Realm `init` takes a significant amount of time

[dlbuckley]

How frequently does the bug occur?

Always

Description

Overview

We are currently investigating app launch times and have realised that Realm is taking up a significant portion (80-90%) of the time it takes to launch our app. As it stands we initialise Realm in our app delegate as we need to access data from it very early in our app lifecycle.

I started a discussion topic here but didn't receive any feedback thus far.

From my experimentation it appears that there is a lot happening in Realm init and as our app grows so does the time it takes to run init and in turn, our launch times. It's so long now that I'm wondering if there is an issue with initialisation within Realm, hence raising a bug report.

To give an idea of how bad this can get; in the real world we are seeing times around the 4 seconds mark in some cases which isn't great for the user experience. See the discussion thread for more information.

Stacktrace & log output

No response

Can you reproduce the bug?

Always

Reproduction Steps

See the discussion thread: #8194

Version

10.37.2

What Atlas Services are you using?

Local Database only

Are you using encryption?

No

Platform OS and version(s)

iOS 16.2

Build environment

Xcode version: 14.2
Dependency manager and version: SPM

[dianaafanador3]

HI @dlbuckley did you tried the suggestions on the discussion thread #8194 (reply in thread). Let us know how that worked for you.

[dlbuckley]

@dianaafanador3 we had a longer easter weekend here but I've now responded in the discussions thread.

I'm wondering if there is a possibility to cache the resolved schema between schema versions to shortcut the amount of work done by reflecting the model objects on initialisation. After looking at whats happening in Instruments this seems to be where the majority of the time is spent. I've not dived into the realm code to see if there are any possibilities to do this, but I would love to hear some thoughts on the possibilities.

[dianaafanador3]

HI @dlbuckley checking with you on this, did you made the improvements you proposed on the discussion thread, this helped you getting Realm initialised faster?. Please share your findings.
From our side until we don't change our schema discovery logic there are not gonna be many changes, and this is not gonna happen until Swift release something which would help us solve this, but we'll keep you updated on any changes.

[dlbuckley]

@dianaafanador3 In short, no. Realm's limiting factor for startup times is its automatic scheme discovery at runtime, there isn't much we can do about it. I was hoping that someone would have engaged in the discussions in and around schema caching which I mentioned, but there was no response to that suggestion.

We looked into using Realm's async initialisation but it would require a large amount of work to refactor our app to be able to use it. So much of our logic relies on our persistence solution (Realm) being set up and ready to go upon app launch it would mean deferring large swathes of logic and managing state around the app lifecycle events which is not something we are going to invest in right now. We did some 'light' testing around async initialisation and noted that while we gained control back from Realm faster, the overall startup times for realm increased by another 100-200ms which defeated the point of this endeavour. This finding tipped the balance away from investing in this direction.

As part of improving the general architecture of our app we are working to isolate common Realm operations and hope there are improvements to Realm's schema discovery speed in future. But if not the changes will give us an opportunity to move away from Realm to another solution.

As a closing comment; Realm is generally pretty good (even if it does have some quirks), but it seems most suited to hobby apps. Our Android app had to move away from Realm a year or two ago as they had too many issues as it grew and now our iOS app might have to consider the same. I would love to see some focus in improving Realm to scale with apps rather than becoming a blocker, but for now thats not quite the case.