5 ms·

What are the benefits of a system like this? More available reads at the expense of terribly slow write locks?

by toolz 11y ago

ahachete 11y ago
pg_paxos should be seen as a building block for other systems/applications (like this example of a distributed execution queue). As such, and as highlighted in the post, it provides several features not present in other distributed systems, such as those based on asynchronous replication (which are, indeed, most of them): - Conflict-free. No need to apply conflict resolution. - Synchronous. No eventual consistency. This is strong consistency (with C in the CAP meaning). - Master-less. No need to elect a master. All nodes are treated equal. Paxos introduces higher latency and lower throughput, so it is best used for low bandwith applications, like metadata management. But many applications may benefit from the very strong guarantees that Paxos in general and pg_paxos in particular, provide.
- mtanski 11y ago
  > Master-less. No need to elect a master. All nodes are treated equal. Is it really masterless or do the nodes elect a master using Paxos to hold an election?
  - misframer 11y ago
    You don't need Paxos to elect a master. It's basically part of Paxos itself.
teraflop 11y ago
Benefits as compared to what? Off the top of my head: - Compared to a single Postgres instance, you get better availability for reads and writes - Compared to a master-slave failover process, you get automated failure recovery without the risk of split-brain or lost writes. - Compared to a NoSQL database, you get strong consistency, and all the powerful SQL support that Postgres provides. The cost is higher latency and lower throughput for both reads and writes.
utternerd 11y ago
From their own documentation the use case is reliable replication, and even reads would be horribly slow: "The drawbacks are high latency in both reads and writes and low throughput. Pg_paxos cannot be used for high performance transactional systems. But it can serve very well for low-bandwith, reliable replication use cases."
- dijit 11y ago
  Better to wait for postgresql 9.6 which will have synchronous replication, write latency but not read. http://michael.otacoo.com/postgresql-2/postgres-9-6-feature-highlight-multi-sync-rep/ http://michael.otacoo.com/postgresql-2/postgres-9-6-feature-...
  - ahachete 11y ago
    PostgreSQL supports synchronous replication since 9.1. What 9.6 will have is support for more than one synchronous replicated server. In any case, synchronous replication means that all of the participating servers have to participate in the replication process. If one of them slows down or hangs, replication (and your transaction) does not proceed. Paxos, on the contrary, can proceed when N/2+1 of the nodes are available. That's a huge difference, and it's irrespective of the latency and performance. In other words: while 9.6's synchronous replication is a really welcomed addition, a single miss-behaving node will halt transactions on the cluster, while pg_paxos will continue operating without problems. Both are meant for different use cases.
    - teraflop 11y ago
      This isn't quite true. As described in that blog post, you can configure Postgres to synchronously replicate to N servers but only wait for M responses. With M=N/2+1, you get the same availability as Paxos. The difference is that with Postgres' replication, when the master fails, write operations can't be executed until a new master is promoted. This has to be done carefully, because you want to make sure that no in-flight operations are still happening on the old master (aka STONITH), and that the most up-to-date slave becomes the new master. Paxos avoids the need for manual (or very delicately-automated) failover, at the cost of extra network round-trips and disk syncs on every operation.

What are the benefits of a system like this? More available reads at the expense of terribly slow write locks?

by toolz 11y ago

What are the benefits of a system like this? More available reads at the expense of terribly slow write locks?

pg_paxos should be seen as a building block for other systems/applications (like this example of a distributed execution queue). As such, and as highlighted in the post, it provides several features not present in other distributed systems, such as those based on asynchronous replication (which are, indeed, most of them): - Conflict-free. No need to apply conflict resolution. - Synchronous. No eventual consistency. This is strong consistency (with C in the CAP meaning). - Master-less. No need to elect a master. All nodes are treated equal. Paxos introduces higher latency and lower throughput, so it is best used for low bandwith applications, like metadata management. But many applications may benefit from the very strong guarantees that Paxos in general and pg_paxos in particular, provide.

mtanski 11y ago
> Master-less. No need to elect a master. All nodes are treated equal. Is it really masterless or do the nodes elect a master using Paxos to hold an election?
- misframer 11y ago
  You don't need Paxos to elect a master. It's basically part of Paxos itself.

teraflop 11y ago

Benefits as compared to what? Off the top of my head: - Compared to a single Postgres instance, you get better availability for reads and writes - Compared to a master-slave failover process, you get automated failure recovery without the risk of split-brain or lost writes. - Compared to a NoSQL database, you get strong consistency, and all the powerful SQL support that Postgres provides. The cost is higher latency and lower throughput for both reads and writes.

utternerd 11y ago

From their own documentation the use case is reliable replication, and even reads would be horribly slow: "The drawbacks are high latency in both reads and writes and low throughput. Pg_paxos cannot be used for high performance transactional systems. But it can serve very well for low-bandwith, reliable replication use cases."

dijit 11y ago
Better to wait for postgresql 9.6 which will have synchronous replication, write latency but not read. http://michael.otacoo.com/postgresql-2/postgres-9-6-feature-highlight-multi-sync-rep/ http://michael.otacoo.com/postgresql-2/postgres-9-6-feature-...
- ahachete 11y ago
  PostgreSQL supports synchronous replication since 9.1. What 9.6 will have is support for more than one synchronous replicated server. In any case, synchronous replication means that all of the participating servers have to participate in the replication process. If one of them slows down or hangs, replication (and your transaction) does not proceed. Paxos, on the contrary, can proceed when N/2+1 of the nodes are available. That's a huge difference, and it's irrespective of the latency and performance. In other words: while 9.6's synchronous replication is a really welcomed addition, a single miss-behaving node will halt transactions on the cluster, while pg_paxos will continue operating without problems. Both are meant for different use cases.
  - teraflop 11y ago
    This isn't quite true. As described in that blog post, you can configure Postgres to synchronously replicate to N servers but only wait for M responses. With M=N/2+1, you get the same availability as Paxos. The difference is that with Postgres' replication, when the master fails, write operations can't be executed until a new master is promoted. This has to be done carefully, because you want to make sure that no in-flight operations are still happening on the old master (aka STONITH), and that the most up-to-date slave becomes the new master. Paxos avoids the need for manual (or very delicately-automated) failover, at the cost of extra network round-trips and disk syncs on every operation.