This article fits in the PostgreSQL Concurrency series, where we installed a tweeter like application schema and had all the characters from Shakespeare’s A Midsummer Night’s Dream tweet their own lines in our database in PostgreSQL Concurrency: Data Modification Language.
A previous article in the series covered how to manage concurrent retweets in an efficient way: Computing and Caching, where we learn how to maintain a cache right in your PostgreSQL database, thanks for materialized views. We even went as far as maintaining an external cache in another application layer using PostgreSQL LISTEN and NOTIFY features and a Golang application.
Today’s article is going to address concurrency in the context of updating data in a batch. This activity is quite common, as soon as your system is connected to other systems either internally or with external providers. While it’s pretty easy to ingest new data, and easy enough to update data from an external source when nothing happens in your database, doing the operation safely with concurrent activity is more complex. Once more though, PostgreSQL comes with all the tooling you need to handle that situation.
Table of Contents
Batch Update, MoMA Collection
The Museum of Modern Art (MoMA) Collection hosts a database of the museum’s collection, with monthly updates. The project is best described in their own words:
MoMA is committed to helping everyone understand, enjoy, and use our collection. The Museum’s website features 75,112 artworks from 21,218 artists. This research dataset contains 131,585 records, representing all of the works that have been accessioned into MoMA’s collection and cataloged in our database. It includes basic metadata for each work, including title, artist, date made, medium, dimensions, and date acquired by the Museum. Some of these records have incomplete information and are noted as “not Curator Approved.”
Using git and git lfs commands, it’s possible to retrieve versions of the artist collection for the last months. From one month to the next, lots of the data remains unchanged, and some is updated.
begin; create schema if not exists moma; create table moma.artist ( constituentid integer not null primary key, name text not null, bio text, nationality text, gender text, begin integer, "end" integer, wiki_qid text, ulan text ); \copy moma.artist from 'artists/artists.2017-05-01.csv' with csv header delimiter ',' commit;
Now that we have loaded some data, let’s have a look at what we have:
select name, bio, nationality, gender from moma.artist limit 6;
Here are some of the artists being presented at the MoMA:
name │ bio │ nationality │ gender ═════════════════╪═════════════════════╪═════════════╪════════ Robert Arneson │ American, 1930–1992 │ American │ Male Doroteo Arnaiz │ Spanish, born 1936 │ Spanish │ Male Bill Arnold │ American, born 1941 │ American │ Male Charles Arnoldi │ American, born 1946 │ American │ Male Per Arnoldi │ Danish, born 1941 │ Danish │ Male Danilo Aroldi │ Italian, born 1925 │ Italian │ Male (6 rows)
Updating the Data
After having successfully loaded the data from May, let’s say that we have received an update for June. As usual with updates of this kind, we don’t have a diff, rather we have a whole new file with a new content.
A batch update operation is typically implemented that way:
Load the new version of the data from file to a PostgreSQL table or a temporary table.
Use the update command ability to use join operations to update existing data with the new values.
Use the insert command ability to use join operations to insert new data from the batch into our target table.
Here’s how to write that in SQL in our case:
begin; create temp table batch ( like moma.artist including all ) on commit drop; \copy batch from 'artists/artists.2017-06-01.csv' with csv header delimiter ',' with upd as ( update moma.artist set (name, bio, nationality, gender, begin, "end", wiki_qid, ulan) = (batch.name, batch.bio, batch.nationality, batch.gender, batch.begin, batch."end", batch.wiki_qid, batch.ulan) from batch where batch.constituentid = artist.constituentid and (artist.name, artist.bio, artist.nationality, artist.gender, artist.begin, artist."end", artist.wiki_qid, artist.ulan) <> (batch.name, batch.bio, batch.nationality, batch.gender, batch.begin, batch."end", batch.wiki_qid, batch.ulan) returning artist.constituentid ), ins as ( insert into moma.artist select constituentid, name, bio, nationality, gender, begin, "end", wiki_qid, ulan from batch where not exists ( select 1 from moma.artist where artist.constituentid = batch.constituentid ) returning artist.constituentid ) select (select count(*) from upd) as updates, (select count(*) from ins) as inserts; commit;
Our batch update implementation follows the specifications very closely. The ability for the update and insert SQL commands to use join operations are put to good use, and the returning clause allows to display some statistics about what’s been done.
Also, the script is careful enough to only update those rows that actually have changed thanks to using a row comparator in the update part of the CTE.
