Acquiring database lock - Only one instance of ssb-browser is allowed to run at a time.

[nichoth]

I get this error when starting this simple demo, not sure why -- https://github.com/nichoth/ssb-browser-demo

require('./ssb-singleton-setup')
const ssbSingleton = require('ssb-browser-core/ssb-singleton')

var [err, SSB] = ssbSingleton.getSSB()
console.log('in herererer', err, SSB)

[arj03]

What do you mean by error? I should say so temporarily until it makes sure that it is the "leader" and can run. You can try and look in ssb-browser-demo to see how it is used.

[nichoth]

Thank you @arj03 . What i mean by error -- the err variable is set to "Acquiring database lock - Only one instance of ssb-browser is allowed to run at a time.", and the SSB variable is null. I don't know why as I only have the one process that is running ssb-browser.

I have been looking at ssb-browser-demo actually, the line with the error in my app is equivalent to https://github.com/arj03/ssb-browser-demo/blob/master/ui/browser.js#L219 in the browser-demo.

[KyleMaas]

Because of the way that the mutex stuff works, there is a short timeout when the singleton system first loads where it checks if any other windows are holding the lock. (Reason being that using localStorage for mutexes is about the only thread-safe way to do it across different windows/tabs in a widely-supported cross-platform manner, and we have a timeout system to watch for a dead controller and migrate to a different one, so there are hoops to jump through.) If you're calling getSSB() within that initial timeout, you'll get that error. As an alternative, you can use getSSBEventually() or getSimpleSSBEventually(), both of which call a callback function when a lock can be acquired. You can also register callbacks with onSuccess() which are called when a lock is acquired.

[nichoth]

Thanks @KyleMaas . I'm trying getSSBEventually, -- https://github.com/nichoth/ssb-browser-demo/blob/main/src/index.js#L4 -- however I'm uncertain of the signature of this function. It's returning another error:

Uncaught TypeError: ssbCheckCB is not a function

It looks like this is the place throwing the error -- https://github.com/arj03/ssb-browser-core/blob/master/ssb-singleton.js#L117 , but I'm not sure of what ssbCheckCB should be

[KyleMaas]

@nichoth I wrote up some docs for how SSB Singleton is supposed to work. See:

https://github.com/arj03/ssb-browser-core/tree/ssb-singleton-docs

You might want to get started with getSimpleSSBEventually(). It is a little simpler to use, covers most use cases, and only has two parameters:

• isRelevantCB - Function, callback function which is called with zero arguments and is expected to return a boolean value of whether or not the caller still needs an SSB object. This can be used, for example, to bail on running calls to getSSBEventually when a Vue component using it has been unloaded, so we don't waste resources retrying forever.
• resultCB - Function, function which takes two arguments (err, SSB) which is called when either there's an error, a timeout, or we successfully acquired an SSB and it has been declared suitable by ssbCheckCB. Basically the only situation this is not called for an end result is if isRelevantCB returns false.

[KyleMaas]

And in your defense, as much as we've tried to keep SSB Singleton easy to use where it requires minimal changes to code to port to a concurrency-safe version, it's still not exactly intuitive to use. Safe concurrency's hard. And for each bit of complexity in SSB Singleton, there's generally been some weird edge case that needed it. So if you have any other questions, feel free to ask.

Also, there were two main use cases we needed to cover:

• Synchronous - we want to try to get an SSB object, and if one is not presently available, we want to know right away and fail right away because what we're doing just isn't that important or is otherwise incompatible with an asynchronous model. This is what getSSB() is for.
• Asynchronous - what we're doing is important, and we would prefer to wait for an SSB object if it's possible to get one. This is what getSSBEventually() and getSimpleSSBEventually() are for.

[nichoth]

thanks @KyleMaas it works now

require('./ssb-singleton-setup')
const ssbSingleton = require('ssb-browser-core/ssb-singleton')

// (isRelevantCB, resultCB)
ssbSingleton.getSimpleSSBEventually(() => true, function (err, SSB) {
    console.log('aaaaaa', err, SSB)
})

ssbSingleton.onError(function (err) {
    console.log('errrrr', err)
})
ssbSingleton.onSuccess(function () {
    console.log('success', arguments)
})