diff --git a/system/jlib/CMakeLists.txt b/system/jlib/CMakeLists.txt
index 9fa8175eabc..1a88f03d3a1 100644
--- a/system/jlib/CMakeLists.txt
+++ b/system/jlib/CMakeLists.txt
@@ -223,6 +223,9 @@ include_directories (
          ${CMAKE_BINARY_DIR}/oss
     )
 
+# ensure httplib uses poll rather than select - otherwise it fail if too many sockets have been opened.
+ADD_DEFINITIONS( -DCPPHTTPLIB_USE_POLL )
+
 ADD_DEFINITIONS( -D_USRDLL -DJLIB_EXPORTS )
 HPCC_ADD_LIBRARY( jlib SHARED ${SRCS} ${INCLUDES} )
 
diff --git a/system/metrics/sinks/prometheus/CMakeLists.txt b/system/metrics/sinks/prometheus/CMakeLists.txt
index c2454e1eadc..f2a89ad3f2d 100644
--- a/system/metrics/sinks/prometheus/CMakeLists.txt
+++ b/system/metrics/sinks/prometheus/CMakeLists.txt
@@ -30,6 +30,9 @@ include_directories(
         ${HPCC_SOURCE_DIR}/system/httplib
 )
 
+# ensure httplib uses poll rather than select - otherwise it fail if too many sockets have been opened.
+ADD_DEFINITIONS( -DCPPHTTPLIB_USE_POLL )
+
 SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${STRICT_CXX_FLAGS}")
 ADD_DEFINITIONS(  -DPROMETHEUSSINK_EXPORTS )
 HPCC_ADD_LIBRARY( hpccmetrics_prometheussink SHARED ${srcs} )