One challenge to be aware of is that when you use multiple data centers, the users can make changes in either data center and those changes will propagate to the other data center. That is, there is no concept of a “read-only data center” in Cassandra. That may be fine, but some organizations want to grant access to the data for analytics but don’t want those teams to be able to modify the original data. You can, in some cases, restrict the write access through user/role permissions (the analytics team only has read access to that table), but that may not work depending on your use case (but it usually does work).
One comment from Benjamin’s comment below. There is one scenario where the Trigger could guarantee the data makes it to both tables, specifically if both tables reside in the same keyspace and have the same partition key(s). Mutations in the same keyspace on tables that have the same partition key are internally to Cassandra merged into a single internal Mutation and always applied atomically. So, if you had an exactly same schema for your second table and it resides in the same keyspace (mytable and mytable_analytics, say, both in mykeyspace) your trigger could duplicate the mutation to the source table to be an exact copy into the second table and Cassandra will apply these both atomically (they both succeed or they both fail - never just one). In this scenario, the analytics team could modify data in the second table and not effect the data in the source table. ---->Brian > On Dec 15, 2020, at 7:38 AM, pauloricardomg <pauloricard...@gmail.com> wrote: > > To extend Paul's point, datacenters in cassandra are logical concepts which > may be useful for your use case and do not necessarily need to be > represented by physical data centers. > > The presentation mentioned by Andrew, while helpful, covers some concepts > which are specific to Hadoop and may be outdated in more recent versions of > Cassandra. > > I'd recommend two more recent presentations on the multi-DC topic: > - > https://www.slideshare.net/DataStax/apache-cassandra-multidatacenter-essentials-julien-anguenot-iland-internet-solutions-c-summit-2016 > - > https://www.slideshare.net/DataStax/operations-consistency-failover-for-multidc-clusters-alexander-dejanovski-the-last-pickle-cassandra-summit-2016 > > Finally, if you have any more questions on this I'd recommend you send them > to the u...@cassandra.apache.org mailing list as this mailing list ( > dev@cassandra.apache.org) is related to the project development of > Cassandra. > >> Em ter., 15 de dez. de 2020 às 09:28, Greg Oliver >> <gol...@microsoft.com.invalid> escreveu: >> >> Can't see it in the email. What's the slide #? >> >> From: Andrew Cobley (Staff) <a.e.cob...@dundee.ac.uk> >> Sent: Tuesday, December 15, 2020 12:26 PM >> To: dev@cassandra.apache.org >> Subject: [EXTERNAL] Re: Triggers >> >> Yes that's right. I remember this illustration: >> >> [Diagram Description automatically generated] >> >> >> From this presentation: >> >> https://www.slideshare.net/rastrick/presentation-12982302< >> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.slideshare.net%2Frastrick%2Fpresentation-12982302&data=04%7C01%7Cgolive%40microsoft.com%7Cb752cef4803d4deae42b08d8a0f4acd5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637436320031889415%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=itM2Kb7eQqOSPO09fGiuHXJPVjW6C51qXCMYq985WaE%3D&reserved=0 >>> >> >> Might help. >> >> Andy >> >> [University of Dundee shield logo]< >> https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fuod.ac.uk%2Fsig-home&data=04%7C01%7Cgolive%40microsoft.com%7Cb752cef4803d4deae42b08d8a0f4acd5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637436320031899372%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=WrafubTszeH9pttSA%2BYVqlfYrcPkTfyIVVZXp7BEq8s%3D&reserved=0 >>> >> >> >> Andy Cobley >> Senior Lecturer, Program Director Data Science and Data Engineering MSc >> School of Science and Engineering, University of Dundee >> +44 (0)1382 385078 (Not at present) | a.e.cob...@dundee.ac.uk<mailto: >> a.e.cob...@dundee.ac.uk> >> [University of Dundee Facebook]< >> 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dev@cassandra.apache.org<mailto:dev@cassandra.apache.org> < >> dev@cassandra.apache.org<mailto:dev@cassandra.apache.org>> >> Subject: Re: Triggers >> Hi Greg, >> >> Andy is talking about Cassandra datacenters, which can easily be co >> located in the same physical datacenter. >> >> Paul >> >>> On 15 Dec 2020, at 12:10, Greg Oliver <gol...@microsoft.com.INVALID >> <mailto:gol...@microsoft.com.INVALID>> wrote: >>> >>> That's great in theory, but what if your customer is a national >> government (they require their data to remain within their borders) and >> there aren't enough DC's in nation to support multiple DC data distribution? >>> >>> To get the throughput needed (say - if the government announces a new >> program and 30M people try to sign up at the same time) CQRS seems a likely >> part of the solution. >>> >>> With Cassandra (and I'm definitely new to it), as I learn more it looks >> like a set of materialized views might be a way to achieve the goal. >>> >>> Thoughts? >>> >>> From: Andrew Cobley (Staff) <a.e.cob...