Consistency

Consistency

1. Update consistency

   • Consistency maintenance methods
     • Optimistic
       • Detects conflicts
       • Solves conflicts
     • Pessimistic
       • Prevents conflicts
       • Could generate deadlocks
   • Write-write conflict
     • Several users updating
       • Same data
       • At the same time

2. Read consistency

   • Read-Write conflict
     • User reads while another user writes
   • Logical consistency
     • Solution
       • Transactions
       • Aggregates
   • Read-your-writes consistency
     • Upgrade commitment
   • Session consistency
     • Read-your-writes consistency in session
   • Replication consistency
     • Reading of
       • Same data
       • Same value
       • Different replicas

3. Quorums

   • More nodes to service a request
     • Reduces inconsistence

4. Relaxing Durability

   • Use of in-memory
   • Occasional writing to disk
   • Agile management of requests

5. Relaxing consistency

   • CAP theorem
     • Properties
       • Partition tolerance
         • Cluster
           • Still working after deterioration
       • Consistency
       • Availability
         • Sure response from node
     • Only two properties