2017-01-12 00:50:53,705 DEBUG BaseHTTPSTransportConnector - Unable to execute HTTP POST due to IOException.Attempt=1; MaxRetries=5; Exception:Timeout waiting for connection from pool ++++ When publishing in bulk, the following message may be encountered in publisher logs.

2017-01-12 09:06:49,322 ERROR BaseHTTPSTransportConnector - Unable to execute File retrieval on destination: 
https://xyz.prod.tridion.sdlproducts.com:8886/httpupload org.apache.http.conn.ConnectionPoolTimeoutException: 
Timeout waiting for connection from pool at org.apache.http.impl.conn.PoolingClientConnectionManager.leaseConnection
(PoolingClientConnectionManager.java:226) at org.apache.http.impl.conn.PoolingClientConnectionManager$1.getConnection
(PoolingClientConnectionManager.java:195) at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:423) at 
org.apache.http.impl.client.AbstractHttpClient.doExecute(AbstractHttpClient.java:882) at 
org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:82) at 
org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:107) at 
com.tridion.transport.connection.connectors.BaseHTTPSTransportConnector.retrieveFileIfPresent(BaseHTTPSTransportConnector.java:235) at 
com.tridion.transport.connection.connectors.DiscoveryServiceTransportConnector.retrieveFileIfPresent(DiscoveryServiceTransportConnector.java:126) at 
com.tridion.transport.connection.connectors.TransportPoolConnector.retrieveFileIfPresent(TransportPoolConnector.java:80) at 
com.tridion.transport.workers.EndpointMetaWorker.getMetadata(EndpointMetaWorker.java:54) at 
com.tridion.transport.DestinationController.run(DestinationController.java:219) at java.lang.Thread.run(Unknown Source)

 

Apply hotfix CD_8.1.1.3656.zip to introduce additional parameters to raise number of connections and timeouts on the Content Manager and Publisher severs.
The above will add options to configure Connection settings - MaxConnectionsPerRoute and HttpConnectionTimeout for the Transport Service.

For Web 8.5, apply hotfix CD_8.5.0.3725 if issue is seen.  Submit a SDL Support ticket if issue persists afterwards.

To download hotfix, see KB article Where can I find hotfixes for SDL Tridion Sites/Web?

Insufficient number of connections and timeout settings configured for the current volume of batch publishing.