Do not execute RELEASE_SCRIPT lock failed with one client

[ievgen-kapinos]

Hi !

First of all, thank you for creating so nice library and amazing documentation.

Setup

I'm running Node.js Web application in Kubernetes. Depends on traffic I have 8-30 pods. I do not have Redis Cluster and have only one redis client in code. I'm using redlock (5.0.0-beta.2). So my setup looks like this.

const redisClient = new Redis('my-redis-url', {
    enableOfflineQueue: false, // Do not collect commands in pending queue. Throw exception, if redis not available.
    maxRetriesPerRequest: 0, // Do not retry command. Throw exception, if redis not available.
});
     
const redlock = new Redlock([redisClient], {
   retryCount: 0, // One attempt or throw exception
   automaticExtensionThreshold: 5000,
})

I need lock to select only one (primary) pod to execute background job which updates cache. All other pods(secondary) retry to acquire lock, if failed then sleep for one minute. I'm using this approach:

await redlock.using(['my-backgroung-job-lock'], 15000, async signal => {
   // business code when lock is acquired
}

Investigation

All good on primary pod. I have 6 evalsha requests per minute (60000/ (15000-5000)) = 6

But on secondary pods amount of evalsha requests is looks to big. I expect only one request per pod, but have two. I've investigated code and found this part.

node-redlock/src/index.ts

Lines 330 to 333 in 5138813

	} catch (error) {
	// If there was an error acquiring the lock, release any partial lock
	// state that may exist on a minority of clients.
	await this._execute(this.scripts.releaseScript, resources, [value], {

This means that each secondary pod executes ACQUIRE_SCRIPT - that is expected. But if it fails, then RELEASE_SCRIPT executed unconditionally.

Proposal

We can improve performance (decrease amount of requests to Redis) and do not execute RELEASE_SCRIPT when:

• number of clients is one
• ACQUIRE_SCRIPT returns zero