Intercept traffic

[mostafa]

Add a way to influence the behavior of the code in server, proxy and client via current and future hooks, like how onConfigLoaded updates the global config. For example, if certain plugins, like a caching plugin, is to be introduced, it should be able to intercept and terminate the connection to the database and return the result from the cache. This is not possible right now.

Tasks

• Redesign hook system #91 (this has to be done at a later stage)
  • Cleanup interfaces #94
  • Slight refactoring of the hook system plus adding termination logic #96
• Dependency injection #93 (feels superfluous)

[mostafa]

Update

After changes to the code in #94 and #96, it is possible to terminate the connection before the request even reaches the database server. To achieve this, the plugins needs to have the OnTrafficFromClient RPC endpoint and also expose the hook in their config. Then they can achieve connection termination by setting two variables in the return value of the RPC endpoint:

return structpb.NewStruct(map[string]interface{}{
    "terminate": true,
    "response":  base64.StdEncoding.EncodeToString([]byte{'X', 0, 0, 0, 4}),
})

The terminate variable instructs the proxy to stop sending the received data (from client) to the server. The response variable will be sent back to the client, instead. If there is no response in the return value, the connection from the client will close abruptly.