Today, like the past few days, we have had some downtime. Apparently some script kids are enjoying themselves by targeting our server (and others). Sorry for the inconvenience.
Most of these ‘attacks’ are targeted at the database, but some are more ddos-like and can be mitigated by using a CDN. Some other Lemmy servers are using Cloudflare, so we know that works. Therefore we have chosen Cloudflare as CDN / DDOS protection platform for now. We will look into other options, but we needed something to be implemented asap.
For the other attacks, we are using them to investigate and implement measures like rate limiting etc.
What if your application has to know a state? Say for certain write requests, only one instance is allowed to process those as it needs a cache that it can somewhat consistently rely on?
(Granted, I wouldn’t know why something like Lemmy needs that. But we had that problem at work, and it was a pain to solve while also supporting multiple app instances.)
In that case, I’d use a message queue. Rabbitmq, or I use Pulsar at work - multiple subscribers (using the same subscription name) to one queue of messages that need to be processed. One worker picks it up, processes it, and marks the message as processed. The worker either passes it into a different queue for further processing, or persists it to the DB.
The nice thing with this is when using the Pulsar paradigm, you can have multiple subscriptions to the same message queue, each one carrying its own state as to which messages are processed or not. So say I get one message from an external system, have one system that is processing it right now, and need to add a second system. In that case I just use a different subscription name for the second system, and it works independently of the first with no issues.
Distributed lock of any form would work. Memcache, redis, etcd, read access mechanism in an MQ…etc. Only one process would work on whatever it as a time. Simple.