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File upload backends

Zulip in production supports a couple different backends for storing files uploaded by users of the Zulip server (messages, profile pictures, organization icons, custom emoji, etc.).

The default is the LOCAL_UPLOADS_DIR backend, which just stores files on disk in the specified directory on the Zulip server. Obviously, this backend doesn't work with multiple Zulip servers and doesn't scale, but it's great for getting a Zulip server up and running quickly. You can later migrate the uploads to S3 by following the instructions here.

We also support an S3 backend, which uses the Python boto library to upload files to Amazon S3 (or an S3-compatible block storage provider supported by the boto library).

S3 backend configuration

Here, we document the process for configuring Zulip's S3 file upload backend. To enable this backend, you need to do the following:

In the AWS management console, create a new IAM account (aka API user) for your Zulip server, and two buckets in S3, one for uploaded files included in messages, and another for user avatars. You need two buckets because the "user avatars" bucket is generally configured as world-readable, whereas the "uploaded files" one is not.
Set s3_key and s3_secret_key in /etc/zulip/zulip-secrets.conf to be the S3 access and secret keys for the IAM account. Alternately, if your Zulip server runs on an EC2 instance, set the IAM role for the EC2 instance to the role.
Set the S3_AUTH_UPLOADS_BUCKET and S3_AVATAR_BUCKET settings in /etc/zulip/settings.py to be the names of the S3 buckets you created (e.g. "exampleinc-zulip-uploads").
Comment out the LOCAL_UPLOADS_DIR setting in /etc/zulip/settings.py (add a # at the start of the line).
If you are using a non-AWS block storage provider, you need to set the S3_ENDPOINT_URL setting to your endpoint url (e.g. "https://s3.eu-central-1.amazonaws.com").

For certain AWS regions, you may need to set the S3_REGION setting to your default AWS region's code (e.g. "eu-central-1").
Finally, restart the Zulip server so that your settings changes take effect (/home/zulip/deployments/current/scripts/restart-server).

It's simplest to just do this configuration when setting up your Zulip server for production usage. Note that if you had any existing uploading files, this process does not upload them to Amazon S3; see migration instructions below for those steps.

S3 local caching

For performance reasons, Zulip stores a cache of recently served user uploads on disk locally, even though the durable storage is kept in S3. There are a number of parameters which control the size and usage of this cache, which is maintained by nginx:

s3_memory_cache_size controls the in-memory size of the cache index; the default is 1MB, which is enough to store about 8 thousand entries.
s3_disk_cache_size controls the on-disk size of the cache contents; the default is 200MB.
s3_cache_inactive_time controls the longest amount of time an entry will be cached since last use; the default is 30 days. Since the contents of the cache are immutable, this serves only as a potential additional limit on the size of the contents on disk; s3_disk_cache_size is expected to be the primary control for cache sizing.

These defaults are likely sufficient for small-to-medium deployments. Large deployments, or deployments with image-heavy use cases, will want to increase s3_disk_cache_size, potentially to be several gigabytes. s3_memory_cache_size should potentially be increased, based on estimating the number of files that the larger disk cache will hold.

You may also wish to increase the cache sizes if the S3 storage (or S3-compatible equivalent) is not closely located to your Zulip server, as cache misses will be more expensive.

S3 bucket policy

The best way to do the S3 integration with Amazon is to create a new IAM user just for your Zulip server with limited permissions. For both the user uploads bucket and the user avatars bucket, you'll need to adjust the S3 bucket policy.

The file uploads bucket should have a policy of:

{
    "Version": "2012-10-17",
    "Id": "Policy1468991802320",
    "Statement": [
        {
            "Sid": "Stmt1468991795370",
            "Effect": "Allow",
            "Principal": {
                "AWS": "ARN_PRINCIPAL_HERE"
            },
            "Action": [
                "s3:GetObject",
                "s3:DeleteObject",
                "s3:PutObject"
            ],
            "Resource": "arn:aws:s3:::BUCKET_NAME_HERE/*"
        },
        {
            "Sid": "Stmt1468991795371",
            "Effect": "Allow",
            "Principal": {
                "AWS": "ARN_PRINCIPAL_HERE"
            },
            "Action": "s3:ListBucket",
            "Resource": "arn:aws:s3:::BUCKET_NAME_HERE"
        }
    ]
}

The file-uploads bucket should not be world-readable. See the documentation on the Zulip security model for details on the security model for uploaded files.

However, the avatars bucket is intended to be world-readable, so its policy should be:

{
    "Version": "2012-10-17",
    "Id": "Policy1468991802321",
    "Statement": [
        {
            "Sid": "Stmt1468991795380",
            "Effect": "Allow",
            "Principal": {
                "AWS": "ARN_PRINCIPAL_HERE"
            },
            "Action": [
                "s3:GetObject",
                "s3:DeleteObject",
                "s3:PutObject"
            ],
            "Resource": "arn:aws:s3:::BUCKET_NAME_HERE/*"
        },
        {
            "Sid": "Stmt1468991795381",
            "Effect": "Allow",
            "Principal": {
                "AWS": "ARN_PRINCIPAL_HERE"
            },
            "Action": "s3:ListBucket",
            "Resource": "arn:aws:s3:::BUCKET_NAME_HERE"
        },
        {
            "Sid": "Stmt1468991795382",
            "Effect": "Allow",
            "Principal": {
                "AWS": "*"
            },
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::BUCKET_NAME_HERE/*"
        }
    ]
}

Migrating from local uploads to Amazon S3 backend

As you scale your server, you might want to migrate the uploads from your local backend to Amazon S3. Follow these instructions, step by step, to do the migration.

First, set up the S3 backend in the settings (all the auth stuff), but leave LOCAL_UPLOADS_DIR set -- the migration tool will need that value to know where to find your uploads.
Run ./manage.py transfer_uploads_to_s3. This will upload all the files from the local uploads directory to Amazon S3. By default, this command runs on 6 parallel processes, since uploading is a latency-sensitive operation. You can control this parameter using the --processes option.
Once the transfer script completes, disable LOCAL_UPLOADS_DIR, and restart your server (continuing the last few steps of the S3 backend setup instructions).

Congratulations! Your uploaded files are now migrated to S3.

Caveat: The current version of this tool does not migrate an uploaded organization avatar or logo.

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