【Distributed System】Replication - Leader-based Replication

Leader-based Replication (Active/passive or Master–slave Replication)

Each node that stores a copy of the database is called a replica.

Every write to the database needs to be processed by every replica; otherwise, the replicas would no longer contain the same data. The most common solution for this is called leader-based replication (also known as active/passive or master–slave replication)

One of the replicas is designated the leader (also known as master or primary). When clients want to write to the database, they must send their requests to the leader, which first writes the new data to its local storage.
The other replicas are known as followers (read replicas, slaves, secondaries, or hot standbys). Whenever the leader writes new data to its local storage, it also sends the data change to all of its followers as part of a replication log or change stream. Each follower takes the log from the leader and updates its local copy of the database accordingly, by applying all writes in the same order as they were processed on the leader.
When a client wants to read from the database, it can query either the leader or any of the followers. However, writes are only accepted on the leader (the followers are read-only from the client’s point of view).

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Posted by Wei on 2022-01-28, Last Modified on 2025-02-04

【Distributed System】Data Flow

Dataflow Through Databases

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Posted by Wei on 2022-01-27, Last Modified on 2025-04-23

【MySQL】常用 SQL 语句（Common SQL）- Display

显示DBs and Tables

mysql> show databases;
+----------------------------------+
| Database                         |
+----------------------------------+
| entry_task2_db                   |
| information_schema               |
| mysql                            |
| performance_schema               |
| sw_benchmark_test                |
| sw_benchmark_test_oltp_read_only |
| sw_test                          |
| sys                              |
+----------------------------------+

mysql> use sw_test;

mysql> SHOW TABLES;
+--------------------------------------------------+
| Tables_in_sw_test                                |
+--------------------------------------------------+
| logistic_promotion_seller_promo_budget_tab       |
| logistic_promotion_seller_promo_budget_usage_tab |
| logistic_promotion_seller_promo_channel_map_tab  |
| logistic_promotion_seller_promo_rule_map_tab     |
| logistic_promotion_seller_promo_tab              |
| seller_grouped_rule_budget_tab_00000004          |
| seller_grouped_rule_budget_usage_tab_00000005    |
| tb_person                                        |
| user                                             |
+--------------------------------------------------+
9 rows in set (0.00 sec)

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Posted by Wei on 2022-01-27, Last Modified on 2023-09-20

【Engineering】How to Write Code

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Posted by Wei on 2022-01-23, Last Modified on 2025-04-23

【Engineering】Logging

Log Levels

The log level is used to denote the severity of each event in the system. In most logging frameworks, the following levels are available.

FATAL: denotes very severe error events that will presumably lead the application to abort. Usually, these can end up in catastrophic failures.
ERROR: denotes error events that might still allow the application to continue running, with reduced capabilities in the affected paths.
WARN: denotes less-harmful events than errors. Usually, they do not lead to any degradation of capabilities or complete failure of the application. However, they are still red flags and must be investigated.
INFO: denotes the important event banners and informational messages in the application behaviour.
DEBUG: denotes specific and detailed information, mainly used for debugging purposes. These logs help us step through the code.
TRACE: denotes most low-level information like stack traces of code to provide the most information on a certain event/context. These logs help us inspect the variable values and full error stacks.

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