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Preview Generator

Nextcloud app that allows admins to pre-generate previews. The app listens to edit events and stores this information. Once a cron job is triggered it will start preview generation. This means that you can better utilize your system by pre-generating previews when your system is normally idle and thus putting less load on your machine when the requests are actually served.

This app is primarily meant for small Nextcloud servers running on cheap hardware where on-demand generation of previews is not quick enough. The app effectively trades higher disk usage for quicker previews and this trade-off should be considered carefully. The previous reasoning of resource exhaustion caused by too many concurrent preview requests is not a concern anymore as there is a configurable limit in modern versions of Nextcloud.

The app does not replace on demand preview generation so if a preview is requested before it is pre-generated it will still be shown.

How to install

  • Install directly from within your Nextcloud from the app store
  • Clone this repository into your Nextcloud app folder

How to use the app

  1. Install the app
  2. Enable the app
  3. Run ./occ preview:generate-all once after installation.
  4. Add a (system) cron job for ./occ preview:pre-generate
  • I run it every 10 minutes

Known issues

  • The app does not work with encryption enabled

How does the app work

  1. Listen to events that a file has been written or modified and store it in the database
  2. On cron run request previews for the files that have been written or modified

If a preview already exists at step 2 then requesting it is really cheap. If not it will be generated. Depending on the sizes of the files and the hardware you are running on the time this takes can vary.

Commands

preview:generate-all [--path=PATH ...] [user_id ...]

Loop over all files and try to generate previews for them. If one or multiple user ids are supplied it will just loop over the files of those users. You can also limit the generation to one or more paths using --path="/[username]/files/[folder path]", e.g. --path="/alice/files/Photos". Note that all given user_ids are ignored if at least one path is specified.

preview:pre-generate

Do the actual pre-generation. This means only for new or modified files (since the app was enabled or the last pre-generation was done).

Use <command> -vv to get a more verbose output if you are interested to see which files are being processed.

Available configuration options

The value of each option can either be a list of sizes separated by spaces or an empty string. Setting an empty string will simply skip those kinds of previews. Deleting or not setting a config will use a built-in default list of values for those previews.

  • Preview sizes must be a power of 4! Other sizes are silently ignored.
  • The smallest possible size is 64.
  • The max size is determined by your preview_max_x and preview_max_y settings in config.php.

occ config:app:set --value="64 256" previewgenerator squareSizes

Cropped square previews which are mostly used in the list and tile views of the files app.

occ config:app:set --value="256 4096" previewgenerator squareUncroppedSizes

Will retain the aspect ratio and try to maximize either width or height.

occ config:app:set --value="64 256 1024" previewgenerator widthSizes

Will retain the aspect ratio and use the specified width. The height will be scaled according to the aspect ratio.

occ config:app:set --value="64 256 1024" previewgenerator heightSizes

Will retain the aspect ratio and use the specified height. The width will be scaled according to the aspect ratio.

FAQ

Why can't I use webcron?

Preview generation can be a very long running job. Thus we need a system that does not time out.

I get "Command already running"

Yes this happens when the pre-generate command crashes. No worries the lock will be released after 30 minutes of inactivity from the app. So go grab a cookie.

What if I'm using Nextcloud All In One?

Follow these instructions

I don't want to generate all the preview sizes

The following options are recommended if you only want to generate a minimum set of required previews. This should include all previews requested by the files, photos and activity apps.

./occ config:app:set --value="64 256" previewgenerator squareSizes
./occ config:app:set --value="256 4096" previewgenerator squareUncroppedSizes
./occ config:app:set --value="" previewgenerator widthSizes
./occ config:app:set --value="" previewgenerator heightSizes

This will only generate:

  • Cropped square previews of: 64x64 and 256x256
  • Aspect ratio previews with a max width or max height of: 256 and 4096

I get "PHP Fatal error: Allowed memory size of X bytes exhausted"

You need to increase the memory allowance of PHP, by default it is 128 MB. You do that by changing the memory_limit in the php.ini file.

If you use the docker container you need set the environment variable PHP_MEMORY_LIMIT instead.

I want to skip a folder and everything in/under it

Add an empty file with the name .nomedia in the folder you wish to skip. All files and subfolders of the folder containing .nomedia will also be skipped.

I want to reset/regenerate all previews

WARNING: This is not supported but it has been confirmed to work by multiple users. Proceed at your own risk. Always keep backups around.

  1. Remove the folder your-nextcloud-data-directory/appdata_*/preview
  2. Optional: change parameters preview_max_x and preview_max_y in config.php (e.g., to 512), and change the previewgenerator app parameters heightSizes, squareSizes and widthSizes as per the README (or better yet, to a low value each, e.g. 512, 256 and 512 respectively)
  3. Run occ files:scan-app-data (this will reset generated previews in the database)
  4. Run occ preview:generate-all [user-id] (this will run very fast if you did step 2)