@dundee.ac.uk<mailto: >> a.e.cob...@dundee.ac.uk>> >>> Sent: Tuesday, December 15, 2020 11:57 AM >>> To: dev@cassandra.apache.org<mailto:dev@cassandra.apache.org> >>> Subject: [EXTERNAL] Re: Triggers >>> >>> I may be wrong, but isn't the correct pattern for this to use two data >> centres? You write to one data centre, replicate to the other and read >> from that one. Or am misunderstanding ? >>> >>> Andy >>> >>> >>> [University of Dundee shield logo]< >> https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fuod.ac.uk%2Fsig-home&data=04%7C01%7Cgolive%40microsoft.com%7Ca5f6a2b17aea4b0f1e6508d8a0f0931a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637436302442412850%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=Ixrehte7KrUWOrVJEAEut%2FhQL2E2Ug0aRZbt6nFases%3D&reserved=0 >> < >> https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fuod.ac.uk%2Fsig-home&data=04%7C01%7Cgolive%40microsoft.com%7Cb752cef4803d4deae42b08d8a0f4acd5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637436320031939190%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=nYMTRgqGct928YpEp0ymbzz5u2qhtvvre3MD461qTN8%3D&reserved=0 >>>> >>> >>> >>> Andy Cobley >>> Senior Lecturer, Program Director Data Science and Data Engineering MSc >>> School of Science and Engineering, University of Dundee >>> +44 (0)1382 385078 (Not at present) | a.e.cob...@dundee.ac.uk<mailto: >> a.e.cob...@dundee.ac.uk<mailto:a.e.cob...@dundee.ac.uk% >> 3cmailto:a.e.cob...@dundee.ac.uk>> >>> [University of Dundee Facebook]< >> https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fuod.ac.uk%2Fsig-fb&data=04%7C01%7Cgolive%40microsoft.com%7Ca5f6a2b17aea4b0f1e6508d8a0f0931a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637436302442412850%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=XMZmW3%2BbXmyJaVSDvAWPAe8e2y7ubJw%2BR7p2XBJJXVM%3D&reserved=0 >> < >> 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https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fuod.ac.uk%2Fsig-cvc&data=04%7C01%7Cgolive%40microsoft.com%7Cb752cef4803d4deae42b08d8a0f4acd5%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637436320031979023%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=JGmKjzYxEKUXANjvr2kE06R6hxO3n9lArajgiXsswEg%3D&reserved=0 >>>> >>> >>> >>> From: Benjamin Lerer <benjamin.le...@datastax.com<mailto: >> benjamin.le...@datastax.com<mailto:benjamin.le...@datastax.com% >> 3cmailto:benjamin.le...@datastax.com>>> >>> Date: Tuesday, 15 December 2020 at 11:50 >>> To: dev@cassandra.apache.org<mailto:dev@cassandra.apache.org<mailto: >> dev@cassandra.apache.org%3cmailto:dev@cassandra.apache.org>> < >> dev@cassandra.apache.org<mailto:dev@cassandra.apache.org<mailto: >> dev@cassandra.apache.org%3cmailto:dev@cassandra.apache.org>>> >>> Subject: Re: Triggers >>> Hi Greg, >>> >>> Things are more tricky in an eventually consistent distributed system >> than >>> they are in a relational database. Even if the C* triggers were perfect >>> (and they are not) and your write and read tables were exactly the same, >>> there is no guarantee that all the updates created by the trigger from >> the >>> original mutations will be successfully delivered to your other table and >>> there are no entropy mechanisms to repair those problems. Overtime the >> data >>> in your write and read tables will just start to diverge. >>> >>> On Mon, Dec 14, 2020 at 2:02 PM Greg Oliver <gol...@microsoft.com.invalid >> <mailto:gol...@microsoft.com.invalid<mailto:gol...@microsoft.com.invalid% >> 3cmailto:gol...@microsoft.com.invalid>>> >>> wrote: >>> >>>> Hi all, >>>> >>>> My customer wants to use Cassandra together with the CQRS pattern. This >> is >>>> to say, they want to separate reads and writes to different tables, >>>> potentially in different keyspace or database. >>>> >>>> In my experience with relational databases I would set up a trigger on >> the >>>> "write" table such that on new row & update row events, a similar row >> would >>>> be inserted into the "read" table. >>>> >>>> I found a few examples of setting up a trigger on a Cassandra table and >>>> have replicated that on my system. But in reading the various Stack >>>> Overflow posts on the topic a persistent message saying "don't do it >> unless >>>> you really know what you're doing" pops up. >>>> >>>> Why? What are the cases for and against using triggers in Cassandra? >> What >>>> are the edge cases to avoid? What is the happy path? >>>> >>>> Thanks, >>>> Greg >>>> >>> >>> The University of Dundee is a registered Scottish Charity, No: SC015096 >> >> >> --------------------------------------------------------------------- >> To unsubscribe, e-mail: dev-unsubscr...@cassandra.apache.org<mailto: >> dev-unsubscr...@cassandra.apache.org> >> For additional commands, e-mail: dev-h...@cassandra.apache.org<mailto: >> dev-h...@cassandra.apache.org> >> >> The University of Dundee is a registered Scottish Charity, No: SC015096 >> --------------------------------------------------------------------- To unsubscribe, e-mail: dev-unsubscr...@cassandra.apache.org For additional commands, e-mail: dev-h...@cassandra.apache.org
