Cutover with Writes Stopped but Reads Continued

For some applications, browsing services may remain available during cutover to ensure a smooth user experience, while write operations are temporarily halted to ensure data consistency. This is useful, for example, for some e-commerce apps. During such a cutover, customers can still browse the website but cannot place orders. If they do play an order, a friendly reminder is displayed telling them the system is upgrading and will recover at 4:00 a.m., for instance.

Comparing different ways of cutover with writes stopped but reads continued

Four options are available:

**Table 1** Four ways of cutting over a system with writes stopped but reads continued
Solution	Description	Scenarios	Operation Complexity	Changes Required
Gateway blocking	The service gateway blocks write requests but allow read requests at the access layer.	A unified gateway blocks write requests and returns friendly reminders for write requests.	Easy to use	No changes required
Blocking writes while allowing read requests	Shut down the write service or the corresponding interface, but keep the read service or the corresponding interface alive.	Read/write splitting is enabled for applications, so that each service allows only read or write operations.	Easy to use	No changes required
Enable read/write splitting for applications first	Modify the application code to enable read/write splitting.	Read/write splitting is not enabled for applications.	Complex	Heavy workload
Revoke write permissions at the middleware and data layers	Revoke the write permissions for related accounts at the middleware and data layers.	When write permissions are revoked, the application system will report errors. Minor changes are needed to handle the error.	Easy to use	Minor changes required

Gateway blocking
The service gateway (such as Gatekeeper, Zuul, and Kong) blocks write requests while allowing read requests. For example, the Gatekeeper gateway can block POST requests and allow only GET requests to pass through. This can be achieved by setting packet handling rules on Gatekeeper.
Figure 1 Gateway blocking

Disabling the write service
If read/write splitting is enabled, shut down the write service or the corresponding interface, but keep the read service or the corresponding interface alive. This makes applications read-only.

Figure 2 Shutting down the write service

App refactoring
Modify the application code to enable read/write splitting. Then use the solution above to enable read-only service.

Figure 3 Application refactoring

Configure read-only at the middleware and data layers
Revoke the write permissions for related accounts at the middleware and data layers.

Figure 4 Configuring read-only at the middleware and data layers