Since channels_redis does not support dedicated read-redis instances, the
autoupdate message may be received before the data was replicated. All workers
read the autoupdate message from the write host, so there is a race between
getting this message and a finished replication. For large payloads, the
replication is slower in the most cases (even more in a distributed setup, where
the master and replica are on different nodes). The easy way is to wait for
replication. But there is one difficulty: The number of replicas has to be
known. There is a new settings-variable "AMOUNT_REPLICAS" which defaults to 1.
It needs to be set correctly! If it is too high, every autoupdate will be
delayed by 1 seconds because of a timeout witing for non-existent replicas. If
it is too low, some autoupdates may be wrong (and not detectable by the client!)
becuase of reading from non-synchronised relicas.
The other possibility is to fork channel_redis and add the feature of a
read-only redis. This ould help, because on a single redis instance all commands
are ordered: First, the data is synced, then the autoupdate message. Attention:
This means, if redis-replicas are scaled up, one must make sure to read from the
same instance. I think this is not possible in the way how dockers overlay
networks work. The only way would be to open one connection and reuse the
connection from channels_redis in OpenSlides. This would mean a heavy
integration of channels_redis (meaning including the source code in our repo).
For the first fix, this one is easy and should work.
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