Let’s continue to dive in PostgreSQL Concurrency. In the previous article of the series, Modeling for Concurrency, we saw how to model your application for highly concurrent activity. It was a follow-up to the article entitled PostgreSQL Concurrency: Isolation and Locking, which was a primer on PostgreSQL isolation and locking properties and behaviors.
Today’s article takes us a step further and builds on what we did in the previous articles in our series. After having 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, and having had them like a retweet a lot in PostgreSQL Concurrency: Isolation and Locking, it’s time to think about how to display our counters in an efficient way.
In this article, we’re going to think about when we should compute results and when we should cache them for instant retrieval, all within the SQL tooling. The SQL tooling for handling cache is a MATERIALIZED VIEW, and it comes with cache invalidation routines, of course.
Computing and Caching in SQL
There’s a pretty common saying:
There are only two hard things in computer science: cache invalidation and naming things.
— Phil Karlton
More about that saying can be read at the Two Hard Things page from Martin Fowler, who tries to track it back to its origins.
It is time that we see about how to address the cache problems in SQL. Creating a set of values for caching is of course really easy as it usually boils down to writing a SQL query. Any SQL query executed by PostgreSQL uses a snapshot of the whole database system. To create a cache from that snapshot, the simplest way is to use the create table as command.
create table tweet.counters as
select count(*) filter(where action = 'rt')
- count(*) filter(where action = 'de-rt')
as rts,
count(*) filter(where action = 'fav')
- count(*) filter(where action = 'de-fav')
as favs
from tweet.activity
join tweet.message using(messageid);
Now we have a tweet.counters table that we can use whenever we need the numbers of rts or favs from a tweet message. How do we update the counters? That’s the cache invalidation problem quoted above, and we’ll come to the answer by the end of this article!
Views
Views allow integrating server-side computations in the definition of a relation. The computing still happens dynamically at query time and is made transparent to the client. When using a view, there’s no problem with cache invalidation, because nothing gets cached away.
create view tweet.message_with_counters
as
select messageid,
message.userid,
message.datetime,
message.message,
count(*) filter(where action = 'rt')
- count(*) filter(where action = 'de-rt')
as rts,
count(*) filter(where action = 'fav')
- count(*) filter(where action = 'de-fav')
as favs,
message.location,
message.lang,
message.url
from tweet.activity
join tweet.message using(messageid)
group by message.messageid, activity.messageid;
Given this view, the application code can query tweet.message_with_counters and process the same relation as in the first normalized version of our schema. The view hides the complexity of how to obtain the counters from the schema.
select messageid,
rts,
nickname
from tweet.message_with_counters
join tweet.users using(userid)
where messageid between 1 and 6
order by messageid;
We can see that I played with generating some retweets in my local testing, done mainly over the six first messages:
messageid │ rts │ nickname
═══════════╪════════╪══════════════
1 │ 20844 │ Duke Theseus
2 │ 111345 │ Hippolyta
3 │ 11000 │ Duke Theseus
5 │ 3500 │ Duke Theseus
6 │ 15000 │ Egeus
(5 rows)
That view now embeds the computation details and abstracts them away from the application code. It allows having several parts of the application deal with the same way of counting retweets and favs, which might come to be quite important if you have different backends for reporting, data analysis, and user analytics products that you’re selling, or using it to sell advertising, maybe. It might even be that those parts are written in different programming languages, yet they all want to deal with the same numbers, a shared truth.
The view embeds the computation details, and still it computes the result each time it’s referenced in a query.
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!
Materialized Views
It is easy enough to cache a snapshot of the database into a permanent relation for later querying thanks to PostgreSQL implementation of materialized views:
create schema if not exists twcache;
create materialized view twcache.message
as select messageid, userid, datetime, message,
rts, favs,
location, lang, url
from tweet.message_with_counters;
create unique index on twcache.message(messageid);
As usual, read the PostgreSQL documentation about the command CREATE MATERIALIZED VIEW for complete details about the command and its options.
The application code can now query twcache.message instead of tw.message and get the extra pre-computed columns for rts and favs counter. The information in the materialized view is static: it is only updated with a specific command. We have effectively implemented a cache in SQL, and now we have to solve the cache invalidation problem: as soon as a new action (retweet or favorite) happens on a message, our cache is wrong.
Now that we have created the cache, we run another benchmark with 100 workers doing each 100 retweets on messageid 3:
CL-USER> (concurrency::concurrency-test 100 100 3)
Starting benchmark for updates
Updating took 8.132917 seconds, did 10000 rts
Starting benchmark for inserts
Inserting took 6.684597 seconds, did 10000 rts
Then we query our cache again:
select messageid,
rts,
nickname,
substring(message from E'[^\n]+') as first_line
from twcache.message
join tweet.users using(userid)
where messageid = 3
order by messageid;
We can see that the materialized view is indeed a cache, as it knows nothing about the last round of retweets that just happened:
messageid │ rts │ nickname │ first_line
═══════════╪══════╪══════════════╪══════════════════
3 │ 1000 │ Duke Theseus │ Go, Philostrate,
(1 row)
Of course, as every PostgreSQL query uses a database snapshot, the situation when the counter is already missing actions already happens with a table and a view already. If some insert are committed on the tweet.activity table while the rts and favs count query is running, the result of the query is not counting the new row, which didn’t make it yet at the time when the query snapshot had been taken. Materialized view only extends the cache time to live, if you will, making the problem more obvious.
To invalidate the cache and compute the data again, PostgreSQL implements the refresh materialized view command:
refresh materialized view concurrently twcache.message;
This command makes it possible to implement a cache invalidation policy. In some cases, a business only analyses data up to the day before, in which case you can refresh your materialized views every night: that’s your cache invalidation policy.
Once the refresh materialized view command has been processed, we can query the cache again. This time, we get the expected answer:
messageid │ rts │ nickname │ first_line
═══════════╪═══════╪══════════════╪══════════════════
3 │ 11000 │ Duke Theseus │ Go, Philostrate,
(1 row)
In the case of instant messaging such as Twitter, maybe the policy would require rts and favs counters to be as fresh as five minutes ago rather than yesterday. When the refresh materialized view command runs in less than five minutes then implementing the policy is a matter of scheduling that command to be executed every five minutes, using for example the cron Unix task scheduler.
Conclusion
Once more, PostgreSQL makes it very easy to solve a complex problem. Managing a cache becomes a matter of using two commands: CREATE MATERIALIZED VIEW to initialize the cache structure, and then REFRESH MATERIALIZED VIEW to implement your cache invalidation policy.
Reminder: everywhere you are caching values, have an explicit cache invalidation policy and a tool to invalidate your cache manually, in case of emergency cache cleaning situations. Also, your software should be able to bypass the cache easily. Caching is efficient, and it comes with complexities that need to be handled.
