> Since connection poolers reuse connections between clients, the connection state of one client “leaks” into the connection state of another.
Wow this is very bad. This actually happens in typical Postgres setups?
Wow this is very bad. This actually happens in typical Postgres setups?
in pgbouncer the connection is reset via a customisable command [0] which should reset the connection to a clean state.
[0] https://www.pgbouncer.org/config.html#server_reset_query
Example: legacy client A connects to MySQL via the bouncer and says 'I want all of our conversations to use latin-1, not utf-8'. This changes the character set that MySQL parses queries with and returns responses in. The legacy client does some queries and then disconnects.
Now a new client connects to MySQL, and the bouncer just assigns it to the still-open connection from before. The new client is fully UTF-8 compatible and since this is the default for our database it doesn't explicitly say so; it just assumes that UTF-8 is the way to go. Unfortunately, the database server is still thinking in latin-1, meaning that if this new client sends UTF-8 data it will be parsed as latin-1; latin-1 is a subset of UTF-8, meaning that queries will actually work fine unless they need to use a character outside of latin-1, in which case they will get an error, or corrupted data, from the server.
The only solutions around this are:
1. Ensure that every client is using the same settings; if your database is for a single app that uses the same ORM, then this is automatic.
2. Ensure that every client is always explicit about everything it might need to change e4very time, so that every UTF-8 client explicitly sets UTF-8 connections even when that's the default; clients that need utf8mb4 ask for it explicitly and clients that can't handle it ask for something else. One way of ensuring this happens is to configure the server (or the bouncer) to use defaults which are not valid for anyone, or which are going to cause errors frequently and not rarely (e.g. setting the default character set to 7-bit swedish, which would cause frequent errors).
3. Use a bouncer which can either disallow these changes or detect and revert them after the original client has disconnected. I'm not sure if this exists for MySQL at least.
4. Use separate bouncers for each application that might be different (extension of #1); in other words, instead of having a bouncer or set of bouncers for each pool of database servers, you have them for each application; your web app gets one, your legacy reporting tool gets one, your ODBC connector gets one, and so on.
It's kind of a huge mess in theory; in practice, a lot of installations fall into the #1 case so it never matters, but that makes the occasional instance where it does matter extremely difficult to debug.
I wonder if clients send something equivalent to a User-Agent, such that the connection pooler could assign them to different pools automatically.
Every new version of your app has the potential to change behavior in a way that would affect the previous version if the connection was recycled during a progressive rollout.
But I don’t think I would want to create a real database user for every version of the app.
I suppose the connection pooler could map versioned users to the same real user, and use separate pools, but a dedicated UA field is probably better.
Why not? Database users are (usually) not expensive, and with groups you can give access to a group you just add the user to.
Adding this logic to the connection pooler seems more complicated.
Also because it doesn’t really concern the database, it concerns the pooler.
Connection poolers already maintain multiple pools, it would not be complicated at all.
I believe ProxySQL does exactly that:
* https://proxysql.com/documentation/mysql-prepared-statements...