How to start a custom projection off of $all from a specific position?

Is there a way so start a projection off of the $all stream from a specific prepare/commit position? Is it possible to write to a projection’s checkpoint stream? If so, what is the name of that stream?

Looks like the name of the checkpoint stream is $projections-{ProjectionName}-checkpoint. If an event is written to this stream and the projection started, would it continue based on this position?

Was responding to ither with checkpoibt stream…

This is possible and yes you would have to update this stream then start the projection. I would have to verify there is no state caching logic but feasibly yes this could work.

A few issues.

  1. it would need to be a fully valid checkpoint (foreachstream as example is different)
  2. no promise that things here wont change (this is way past any public contract)

Greg

OK cool, I’ll try. This is a one-off, temporary measure and I don’t expect to do this again. The use-case is to alleviate slow linked category projections by starting a copy projection where the category projection left off. This is all temporary as we place projections into a separate eventstore.

Will you be writing to the $ streams? Remember that you cant go backnonce youve done that unless you jump through a lot of hoops to set things needed by projections…

Will you be writing to the $ streams? Remember that you cant go backnonce youve done that unless you jump through a lot of hoops to set things needed by projections…

OK cool, I’ll try. This is a one-off, temporary measure and I don’t expect to do this again. The use-case is to alleviate slow linked category projections by starting a copy projection where the category projection left off. This is all temporary as we place projections into a separate eventstore.

Was responding to ither with checkpoibt stream…

This is possible and yes you would have to update this stream then start the projection. I would have to verify there is no state caching logic but feasibly yes this could work.

A few issues.

  1. it would need to be a fully valid checkpoint (foreachstream as example is different)
  2. no promise that things here wont change (this is way past any public contract)

Greg

Is there a way so start a projection off of the $all stream from a specific prepare/commit position? Is it possible to write to a projection’s checkpoint stream? If so, what is the name of that stream?

Looks like the name of the checkpoint stream is $projections-{ProjectionName}-checkpoint. If an event is written to this stream and the projection started, would it continue based on this position?

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Yes, I plan to write to the stream $projections-{ProjectionName}-checkpoint where {ProjectionName} is the name of a new JS projection I’m creating to take over the $by_category projection. I’ll stop all projections, then take the last event in $projections-$by_category-checkpoint and write it to $projections-{ProjectionName}-checkpoint. Then I will start the new projection, leaving $by_category off. After this, the only projection running will be this new projection. Its a simple projection that will just do the same thing as the $by_category projection except it will do copyTo for specific stream prefixes.