4 ms·

Postgres will NEVER lose an increment if two transactions try to swap the same counter at the same time, even if denormalized or distributed reads. ACID means "

Postgres will NEVER lose an increment if two transactions try to swap the same counter at the same time, even if denormalized or distributed reads. ACID means "doesn't lose data" Postgresql with read replicas is ACID-Consistent transactions because there is a single writer process to prevent data loss. Only the reads are distributed and thus cap-AP eventually consistent.

elvinyung 9y ago
What happens if a user makes an update to their data, and then my "single writer" master crashes, and I failover to a hot standby that doesn't have the new transaction? And then what happens if the user tries to update their data again?
- dustingetz 9y ago
  the D in ACID addresses this. Your standby transactor process is attached to the same storage as the previous transactor (EBS block or dynamodb or whatever cloud HA storage people use these days). If they don't share storage, it's not a single writer.
  - elvinyung 9y ago
    You're not actually saying that PostgreSQL itself is ACID. You are essentially asserting that a precondition for ACID in Postgres is that the underlying storage medium (or at least the bits on them) will never suffer permanent failures, and has infinite 9's of availability everywhere in the world, instantly. Then, and only then, can PostgreSQL provide anything resembling seamless consistency. Sorry, no dice. This is not how distributed systems work in the world outside the walled gardens. I'm a huge proponent of the cloud, but I'm not a huge proponent of things that only work if you rely on the cloud.
    - dustingetz 9y ago
      a black hole might swallow the solar system and thus our ACID writer process lost a write? No, you get to choose how durable you would you like your D. Don't acknowledge the transaction as successful until you successfully commit to storage orbiting alpha centauri. non-ACID writers like Mongo lose writes routinely, every day, often without anybody noticing. There are no guarantees whatsoever about data integrity. ACID writers lose data in 6-sigma catastrophic failures only, and you can control how many sigmas by tuning durability. You asked a question about HA configurations so I answered in terms of HA configurations. Serving durable HA storage is literally what dynamo/riak is for. "The write operation is durable. dw (durable write quorum) is a configuration setting used to specify how many replicas to commit to durable storage before returning a successful response." https://stackoverflow.com/questions/22736821/riak-database-and-acid https://stackoverflow.com/questions/22736821/riak-database-a... Anyway I'm interested in continuing to discuss this if you are because this is a subject I am beginning to write about as part of my startup. so this conversation helps me. Perhaps there is a better medium though if you would like to continue?

elvinyung 9y ago

What happens if a user makes an update to their data, and then my "single writer" master crashes, and I failover to a hot standby that doesn't have the new transaction? And then what happens if the user tries to update their data again?

dustingetz 9y ago
the D in ACID addresses this. Your standby transactor process is attached to the same storage as the previous transactor (EBS block or dynamodb or whatever cloud HA storage people use these days). If they don't share storage, it's not a single writer.
- elvinyung 9y ago
  You're not actually saying that PostgreSQL itself is ACID. You are essentially asserting that a precondition for ACID in Postgres is that the underlying storage medium (or at least the bits on them) will never suffer permanent failures, and has infinite 9's of availability everywhere in the world, instantly. Then, and only then, can PostgreSQL provide anything resembling seamless consistency. Sorry, no dice. This is not how distributed systems work in the world outside the walled gardens. I'm a huge proponent of the cloud, but I'm not a huge proponent of things that only work if you rely on the cloud.
  - dustingetz 9y ago
    a black hole might swallow the solar system and thus our ACID writer process lost a write? No, you get to choose how durable you would you like your D. Don't acknowledge the transaction as successful until you successfully commit to storage orbiting alpha centauri. non-ACID writers like Mongo lose writes routinely, every day, often without anybody noticing. There are no guarantees whatsoever about data integrity. ACID writers lose data in 6-sigma catastrophic failures only, and you can control how many sigmas by tuning durability. You asked a question about HA configurations so I answered in terms of HA configurations. Serving durable HA storage is literally what dynamo/riak is for. "The write operation is durable. dw (durable write quorum) is a configuration setting used to specify how many replicas to commit to durable storage before returning a successful response." https://stackoverflow.com/questions/22736821/riak-database-and-acid https://stackoverflow.com/questions/22736821/riak-database-a... Anyway I'm interested in continuing to discuss this if you are because this is a subject I am beginning to write about as part of my startup. so this conversation helps me. Perhaps there is a better medium though if you would like to continue?