This is the method on an
IConnectionPool that creates a view of all the live nodes in the cluster that the client
knows about. Different connection pool implementations can decide on the view to return, for example,
SingleNodeConnectionPoolis only ever seeded with and hence only knows about one node
StickyConnectionPoolcan return a view of live nodes with the same starting position as the last live node a request was made against
SniffingConnectionPoolreturns a view with a changing starting position that wraps over on each call
CreateView is implemented in a lock free thread safe fashion, meaning each callee gets returned
its own cursor to advance over the internal list of nodes. This to guarantee each request that needs to
fall over tries all the nodes without suffering from noisy neighbours advancing a global cursor.
Here we have setup a Static connection pool seeded with 10 nodes. We force randomization OnStartup to false so that we can test the nodes being returned are in the order we expect them to be.
var uris = Enumerable.Range(9200, NumberOfNodes).Select(p => new Uri("http://localhost:" + p)); var staticPool = new StaticConnectionPool(uris, randomize: false); var sniffingPool = new SniffingConnectionPool(uris, randomize: false); this.AssertCreateView(staticPool); this.AssertCreateView(sniffingPool);
So what order do we expect? Imagine the following:
- Thread A calls
CreateView()first without a local cursor and takes the current value from the internal global cursor, which is
- Thread B calls
CreateView()second without a local cursor and therefore starts at
- After this, each thread should walk the nodes in successive order using their local cursor. For example, Thread A might get 0,1,2,3,5 and thread B will get 1,2,3,4,0.
var startingPositions = Enumerable.Range(0, NumberOfNodes) .Select(i => pool.CreateView().First()) .Select(n => n.Uri.Port) .ToList(); var expectedOrder = Enumerable.Range(9200, NumberOfNodes); startingPositions.Should().ContainInOrder(expectedOrder);
What the above code just proved is that each call to
CreateView() gets assigned the next available node.
Lets up the ante:
NumberOfNodes * 2threads
- On each thread, call
NumberOfNodes * 10times using a local cursor.
We’ll validate that each thread sees all the nodes and that they wrap over, for example, after node 9209 comes 9200 again
var threadedStartPositions = new ConcurrentBag<int>(); var threads = Enumerable.Range(0, 20) .Select(i => CreateThreadCallingCreateView(pool, threadedStartPositions)) .ToList(); foreach (var t in threads) t.Start(); foreach (var t in threads) t.Join();
Each thread reported the first node it started off. Let’s make sure we see each node twice
because we started
NumberOfNodes * 2 threads
var grouped = threadedStartPositions.GroupBy(p => p).ToList(); grouped.Count.Should().Be(NumberOfNodes); grouped.Select(p => p.Count()).Should().OnlyContain(p => p == 2);