Ever wonder how replication happens between Master and Replica? How changes on Master propagates to Replica?
This is a short thread of how it happens
#systemdesign #distributedsystems
🧵
This is a short thread of how it happens
#systemdesign #distributedsystems
🧵
Any write operation happening on the Master is logged in the Replication log file as an event. The format in which these events are logged in the Log file is called Replication Format.
The two common Replication formats:
- Statement-based format
- Row-based format
The two common Replication formats:
- Statement-based format
- Row-based format
✨ Statement-based Format
The Master records the operation as an event in its log, and when the Replica reads this log, it executes the same operation on its copy of data.
This way, the operation on the Master is executed on the Replica, which keeps it in sync with the Master.
The Master records the operation as an event in its log, and when the Replica reads this log, it executes the same operation on its copy of data.
This way, the operation on the Master is executed on the Replica, which keeps it in sync with the Master.
UPDATE tasks SET is_done = true WHERE user_id = 53;
is logged as
UPDATE tasks SET is_done = true WHERE user_id = 53;
is logged as
UPDATE tasks SET is_done = true WHERE user_id = 53;
👉 Advantages of Statement-based Replication:
- Smaller log files
- Log files can be used to audit the database
- Smaller log files
- Log files can be used to audit the database
👉 Disadvantages of Statement-based Replication:
- Non-deterministic operations like RAND(), UUID(), will yield different values on Master and Replica
- The Replica lag depends on the load and concurrent queries executing during replication.
- Non-deterministic operations like RAND(), UUID(), will yield different values on Master and Replica
- The Replica lag depends on the load and concurrent queries executing during replication.
✨ Row-based Format
The Master logs the updates on the individual data item instead of the operation.
When the Replica reads this log, it updates its copy of the data by applying the changes on its data items. This way the Replica remains in sync with the Master.
The Master logs the updates on the individual data item instead of the operation.
When the Replica reads this log, it updates its copy of the data by applying the changes on its data items. This way the Replica remains in sync with the Master.
UPDATE tasks SET is_done = true WHERE user_id = 53;
is logged as
tasks:121 is_done=true
tasks:142 is_done=true
tasks:643 is_done=true
tasks:713 is_done=true
tasks:862 is_done=true
is logged as
tasks:121 is_done=true
tasks:142 is_done=true
tasks:643 is_done=true
tasks:713 is_done=true
tasks:862 is_done=true
👉 Advantages
- changes can be safely and predictably applied on the Replica
- locks are fewer and shorter
- changes can be safely and predictably applied on the Replica
- locks are fewer and shorter
👉 Disadvantages
- If an operation affects 5000 rows, the Master would create 5000 entries in the log file
- longer lock taken during logging affects the throughput
- If an operation affects 5000 rows, the Master would create 5000 entries in the log file
- longer lock taken during logging affects the throughput
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