Is something along the lines of nested memoization needed here?
- by Daniel
System.Transactions notoriously escalates transactions involving multiple connections to the same database to the DTC. The module and helper class, ConnectionContext, below are meant to prevent this by ensuring multiple connection requests for the same database return the same connection object. This is, in some sense, memoization, although there are multiple things being memoized and the second is dependent on the first. Is there some way to hide the synchronization and/or mutable state (perhaps using memoization) in this module, or perhaps rewrite it in a more functional style?
(It may be worth nothing that there's no locking when getting the connection by connection string because Transaction.Current is ThreadStatic.)
type ConnectionContext(connection:IDbConnection, ownsConnection) =
member x.Connection = connection
member x.OwnsConnection = ownsConnection
interface IDisposable with
member x.Dispose() = if ownsConnection then connection.Dispose()
module ConnectionManager =
let private _connections = new Dictionary<string, Dictionary<string, IDbConnection>>()
let private getTid (t:Transaction) = t.TransactionInformation.LocalIdentifier
let private removeConnection tid =
let cl = _connections.[tid]
for (KeyValue(_, con)) in cl do
con.Close()
lock _connections (fun () -> _connections.Remove(tid) |> ignore)
let getConnection connectionString (openConnection:(unit -> IDbConnection)) =
match Transaction.Current with
| null -> new ConnectionContext(openConnection(), true)
| current ->
let tid = getTid current
// get connections for the current transaction
let connections =
match _connections.TryGetValue(tid) with
| true, cl -> cl
| false, _ ->
let cl = Dictionary<_,_>()
lock _connections (fun () -> _connections.Add(tid, cl))
cl
// find connection for this connection string
let connection =
match connections.TryGetValue(connectionString) with
| true, con -> con
| false, _ ->
let initial = (connections.Count = 0)
let con = openConnection()
connections.Add(connectionString, con)
// if this is the first connection for this transaction, register connections for cleanup
if initial then
current.TransactionCompleted.Add
(fun args ->
let id = getTid args.Transaction
removeConnection id)
con
new ConnectionContext(connection, false)
