Liveness and readiness probes

If you want to test the readiness and the liveness of nevisProxy, you can use the Lua filter (LuaFilter). The sample code in the following code block illustrates how to configure two Lua filters for livenes or readiness probes within nevisProxy. The name of the two configured Lua filters is LivenessLuaFilter and ReadinessLuaFilter.

You find this example in the delivered nevisproxy package under: /opt/nevisproxy/examples/various/LuaFilter_liveness_and_readiness_probe.example

Readiness and Liveness Probes with the LuaFilter

<filter>
    <filter-name>LivenessLuaFilter</filter-name>
    <filter-class>ch::nevis::isiweb4::filter::lua::LuaFilter</filter-class>
    <init-param>
        <param-name>Script.InputHeaderFunctionName</param-name>
        <param-value>inputHeaders</param-value>
    </init-param>
    <init-param>
        <param-name>Script</param-name>
        <param-value>
            function inputHeaders(req, resp)
                resp:setHeader("Content-Type", "application/json")
                resp:setBody("{ \"status\" : \"UP\" }")
                resp:send(200)
            end
        </param-value>
    </init-param>
</filter>
 
<filter>
    <filter-name>ReadinessLuaFilter</filter-name>
    <filter-class>ch::nevis::isiweb4::filter::lua::LuaFilter</filter-class>
    <init-param>
        <param-name>Script.InputHeaderFunctionName</param-name>
        <param-value>inputHeaders</param-value>
    </init-param>
    <init-param>
        <param-name>Script</param-name>
        <param-value>
            package.path = package.path .. ";@PKG_VAR@/@PKG_INSTANCE@/lib/?.lua"
            local GlobalStore = require "GlobalStore"
            local databaseServlets = { "MySQLSessionStoreServlet1", "MySQLSessionStoreServlet2" }
            local databasePingName = "DatabasePing"
            local backendsServlets = { "HttpConnectorServlet1", "HttpConnectorServlet2" }
            function testDatabaseServlets(req)
                local tracer = req:getTracer()
                local ret = true
                  
                for i,servlet in ipairs(databaseServlets) do
                    tracer:debug("Testing servlet '" .. servlet .. "'")
                    local store = GlobalStore:new(servlet)
                    local value = store:get(databasePingName)
                    if not value then
                        store:set(databasePingName, "OK", 3600)
                        value = store:get(databasePingName)
                    end
                    if value ~= "OK" then
                        tracer:error("The servlet '" .. servlet .. "' seems to be down")
                        ret = false;
                    else
                        tracer:debug("Servlet '" .. servlet .. "' is up and running")
                    end
                end
                return ret
            end
            function testBackends(req)
                local tracer = req:getTracer()
                local ret = true
                  
                for i,servlet in ipairs(backendsServlets) do
                    tracer:debug("Testing servlet '" .. servlet .. "'")
                    local pingRequest = nevis.filter.lua.request.new()
                    pingRequest:setRequestUri("/ping")
                    pingRequest:setMethod("GET")
                    local pingResponse = nevis.filter.lua.response.new()
                    local dispatcher = req:getDispatcher(servlet)
                    dispatcher:forward(pingRequest, pingResponse)
                    local pingStatusCode = pingResponse:getStatus()
                    if pingStatusCode ~= 200 then
                        tracer:error("The servlet '" .. servlet .. "' seems to be down")
                        ret = false;
                    else
                        tracer:debug("Servlet '" .. servlet .. "' is up and running")
                    end
                end
                return ret
            end
            function inputHeaders(req, resp)
                local tracer = req:getTracer()
                local dbReady
                local dbackendReady
                 
                if pcall(function()
                    dbReady = testDatabaseServlets(req)
                    dbackendReady = testBackends(req)
                    end)
                then
                    resp:setHeader("Content-Type", "application/json")
                    if not dbReady then
                        resp:setBody("{ \"status\" : \"DOWN\" }")
                        resp:send(503)
                    elseif not dbackendReady then
                        resp:setBody("{ \"status\" : \"DOWN\" }")
                        resp:send(503)
                    else
                        resp:setBody("{ \"status\" : \"UP\" }")
                        resp:send(200)
                    end
                else
                    tracer:notice("An exception occured")
                    resp:setHeader("Content-Type", "application/json")
                    resp:setBody("{ \"status\" : \"DOWN\" }")
                    resp:send(503)
                end
            end
        </param-value>
    </init-param>
</filter>

• The LivenessLuaFilter returns the status code "200" to the frontend if the proxy is alive. If the LivenessLuaFilter is not triggered, the proxy is not alive.
• The ReadinessLuaFilter returns the status code "200" if the configured backends and the MySQLSessionStoreServlet (in the LuaFilter) are up and running ("ready").

In the sample code, the ReadinessLuaFilter first checks the MySQLSessionStoreServlets "MySQLSessionStoreServlet1" and "MySQLSessionStoreServlet2" by trying to add an entry to both servlets. To avoid too many update calls, the entry is added once per hour. The ReadinessLuaFilter also checks the HttpConnectorServlets "HttpConnectorServlet1" and "HttpConnectorServlet2", by calling the URL "/ping". Note that "/ping" is just an example. The correct URL depends on the backend itself.

To map both LuaFilters to a URL, and to ensure that the proxy will process the requests, you can place a DefaultServlet at the end of the filter chain. This servlet will never be reached, but this is no problem, as the LuaFilters will in any case send an answer to the frontend.