Multiple endpoints for connection.

ClickHouse / clickhouse-cpp

C++ client library for ClickHouse

Apache License 2.0

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Multiple endpoints for connection. #310

Closed MikhailBurdukov closed 1 year ago

MikhailBurdukov commented 1 year ago

Specify multiple hosts for connection, to provide fault tolerance.

.SetHosts({ getEnvOrDefault("CLICKHOUSE_HOST",      "asasdasd"), 
                            getEnvOrDefault("CLICKHOUSE_HOST",      "localhost"),
                            getEnvOrDefault("CLICKHOUSE_HOST",      "noalocalhost"), 
                            getEnvOrDefault("CLICKHOUSE_HOST",      "localhost"), 
                           })

The endpoint is selecting according to EndpointsIterationAlgorithm, rn only RoundRobin approach is implemented.

Ref #139

MikhailBurdukov commented 1 year ago

In the last commit, a behavior was added that allows users to reset the connection to a connected endpoint, which is described here. I also checked manually that the client is trying to reconnect to the same endpoint using options_.send_retries, but I don't know how to implement this in the test, because as far as I understand, there are no tests that allow you to manipulate the connection or the `ClickHouse-server' directly. Does anyone have some ideas how to test such situation?

MikhailBurdukov commented 1 year ago

Discussed in direct messages:

[x] Connection format: N times to all hosts. Example with 3 endpoints and N = 2 : 1- 2 -3 -1 - 2 -3.
[x] Iterator must have only one method: virtual Endpoint Next() = 0; and it is an endless generator of endpoints.
[x] The logic of connection control in Client Impl. Number of connection attempts = endpoints.size() * send_retry
[x] If the host has become unavailable, we first try to recreate the connection with it, if it did not work, then we try to find a new one.

All this is done in the last commit