boundary/internal/oplog

oplog

oplog is a package for writing operation log (oplog) entries for the purpose of replication and verification of the data stored in the Watchtower RDBMS.

• oplog
  • Usage
  • TBD/TODO
  • oplog entry
  • oplog tables
  • oplog optimistic locking using tickets

Usage

// you must init the ticket in its own transaction.  You only need
// to init a ticket once in the database.  It doesn't need to happen for 
// every connection.  Once it's persistent in the database, you can simply Get it.
initTx := db.Begin()
ticketer := &GormTicketer{Tx: initTx}
err = ticketer.InitTicket("users")
// if there's no error, then commit the initialized ticket
initTx.Commit()

userCreate := oplog_test.TestUser{
  TestUser: oplog_test.TestUser{
    Name: userName,
  },
}
tx := db.Begin()
// write the user to the database
tx.Create(&userCreate)

ticketer = &GormTicketer{Tx: db}

// get a ticket for writing users to the oplog
ticket, err := ticketer.GetTicket("users")

// create an entry for the oplog with the entry's metadata (likely the entry's Scope)
newLogEntry := NewEntry(
  "test-users",
  []Metadata{
    Metadata{
      Key:   "deployment",
      Value: "amex",
    },
    Metadata{
      Key:   "project",
      Value: "central-info-systems",
    },
  },
  cipherer, // wrapping.Wrapper
  ticketer,
)

// write an entry with N messages (variadic parameter) in the order they were sent to the database 
_, err = newLogEntry.WriteEntryWith(
    context.Background(), 
    &GormWriter{tx}, 
    ticket,
    &Message{Message: &userCreate, TypeName: "user", OpType: OpType_CREATE_OP},
)
// if there's an error writing the oplog then roll EVERYTHING back
if err != nil {
    tx.Rollback()
}
// no err means you can commit all the things.
tx.Commit()

TBD/TODO

We need to discuss and decide how Watchtower is going to handle the following oplog things:

• SQL migrations: you'll find the package's SQL migrations under: ./migrations/postgres We need to decide how Watchtower will manage migrations across the system and we will likely need to reference this package's migrations somehow.

oplog entry

  Example Oplog Entry for the Target Aggregate      
┌────────────────────────────────────────────────┐  
│                        FIFO with []byte buffer │  
│                                                │  
│┌─Msg 4────┐┌─Msg 3────┐┌─Msg 2────┐            │  
││          ││          ││          │            │  
││  Tags    ││  Host    ││ HostSet  │            │  
││┌────────┐││┌────────┐││┌────────┐│            │  
│││        ││││        ││││        ││            │  
│││        ││││        ││││        ││            │  
│││  Tag   ││││  Host  ││││HostSet ││            │  
│││protobuf││││protobuf││││protobuf││            │  
│││        ││││        ││││        ││            │  
│││        ││││        ││││        ││            │  
││└────────┘││└────────┘││└────────┘│            │  
││┌────────┐││┌────────┐││┌────────┐│            │  
│││        ││││        ││││        ││            │  
│││typeName││││typeName││││typeName││            │  
│││  Tag   ││││  Host  ││││HostSet ││            │  
││└────────┘││└────────┘││└────────┘│            │  
││┌────────┐││┌────────┐││┌────────┐│            │  
│││        ││││        ││││        ││            │  
│││ OpType ││││ OpType ││││ OpType ││            │  
│││ Create ││││ Create ││││ Create ││            │  
││└────────┘││└────────┘││└────────┘│            │  
│└──────────┘└──────────┘└──────────┘            │  
└────────────────────────────────────────────────┘  

oplog tables

                  oplog tables:                   
      as the diagram shows, we can split the      
      oplog_entries into multiple tables if       
             needed for performance.              
                                                  
┌────────────────┐                                
│┌───────────────┴┐                               
││┌───────────────┴┐            ┌────────────────┐
│││ oplog_entries  │            │  oplog_ticket  │
││├────────────────┤            ├────────────────┤
│││id              │╲           │id              │
│││aggregate_name  │──┼───────┼ │aggregate_name  │
└┤│data            │╱           │version         │
 └┤                │            │                │
  └────────────────┘            └────────────────┘
          ┼                                       
          │                                       
          │                                       
          ┼                                       
         ╱│╲                                      
 ┌────────────────┐                               
 │ oplog_metadata │                               
 ├────────────────┤                               
 │id              │                               
 │entry_id        │                               
 │key             │                               
 │value           │                               
 └────────────────┘                               

oplog optimistic locking using tickets

    Alice's                        Database                                               
  transaction                                                          Bob's transaction  
                                                                                          
     │                                 │                                      │           
     │─────────BEGIN──────────────────▶│                                      │           
     │                                 │◀───────────────BEGIN─────────────────┤           
     │                     ┌───────────┴───────────┐                          │           
     │                     │   ticket version:1    │                          │           
     │                     └───────────┬───────────┘                          │           
     │       Select oplog-ticket for   │                                      │           
     ├──────────────"Target"──────────▶│        Select oplog-ticket for       │           
     │                                 │◀──────────────"Target"───────────────┤           
     │                                 │                                      │           
     │      Write to Tables in         │                                      │           
     ├───────Target Aggregate─────────▶│                                      │           
     │                                 │         Write to Tables in           │           
     │                                 │◀─────────Target Aggregate────────────┤           
     │                                 │                                      │           
     │                                 │                                      │           
     │      Update Ticket              │                                      │           
     ├────Version = 2 where───────────▶│                                      │           
     │       Version = 1     ┌─────────┴─────────┐                            │           
     │                       │ ticket version: 2 │                            │           
     │                       └─────────┬─────────┘                            │           
     │                                 │              Update Ticket           │           
     │                                 │◀───────────Version = 2 where─────────┤           
     │                                 │               Version = 1       ┌────┴──────────┐
     │                                 │                                 │ update failed │
     ├────────────Commit──────────────▶│                                 └────┬──────────┘
     │                                 │                                      │           
     │                                 │                                      │           
     │                                 │                                      │           
     │                                 │◀────────────Rollback─────────────────┤           
     │                                 │                                      │           
     │                                 │                                      │           
     ```