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Exporting data from a large multi-realm Zulip server

Draft status

This is a draft design document considering potential future refinements and improvements to make large migrations easier going forward, and is not yet a set of recommendations for Zulip systems administrators to follow.

Overview

Zulip offers an export tool, management/export.py, which works well to export the data for a single Zulip realm, and which is your best choice if you're migrating a Zulip realm to a new server.

This document supplements the explanation in management/export.py, but here we focus more on the logistics of a big conversion of a multi-realm Zulip installation. (For some historical perspective, this document was originally begun as part of a big Zulip cut-over in summer 2016.)

There are many major operational aspects to doing a conversion. I will list them here, noting that several are not within the scope of this document:

  • Get new servers running.
  • Export data from the old DB.
  • Export files from Amazon S3.
  • Import files into new storage.
  • Import data into new DB.
  • Restart new servers.
  • Decommission old server.

This document focuses almost entirely on the export piece. Issues with getting Zulip itself running are out of scope here; see the production installation instructions. As for the import side of things, we only touch on it implicitly. (My reasoning was that we had to get the export piece right in a timely fashion, even if it meant we would have to sort out some straggling issues on the import side later.)

Exporting multiple realms' data when moving to a new server

The main exporting tools in place as of summer 2016 are below:

  • We can export single realms (but not yet limit users within the realm).
  • We can export single users (but then we get no realm-wide data in the process).
  • We can run exports simultaneously (but have to navigate a bunch of /tmp directories).

Things that we still may need:

  • We may want to export multiple realms simultaneously.
  • We may want to export multiple single users simultaneously.
  • We may want to limit users within realm exports.
  • We may want more operational robustness/convenience while doing several exports simultaneously.
  • We may want to merge multiple export files to remove duplicates.

We have a few major classes of data. They are listed below in the order that we process them in do_export_realm():

Public Realm Data

Realm/RealmDomain/RealmEmoji/RealmFilter/DefaultStream.

Cross Realm Data

Client/zerver_userprofile_cross_realm

This includes Client and three bots.

Client is unique in being a fairly core table that is not tied to UserProfile or Realm (unless you somewhat painfully tie it back to users in a bottom-up fashion though other tables).

Disjoint User Data

UserProfile/UserActivity/UserActivityInterval/UserPresence.

Recipient Data

Recipient/Stream/Subscription/Huddle.

These tables are tied back to users, but they introduce complications when you try to deal with multi-user subsets.

File-related Data

Attachment

This includes Attachment, and it references the avatar_source field of UserProfile. Most importantly, of course, it requires us to grab files from S3. Finally, Attachment's m2m relationship ties to Message.

Message Data

Message/UserMessage

Summary

Here are the same classes of data, listed in roughly decreasing order of riskiness:

  • Message Data (sheer volume/lack of time/security)
  • File-Related Data (S3/security/lots of moving parts)
  • Recipient Data (complexity/security/cross-realm considerations)
  • Cross Realm Data (duplicate ids)
  • Disjoint User Data
  • Public Realm Data

(Note the above list is essentially in reverse order of how we process the data, which isn't surprising for a top-down approach.)

The next section of the document talks about risk factors.

Risk Mitigation

Generic considerations

We have two major mechanisms for getting data:

Top Down

Get realm data, then all users in realm, then all recipients, then all messages, etc.

The problem with the top-down approach will be filtering. Also, if errors arise during top-down passes, it may be time consuming to re-run the processes.

Bottom Up

Start with users, get their recipient data, etc.

The problems with the bottom up approach will be merging. Also, if we run multiple bottom-up passes, there is the danger of duplicating some work, particularly on the message side of things.

Approved Transfers

We have not yet integrated the approved-transfer model, which tells us which users can be moved.

Risk factors broken out by data categories

Message Data

  • models: Message/UserMessage.
  • assets: messages-*.json, subprocesses, partial files

Rows in the Message model depend on Recipient/UserProfile.

Rows in the UserMessage model depend on UserProfile/Message.

The biggest concern here is the sheer volume of data, with security being a close second. (They are interrelated, as without security concerns, we could just bulk-export everything one time.)

We currently have these measures in place for top-down processing:

  • chunking
  • multi-processing
  • messages are filtered by both sender and recipient

File Related Data

  • models: Attachment
  • assets: S3, attachment.json, uploads-temp/, image files in avatars/, assorted files in uploads/, avatars/records.json, uploads/records.json, zerver_attachment_messages

When it comes to exporting attachment data, we have some minor volume issues, but the main concern is just that there are lots of moving parts:

  • S3 needs to be up, and we get some metadata from it as well as files.
  • We have security concerns about copying over only files that belong to users who approved the transfer.
  • This piece is just different in how we store data from all the other DB-centric pieces.
  • At import time we have to populate the m2m table (but fortunately, this is pretty low risk in terms of breaking anything.)

Recipient Data

  • models: Recipient/Stream/Subscription/Huddle
  • assets: realm.json, (user,stream,huddle)_(recipient,subscription)

This data is fortunately low to medium in volume. The risk here will come from model complexity and cross-realm concerns.

From the top down, here are the dependencies:

  • Recipient depends on UserProfile
  • Subscription depends on Recipient
  • Stream currently depends on Realm (but maybe it should be tied to Subscription)
  • Huddle depends on Subscription and UserProfile

The biggest risk factor here is probably just the possibility that we could introduce some bug in our code as we try to segment Recipient into user, stream, and huddle components, especially if we try to handle multiple users or realms. I think this can be largely mitigated by the new Config approach.

And then we also have some complicated Huddle logic that will be customized regardless. The fiddliest part of the Huddle logic is creating the set of unsafe_huddle_recipient_ids.

Last but not least, if we go with some hybrid of bottom-up and top-down, these tables are neither close to the bottom nor close to the top, so they may have the most fiddly edge cases when it comes to filtering and merging.

Recommendation: We probably want to get a backup of all this data that is very simply bulk-exported from the entire DB, and we should obviously put it in a secure place.

Cross Realm Data

  • models: Client
  • assets: realm.json, three bots (notification/email/welcome), id_maps

The good news here is that Client is a small table, and there are only three special bots.

The bad news is that cross-realm data complicates everything else, and we have to avoid database ID conflicts.

If we use bottom-up approaches to load small user populations at a time, we may have merging issues here. We will need to consolidate IDs either by merging exports in /tmp or handle it at import time.

For the three bots, they live in zerver_userprofile_crossrealm, and we re-map their IDs on the new server.

Recommendation: Do not sweat the exports too much. Deal with all the messiness at import time, and rely on the tables being really small. We already have logic to catch Client.DoesNotExist exceptions, for example. As for possibly missing messages that the welcome bot and friends have sent in the past, I am not sure what our risk profile is there, but I imagine it is relatively low.

Disjoint User Data

  • models: UserProfile/UserActivity/UserActivityInterval/UserPresence
  • assets: realm.json, password, api_key, avatar salt, id_maps

On the DB side this data should be fairly easy to deal with. All of these tables are basically disjoint by user profile ID. Our biggest risk is remapped user ids at import time, but this is mostly covered in the section above.

We have code in place to exclude password and api_key from UserProfile rows. The import process calls set_unusable_password().

Public Realm Data

  • models: Realm/RealmDomain/RealmEmoji/RealmFilter/DefaultStream
  • asserts: realm.json

All of these tables are public (per-realm), and they are keyed by realm ID. There is not a ton to worry about here, except possibly merging if we run multiple bottom-up jobs for a single realm.