zulip/docs/subsystems/conversion.md

# 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](../index.html#zulip-in-production).
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.