache | 10000 |
| mysql-mirror_max_concurrency | 16 |
| mysql-mirror_max_queue_length | 32000 |
| mysql-default_max_latency_ms | 1000 |
| mysql-query_processor_iterations | 0 |
| mysql-query_processor_regex | 1 |
| mysql-set_query_lock_on_hostgroup | 0 |
| mysql-reset_connection_algorithm | 2 |
| mysql-auto_increment_delay_multiplex | 5 |
| mysql-auto_increment_delay_multiplex_timeout_ms | 10000 |
| mysql-long_query_time | 1000 |
| mysql-query_cache_size_MB | 256 |
| mysql-poll_timeout_on_failure | 100 |
| mysql-keep_multiplexing_variables | tx_isolation,transaction_isolation,version |
| mysql-kill_backend_connection_when_disconnect | true |
| mysql-client_session_track_gtid | true |
| mysql-session_idle_show_processlist | true |
| mysql-show_processlist_extended | 0 |
| mysql-query_digests | true |
| mysql-query_digests_lowercase | false |
| mysql-query_digests_replace_null | false |
| mysql-query_digests_no_digits | false |
| mysql-query_digests_normalize_digest_text | false |
| mysql-query_digests_track_hostname | false |
| mysql-query_digests_keep_comment | false |
| mysql-servers_stats | true |
| mysql-default_reconnect | true |
| mysql-ssl_p2s_ca | |
| mysql-ssl_p2s_capath | |
| mysql-ssl_p2s_cert | |
| mysql-ssl_p2s_key | |
| mysql-ssl_p2s_cipher | |
| mysql-ssl_p2s_crl | |
| mysql-ssl_p2s_crlpath | |
| mysql-init_connect | |
| mysql-ldap_user_variable | |
| mysql-add_ldap_user_comment | |
| mysql-default_tx_isolation | READ-COMMITTED |
| mysql-default_session_track_gtids | OFF |
| mysql-connpoll_reset_queue_length | 50 |
| mysql-min_num_servers_lantency_awareness | 1000 |
| mysql-aurora_max_lag_ms_only_read_from_replicas | 2 |
| mysql-stats_time_backend_query | false |
| mysql-stats_time_query_processor | false |
| mysql-query_cache_stores_empty_result | true |
| mysql-threads | 4 |
| mysql-max_connections | 2048 |
| mysql-default_query_delay | 0 |
| mysql-default_query_timeout | 36000000 |
| mysql-have_compress | true |
| mysql-poll_timeout | 2000 |
| mysql-interfaces | 0.0.0.0:6033 |
| mysql-default_schema | information_schema |
| mysql-stacksize | 1048576 |
| mysql-server_version | 5.7.22 |
| mysql-connect_timeout_server | 3000 |
| mysql-monitor_username | proxysql |
| mysql-monitor_password | iD!^^EjU#Yxr5$p |
| mysql-monitor_history | 600000 |
| mysql-monitor_connect_interval | 60000 |
| mysql-monitor_ping_interval | 10000 |
| mysql-monitor_read_only_interval | 1500 |
| mysql-monitor_read_only_timeout | 500 |
| mysql-ping_interval_server_msec | 120000 |
| mysql-ping_timeout_server | 500 |
| mysql-commands_stats | true |
| mysql-sessions_sort | true |
| mysql-connect_retries_on_failure | 10 |
| mysql-server_capabilities | 569899 |
+----------------------------------------------------------------------+--------------------------------------------+
196 rows in set (0.00 sec)
3.read_only监控
检查mysql_replication_hostgroups表中所有节点的read_only值，并记录到mysql_server_read_only_log。如果read_only=1，表示只读，是一个slave，这样的节点将会自动移入reader_hostgroup中，如果read_only=0，表示可写，可能是master，这样的节点将会自动移入writer_hostgroup中。
4.replication lag监控
对mysql_servers表中所有配置了max_replication_lag的后端slave节点都检查复制延迟，通过show slave status返回结果中的Seconds_Behind_Master字段，判断slave和master之间的延迟程度，并记录到mysql_server_replication_lag_log表中。
如果Seconds_Behind_Master > max_replication_lag，表示该slave延迟很严重，ProxySQL会自动避开这种slave节点，直到Seconds_Behind_Master < max_replication_lag。
Monitor监控上述指标时，会使用MySQL节点上的用户连接到后端节点，所以，需要先在后端节点上创建负责监控的用户。监控connect、ping和read_only这3项指标时，该用户只需具有USAGE权限，如果要监控replication lag指标，则需要replication client权限。