Send.timeout - #20
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Only affects elysium_buffer_sup.erl.
Elysium_channel is a replacement for seestar_session which avoids using the multiplexing logic for Cassandra queries. This logic is brittle in the face of failure, and the elysium approach avoids reusing a Cassandra connection for multiple simultaneous requests. The channel is essentially a copy of seestar_session with some code removed, so the compilation environment needed to handle R16 vs R17 datatypes, thus the new elysium_compile_utils module. Epocxy was upgraded to 0.9.8 mostly to allow the introduction of synchronization barriers for testing purposes. As of yet, there is no real dependency on 0.9.8 but other projects which use elysium needed the latest version. Seestar was upgraded to tigertext 0.1.1 which is a fork of iamaleksey/seestar master. The introduction of elysium_channel is incompatible with seestar_session in that mixing the two types of sessions is not a good idea. The changes tigertext wanted for seestar_session (send timeouts and a reduced receive timeout) can only be applied properly in a world of single requests on a socket. The multiplexing approach must wait forever because it cannot distinguish timeouts on all the outstanding requests. The brittleness in seestar is due to the fact that all the pending requests are kept in a gen_server state, with overflow going to an ets table. If the seestar_session gen_server goes down it will take all the requests that are waiting for replies with it. Worse, since the gen_server owns the ets table, it will also take down all the pending requests that haven't been sent to Cassandra. In process Cassandra requests have to be lost in any case since the socket will be closed, but the untransmitted requests should still be able to be managed. In the end, having monoplexed requests makes recoverability and resilience easier, plus it allows us to rely on normal gen_server and gen_tcp:send timeouts to identify when the connection to Cassandra is not working normally.
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Introduce elysium_buffering_strategy behaviour
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