Finally, note the usage of an anti-join in the insert part of the CTE in order to only insert data we didn’t have already.
Here’s the result of running this batch update script:
BEGIN CREATE TABLE COPY 15186 updates │ inserts ═════════╪═════════ 35 │ 21 (1 row) COMMIT
An implicit assumption has been made in the creation of this script. Indeed, it considers the constituentid from MoMA to be a reliable primary key for our data set. This assumption should, of course, be checked before deploying such an update script to production.
This article is extracted from my book The Art of PostgreSQL, which teaches SQL to developers so that they may replace thousands of lines of code with very simple queries. The book has a full chapter about Data Manipulation and Concurrency Control in PostgreSQL, including caching with materialized views, check it out!
While in this solution the update or insert happens in a single query, which means using a single snapshot of the database and a within a transaction, it is still not prevented from being used concurrently. The tricky case happens if your application were to run the query above twice at the same time.
What happens is that as soon as the concurrent sources contain some data for the same primary key, you get a duplicate key error on the insert. As both the transactions are concurrent, they are seeing the same target table where the new data does not exists, and both will conclude that they need to insert the new data into the target table.
There are two things that you can do to avoid the problem. The first thing is to make it so that you’re doing only one batch update at any time, by building your application around that constraint.
A good way to implement that idea is with a manual lock command as explain in the explicit locking documentation part of PostgreSQL:
LOCK TABLE target IN SHARE ROW EXCLUSIVE MODE;
That lock level is not automatically acquired by any PostgreSQL command, so the only way it helps you is when you’re doing that for every transaction you want to serialize. When you know you’re not at risk (that is, when not playing the insert or update dance), you can omit that lock.
Another solution is using the new in PostgreSQL 9.5 on conflict clause for the insert statement.
On Conflict Do Nothing
When using PostgreSQL version 9.5 and later, it is possible to use the on conflict clause of the insert statement to handle concurrency issues, as in the following variant of the script we already saw. Here’s a diff of the first update script and the second one, that handles concurrency conflicts:
--- artists.update.sql 2017-09-07 23:54:07.000000000 +0200 +++ artists.update.conflict.sql 2017-09-08 12:49:44.000000000 +0200 @@ -5,11 +5,11 @@ like moma.artist including all ) on commit drop; -\copy batch from 'artists/artists.2017-06-01.csv' with csv header delimiter ',' +\copy batch from 'artists/artists.2017-07-01.csv' with csv header delimiter ',' with upd as ( update moma.artist set (name, bio, nationality, gender, begin, "end", wiki_qid, ulan) @@ -41,10 +41,11 @@ ( select 1 from moma.artist where artist.constituentid = batch.constituentid ) + on conflict (constituentid) do nothing returning artist.constituentid ) select (select count(*) from upd) as updates, (select count(*) from ins) as inserts;
Notice the new line on conflict (constituentid) do nothing. It basically implements what it says: if inserting a new row causes a conflict, then the operation for this row is skipped.
The conflict here is a primary key or a unique violation, which means that the row already exists in the target table. In our case, this may only happen because a concurrent query just inserted that row while our query is in flight, in between its lookup done in the update part of the query and the insert part of the query.
PostgreSQL implements several facilities that we can rely on to maintain an application cache for data that changes often:
In Computing and Caching, we saw how to use a MATERIALIZED VIEW to get a fixed snapshot of the data, and then easily REFRESH it to implement our cache invalidation policy.
This technique is well adapted to use cases where you want to rebuild your cache every once in a while, maybe every night, or several times a day, down to maybe every five minutes if the refreshing of the cache is really fast.
In PostgreSQL Event Based Processing we saw how to use TRIGGERs to maintain a transactionally correct cache, and the impact of such a choice on the scalability properties of your database backend.
This solution is well suited to use case where the application only receives a small amount of UPDATE traffic, and quite far apart, and can’t tolerate any lag when using the cache.
In PostgreSQL LISTEN/NOTIFY we saw how to build an online cache maintenance service with PostgreSQL’s advanced notification features.
This solution is well suited to use cases where a small amount of lag can be tolerated, up to maybe some seconds, most typically measured in the hundreds of milliseconds.
Adding to this cache management facilities, today’s article shows how to manage batch updates to your data set in a cincurrency safe way, thanks to the INSERT … ON CONFLICT … DO UPDATE, documented as part of the INSERT command.
Again, we see that core PostgreSQL features allow application developers to build exactly the facility they need. PostgreSQL really is YeSQL!