Cache and Replay Messages without using node red contrib infocache

Update 2023-01-10: uib-cache went live with uibuilder v5 as expected and has been improved since then. Caching leverages Node-RED's native context storage and so can be persistent across restarts if desired. Some of the other uibuilder nodes can also now use the same caching capabilities. uib-cache will continue to improve and so it strongly recommended that you use it rather than creating your own scheme.

Update 2022-01-19: When v5 goes live, a new node uib-cache will be available which could be used instead of a function node.

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If you want to cache and replay messages without another node, here is a very simple example flow.

Function

This is the function that is key. As always, it is up to you to keep track of how much memory is being consumed. The replayed messages will go to the client that requested them if it can identify it.

The function only keeps the last 1 message of each topic and this version has no method to clear the cache.

// Expects input msgs with topic set

// saved context
var homeMsgs = context.get('homeMsgs') || {}

// Replay cache if requested
if ( msg.hasOwnProperty('cacheControl') && msg.cacheControl === 'REPLAY' ) {
    for (var topic in homeMsgs) {
        // Only send to a single client if we can
        if ( msg.hasOwnProperty('_socketId') )
            node.send({
                "topic": topic, 
                "payload": homeMsgs[topic],
                "_socketId": msg._socketId
            })
        else
            node.send({
                "topic": topic, 
                "payload": homeMsgs[topic]
            })
    }
    return null
}

// ignore cacheControl and uibuilder control messages
if ( msg.hasOwnProperty('cacheControl') || msg.hasOwnProperty('uibuilderCtrl') ) return null

// Keep the last msg.payload by topic
homeMsgs[msg.topic] = msg.payload

// save context for next time
context.set('homeMsgs', homeMsgs)

return msg;

Example Flow

Note that the flow doesn't contain the MQTT-in node that drives it. Adapt to your own requirements.

[{"id":"401897b8.9931e8","type":"uibuilder","z":"9974253c.de8db8","name":"","topic":"","url":"home","fwdInMessages":false,"allowScripts":true,"allowStyles":true,"debugFE":false,"copyIndex":true,"x":490,"y":700,"wires":[["d0dec491.704248"],["bbc35ca3.97343","ff280acc.207098"]]},{"id":"d0dec491.704248","type":"debug","z":"9974253c.de8db8","name":"home data out","active":true,"console":"false","complete":"true","x":680,"y":680,"wires":[]},{"id":"889a6459.17d2f8","type":"function","z":"9974253c.de8db8","name":"","func":"// Expects input msgs with topic set\n\n// saved context\nvar homeMsgs = context.get('homeMsgs') || {}\n\n// Replay cache if requested\nif ( msg.hasOwnProperty('cacheControl') && msg.cacheControl === 'REPLAY' ) {\n    for (var topic in homeMsgs) {\n        // Only send to a single client if we can\n        if ( msg.hasOwnProperty('_socketId') )\n            node.send({\n                \"topic\": topic, \n                \"payload\": homeMsgs[topic],\n                \"_socketId\": msg._socketId\n            })\n        else\n            node.send({\n                \"topic\": topic, \n                \"payload\": homeMsgs[topic]\n            })\n    }\n    return null\n}\n\n// ignore cacheControl and uibuilder control messages\nif ( msg.hasOwnProperty('cacheControl') || msg.hasOwnProperty('uibuilderCtrl') ) return null\n\n// Keep the last msg.payload by topic\nhomeMsgs[msg.topic] = msg.payload\n\n// save context for next time\ncontext.set('homeMsgs', homeMsgs)\n\nreturn msg;","outputs":1,"noerr":0,"x":310,"y":700,"wires":[["401897b8.9931e8","b4850396.c9aef"]]},{"id":"bbc35ca3.97343","type":"debug","z":"9974253c.de8db8","name":"home controls","active":true,"console":"false","complete":"true","x":680,"y":720,"wires":[]},{"id":"e35dc750.bbfbe8","type":"link in","z":"9974253c.de8db8","name":"home-replay","links":["ff280acc.207098"],"x":115,"y":700,"wires":[["889a6459.17d2f8"]]},{"id":"ff280acc.207098","type":"link out","z":"9974253c.de8db8","name":"home-controls","links":["e35dc750.bbfbe8"],"x":615,"y":760,"wires":[]},{"id":"b4850396.c9aef","type":"debug","z":"9974253c.de8db8","name":"input messages","active":true,"console":"false","complete":"true","x":480,"y":640,"wires":[]}]