MMKV storage file grows until app crash

[ryskin]

The library is great but one crucial issue doesn't give me chance to use it.

And it grows by stop 64Mb, 128Mb etc.
If I use slow FS storage, the file size never becomes more than 15Mb. I also used the console log to check the size of the string.

It's working really fast and I want to use that, but with every rewriting, it becomes more significant. I checked key is always stable.

"react-native-mmkv": "^2.4.3",
"react-native": "0.66.3",

[mrousavy]

How often are you writing to MMKV? It's not a database, it's a memory map key value storage.

[ryskin]

How often are you writing to MMKV? It's not a database, it's a memory map key value storage.

I'm writing when my state change with a few seconds of throttling. I use it with redux-persist.
But why does the size of the file grow? As I mentioned before, I use FS storage, which is significantly slower, but the file size increases up to 15Mb. I use only one key.

[ramirobg94]

i have the same "bug"

Check the explanation here ammarahm-ed/react-native-mmkv-storage#286

Seems that the Key Value doesn't overwrite the value, just add a new row into the storage. @mrousavy

Please @ryskin let me know if you solved the problem

[mark-careplanner]

Any further thoughts on this? We are seeing a similar issue

[mrousavy]

Possibly related? #397

[aLemonFox]

@mrousavy to clear things up, does mmkv insert a new row for every setter, without overwriting the previous value? If so, how can we properly clear 'old' entries?

[ice-cap0]

@aLemonFox have you tried removing the previous entry before setting the new one?

[JacobFJ]

@mrousavy Confirming this contributes to my app crashes. The memory is hogging in Android 12 Go edition which typically 2GB or less ram.

[ramirobg94]

@shawarmaz this is dangerous because you can get into two cases:

1 -> async delete, then async write -> maybe write is too slow so there is no advantage to using mmkv
2 -> race condition -> The new data can be deleted

I see the point on how this memory is designed but should have some cleaning method to reduce the memory consumption, there is no point in keeping dozens of versions (Is there any method to recover previous versions? I think that no, so it is pointless)

[ice-cap0]

@ramirobg94 neither of those cases apply because the calls are sync. To be clear that means not async.

[mrousavy]

This is apparently how Tencent/MMKV is designed.

[arthurgeron-work]

Checking if it exists and deleting before set seems to have worked; I've used patch-package, but I can open a pr with the fix:

diff --git a/node_modules/react-native-mmkv/lib/module/MMKV.js b/node_modules/react-native-mmkv/lib/module/MMKV.js
index 8856aea..9cbe764 100644
--- a/node_modules/react-native-mmkv/lib/module/MMKV.js
+++ b/node_modules/react-native-mmkv/lib/module/MMKV.js
@@ -45,6 +45,12 @@ export class MMKV {
   }
   set(key, value) {
     const func = this.getFunctionFromCache('set');
+    const contains = this.getFunctionFromCache('contains');
+    if (key && contains(key)) {
+      const _delete = this.getFunctionFromCache('delete');
+      _delete(key);
+    }
     func(key, value);
     this.onValuesChanged([key]);
   }

You could also just leave the delete call there before set to avoid increasing the time this function takes to finish:

diff --git a/node_modules/react-native-mmkv/lib/module/MMKV.js b/node_modules/react-native-mmkv/lib/module/MMKV.js
index 8856aea..11b43da 100644
--- a/node_modules/react-native-mmkv/lib/module/MMKV.js
+++ b/node_modules/react-native-mmkv/lib/module/MMKV.js
@@ -45,6 +45,10 @@ export class MMKV {
   }
   set(key, value) {
     const func = this.getFunctionFromCache('set');
+    if (key) {
+      const _delete = this.getFunctionFromCache('delete');
+      _delete(key);
+    }
     func(key, value);
     this.onValuesChanged([key]);
   }

[ice-cap0]

I don’t think this should be merged, maybe just a doc update. You can just wrap the method yourself.

[arthurgeron-work]

I don't think this should be left as is, having it fill up memory is a serious issue, having it go a little slower is much better than having it crash your app.