Suppose we have some data in a production OLTP database, and we need to send it to some OLAP database. This post describes one of the simplest approaches, and how to make it productional enough to rely on.

For every table t, we need to:

• introduce a new field, updated_at
• introduce a new index on that field, so we can get the records that changed after a certain updated_at.

For example,

ALTER TABLE t
NOT NULL
DEFAULT CURRENT_TIMESTAMP(3)
ON UPDATE CURRENT_TIMESTAMP(3),


We add the ON UPDATE CURRENT_TIMESTAMP so that we don't need to deal with any application code to give us what we need.

We add the idx__updated_at to support a new query pattern:

SELECT *
FROM t
WHERE updated_at >= :lastCutoff
ORDER BY updated_at
LIMIT :limit


Here, we're basically going to fetch the records that have been updated since the last time we ran the query, up to a given limit. The limit is an important mechanism, because it lets us paginate over the changed records. Bounding the amount of work we do for a single fetch lets us bound the expected execution time, so we can find out more aggressively when things are broken.

This consumes a single "page" of data. We will need to repeatedly execute this query to emit new records as they are updated.

## non-strict inequality

The astute reader might have noticed we used >= in the lastCutoff portion of the query; this implies that we might see duplicates. But it is also important, because in a given fetch, we might not have consumed all records with that timestamp. For example, consider

rec(id=1, updated_at=1)
rec(id=2, updated_at=2)
rec(id=3, updated_at=3)
rec(id=4, updated_at=3)
rec(id=5, updated_at=4)


consumed from the start with lastCutoff=0 and limit=3. The first fetch returns

rec(id=1, updated_at=1)
rec(id=2, updated_at=2)
rec(id=3, updated_at=3)


So if the next fetch query were to be

SELECT *
FROM t
WHERE updated_at > 3
ORDER BY updated_at
LIMIT 3


we would skip the id=4 record.

## wedge possibility

The non-strict inequality induces a different problem, though. Consider

rec(id=1, updated_at=1)
rec(id=2, updated_at=1)
rec(id=3, updated_at=1)
rec(id=4, updated_at=1)
rec(id=5, updated_at=1)


consumed with lastCutoff=0 and limit=3. The first fetch again returns the first three records, but the next query will be

SELECT *
FROM t
WHERE updated_at >= 1
ORDER BY updated_at
LIMIT 3


which will again return the first three records and therefore fail to make progress -- it has become wedged.

There are a variety of resolutions to this process, including:

1. don't actually resolve it, just

a. pick a relatively large limit

b. pick a relatively granular updated_at.

c. just rely on the monitoring provisions below to update these as needed.

2. use an adaptive limit that temporarily increases limit if it detects a lack of progress

3. use a more feed-like mechanism instead

## fixing duplicates

Whether or not we observe duplicates from the precise fetch mechanism, we can also observe duplicates from a sequence of updates like

rec(id=1, updated_at=1, value="t")
rec(id=1, updated_at=2, value="u")
rec(id=1, updated_at=3, value="v")


If observing the intermediate results is inconsequential, we can easily deduplicate these by picking the record with the greatest updated_at value.

## missing intermediate updates / only guarantees LAST update

It should be pretty clear that this mechanism does not necessarily deliver all updates. If my first fetch runs at t>3 against this history of rec values:

rec(id=1, updated_at=1, value="t")
rec(id=1, updated_at=2, value="u")
rec(id=1, updated_at=3, value="v")


the database will only actually have a single record:

rec(id=1, updated_at=3, value="v")


and therefore, we will never emit the value=t or value=u records!

In a lot of systems, this can be ok! But if it's not, we must consider either a different mechanism or a different data model. As an example, a different data model could store

rec_update(id=101, rec_id=1, updated_at=1, value="t")
rec_update(id=102, rec_id=1, updated_at=2, value="u")
rec_update(id=103, rec_id=1, updated_at=3, value="v")


## monitoring

This being an online system, we should define some appropriate correctness / monitoring criteria.

One possible correctness property of this approach is

the most recently processed updated_at is sufficiently recent

but this might not be appropriate if the table being replicated can have long gaps between updates. A slightly better variant might be

the most recent successfully poll operation occurred sufficiently recently

This is good to ensure that we're making progress and that our process hasn't broken. But even if our polling process is working, it might be that the limit or polling frequency are simply not high enough to keep up! To account for that, we can instead check that

the number of records after lastCutoff is sufficiently small

This works ok, except 1) we need cron-style monitoring for this process as well, and 2) if we are very behind, such a count might be prohibitively expensive. We can lessen the overhead of (2) by changing to:

the difference between the max updated_at in the table & the lastCutoff is sufficiently small

Here, the index on updated_at makes the query effectively constant-time, which is great! But it does sacrifice the knowledge of how many records we're behind by, which is often a better indication of how much work we need to do to catch up. One intermediate approach is executing a subquery with a LIMIT clause, so we can obtain precise backlog figures when it is cheap enough to do so, and otherwise tops out at that limit. I've seen those limits set between 1000 and 100000 in various production systems I've worked with, and they have been pretty effective.

## comparison with feeds

This mechanism bears a lot of similarity to MySQL feeds, but with a few major differences:

1. this polling approach is much simpler than feeds because it doesn't need the feed_sync_id assignment process or any API
2. presence of the API layer in the feed approach make it possible to perform transformations on the exposed records, while there is no such capability in this approach
3. the feed approach avoids the "wedge possibility" problem since it is impossible to have multiple records with the same feed_sync_id