What causing unreachable errors in Google PubSub? - google-cloud-platform

Question

i'm running application which consists of Google Cloud Functions, triggered by PubSub Topics, so basically they're communicating to each other via Google PubSub.
The problem is, it can struggle sometimes and show delays when publishing messages up to 9s or more. I checked the Metrics Explorer and found out that when high delays it shows next errors:
unreachable_5xx_error_500
unreachable_no_response
internal_rejected_error
unreachable_5xx_error_503
url_4xx_error_429
Here is the chart showing delays:
Code example of publishing message inside Google Cloud Function:
const {PubSub} = require('#google-cloud/pubsub');
const pubSubClient = new PubSub();
async function publishMessage() {
const topicName = 'my-topic';
const dataBuffer = Buffer.from(data);
const messageId = await pubSubClient.topic(topicName).publish(dataBuffer);
console.log(`Message ${messageId} published.`);
}
publishMessage().catch(console.error);
Code example of function triggered by PubSub Topic:
exports.subscribe = async (message) => {
const name = message.data
? Buffer.from(message.data, 'base64').toString()
: 'World';
console.log(`Hello, ${name}!`);
}
And i think this errors causing delays. I didn't find anything on this on the internet, so i hope you can explain what causing this errors and why and probably can help with this.

Answer 1

As it was discussed in the comments, there are some changes and workarounds that can be done to solve or reduce the problem.
At first, as can be found in this guide, PubSub tries to gather multiple messages before delivering it. In other words, it tries to delivery many messages at once. In this specific case to achieve a more realistic real time scenario, should be specified a batch size of 1, which would cause PubSub to delivery every message separately. This batch size can be specified using the maxMessages property in the publisher object creation like in the code below. Besides that, the maxMilliseconds property can be used to specify the maximum latency allowed.
const batchPublisher = pubSubClient.topic(topicName, {
batching: {
maxMessages: maxMessages,
maxMilliseconds: maxWaitTime * 1000,
},
});
In the discussion it was also noticed that the problem is probably related to the Cloud Function's cold-start which makes the latency bigger for this application due to its architecture. The workaround for solving this part of the problem was inserting a Node JS server in the architecture to trigger the functions using PubSub.