Automatically retry an action after an error

Problem

You want to retry an action after an error is thrown.

Solution

Use the Catch node to receive the error and connect it back to the node that needs to retry the action.

Example

Flow JSON

[{"id":"27e61f12.c1a15","type":"inject","z":"fc046f99.4be08","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":100,"y":320,"wires":[["d7d08440.31b678"]]},{"id":"d7d08440.31b678","type":"function","z":"fc046f99.4be08","name":"Random error","func":"// Randomly throw an error rather than\n// pass on message.\nif (Math.random() < 0.5) {\n   node.error(\"a random error\", msg);\n} else {\n    return msg;\n}","outputs":1,"noerr":0,"x":320,"y":320,"wires":[["f22b1e9a.5d89b"]]},{"id":"f22b1e9a.5d89b","type":"debug","z":"fc046f99.4be08","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":510,"y":320,"wires":[]},{"id":"2166290d.98d736","type":"delay","z":"fc046f99.4be08","name":"","pauseType":"delay","timeout":"2","timeoutUnits":"seconds","rate":"1","nbRateUnits":"1","rateUnits":"second","randomFirst":"1","randomLast":"5","randomUnits":"seconds","drop":false,"x":240,"y":380,"wires":[["d7d08440.31b678"]]},{"id":"139b836e.7950ed","type":"catch","z":"fc046f99.4be08","name":"","scope":["d7d08440.31b678"],"uncaught":false,"x":90,"y":380,"wires":[["2166290d.98d736","9c8ab214.0ecaa"]]},{"id":"9c8ab214.0ecaa","type":"debug","z":"fc046f99.4be08","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"error","targetType":"msg","x":240,"y":440,"wires":[]}]

Discussion

Some errors are transitory and an action simply needs to be retried in order to succeed. Alternatively, there may be some remedial action needed before retrying.

In the example flow, a Function simulates a random error - there is a 50% chance it will throw an error rather than pass on the message.

The Catch receives the error which passes the message back to the Function node to retry. It also includes a Delay node as, in some circumstances, it is suitable to wait for a short interval before retrying.