In my previous post, I asked about designing a document DB, and brought up the issue of concurrency, along with a set of questions that effect the design of the system:
- What concurrency alternatives do we choose?
We have several options. Optimistic and pessimistic concurrency are the most obvious ones. Merge concurrency, such as the one implemented by Rhino DHT, is another. Note that we also have to handle the case where we have a conflict as a result of replication.
I think that it would make a lot of sense to support optimistic concurrency only. Pessimistic concurrency is a scalability killer in most system. As for conflicts as a result of concurrency, Couch DB handles this using merge concurrency, which may be a good idea after all. We can probably support both of them pretty easily.
It does cause problems with the API, however. A better approach might be to fail reads of documents with multiple versions, and force the user to resolve them using a different API. I am not sure if this is a good idea or a time bomb. Maybe returning the latest as well as a flag that indicate that there is a conflict? That would allow you to ignore the issue.
- What about versioning?
In addition to the Document ID, each document will have an associated version. The Document Id is a UUID, which means that it can be generated at the client side. Each document is also versioned by the server accepting it. The version syntax follow the following format: [server guid]/[increasing numeric id]/[time].
That will ensure global uniqueness, as well as giving us all the information that we need for the document version.
More posts in "Designing a document database" series:
- (17 Mar 2009) What next?
- (16 Mar 2009) Remote API & Public API
- (16 Mar 2009) Looking at views
- (15 Mar 2009) View syntax
- (14 Mar 2009) Aggregation Recalculating
- (13 Mar 2009) Aggregation
- (12 Mar 2009) Views
- (11 Mar 2009) Replication
- (11 Mar 2009) Attachments
- (10 Mar 2009) Authorization
- (10 Mar 2009) Concurrency
- (10 Mar 2009) Scale
- (10 Mar 2009) Storage