When using UPA the terminology in the RFA logs makes a bit more sense. Basically your app received a TCP ACK from server76 then sends an RSSL handshake, whilst attempting to send those first few bytes of information the socket was closed and thus the "channel flush" failed. The "flush" command is used to send buffered messages on the wire, you will rarely see a "channel write" failed.

I would like to see a test with only "server09, server10" in the "serverList" to verify if the API can connect at all. If these servers are skipped it is an implication that they cannot be resolved to an IPv4 target address.

It is highly recommended to reproduce the scenario with the latest 8.0 or 7.6 emergency loads to verify the exhibited behaviour is not an actual defect that has already been resolved.

Also, try using the IPv4 address instead of the hostname.

You can null route or reject the target server IP addresses to temporarily test the cold standby failover mechanism, e.g.

echo -n "Blocking connectivity to ads1"
route add -host ads1 reject
read -p " ... "
route delete -host ads1 reject

Thanks for your answer.

The day after this issue, it worked properly with the 2 last servers:

[09:13:07]: (ComponentName) Static: (Severity) Warning: RSSL Channel closing on connection "Default::Connection_Rssl_Sub server75:14002" due to "channel read failure"
[09:13:07]: (ComponentName) Static: (Severity) Information: Connection Status Changed, RSSL_Cons_Connection "Default::Connection_Rssl_Sub server75:14002" State: "Down" StatusCode: "None" StatusText: Connection down
[09:13:07]: (ComponentName) Static: (Severity) Information: RSSL_Cons_Connection "Default::Connection_Rssl_Sub server76:14002" will be tried in 100 milliseconds
[09:13:29]: (ComponentName) Static: (Severity) Warning: RSSL Channel closing on connection "Default::Connection_Rssl_Sub server76:14002" due to "channel disconnect"
[09:13:29]: (ComponentName) Static: (Severity) Information: Connection Status Changed, RSSL_Cons_Connection "Default::Connection_Rssl_Sub server76:14002" State: "Down" StatusCode: "None" StatusText: Connection down
[09:13:29]: (ComponentName) Static: (Severity) Information: RSSL_Cons_Connection "Default::Connection_Rssl_Sub server09:14002" will be tried in 100 milliseconds
[09:13:29]: (ComponentName) Static: (Severity) Information: RSSL channel init in progress for connection "Default::Connection_Rssl_Sub server09:14002".
[09:13:29]: (ComponentName) Static: (Severity) Information: RSSL channel connected for connection "Default::Connection_Rssl_Sub server09:14002".
[09:13:29]: (ComponentName) Static: (Severity) Information: RSSL channel on connection "Default::Connection_Rssl_Sub server09:14002" has negotiated ping timeout set to: 30000
[09:13:29]: (ComponentName) Static: (Severity) Information: Connection Status Changed, RSSL_Cons_Connection "Default::Connection_Rssl_Sub server09:14002" State: "Up" StatusCode: "None" StatusText: Connection up
[09:13:33]: (ComponentName) Static: (Severity) Information: RSSL Channel closing on connection "Default::Connection_Rssl_Sub server09:14002" due to "requested disconnect"
[09:16:51]: (ComponentName) Static: (Severity) Error: OMMConsumer::registerClient() has been called with an item request prior to login request. Login request is expected first.

If I try to null route the target server IP address, it works properly and it switches to the next server. In this example, the server list is mkdppsusr01d, mkdppsbck01d. The null route is configured for mkdppsusr01d only:

[16:29:48,865]: (ComponentName) Static: (Severity) Information: The RSSL_Cons_Adapter initialization succeeded
[16:29:48,865]: (ComponentName) Static: (Severity) Information: Using the following configuration for RSSL_Cons_Connection_Manager "Default::Connection_SSLED_RMDS_Dict": requestQueueReadThreshold = 1, watchListTableSize = 10000, requestTimeout = 45000, itemPostTimeout = 15000, payloadCacheEnabled = False, payloadCacheLoadFileDictionary = True, payloadCacheDictionaryName = RDMFieldDictionary, payloadCacheDictionaryPerService = False, serverList = { mkdppsusr01d, mkdppsbck01d }, Connection capabilities: OMMMsgConnCapability
[16:29:48,866]: (ComponentName) Static: (Severity) Warning: Batch request size may be limited due to throttling on RSSL_Cons_Connection "Default::Connection_SSLED_RMDS_Dict mkdppsusr01d:14002"
[16:29:48,867]: (ComponentName) Static: (Severity) Information: Using the following configuration for RSSL_Cons_Connection "Default::Connection_SSLED_RMDS_Dict mkdppsusr01d:14002": hostName = "mkdppsusr01d", rsslPort = "14002", interfaceName = "localhost", compressionType = 0, channelBlocking = false, recvBufSize = 65535, sendBufSize = 65535, writeBufferMaxFragmentLength = 6144, connectionWaitTimeout = 30000, connectionPingTimeout = 30000, guaranteedOutputBuffers = 400, numInputBuffers = 5, tcp_nodelay = True, throttleEnabled = True, throttleType = count, throttleMaxCount = 250, throttleBatchCount = 10, traceMsg = False, traceMsgToFile = False, dictionaryRequestTimeout = 45
[16:29:48,966]: (ComponentName) Static: (Severity) Information: RSSL channel init in progress for connection "Default::Connection_SSLED_RMDS_Dict mkdppsusr01d:14002".
[16:29:48,968]: (ComponentName) Static: (Severity) Information: RSSL channel connected for connection "Default::Connection_SSLED_RMDS_Dict mkdppsusr01d:14002".
[16:29:48,968]: (ComponentName) Static: (Severity) Information: RSSL channel on connection "Default::Connection_SSLED_RMDS_Dict mkdppsusr01d:14002" has negotiated ping timeout set to: 30000
[16:29:48,969]: (ComponentName) Static: (Severity) Information: Connection Status Changed, RSSL_Cons_Connection "Default::Connection_SSLED_RMDS_Dict mkdppsusr01d:14002" State: "Up" StatusCode: "None" StatusText: Connection up
[16:29:48,988]: Received an event for the connection with name 'Connection_SSLED_RMDS_Dict', with type 'RSSL', with state up, associated to the status text 'Connection up' and status code None 
[16:29:48,989]: (ComponentName) Static: (Severity) Warning: Connection "Connection_SSLED_RMDS_Dict" does not support views. View requests will be converted to full item requests for this connection.
[16:29:48,990]: (ComponentName) Static: (Severity) Warning: Connection "Connection_SSLED_RMDS_Dict" does not support Pause/Resume. Pause and Resume requests for this connection will be ignored.
[16:29:48,990]: (ComponentName) Static: (Severity) Warning: Connection "Connection_SSLED_RMDS_Dict" does not support optimized Pause/Resume. Optimized Pause/Resume requests will be converted to individual item Pause/Resume requests for this connection.

Regards.