Context

The ExecutionContext class provides methods to visit and handle context data beyond js runtime. It is used in fetch handling. For example:

export default {
    // ctx is the ExecutionContext object
    async fetch(request, env, ctx){
        return new Response("Hello, World!");
    }
}

waitUntil

waitUntil is a method that is called when the request is finished. It is used to handle other tasks beyond blocking http response. It accept a Promise as an argument. The runtime will continue to wait for the Promise to resolve even if the response is already sent.

waitUntil is common used to:

• Push request message to logging service, or anaylysis service.
• Push items into cache.

waitUntil can be called multiple times. The added promises will be executed totally, no matter one of them is rejected.

export default {
  async fetch(request, env, ctx) {
    const log_message = async function(){
        // In real use, you can push the message to a logging service
        console.log("Someone viewed this page!",request.url);
    };
    ctx.waitUntil(log_message());
    return new Response("Hello, World!");
  },
};

WARNING

It's no way to expose failed message if the promise is rejected in waitUntil. You need to monitor the case with relative service. Such as monitor messages counts in logging service that compares with the view counts.